Abstract. The lines of research in the field of inorganic chemistryThe lines of research in the field of inorganic chemistry
that are closely related to the design of novel functional materialsthat are closely related to the design of novel functional materials
are analysed. Special attention is given to areas of inorganicare analysed. Special attention is given to areas of inorganic
materials chemistry that have been developed most intensely atmaterials chemistry that have been developed most intensely at
the Division of Inorganic Chemistry of the Department ofthe Division of Inorganic Chemistry of the Department of
Chemistry and at the Department of Materials Science of theChemistry and at the Department of Materials Science of the
Moscow State University over the past decade. The bibliographyMoscow State University over the past decade. The bibliography
includes 135 references.includes 135 references.
I. Introduction
Much of the actual knowledge which constitutes the basis of
modern inorganic chemistry was obtained rather long ago; how-
ever, in the second half of the 20th century inorganic chemistry
faded into the background of extraordinary achievements in
organic chemistry and the chemistry of living systems. The
progress in electronic engineering,1 photonics,2 sensorics 3 and
spintronics,4 which is impossible without the development of new
generations of functional materials, has resulted in a kind of
renaissance in inorganic chemistry in the past 10  15 years.
An active search for ways of utilisation of virtually all
chemical elements systematised as the Periodic Table aimed at
the development of novel materials and technologies is under way.
The increasing applications of the most recently discovered
chemical elements, e.g., rhenium, technetium and francium, to
say nothing of plutonium, americium and other actinides, provide
illustrative examples.
The transition from chemical elements to materials is
extremely complicated, and even the development of methods of
combinatorial chemistry 5, 6 does not allow one to master a multi-
tude of combinations of the elements. Just for elements having
stable isotopes, the number of such combinations exceeds 761023.
This increases by many orders of magnitude considering that the
majority of the existing functional materials are developed on the
basis of metastable (rather than thermodynamically stable) states
of substances which are indefinably numerous for each particular
compound with a fixed composition.
Obviously, the method of random enumeration of composi-
tions is inefficient in the design of novel materials, and a
significantly more pronounced effect is produced owing to the
use of various approaches based, in particular, on the progress in
inorganic chemistry. Despite the well-established traditions in the
development of inorganic chemistry in Russia, the range of
research in the field of inorganic synthesis in our country has
recently narrowed considerably due to the limited experimental
facilities of many research groups wich have no access to modern
high-precision equipment, such as electron microscopes, synchro-
tron radiation sources, squid magnetometers, high-resolution
NMR spectrometers, etc. This is all the more distressing taking
into account the outstanding contribution of Russian scientists to
the development of modern inorganic chemistry. Suffice it to
mention the investigations by D I Mendeleev, N S Kurnakov,
L A Chugaev and I I Chernyaev and more recent studies by
A V Novoselova, I V Tananaev, V I Spitsyn and many others.
At present, the inorganic chemistry of materials is being
developed especially intensively at such research centres as the
N S Kurnakov Institute of General and Inorganic Chemistry of
the RAS (Moscow), the A V Nikolaev Institute of Inorganic
Chemistry of the Siberian Branch of the RAS (Novosibirsk), the
Institute of Solid State Chemistry of the Urals Branch of the RAS
(Ekaterinburg), the I V Grebenshchikov Institute of Silicate
Chemistry of the RAS (Saint-Petersburg) as well as by divisions
of inorganic chemistry of the largest classical and technical
Russian universities including the M V Lomonosov Moscow
State University (MSU).
In writing the present review and other reviews published in
this issue of the Journal, the authors did not set themselves the
task encompassing all the trends in contemporary inorganic
chemistry, which is impossible in principle. They have consciously
confined the range of scientific problems to those directly related
to the design of novel functional materials and appropriate to
their own research experience gained at the Division of Inorganic
Chemistry of the Department of Chemistry and the Department
of Materials Science of MSU over the past 10  15 years.
It may be hoped that despite the inevitable subjectivity, the
reviews presented in this issue will provide an insight into modern
Yu D Tretyakov Department of Chemistry and Department of Materials
Science, M V Lomonosov Moscow State University, Leninskie Gory,
119992 Moscow, Russian Federation. Fax (7-095) 939 09 98.
Tel. (7-095) 939 20 74. E-mail: yudt@inorg.chem.msu.ru
Received 9 March 2004
Uspekhi Khimii 73 (9) 899  916 (2004); translated by R L Birnova
DOI 10.1070/RC2004v073n09ABEH000914
Development of inorganic chemistry as a fundamental for the design of
new generations of functional materials
Yu D Tretyakov
Contents
I. Introduction 831
II. Chemistry of ionic and molecular precursors 832
III. Crystal chemical design of novel inorganic compounds and preparation of materials based on them 834
IV. Inorganic nano- and supramolecular chemistry 835
V. Chemistry of compounds with variable compositions 836
VI. Chemistry of elements in unusual oxidation states 837
VII. Chemistry of inorganic biomaterials 838
VIII. Inorganic synthesis of functional materials using sonication, hydrothermal treatment and microwave irradiation 841
IX. Conclusion 844
Russian Chemical Reviews 73 (9) 831  846 (2004) # 2004 Russian Academy of Sciences and Turpion Ltd
inorganic chemistry as a fundamental for the design of new
generations of functional materials. The main research trends
touched upon in the present review are as follows:
 chemistry of ionic and molecular precursors;
 crystal chemical design of novel inorganic compounds and
design of materials based on them;
 inorganic nano- and supramolecular chemistry;
 chemistry of non-stoichiometric compounds;
 chemistry of elements in unusual oxidation states;
 inorganic chemistry of biomaterials and implants;
 inorganic synthesis of functional materials using ultra-
sonic, hydrothermal and microwave methods.
II. Chemistry of ionic and molecular precursors
The design of materials with a definite range of structure-depend-
ent properties on the basis of chemical compounds (rather than
merely the synthesis of chemical compounds) is the ultimate goal
for a wide range of inorganic chemists engaged in the development
and perfection of functional materials. Such properties depend on
the real structure of a material beginning with the structure of the
unit cell and ending with the peculiarities of polycrystalline
(ceramic) structures with their characteristic defects (dislocations,
crystallite boundaries, pores and cracks). Different structural
levels are mutually related. The possibility of control over such
relationships is considered in reviews 7, 8 within the framework of
approaches of non-linear dynamics. The latter can be used for the
description of the evolution of physicochemical systems under
non-equilibrium conditions determined by specific interactions of
these systems with the environment. It has been shown 9 that some
factors, among which the unambiguity and reproducibility of the
original states of chemical precursors are critical, should be taken
into consideration in the optimisation of the reaction zone for the
synthesis of materials. The more complex are the systems the more
difficult this is to attain. Here, we define the complexity of a
system as both its chemical and phase compositions and activities
of its precursors determined by their chemical and thermal
prehistories, i.e., the effect of `topochemical memory'.10
If a material, which is the target product of chemical and phase
conversions, is multicomponent, additional requirements for
homogeneity and ability to preserve it upon transition to the
target product are imposed on its precursors. It is for this reason
that solid-phase synthesis, which seems to be technologically the
simplest and has been widely employed previously for the manu-
facture of magnetic dielectrics and piezoceramics, is fairly ineffec-
tive in the preparation of many functional materials. Thus the
synthesis of manganese  zinc ferrites with high magnetic perme-
ability was carried out by the reaction:
1/3 MnCO3 + 2/3 ZnO + Fe2O3 = Mn0.33Zn0.67Fe2O4 + 1/3 CO2.
It was found, however, that in many cases the combination of
traditional processes, such as mechanical blending, compaction of
precursor mixtures and high-temperature annealing (*1200 8C),
does not allow preparation of a single-phase product. An increase
in temperature results in inevitable losses of volatile zinc oxide and
dissociation of iron(III) oxide to form magnetite. These processes
significantly deteriorate the composition and properties of the
product due to considerable magnetic and dielectric losses,
especially at ultrahigh frequencies. Repeated grinding and heating
at constant temperatures lead to further contamination of the
product with the mill materials, which markedly decreases the
magnetic permeability of the target ferrites.
The problem of reproducibility and enhancement of func-
tional properties of solid-phase synthetic products can be over-
come by combining the solid-phase synthesis and thermolysis. In
this case, mechanical mixtures or co-precipitated salts of various
oxoacids 11 including sulfates, carbonates, nitrates, oxalates, ace-
tates, citrates, formates and stearates are used as chemical
precursors. The use of salt solid solutions obtained under equili-
brium or non-equilibrium conditions was next step forward. It has
been found 12, 13 that schoenite-type salts of the general formula
M2+A+(RO4)2
. 6 H2O (M = Mg, Mn, Ni, Co, Fe, Cu, Zn; A is an
alkali metal or NH4; R = S, Se, Te, Cr) or alum-type salts
A+M3+(SO4)2
. H2O (where A is an alkali element or NH4;
M = Fe, Cr, Co) are the most convenient for equilibrium crystal-
lisation.
The ability to form continuous solid solutions at any ratios of
the reaction components and good solubility in water, which
decreases drastically upon cooling, were used as a basis for the
development of original methods for the preparation of highly
homogenous isomorphic salt mixtures. These methods are based
on isoconcentration evaporation of the solvent or isoconcentra-
tion elimination of micro-oversaturation of the solution.
The practical implementation of these processes has become
possible due to extensive studies 12 of equilibrium diagrams of
ternary (salt 1  salt 2  water) and more complex systems and
determination of thermodynamic distribution coefficients of salt
components between co-existing liquid and solid phases aimed at
selecting the conditions for equilibrium crystallisation of salt solid
solutions the cationic compositions of which are identical to those
of the ferrites to be synthesised. The transition from the salt
precursor to the target product occurs upon thermolysis using a
reaction of the type:
(Ni1/3Fe2/3)SO4
. (NH4)2SO4
. 6 H2O =
= 1/3 NiFe2O4 + 2 SO2 + 2 NH3 + 3/2 O2 + 7 H2O.
The conditions of this reaction predetermine, to a large extent, the
feasibility of preparation of a polycrystalline material with a
preset ceramic structure and structure-dependent properties.
The cryochemical method may be regarded as a versatile
procedure for the preparation of salt-like multicomponent pre-
cursors. The cryochemical technology 13  15 is based on a balanced
combination of heating and cooling processes, the latter being
used to prevent the uncontrollable changes in intermediate and
target products (Fig. 1). Rapid freezing (e.g., in liquid nitrogen) of
solutions containing metal salts as constituents of materials to be
synthesised and dispersed in the form of mono- or polydispersed
drops is a crucial step in the cryochemical technology. Frozen
Cryocrystallisation
Dispersion of solutions
Preparation of solutions
Cryoimpregnation
Moulding, sintering, thermal treatment
Dehydration of
the salt product
Freeze-dryingCryoextraction
Cryogrinding
Cryo-
precipitation
Thermal decomposition
Figure 1. Processes of cryochemical technology.
832 Yu D Tretyakov
solutions of salts in the form of cryogranules are usually subject to
freeze-drying; its optimum conditions provide the preparation of
highly homogenous salt precursors. The rate of cryocrystallisation
exceeds that of solid phase formation by several orders of
magnitude when other known non-equilibrium methods (e.g.,
solvent replacement by desalting from aqueous solutions with
non-aqueous solvents, such as acetone, ethanol or methanol) are
used for the preparation of salt precursors.
The wide application of cryochemical technology was pre-
ceded by fundamental investigations of cryocrystallisation, freeze-
drying, cryoprecipitation, cryoextraction, cryodispersion and
some original cryocalorimetric procedures.13  16 The cryochem-
ical synthesis of salt precursors underlies the preparation of
various functional materials (Fig. 2). Dispersed powders prepared
by thermal decomposition of salt precursors are distinguished by
microplasticity, high reactivity and sintering ability in addition to
their chemical homogeneities.
The so-called topochemical memory, i.e., the ability of solid-
phase chemical conversion products to store (`to remember')
information about their origin and to transfer it upon subsequent
topochemical reactions, is a characteristic feature of salt-like ionic
precursors. A real structure, which is usually formed under non-
equilibrium conditions and affects significantly the properties of
functional materials obtained as polycrystalline ceramics, seems
to be the most probable candidate for `storing' and `delivering'
information in solids.10
Studies related to the chemistry of molecular precursors are
extremely important for the design of thin-film materials by
molecular deposition. In order to solve the global problem of the
increase in the density of information recording in silicon chips, it
is necessary to replace the `natural' oxide (SiO2) by oxides with
higher dielectric constants. The most probable candidates are
HfO2, ZrO2 and Ln2O3. Preparation of films of such oxides on the
surface of silicon by MOCVD (Metal-Organic Chemical Vapour
Deposition) requires development of molecular precursors pos-
ssessing high volatilities and relative thermal stabilities as well as
abilities to undergo hydrolysis, pyrohydrolysis or other decom-
position reactions leading to the formation of amorphous oxides
on the surface of silica. The main idea realised at the Laboratory
of Coordination Chemistry of MSU 17 consists of the use of novel
molecular precursors based on metal complexes with complete
saturation of the coordination sphere with ligands that form
relatively weak donor  acceptor bonds. Saturation (and, as a
consequence, volatility) was provided by the introduction of an
additional neutral ligand (e.g., phenanthroline); charged ligands
were chosen on the basis of quantum-chemical calculations of the
energies of the metal  ligand bonds.
Complexes with organic ligands were also successfully used
for the solution of yet another vital task, viz., preparation of
thermoprotective (barrier) coatings for gas-turbine unit blades. It
was shown 18 that deposition of coatings from vapours of b-di-
ketonates of appropriate metals using ZrO2 (Y2O3) is the most
efficient approach which allows radical reduction of the produc-
tion cost owing to the use of inexpensive acetylacetonates. As for
the problem of insufficient volatility of polymeric Y(acac)3, it is
solved by co-evaporation with volatile Zr(acac)4, its small por-
tions being fed gradually into the evaporator. The well-known
synergistic effect is observed, i.e., joint evaporation is more
complete than separate.
A series of novel REE mixed-ligand complexes including
Tb(III) complexes with some aromatic acid derivatives to be used
as luminescent materials for an organic light emitting diode
(OLED) have been synthesised. An OLED represents a multilayer
unit constructed using planar technology. The material of the
working layer should not only possess bright electroluminescence,
but also be temperature-resistant (since OLED is spontaneously
heated during operation) and form smooth films upon deposition
on the surface of the substrate, i.e., be readily soluble in organic
solvents, to moisten the surface of the anodic layer and be resistant
to cracking. Such luminescent materials are prepared by mixed-
ligand complex formation using neutral ligands.
Spectral characteristics of the terbium salicylate complex
Tb(sal)3
. H2O with triphenylphosphine (TPPO) obtained by the
reaction
are shown in Fig. 3 as an example.
Terbium salicylate is stable at temperatures up to 120 8C,
whereas the complex Tb(sal)3
. (TPPO)2 retains stability up to
Tb(sal)3
. H2O + TPPO Tb(sal)3
. (TPPO)2
EtOH  C6H6
Sorbents
Pigments
Ferrites
Magnetic chalco-
spinel-type
semiconductors
Catalysts
Cryo-
chemical
technology
Piezomaterials
Electrode
materials Porous
ceramics
Solid
electrolytes
Resistive
materials
Composites
(including
dispersoids)
Powders for glass
manufacturing and
single crystal
growing
Pharmaceuticals
Chemical reagents
Figure 2. The main products prepared by cryochemical technology.
450 500 550 600 l /nm
L (rel.u.)
1.2
0.8
0.4
0.0
a
2
1
480 520 560 600 l /nm
L (rel.u.)
0.8
0.4
0.0
b
0 8 16 24 U /V
c
0.8
0.4
0.0
j /mA cm72
1
2
Figure 3. Spectral characteristics of Tb(sal)3
. H2O, Tb(sal)3
. (TPPO)2 (a) and OLED based on Tb(sal)3
. (TPPO)2 (b, c).
(a) Luminescence spectra of Tb(sal)3
. H2O (1) and Tb(sal)3
. (TPPO)2 (2); (b) photo- (1) and electroluminescence (2) spectra of OLED; (c) voltammetric
characteristics of OLED (the luminescence zone is shown in grey).
Development of inorganic chemistry as a fundamental for the design of new generations of functional materials 833
200 8C, the surface roughness of the former being 66 nm, that of
the latter being 9 nm.
One of the latest successful investigations related to molecular
precursors was carried out by Romanov et al.19 These authors
have synthesised a mixed-ligand complex niobium isopropoxide 
dipivaloylmethanate and used it, along with dipivaloylmethanate
potassium tert-butoxide, for the preparation, by the MOCVD
method, of epitaxial films of potassium niobate KNbO3 charac-
terised by extremely high values of non-linear optical and piezo-
electric constants.
III. Crystal chemical design of novel inorganic
compounds and preparation of materials based on
them
Antipov and co-workers 20 have developed a concept of inter-
growth structures and used it in a search for novel oxide materials.
This was achieved through the use of structural design which had
proved to be efficient in the preparation of novel high-temperature
superconductors based on complex copper oxides. The latter
contain structures derived from perovskite (ABO3) or a perov-
skite-like fragment as one of the structural components of inter-
growth structures. The prediction and synthesis of novel
superconductors were carried out by selecting various intergrowth
structures. Such a synthesis is, to a certain extent, similar to that of
organic compounds; however, its realisation demands an analysis
of infinite two-dimensional layers forming structural units rather
than of isolated groups of atoms as in the case of organic
molecules.
The geometric commensurability of structurally different
fragments in the plane ab is a prerequisite for the formation of
intergrowth structures. By varying the cationic compositions of
the units and the formal degree of oxidation of copper ions, one
can vary the geometrical sizes of the units and obtain the desired
compounds using an appropriate synthetic procedure. In predict-
ing the compositions of intergrowth structures, attention should
be given to crystal chemical characteristics of the cations, their
chemical similarities and electron neutrality of the elementary cell
as a whole. The choice of a synthetic procedure is determined by
the chemical peculiarities of the starting compounds and the
degree of oxidation of the cations in the target structure.
This approach was used for the preparation of more than 100
novel complex copper oxides among which about 30 oxides
possess superconducting properties.21
The search for novel superconductors was carried out among
compounds comprising an anion-deficient perovskite-like struc-
ture, viz., An(Cu,M)nO3n7d, and a series of novel oxides was
obtained. The `structural substitution' approach was also used in
the synthesis of novel compounds where prototypes with a layer
sequence corresponding to the formula (CuO2)(M&)(CuO2)
served as the basis of the structure. In the structures of novel
compounds, this fragment was rearranged into the sequence
(CuO2)(A2O2)(CuO2) the (A2O2) block in which has the structure
of fluorite.
Crystal chemical simulation has made it possible to predict the
existence and to synthesise a new family of superconductors, viz.,
HgBa2Can71CunO2n+2+d with record values of Tc. The
transition temperatures for superconduction of the phases
HgBa2Can71CunO2n+2+d depend on the content of oxygen (d)
and the number of CuO2 layers (n). A progressive increase in Tc on
going from the first member of the homologous family (97 K) to
the second (127 K) and third (135 K) members is followed by its
decrease for the fourth (127 K), fifth (110 K) and sixth (107 K)
members. The Tc value for HgBa2Ca2Cu3O8+d at 23.5 GPa was
found to be equal to 157 K. The possibility, in principle, of a
transition into the superconducting state at Tc = 150  160 K at
atmospheric pressure has been demonstrated for compounds with
the Cu7O distances corresponding to those in the
HgBa2Can71CunO2n+2+d complexes under external pressure.
The fact that the anionic sublattice may contain fluorine atoms
along with oxygen atoms significantly expands the area of a search
for Cu-containing oxofluoride superconductors. Xenon difluor-
ide, which was employed for the first time as a mild fluorinating
agent by Abakumov,22 has a number of advantages over tradi-
tional fluorine sources. First, XeF2 is solid at room temperature,
which permits its precise quantification to attain a required level
of fluorination and desired copper oxidation state; second, XeF2 is
less aggressive than gaseous fluorine, which excludes the use of
sophisticated equipment and, third, no reaction products that
could contaminate the target material are formed from XeF2.
Altogether, more than twenty novel cuprates including
superconductors, such as YBa2Cu3O6F2 (Tc = 94 K) and
Y2Ba4Cu7O14F2 (Tc = 62 K), have been synthesised. Special
mention should be made of the synthesis of HgBa2Ca2Cu3O8F0.32;
here, the introduction of fluorine caused anisotropic chemical
compression and a record-high elevation of Tc (138 K).
The method of structural design was successfully used for the
simulation of structures of novel complex manganites that some-
times possess colossal magnetoresistance.23
Epitaxial stabilisation of unstable oxide phases in the form of
thin films developed by Kaul et al.24 is a new in principle approach
to the design of novel materials. Based on the results of thermody-
namic simulation, direct syntheses of the novel phases RBO3
(R = REE; B = Ni, Co, Mn, Fe, In) have been carried out. Some
of them are ferroelectrics (RFeO3 and RMnO3); in others (RNiO3,
RCoO3), a transition `metal  insulator' is possible; (R3Fe5O12) is
a magnetic material. It was found that epitaxial stabilisation can
successfully be used in the technology of preparation of thin-film
materials for microelectronics. This technology was used as a basis
for realisation of the local epitaxy principle during photolitho-
graphic formation of planar heterostructures.
A detailed study of epitaxial ratios of various structural types
has made it possible to significantly expand the potential of
heteroepitaxy. This method was used,24 in particular, for the
design of nanodomain epitaxial structures based on REE man-
ganites characterised by high tunnel magnetoresistance values. It
was shown that nanodomain epitaxial structures are formed as a
result of self-organisation of a system that is crystallised at the
nanolevel. Provided the energetically identical heteroepitaxial
approaches exist, these structures can be used as a basis for the
manufacture of inexpensive, highly sensitive magnetic field sen-
sors.
The fundamental physicochemical problems related to the
design of HTSC materials based on nonstoichiometric phases
R1+xBa27xCu3Oz with predetermined compositions, homogene-
ities and structures conferring desired structure-dependent prop-
erties, are considered in recent reviews.25, 26 TTT diagrams
(Time  Temperature  Transformation) have been developed for
the first time and the efficiency of their use in the design of HTSC
materials has been demonstrated based on the results of system-
atic studies of temperature- and time-dependent changes in these
phases.
The properties of manganites manifesting colossal magneto-
resistance, which make them promising components of various
magnetic sensors and novel magnetic information-recording devi-
ces, depend critically on structural parameters. Compositions
with extremely high sensitivities to the magnetic field, light and
other factors including changes in the isotopic composition have
been obtained for the first time.27, 28 Optimisation of chemical
compositions of manganites was carried out with due regard to the
results of physical simulation of electron transfer processes. The
phases (La17xPrx)0.7CaO3MnO3 in the form of ceramics and thin
epitaxial films have been studied in detail. It was found that
unusually high sensitivities of electric resistances of critical com-
positions to the light and electric and magnetic fields are related to
the competition between the metallic and charge-ordered states.
The material synthesised by Babushkina et al. 28 (x = 0.75)
manifests an anomalously high isotope effect upon the substitu-
tion of 18O for 16O and a transition from the metal-type con-
834 Yu D Tretyakov
ductivity to a dielectric state. The magnetoresistances of film
materials depend on elastic stresses generated upon their epitaxial
fusion with the substrate. Critical compositions of the system
R17xSrxMnO3 are characterised by extremely high magneto-
striction concomitant with a strong magnetocalorific effect. Such
phases present considerable interest for the design of magnetic and
mechanical sensors and Freon-free refrigerating plants having
high coefficients of efficiency.
Needless to say, the ideology of the construction of functional
materials is not confined to a crystal chemical design of novel
inorganic compounds. Suffice it to remember the system of
physicochemical principles formulated by the author of the
present review rather long ago 29 and successfully used for the
optimisation of properties of many functional materials, such as
ferrites, magnetic semiconductors based on chalcospinels, elec-
tronic-ionic conductors of the oxide bronze type, complex silver
chalcogenides and lanthanoidates, ceramic electrolytes with uni-
polar conductivities with respect to anions, cations and hydrogen
as well as piezoceramic materials and flexible piezocomposites
based on them.30 This system comprises the following principles:
periodicity (properties of compounds of elements arranged
according to the increase in their atomic numbers change periodi-
cally);
physicochemical analysis (principles of continuity, conformity
and compatibility of components of an equilibrium system);
limitation of the number of independent parameters of states
(among the multitude of parameters of state that characterise an
equilibrium system comprising material-forming solid phases,
only very few are independent according to the Gibbs' phase rule);
disordering and variability of composition (diverse defects are
formed in crystalline lattices of solids at any temperature different
from the absolute zero, while any solid-phase compounds with
non-molecular types of bonds have variable compositions);
complication of composition (properties of a system change as a
result of its chemical, structural and phase complication);
homogeneity (properties of solid-phase materials are sensitive
to the degree of their homogeneities at both micro- and macro-
levels);
equivalence of disorder sources (under equilibrium conditions,
materials acquire specific defects which ensure a maximum
increase in the entropy at minimum energy expenditures);
similarity of responses to physicochemical effects (necessary
changes in the properties of materials can be achieved under the
influence of various physicochemical factors);
nonequivalence of volume and surface;
metastable diversity (the number of non-equilibrium states of
materials is indefinitely large; these states can be kinetically stable
in solid phases).
The aforementioned principles constitute the system which
defines possible approaches to the solution of material-science
problems from the point of view of a chemist. Initially, no
preference to these principles can be given, they represent a
specific set of alternatives. However, when a specific problem is
concerned, a decisive (key) principle is isolated from the system for
a particular step by analysis of physicochemical conditions and
used for the solution of problems aimed at the practical realisation
of this principle. The next step towards the solution of the entire
problem entails return to the system of principles, isolation and
realisation of the next alternative, and so on.
IV. Inorganic nano- and supramolecular chemistry
In 2000, a national programme for investigations and develop-
ments in the field of nanotechnologies which has the name `The
National Nanotechnology Initiative' was set up in the USA. In the
2001 budget, the financing for its realisation constituted $500 M,
i.e., significantly larger than, for example, the sum allocated for
investigations in the field of high-temperature superconductivity
immediately after its discovery in 1986. This is not surprising,
however, since it is with nanotechnology that the next industrial
revolution is associated. Amazingly, many research groups in
Russia manage to carry on successful research in the field of
nanomaterials and nanotechnologies with financial support which
is three orders of magnitude smaller than that in the USA.
The interest in nanomaterials is associated with their unusual
physical properties which are not characteristic of bulk materials.
For example, a decrease in the size of semiconductor particles
below a certain critical level can lead to the change in the width of a
band gap, the generation of a second harmonic and dimensional
quantisation of energy levels. At the same time, it is known that
the predisposition of free nanoparticles to aggregation restricts
their practical application. In this context, the development of
methods for preparation of nanocomposites, i.e., nanoparticles
incorporated into an inert matrix, which protects the particles
from environmental factors and thus prevents their aggregation,
appears to be of crucial importance.
A promising method of matrix isolation of nanoparticles is
based on chemical modification of lamellar double hydroxides
(LDH)31 with the general formula M2
1xM3
x(OH)2[(Xn7)x/n
.
m H2O]. These compounds are made up of positively charged
hydroxide layers M2
1xM3
x(OH)x
2 and interstitial anions Xn7.
The method for the synthesis of nanocomposites is related to the
chemical modification of these anions. The reaction zone is
surrounded by hydroxide layers, so the conditions for the syn-
thesis of the nanophase are similar to those observed in a two-
dimensional nanoreactor.
The unique characteristics of lamellar double hydroxides open
up broad opportunities for the design of nanocomposite materials
based on them. Magnetic (based on metallic iron, cobalt and
nickel) and semiconducting (based on MS, M = Pb, Zn, Cd)
nanomaterials prepared from anion- and cation-substituted
hydrotalcite, Mg17xAlx(OH)2[(CO3)x/2
. m H2O], using the
above-mentioned method were the subjects of investigation.32  35
Some of the factors influencing the structures and properties of
nanocomposites synthesised have been established. Thus the
dependences of the composition, size distribution, morphology
and anisotropy of the nanostructures on the structure and
composition of LDH used as a lamellar matrix were revealed.
Conditions for chemical modification of substituted LDH also
have a pronounced effect on the composition and structure of the
nanophase formed inside the matrix.
In addition to LDH, a family of silica-based mesoporous
molecular sieves has been recommended as a template for the
synthesis of nanocomposite materials with a nanophase endowed
with predetermined properties.36 The structure of silica is charac-
terised by the presence of a highly ordered system of pores with the
diameters ranging from 20 to 100 #A. It is of note that such specific
features of the aforementioned mesoporous materials as uniform
size distribution of pores, extremely high specific surface
(*1000 m2 g71) and broad variability of a pore size, make them
especially attractive for investigators. By virtue of its ordered
structure, mesoporous SiO2 can be used as a template in the
synthesis of metal and semiconductor nanoparticles as well as
polyaniline and carbon fibres.
Currently, the main attention of specialists in inorganic supra-
molecular chemistry is focused on the design of supramolecular
ensembles manifesting specific properties. Of considerable interest
are ideally ordered ensembles, their structure being determined by
the structure of the backbone of the `host' (receptor), while the
functional properties are determined by the nature of the `guest'
(substrate). The diversity of physical properties is due to structural
peculiarities of the `guest' structures including magnetic ordering
of d1-, d3- and d9-centres, spin transitions in d6-centres, electron
transitions with valence fluctuations and dynamic effects.
The properties related to dynamic effects are employed in the
design of novel thermoelectrical materials. Semiconducting clath-
rates provide an example of compounds that are studied to this
end.37 The crystalline structure of a clathrate includes the back-
bone of the `host' with `guest' atoms with the coordination
numbers of 20 and 24 in its cavities. The `guest' atoms are not
Development of inorganic chemistry as a fundamental for the design of new generations of functional materials 835
bound to the `host' backbone by covalent bonds and oscillate
inside the cage with frequencies ensuring effective scattering of
heat-conducting phonons. This results in a very low heat con-
ductivity (as in the case of glasses). At the same time, the
covalently bound backbone ensures high concentrations and
mobilities of charge carriers, which accounts for high electro-
conductivity. A combination of these two features makes clath-
rates promising compounds for the design of effective
thermoelectric materials. It has been found 38 that abnormal
magnetisation and thermal capacities of tin-containing clathrates
observed at *250 K are not associated with ordinary phase
transitions. These data point to the possibility, in principle, of
independent optimisation of electro- and heat-conducting proper-
ties determined by the structure of the backbone and the dynamics
of guest molecules, respectively.
A new trend in inorganic supramolecular chemistry is related
to controlled self-assembly of ordered supramolecular ensembles
under conditions of high-temperature reactions where reaction
systems are complex and heterophasic and are not subject to
analysis. Such processes are based on the mutual adjustment of the
`host' and the `guest' where the `guest' is a template for the
formation of a definite topology and geometry of the `host'
backbone and it itself changes its parameters in accordance with
the `host' requirements. It has been found 39 that weak `guest 
host' interactions are decisive in such self-assembly. Being limited
by van der Waals forces, these interactions control the reciprocal
arrangement of the `guests' and the particles forming the `host'
backbone in the first step. If the `guest' and the `host' match each
other, the formation of ideally ordered supramolecular ensembles
takes place where hosts are recognised by `guests' of various
compositions and occupy their predetermined specific sites. As an
illustration, Fig. 4 shows the structure [Hg6P4](TiCl6)Cl formed as
a result of self-assembly of TiCl3
6 and Cl7 in the cavities of the
backbone of [Hg6P4]4+.
V. Chemistry of compounds with variable
compositions
The formation of chemical compounds with variable composi-
tions, which have the name `berthollides', was discovered over 100
years ago by N S Kurnakov who studied silicate and metallic
systems. Later, it was found that compositional variability is an
intrinsic property of inorganic solid-phase compounds with ionic
bonds predominating. Depending on the nature of the elements
constituting inorganic compounds, the area of their existence is
either very broad (e.g., titanium and iron monoxides) or, vice
versa, extremely narrow (e.g., alkali metal halides, zinc, lead and
cadmium chalcogenides). In any case, a deviation from stoichi-
ometry is a critical factor in the formation of binary and more
complex crystals.
Using statistical thermodynamics methods, Schottky and
Wagner 40 have established a correlation between the defects of
crystal lattices of inorganic compounds and their non-stoichio-
metries and proved the inevitable existence of the latter in any
ionic crystal. Minor deviations from stoichiometry are usually
described within the framework of disordering models with
formation of non-interacting vacancy-type point defects, incorpo-
rated atoms or antistructural defects. Defect formation is accom-
panied by pronounced changes in electrical, magnetic, optical,
thermal and mechanical properties of solid-phase compounds;
therefore, an analysis of their non-stoichiometries is one of the
most important directions in inorganic chemistry of solids.41
The development of methods for coulometric titration in
reversible electrochemical circuits with solid electrolytes possess-
ing ionic (anionic or cationic) conductivity 42 has led to consid-
erable progress in the study of non-stoichiometry, in particular of
compounds manifesting a narrow range of homogeneity. Quasi-
chemical models of defect formation in inorganic compounds
were successfully employed for the description of functional
properties of semiconductors and ferrites. For example, the results
of studies of non-stoichiometry as a function of oxygen pressure
and temperature using a quasichemical approximation have
provided a clue to the solution of problems related to directed
doping of ferrites.43
Defect formation in ferrospinels MFe2O4 can be described by
the following system of equations:
O eH
+ h
.
, K1 = np,
Fex
Fe Fei
...
+ V HHH
Fe, K2 = [Fei
...
] [V HHH
Fe],
Mx
M Mi
..
+ V HH
M, K3 = [Mi
..
] [V HH
M],
Mx
M + Fex
Fe MH
Fe + FeM
.
, K4 =[MH
Fe] [FeM
.
],
O V HH
M + 2 V HHH
Fe + 4 VO
..
, K5 = [V HH
M] [V HHH
Fe]2 [V O
..
]4,
Ox
O 1/2 O2 + V O
..
+ 2 eH
, K6 = [V O
..
] n2p
1a2
O2
,
Fe2O3 2 Fex
Fe + 3 Ox
O + VO
..
+ V HH
M, K7 = [V O
..
] [V HH
M] a1
Fe2O3
,
3 [Fei
...
] + 2 [Mi
..
] +[FeM
.
] + 2 [VO
..
] + p =[MH
Fe] + 3 [V HHH
Fe] + 2 [V HHH
Fe] + n.
In addition to activity (aFe2O3
), temperature (T ) and partial
oxygen pressure (pO2
), the role of variables is played by the
concentrations of all possible types of point defects, viz., [Mi
..
],
[Fei
...
], [MH
Fe], [FeM
.
], [VO
..
], [V HH
M], [V HHH
Fe], n, p, which can be
calculated as functions of the parameters of state. Whilst the
quasichemical approach is not strictly accurate, its practical utility
leaves no doubt.
Figure 5 shows a defect formation diagram plotted in the
coordinates log[i ]  f (log pO2
) at constant values of T and aFe2O3
,
where [i ] is the concentration of the defects of the ith type. From
the diagram it follows that changes in thermal treatment and
sintering regimes of ferrites can underlie the transitions from
electron to hole conductivity and the predominance of ionic
conductivity for compositions close to stoichiometric.
Hg P Cl
TiCl6
Figure 4. The structure of [Hg6P4](TiCl6)C1 prepared by high-temper-
ature synthesis.
The structure was formed by self-assembly of TiC3
6 and Cl in the cavity
of a three-dimensional cage of [Hg6P4]4+.
836 Yu D Tretyakov
Passage to inorganic compounds with more pronounced non-
stoichiometries casts doubts on the existence of a strict correlation
between the non-stoichiometry index and the concentration of
predominant point defects as is implied by the quasichemical
approximation.
Changes in the structures of binary or more complex crystals
as a result of increasing non-stoichiometries have a different
origin. Sometimes, they are related to the formation of associates,
the simplest being bivacancies, to a spatial ordering of super-
structure-type defects and finally to annihilation of point defects
accompanied by changes in the types of bonds of coordination
polyhedra and formation of the so-called crystallographic-shift
structures of the type of homologous series: MnO3n71 where
n = 8, 9, 10, 11, 12 and 14. However, the concept of discrete
compositional changes resulting necessarily from rearrangements
of coordination polyhedra upon crystallographic shift (CS) is
questionable.
According to Anderson,44 there exist many systems possessing
infinitely adaptable structures, i.e., the phases that maintain a high
level of structural organisation despite continuous composition
changes by virtue of changes in the repetition frequencies of CS
planes or orientation of the latter with the preservation of frag-
ments of the matrix structure rather than involvement of point
defects. The former process is discrete and gives rise to the
formation of various homologues of the same series, while the
latter is continuous. Their combination ensures the continuity of
compositional changes within the same order. An infinitely
adaptable structure was found, for example, in phases with
variable compositions of Y(OF)2.13  2.22, solid solutions
(TiO2  Cr2O3) and cerium  cadmium alloys.
The imperfections of structural patterns, e.g., coherent inter-
growth of sites with different orientations of CS planes, are
predominant defects in the case of non-stoichiometric phases
with a high level of structural organisation. These defects, which
are commonly referred to as Wadsley defects, strongly affect the
properties of solid phases and their behaviour under the action of
various chemical and physical agents.
It is clear that the compositions and structures of non-
stoichiometric compounds do not always correspond to the
equilibrium conditions of their formation. On the contrary, very
often the conditions of synthesis, thermal treatment and existence
of such compounds are far from equilibrium and the net result is
determined by the rates of processes involved. For example, nickel
monoxide prepared by thermolysis of nickel nitrate at moderate
heating temperatures may contain excess oxygen up to the
composition NiO1.5, whereas nickel monoxide synthesised in air
under equilibrium conditions has the composition NiO17x, where
x = f (T, pO2
). At 1373 K and pO2
= 261079 atm, the value of x is
equal to 1.861078.
Undoubtedly, the fundamental principles of chemistry of non-
stoichiometric compounds considered above are valid for any
newly produced functional materials, be they superconducting
cuprates, manganites with superhigh magnetic resistance or
cerates manifesting high proton conductivity. At the same time,
any of these materials have specific features of their own and
demand analysis of their non-stoichiometries as in the case of
semiconducting or ferrite systems.
The design of superconducting materials able to maintain high
levels of critical currents in strong magnetic fields has become
possible owing to the passage from yttrium  barium cuprate
YBa2Cu3Oz with strict cationic stoichiometry to neodymium 
barium cuprate Nd17xBa27xCu3Oz with a broad range of
homogeneity for both oxygen and cations. It was found also that
a change in the cationic composition within the homogeneity zone
is accompanied by evolution of the structure from 123 via 336 to
the superstructure 213.45 The use of a TTT diagram, which
characterises the behaviour of a metastable, slowly evolvable
system, is necessary because of an extremely low rate of cationic
ordering in neodymium  barium cuprates.
VI. Chemistry of elements in unusual oxidation
states
The design of novel functional materials based on chemical
elements in unusual oxidation states is a nontrivial process, at
least because the unusual oxidation state is equivalent to the
instability of a particular state of an element. Hence, the phenom-
enon of the appearance of one or another unusual oxidation state
of a chemical element is insufficient from the practical point of
view, one should also be sure that the lifetime of a chemical
element in an unusual oxidation state is commensurate with the
period of functioning of the material containing this element. A
significant contribution to the chemistry of elements in unusual
oxidation states has been made by researchers of the Institute of
Chemical Physics of the RAS and the Department of Chemistry of
the MSU. The former have succeeded in obtaining Np(VII),
Pu(VII), Am(VII), Cm(VI), Cm(V) (see Ref. 46) and a series of
actinides and lanthanides in lower oxidation states,47 the latter
have demonstrated the possibility of existence of Tm(IV) and
found that the tetrad effect characteristic of lanthanides and
actinides is more significant for `distant' lanthanides than for
`near' lanthanides in solid compounds.48
At first, the values of extremely high oxidation states for
transition metals of Groups 3  11 of the Periodic System correlate
with the group number, but then this correlation is absent, which
is determined by the electronic structure or, more precisely, by
disturbances in the fine balance between binding and loosening
forces in atomic orbitals and increasing instabilities of the
corresponding compounds in their extremal oxidation states.
`Matrix' systems formed upon crystallisation from homoge-
neous media (solutions, melts), co-precipitation or solid-phase
synthesis present the greatest interest for specialists in materials
sciences. Matrix systems containing Fe(IV), Fe(V), Fe(VI) and
Co(VI) were prepared by reactions of binary oxides with dopant
oxides.49
An alternative approach consists of the so-called constitu-
tional substitution where the doping ion occupies the host metal
positions of the matrix; the properties of which predetermine the
oxidation state of the doping ion. Thus Fe(V) is stabilised in
vanadium pentaoxide; this oxidation state unusual for iron is
formed upon dissolution of trivalent iron oxide in a melt of
vanadium pentaoxide. However, the formation of Fe(V) could be
observed only after quenching of the samples into metallic
mercury.
The formation of matrix systems under conditions of oxida-
tive synthesis (sintering of Group 3  11 metal oxides with alkali
eH
Y FeM(71/8)
FeM (71/16)
VO (71/8)
VO (71/4) VO (0)
VO (71/16)
logpO2
Fei (73/8)
Fei (0)
Fei (71/4)
Fei
eH
(73/16)
eH
(1/4)
eH
(71/6)
hH
(1/6)
h (3/16)
h (1/4)
VHH
M(0)
V
HH
M
(1/16)
VHH
M(1/8)
VHH
M(1/4)
FeM(0)
log [i]
Figure 5. A diagram of defect formation in a crystal lattice of a
ferrospinel at constant temperature and constant activity of Fe2O3.
The individual segments of the dependences show the predominant
defects; the numbers in parentheses indicate the slopes.
Development of inorganic chemistry as a fundamental for the design of new generations of functional materials 837
metal peroxides) revealed certain peculiarities of the final prod-
ucts which are determined by the properties of the matrix metal
and the mechanisms of reactions. Thus it was found that the final
products prepared with the use of molybdate and tungstate
matrices contained extrinsic magnetics, i.e., Mo(V)- and W(V)-
containing exchangeable clusters of the ferro- and antiferromag-
netic type. The initial steps of the reactions may involve the
formation of bronze-type derivatives, trace amounts of which
are partly stabilised in final products and elicit the corresponding
magnetic responses.50
Yet another line of research related to the synthesis of bismuth
derivatives in lower oxidation states has been described. 51 The
authors have succeeded in preparation of homonuclear polyca-
tions of the types Bi3
5 , Bi5
8 and Bi5
9 . A fragment of the crystal
structure and the main structural elements of compound
Bi10Nb3Cl18 synthesised upon heating of a Bi  BiCl3  NbCl5
mixture to 550 8C are shown in Fig. 6. In the structure of this
compound, the polycations Bi5
9 are surrounded by octahedral
counteranions NbCl2
6 containing Nb(IV). Usually, it is NbCl
6
ions containing Nb(V) that are more stable.
The crystal structure of compound Ni8Bi8SI synthesised by
Baranov et al.52, 53 contains unusual bimetallic infinitely one-
dimensional cationic columns of [Ni8Bi8SI]
? which are made up
of square nickel antiprisms surrounded by a sheath of bismuth
atoms (Fig. 7). In this compound, nickel and bismuth atoms exist
in unusually low oxidation states, which are responsible for the
formation of such an unusual system of metallic bonds between
the metal atoms. This structure can be regarded as consisting of a
set of `molecular' metal wires (a thread of nickel antiprisms)
enclosed in an insulating matrix (a sheath of bismuth atoms).
According to its physical properties, this compound represents a
one-dimensional metallic conductor and a Pauli paramagnet.
VII. Chemistry of inorganic biomaterials
Natural ageing of biomaterials, the building elements of any living
organism, shortens their lifetime. What is most surprising is that
ageing problems are inherent in such strong, at first glance,
a
b c
Bi
Nb
Cl
Bi2
Bi1Bi1
Bi3
Bi2
Bi3
Bi2
Bi3
Bi1
a
b
Figure 6. A fragment of the crystal structure (a) and the main structural
elements (b, c) of Bi10Nb3Cl18.
a
b
c
Bi2 Bi1
Bi2
Bi1
Bi2Bi1
Bi2
Bi1
I
Bi1 Ni1
Bi2 Ni2
Bi1 Ni1
Bi2 Ni2
Bi1 Ni1
Bi2 Ni2
c
b
aI Bi Ni S
Figure 7. The crystal structure of Ni8Bi8SI.
838 Yu D Tretyakov
natural materials as bone and dental tissues. From the standpoint
of materials science, the bone tissue is a composite material which
comprises both inorganic and organic components,54  57 viz.,
hydroxyapatite Ca10(PO4)6(OH)2 (63 mass %) and the protein
collagen (20 mass %), respectively (Fig. 8). In addition to Ca2+
and PO3
4 , bone tissue contains appreciable amounts of Na+,
Mg2+, K+, Cl7, F7 and CO2
3 ions. The physicochemical
character of the interactions between bone components, which
ensures, in particular, its high strength properties, has not been
finally established yet. Even the use of modern biotechnological
approaches does not permit one to construct a full analogue of the
natural bone tissue. Therefore, the use of artificial materials
(implants) is still the only way to restore functioning of large
regions of damaged bones.
Metal alloys based on low-toxic titanium are used in those
cases where it is required to restore the bones of parts of the body
exposed to heavy cyclic loads (e.g., hip joints). In other cases,
preference is given to ceramic or composite materials based on
calcium phosphates; their chemical compositions are similar to
that of bone.54  58 Among other modern trends in the field of
implant design, special mention should be made of `regeneration',
viz., synthesis and application of materials able to stimulate
regenerative processes in damaged tissues owing to their active
interaction with the organism.54 Such implants cannot be pre-
pared without a thorough knowledge of mechanisms of reactions
occurring in bone tissues in all steps of their evolution.
It is believed that not only the chemical composition, but also
the morphology of synthetic hydroxyapatite (HAP) crystals are
important characteristics that determine the response of the
organism to the exogenous material of an implant. In this context,
an ideal material should possess the chemical composition and
granulometric properties similar to those of a bone biomineral,
e.g., non-stoichiometric hydroxyapatite, Ca107x(HPO4)x.
.(PO4)67x(OH)27x (0 < x < 1). Its crystals have the shapes of
plates (4062065 nm) with an axis c lying in the crystal plane
(sp.gr. P63/m of the hexagonal system; a = b = 9.432 #A,
c = 6.881 #A) (Fig. 9).55
Calcium phosphate-based ceramic materials prepared by
high-temperature treatment are routinely used in clinical prac-
tice.59  62 Strong hydroxyapatite ceramic materials are chemically
inert, which significantly decelerates the resorption of the material
and inhibits the growth of the contact bone tissue. Large specific
surfaces of calcium phosphates prepared by solution methods (up
to 100 m2 g71) make them more reactive in comparison with their
high-temperature counterparts (*1 m2 g71). The results of the
use of calcium phosphate-containing cements and biphasic com-
posite materials suggest that in some cases the use of materials that
could serve as bioapatite `sources' in vivo is a simpler and more
efficient approach (Fig. 10).54, 59, 61
Large-scale production of materials strictly corresponding to
calcium phosphates of biological origin as regards their chemical
and structural characteristics is very problematic. Nevertheless,
there is every reason to believe that chemical and morphological
match of a biomaterial and a bone mineral is one of the basic
principles in the design of novel materials for biomedical pur-
poses. Deviations of the compositions and sizes of artificial
crystals from those of their natural counterparts can be employed
for the directional synthesis of biomaterials with predetermined
bioactivities. The practical implementation of such a programme
requires the elucidation of a quantitative relationship, first,
between chemical composition and micromorphology of calcium
phosphate crystals and parameters of their solution synthesis and,
second, between micromorphological parameters and bioactivity
of the material.
The existing methods for the synthesis of HAP enable the
preparation of highly dispersed powders with particle shapes
varying unpredictably in a rather broad range, viz., from equiaxial
to needle-shaped, depending on conditions.55, 56, 63 Nevertheless,
no explicit and quantitative information about the effects of
parameters of HAP synthesis on the morphology of the target
2
3
4
5
6
7
8
9
10
11
1
Figure 8. The levels of structural organisation of the bone tissue.
(1) Internal bone tissue membrane, (2) intermediate plate, (3) osteocyte
(bone cell), (4) osteon (Haversian system), (5) Haversian plate, (6) outer
plate, (7) the Sharpie fibre, (8) peroosteum, (9) Haversian canal, (10)
lacuna, (11) the Volkmann vessel.
P
Ca
OH
1208 608
a=9.43 #A
1/4
c=6.88 #A
1
3/4
Figure 9. A simplified representation of a unit cell of hydroxyapatite.
1
2
Figure 10. Formation of new bone tissue at the boundary of a cement
implant with the composition 75% Ca3(PO4)2 + 20% Ca4P2O9 +
5% CaHPO4.62 (1) New tissue, (2) implant.
Development of inorganic chemistry as a fundamental for the design of new generations of functional materials 839
product is available. It is noted that samples of calcium phosphate
powders prepared according to one synthetic procedure elicit
different responses of an organism up to a complete rejection of
the material. This brings forth a vast variety of chemical and
materials science-related problems connected with the synthesis of
calcium phosphates with predetermined compositions and micro-
morphological characteristics as well as with prediction of their
bioactivities in vitro and development of methods for the bio-
activity correction by chemical modification of the synthetic
product.
In practice, synthesis of HAP is mainly carried out in aqueous
solutions and the existing methods can be classified into three
main groups: (1) precipitation in solutions with constant or
variable composition; (2) hydrothermal synthesis and (3) hydrol-
ysis of calcium phosphates.55, 56, 58, 63, 64 Precipitation of HAP
from aqueous alkaline solutions yields fine-crystalline calcium
hydroxyapatite.
10 CaX2 + 6 M2HPO4 + 8 MOH = Ca10(PO4)6(OH)2 + 20 MX + 6 H2O,
where M = Na, K, NH4, H; X = Cl, NO3, CH3COO, OH.
The formation of HAP is usually preceded by precipitation of
amorphous calcium phosphate.57, 65 The degree of crystallinity of
the precipitated HAP increases with temperature and duration of
ageing, while its non-stoichiometry, i.e., the deviation of the Ca/P
ratio from 1.67, increases with a decrease in pH, temperature and
duration of synthesis.55 The hydroxyapatite powders prepared by
solution methods have a large specific surface (*100 m2 g71) and
a small size of crystallites (*100  200 nm). The size and the
habitus of crystals depend on a number of factors, such as
duration of crystallisation, temperature, composition of solution,
degree of supersaturation, ionic strength, pH, the order of
addition and mixing rates of reactants.55, 56, 58, 64 The diversity of
morphology of HAP crystals can be attributed to the fact that the
synthesis is performed under strictly non-equilibrium conditions;
the shape of the crystals strongly depends on the presence of
admixtures.65  68 In the presence of carbonate ions in the solution,
carbonate apatite crystals of type A [Ca10(PO4)6(OH)27x.
.(CO3)x/2] or type B [Ca107xNax(PO4)67x(CO3)x(OH)2] differing
in shape (plates or needles) depending on the carbonate content
are formed.
As was mentioned above, the micromorphology of HAP
powders is an important property which largely determines the
areas of application of calcium phosphate-based biomaterials. It is
of note that the choice of experimental conditions for the synthesis
of HAP crystals of predetermined morphology implies knowledge
of the contribution of each parameter (factor) of synthesis to, and
of the result of their mutual action (interaction of factors) on, the
two characteristic sizes of anisotropic HAP crystals, viz., along
and perpendicular to the axis [001]. This information, which has
not been explicitly and quantitatively documented in the literature
concerning the methods of apatite synthesis, is considered in a
review.64 A systematic study of effects of various parameters of
HAP synthesis by precipitation from salt solutions (initial con-
centration of reagents, pH and temperature) on the sizes and
shapes of the crystals formed has been carried out. The morpho-
logical characteristics of HAP crystals were established through
mutual application of a comprehensive X-ray diffraction analysis
and transmission electron microscopy (Fig. 11).
Mathematical models based on the results of statistical
analysis of morphological characteristics of crystals allowed
quantitation of various factors on HAP synthesis by precipitation.
These factors (separately and in combination) determine ionic
equilibria in solutions, since changes in temperature, pH and
initial concentrations of solutes induce changes in the concen-
trations of ionic forms as can be judged from the calculations on
activities of ions and ion pairs. One can state that there is a
tendency towards an increase in the size of HAP crystals as the
activities of the ion associates of CaPO
4 and CaOH+ in solutions
increase. The formation of HAP occurs in two steps. Amorphous
calcium phosphate is formed from ion associates with the compo-
sition Ca9(PO4)6 (the `Pozner clusters') by a sol  gel transition
mechanism in the first, relatively fast step. This has a mesoporous
structure and hydroxyapatite nanocrystals formed in the second
step by an AFC dissolution  HAP precipitation mechanism
inherit porosity.64, 65
The use of hydrolytic reactions of calcium orthophosphates of
the type
3 Ca3(PO4)2 + H2O = Ca9HPO4(PO4)5OH
is yet another convenient approach to the synthesis of HAP
crystals with different micromorphological features (needles,
plates) 55, 63, 69, 70 (Fig. 12).
The sizes of HAP crystals obtained upon heating of solutions
exceed significantly those synthesised by precipitation
(*0.5  5 mm and *20  100 nm, respectively). It was noted
above that materials with particles of submicrometric sizes are
preferable for practical use. If HAP are prepared by hydrolysis of
poorly soluble calcium orthophosphates [CaHPO4,
CaHPO4
. 2 H2O, a- and b-Ca3(PO4)2, Ca8(HPO4)2(PO4)4
.
. 5 H2O, Ca4(PO4)2O], the composition of the solution can be
considered as being quasi-equilibrium, this depending on the
solubility product of the solid phase. Like the majority of hetero-
genous processes, hydrolysis of calcium orthophosphates is char-
acterised by a low rate, since the layer of the product formed
blocks the surface of the starting material. The reaction can be
accelerated by increasing the temperature and/or activating the
b
50 nm
28 32 2y /deg
Intensity /pulse s71
a
(002)
(102)
(210)
(211)
(112)
(300)
(202)
0
400
800
Figure 11. The results of studies of HAP powders using a multiprofile X-ray analysis (a) and transmission electron microscopy (b);64
(a): crystal size 55618 nm, the numbers in parentheses indicate Miller indexes.
840 Yu D Tretyakov
reagent surface by sonication. It was shown that the conversion
rate of a CaHPO4  Ca(OH)2 mixture into HAP upon sonication
of the suspension is 20%  30% per 1 h, which is comparable with
the rate of HAP synthesis from the same mixture at 60 8C. This is
accompanied by the formation of aggregates consisting of spher-
ical hydroxyapatite particles with a diameter of *100  300 nm.64
The biological activities of calcium phosphates can be
increased by increasing the specific surfaces of the powders and
decreasing the sizes of the crystallites as well as by changing the
physicochemical characteristics of the surface. An alternative
approach consists of chemical modification of calcium phos-
phates aimed at preparing materials able to undergo active
resorption upon interaction with body fluids. In this context,
special attention is given to the development of methods for the
synthesis of carbonate-containing hydroxyapatite
Ca1070.5x(PO4)67x(CO3)x(OH)2 as well as silicon-containing
hydroxyapatite, Ca10(PO4)67x(SiO4)x(OH)27x.64, 68 The latter
can be synthesised by precipitation of HAP from solution using
a reaction of amorphous calcium phosphate with tetraethoxysi-
lane in an alkaline medium. It was found that silicon-containing
HAP is more soluble in weakly acidic solutions than non-
substituted hydroxyapatite.64
The very fact of design of materials stimulating osteogenesis
suggests that the nearly half a century-old history of intense
application of biomaterials has led to the comprehension that
regeneration and replacement of bone tissue is an extremely
complex and ambiguous problem. The use of one or another
material depends on both the biomedical characteristics of a bone
defect and, perhaps, the clinical case. Therefore, the solution of
this problem is possible only if a broad range of biomaterials is
available, while the choice of a maximum suitable material, which
meets all the specific requirements, is the key to success.
VIII. Inorganic synthesis of functional materials
using sonication, hydrothermal treatment and
microwave irradiation
Inorganic synthesis of functional materials always requires the
activation of the starting reactants whatever the mechanisms of
chemical transformations involved. The simplest method is heat-
ing; its efficiency is estimated from the activation energy in the
Arrhenius equation. However, this parameter, which is deter-
mined experimentally as a temperature coefficient of the reaction
rate, has a physical meaning only if it is associated with a
particular elementary process which limits the reaction rate in a
given temperature range.
In real systems used in the synthesis of functional materials,
chemical interactions involve at least two fundamental processes,
viz., the chemical reaction itself and transfer of a substance to the
reaction zone. Since the latter occurs owing to diffusion and the
diffusional mobility of components of the solid is determined by
the imperfection of its structure, it can be expected that the defects
influence substantially the conditions of the solid-phase synthesis.
This in turn suggests that the rate and direction of chemical
transformations can be controlled through physical actions,
which occur either simultaneously with the thermal activation or
separately. It is often recommended that high-temperature factors
are excluded, since both the reactants and the reaction products
may be subject to undesirable changes which include:
 dissociation of the solid-phase product of inorganic syn-
thesis, e.g., thermal dissociation of ferrites having a spinel struc-
ture according to the reaction:
6 MFe2O4 4 Fe3O4 + 6 MO + O2 .
The magnetic and dielectric properties of the material are usually
deteriorated by the reaction products;
 changes in the composition of reaction products due to the
loss of volatile components, especially in gaseous media contain-
ing water vapour and CO2. These changes are substantial, e.g., in
the synthesis of solid electrolytes based on alkali metal polyalu-
minates and lithium-containing ferrites.
The efficacy of sonication in reactions occurring in various
reaction systems in the synthesis of materials 71, 72 depends on the
phase state of a system. In liquid-phase systems (solutions, melts),
cavitation, i.e., the formation, oscillation and collapse of micro-
vesicles, is the main result of sonication which leads to the
formation of short-living hot zones with characteristic temper-
atures up to *5000 K and pressures up to 1000 atm. In some
cases, `collapse' of cavitation vesicles is accompanied by forma-
tion of intense microcurrents of fluids and potent shock waves,
which enhance the mass transfer. As a consequence, in physico-
chemical systems (both homogenous and heterogenous) decom-
position of volatile compounds, like metal carbonyls, acceleration
of redox reactions, formation of stable emulsions and suspensions
and disaggregation of particles in colloid solutions take place.
Sonication of liquid-phase systems has proved to be efficient in
classical reactions of organic and inorganic synthesis 73, 74 as well
as in the synthesis of amorphous and crystalline metal, oxide and
carbide nanopowders.75, 76
In solid-phase systems, sonication is efficient only if a certain
power threshold of acoustic vibrations is exceeded concomitantly
with a significant (by one or two orders of magnitude) increase in
the number of dislocations and in the values of effective diffusion
coefficients and the formation of new interphase contacts.77, 78 As
a consequence, sonication of reaction mixtures may change the
reaction mechanism, decrease considerably the apparent activa-
tion energy and the temperature of vigorous interactions of the
reactants.79  81 Acoustic activation of solid-phase processes ena-
bles synthesis of various oxide materials 80, 81 and active forms of
1 mm.
1 mm.
a
b
Figure 12. Micrographs of samples prepared by hydrolysis of
a-Ca3(PO4)2 at 40 (a) and at 100 8C (b).
Development of inorganic chemistry as a fundamental for the design of new generations of functional materials 841
precursors 82 and, in some special cases, directed modulation of
the structure-dependent properties of solid-phase products.83
It has been found 83 that the concentration of structural
defects in sonicated compounds depends non-linearly on the
amplitude of input acoustic vibrations and the temperature. The
defectiveness of samples increases significantly (by more than one
order of magnitude), in a definite temperature interval and at a
definite ultrasound volume, i.e., the acoustic and thermal effects
are synergistic.
At fixed temperatures, a significant (2  3-fold) increase in
effective diffusion coefficients of carbon in steels is observed only
at definite acoustic input vibration amplitudes.84 Similarly, at
fixed ultrasound volume the defectiveness of simple metal oxides
increases anomalously, however, only in a narrow temperature
range.85 It is of note that the temperature range corresponding to
the maximum activation is determined by the chemical nature of
the oxides that undergo sonication. Thus it has been shown that in
high-temperature sonication of iron(III) oxide ultrasonic gener-
ation and annihilation of defects and their thermal annealing
compete.85 The rates of each of these processes depend differently
on temperature; therefore, the defectiveness of a-Fe2O3 and the
corresponding reactivity change non-linearly reaching a maxi-
mum at *800 8C.
Combination of different factors allows one to change both
the kinetics and the mechanisms of solid-phase reactions. Studies
of interactions in the system Fe2O3 + Li2CO3 have shown 79 that
superposition of an acoustic field at definite temperatures induces
changes in the reaction mechanism, viz., the chemical interaction
at the interphase becomes a limiting step, whereas under normal
conditions this reaction is limited by the formation of nuclei and
their progressive diffusion-dependent growth. In turn, the reac-
tion rate increases significantly with a change in the reaction
mechanism and an increase in the mass transfer rate. It is note-
worthy that in this case the temperature, which corresponds to the
maximum effect of the ultrasound, is lower than the temperature
of the onset of the reaction in the system under ordinary
conditions, i.e., sonication allows one to reduce significantly (by
100  150 8C) the temperature of synthesis. Studies of solid-phase
reactions of ferrite formation
have shown 86, 87 that temperatures of the onset of the reaction
decrease by 50  200 8C in the case of sonication, while the time of
synthesis of monophasic ferrites diminishes 1.5  2-fold. A change
in the reaction mechanism is due to a direct formation of the
reaction zone in the acoustic field; its most characteristic feature is
the presence of a well-defined initial step of a sonochemical
reaction mediated by a special mechanism of formation of
intercrystallite contacts in the ultrasonic field.
Of special interest are studies of effects of sonication on
hydrothermal synthesis of simple and complex oxides and chalco-
genides.88 First of all, it has been established 89 that sonication
causes cavitation both in hydrothermal solutions and ordinary
liquid media the intensity of which at 250 8C is 70%  80% of that
observed at 25 8C. Cobalt, titanium and zirconium oxides as well
as ferrites Ni17xZnxFe2O4 were used as subjects in these studies.
Hydrothermal treatment and sonication of a suspension of an
amorphous gel of Co(OH)2 in water at 250 8C yielded the phase
Co3O4; no such phase was formed without sonication. Most
probably, sonolysis of water in hydrothermal solutions gives rise
to hydrogen peroxide which induces the oxidation of Co(II) into
Co(III). The particle sizes of the oxide products of hydrothermal
synthesis in an ultrasonic field are significantly smaller than those
of an analogous product synthesised without sonication (Fig. 13).
Hydrothermal treatment of amorphous gels ZrO2
. n H2O and
TiO2
. n H2O and simultaneous sonication result in a significant
increase in the rates of their crystallisation, whereas sonication of
products of hydrothermal synthesis favours the formation of
thermodynamically more stable phases (Fig. 14).90
Although the idea of large-scale application of microwave
heating for domestic and technological purposes was realised
rather long ago 91 and numerous studies were undertaken in the
past decade on the microwave-induced synthesis and sintering of
functional materials,92, 93 the physicochemical nature of the proc-
esses occurring in microwave synthesis still remains obscure.
Oxides, hydroxides, salts or their mixtures were used as precursors
for various functional materials including ferrites, manganites,
cobaltites, nickelates and cuprates subject to microwave radia-
tion.94  96
According to theoretical concepts on the interaction of micro-
wave radiation with matter,97 effective absorption of energy
requires the presence in the matter of either dipoles capable of
changing their orientation and rotating under the action of
microwave radiation, or free charge carriers able to move in an
applied microwave field. Many inorganic salts possess low elec-
tronic, hole or ionic conductivities. At the same time, water
molecules, which are present in the crystal lattices of crystal
hydrates, and some anions have high dipole moments. Thus, in
the majority of salt systems the most probable mechanism of
absorption of microwave radiation is related to dipoles.
It has been found experimentally 94 that compounds devoid of
chemically bound water (carbonates, nitrates and oxalates of
alkali and alkaline-earth elements) virtually do not adsorb micro-
wave radiation. In contrast, crystal hydrates of inorganic salts
interact with a microwave field, the mode of this interaction being
LiFeO2 + 2 Fe2O3
530  650 8C
LiFe5O8 ,
ZnO + Fe2O3
600  800 8C
ZnFe2O4 ,
NiO + Fe2O3 NiFe2O4
600  800 8C
a
1 mm
1 mm
b
Figure 13. Micrographs of products of hydrothermal syntheses in the
absence (a) and in the presence of an ultrasonic field (b).
842 Yu D Tretyakov
dependent on the chemical nature of the cations (Fig. 15). Later, it
was shown that the decomposition of crystal hydrates in the
microwave field to oxides takes place only if the oxide formation
occurs prior to the removal of all the water present in the system.
Of all the salt crystal hydrates tested in this study,94 only 3d-metal
nitrate crystal hydrates meet this requirement and can be used as
active (with respect to microwave radiation) components in the
synthesis of complex oxide phases. A generalised scheme of
interaction of metal salt crystal hydrates with microwave radia-
tion is shown in Fig. 16. Studies of salt precursors have demon-
strated the expediency of in situ application of microwave
treatment of salt mixtures in solid-phase reactions, since highly
dispersed oxide particles formed upon decomposition of salt
mixtures and distributed uniformly throughout the reaction bulk
react actively with one another.
Specific `non-thermal' effects of microwave radiation caused
by generation of ionic currents on intercrystallite and interaggre-
gate boundaries make a significant contribution to diffusional
processes occurring in the solid-phase reactions; the intensity of
ionic currents increases in highly dispersed systems. Conse-
quently, microwave irradiation of a reaction mixture can be
accompanied by an increase in the rate and a decrease in the
temperature of solid-phase interactions. It is this effect that is
observed in synthesis of functional materials.98  105
However, it would be erroneous to think that microwave-
induced synthesis always makes use of salt precursors able to
convert microwave energy into thermal energy starting from room
temperature. It is known that the energy (P) absorbed by an object
placed into a uniform microwave field is expressed by the
equation:
P = 2pfe0 K tan d |E|2,
where f is the microwave frequency, e0 is the vacuum permittivity,
K is the dielectric constant of the sample, tan d is the dielectric loss
tangent and E is the microwave field intensity in the sample under
assay. The K and d values depend on temperature and on the
nature of the phases used as precursors. For example, the mixture
SrCO3  Co(NO3)2
. 6 H2O  Fe2O3 used in the synthesis of
SrFeCo0.5Oy, a promising electron-ionic conductor, can absorb
low-wavelength energy only in those cases where microwave
irradiation is preceded by ordinary heating as a result of which
the oxidative thermolysis of cobalt nitrate yields Co3O4. The latter
effectively absorbs microwave energy and favours rapid heating of
the entire reaction mixture to temperatures above 1000 8C,
although other components of the reaction mixture (SrCO3,
Fe2O3) are inert in the microwave field and are thermally activated
only upon interaction with Co2O3.106
Non-stoichiometric oxides of the type TiO27x or Ta2O57x,
107
hydroxides or basic salts 95 as well as amorphous carbon used as
soot 108 or a highly dispersed metal 109 can be used as active
components for microwave irradiation. In the manufacture of
compact piezoelectric ceramic materials of the type (Ba,Sr)TiO3
(see Ref. 110) or ion-conducting ceramic materials of the type
Na-b-alumina,111 the powdered precursor with the identical
composition is synthesised by ordinary heating, whereas sintering
is performed in a microwave field.
And, finally, the use of the so-called monomodal microwave
heating 112, 113 based on the propagation of an electromagnetic
beam from a magnetron along a waveguide where a standing wave
is established opens up radically new opportunities for inorganic
synthesis, for it allows one to focus the microwave energy in a
small volume of the reaction mixture and to initiate very rapidly
chemical and structural transformations that are impossible in the
case of ordinary thermal or multimodal microwave heating.
a
20 40 60 2y /deg
M
M
M
M
T
T
T
T
T
T
T
1
2
22 24 26 28 2y /deg
b
1
2
A
A
P
P
IntensityIntensity
Figure 14. X-ray diffraction spectra of products of hydrothermal treat-
ment of ZrO2
. n H2O (a) and TiO2
. n H2O (b) performed at 250 8C for 3
hours without (1) and with (2) sonication.
Designations: T, tetragonal phase of ZrO2; M, monoclinic phase of ZrO2;
A, anatase; P, rutile.
Crystal hydrates of nitrates Crystal hydrates of
formates, acetates,
oxalates; sulfates,
phosphates
of manganese
(Mn2+), iron (Fe3+),
cobalt (Co2+),
copper (Cu2+)
of nickel (Ni2+)
Formation of
oxonitrates
Decomposi-
tion to oxides
Only partial or
only complete
dehydration.
of 3d-elementsof lanthanides of alkali and
alkaline-earth
elements
P
P
P
Figure 15. Interaction of crystal hydrates with microwave radiation. P,
energy of a microwave field.
Development of inorganic chemistry as a fundamental for the design of new generations of functional materials 843
IX. Conclusion
In the past two decades, inorganic chemistry has experienced
impressive changes owing to its integration with physical chem-
istry, physics of solids, organic chemistry and biochemistry and
the use of advanced instrumental methods. The range of subjects
of inorganic chemistry has been extended considerably. Now,
these encompass not only chemical compounds, but also materi-
als, sometimes even those which contain, along with an inorganic
component, organic, polymeric or biopolymeric fragments. At
present, studies of the majority of subjects are carried out at
several levels. In addition to crystal or molecular structures, they
involve electronic and magnetic structures of various compounds,
specific defects of their crystal structures, distribution of micro-
admixtures, structures of interfaces in multicrystalline com-
pounds, nanostructures, micro- and mesopore structures and
surface structures as well as the effects of the aforementioned
organisational levels of a substance on its properties. Special
mention should be made of the unusual extension of the range of
the impact of different kinds of energy on substances aimed at
determining their limit characteristics and the development of
methods for the construction of novel materials.
The spectrum of problems considered in this review does not
encompass all the trends in inorganic chemistry related to the
design of novel functional materials. For example, the problem of
organisation of the reaction zone in the synthesis of materials
considered in the review 7 was mentioned only briefly. Neither was
the problem of the development of microcomposite materials
based on superconducting cuprates 114 discussed in proper detail.
In addition to the functional inorganic materials considered in the
present review, the subjects under study at the Division of
Inorganic Chemistry of the Department of Chemistry and the
Department of Materials Science of MSU include:
 inorganic structures as materials for gas sensors;{
 barrier materials based on BaMO3 (M = Zr, Hf, Th, Ce,
Ti);116, 117
 luminescent nanocomposites based on AIVBVI;118, 119
 low-dimensional structures and superlattices;120, 121
 new generations of semiconductors and semiconductor
heterostructures;122  124
 oxygen-conducting membranes and solid electro-
lytes;125, 126
 materials for lithium current sources;127  129
 magnetic nanocomposites based on glassy matrices;130
 photonic crystals;131
 ceramic pigments based on hydroxyapatites containing
copper ions in an unusual linear coordination.132
It should be mentioned in conclusion that other promising
trends of research related primarily to the development of
molecular electronics,133 spintronics 134 and biocrystal systems 135
require active participation of inorganic chemists. Taking into
account the extreme dynamism shown by the scientific community
towards the design of novel functional materials, it has become
necessary to follow closely the new trends in this field which are
especially successfully documented in journals such as `MRS
Bulletin', `Advanced Materials', `Materials Today' and `Materials
Chemistry'. Unfortunately, no domestic analogues are available
so far.
The author thanks his colleagues at the Division of Inorganic
Chemistry of the Department of Chemistry and the Department
of Materials Science of MSU for valuable assistance in the
preparation of this review for publication. The contributions of
STARTING SALTS
(absorption of microwave radiation
by water of crystallisation)
DEHYDRATED SALTS
(not adsorbing microwave radiation)
SALT SOLUTIONS
(absorption of microwave radiation by
water of crystallisation)
20  30 8C Melting, liberation of water vapour
130  180 8C Hydrolysis, complete elimination of water from a system:
liberation of water vapour and hydrolytic products
INTERMEDIATE PRODUCTS (dehydrated salts,
oxo and hydroxo compounds not absorbing
microwave radiation)
INTERMEDIATE PRODUCTS (oxo and hydroxo
compounds) and OXIDE PHASE (adsorption of microwave
radiation by the oxide phase)
Initial step of oxide
phase formation
The oxide phase is not formed
250  800 8C Heating of the oxide phase,
isolation of gaseous thermolysis products
OXIDE PHASES
(absorption of microwave radiation by the oxide phase)
Figure 16. A scheme of the interaction of metal salt crystal hydrates with microwave radiation.
{ A review devoted to this subject 115 will be published in the next issue of
`Russian Chemical Reviews'.
844 Yu D Tretyakov
Drs A V Lukashin, A G Veresov, A R Kaul and E A Goodilin
are especially highly appreciated.
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