Precursor Methods Goals - decrease diffusion paths, shorten reaction times and temperatures Intimate mixing of components in solution, precipitation, filtration, washing, drying, calcination High degree of homogenization <^ Large contact area <^ Reduction of diffusion distances <^ Faster reaction rates <^ Lower reaction temperatures <^ Metastable phases, smaller grain size, larger surface area 1 Coprecipitation Method Coprecipitation applicable to nitrates, acetates, oxalates, hydroxides, alkoxides, beta-diketonates Requires: similar salt solubilities similar precipitation rates no supersaturation Washing: water, organic solvents Drying: evaporation azeotropic distillation freeze-drying Disadvantage: difficult to prepare high purity, accurate stoichiometric phases if solubilities do not match 2 Coprecipitation Method Spinels oxalates: Zn(C02)2/Fe2[(C02)2]3/H20 1: 1 mixing, H20 evaporation, salts coprecipitation Solid-solution mixing on atomic scale, filter, calcine in air Zn(C02)2 + Fe2[(C02)2]3 -> ZnFe204 + 4CO + 4C02 A1203 Bayer Process bauxite Na°H,P » Al(OH)4 f!% Al(OH)3 m°£a-M203 + Fe(OH)3, Ti02, Si02 BaTi03 BaCl2 + TiOCl2 + 2 H2C204 + 4 H20 + Ln dopants -► BaTiO(C204)2.4H20 + 4 HC1 filtration, washing, drying, calcination @ 730 °C Coprecipitation Method Spinel A1(N03)3 + Mg(N03)2 + H20 freeze-drying gives amorphous mixture, calcination @ 800 °C !!! low T Mg(N03)2 + 2 A1(N03)3 -► MgAl204 + 6 NOx + (10-3x)O2 random Ruby Ion exchange A1(N03)3 + Cr(N03)3 -► Al(OH)3 + Cr(OH)3 sol freeze drying gives solid (Al/Cr)(OH)3 @ LN2 temperature, 5 Pa anealing @ 950 °C for 2.5 h gives solid solution Al2.xCrx03 Zirconia ZrSi04(zircon) + NaOH -► Na2Zr03 + Na2Si03 -► ZrOCl2 Zr(OH)4 / Y(OH)3 ™»^n > nano-Y/Zr02 4 Coprecipitation Method High-Tc Superconductors 1373 K La3+ + Ba2+ + Cu2+ + H2C204 —► ppt -► La185Ba0.15CuO4 Magnetic garnets, tunable magnetic materials Y(N03)3 + Gd(N03)3 + FeCl3 + NaOH -> YxGd3xFe5012 Firing @ 900 °C, 18-24 hrs, pellets, regrinding, repelletizing, repeated firings, removes REFe03 perovskite impurity Isomorphous replacement of Y3+ for Gd3+ on dodecahedral sites, solid solution, similar rare earth ionic radii complete family accessible, 0 < x < 3,2Fe3+ Oh sites, 3Fe3+ Td sites, 3RE3+ dodecahedral sites 5 Pechini and Citrate Gel Method Aqueous solution of metal ions HO- Chelate formation with citric acid Polyesterification with polyfunctional alcohol on heating Further heating leads to resin, transparent glassy gel calcination provides oxide powder Control of stoichiometry by initial reagent ratio Complex compositions, mixture of metal ions Good homogeneity, mixing at the molecular level Low firing temperatures Pechini and Citrate Gel Method r COOK HOOC t:oow { )^ on HOOCCnicH,GOf>H I I OH Chelation Complexation-coordination polymers HOCH2CH2OH Polyesterification polycondensation OH I 2IIOCI I;C11,OCOC J l^CHjCf >f>CH ^H/Hl r t OH I OK I on OH OH 7 Pechini and Citrate Gel Method BaTi03 by conventional powder method at 1200 °C Ba2+ + Ti^Pr^ + citric acid at 650 °C Sc203 + 6 HCOOH -► 2Sc(HCOO)3 + 3 H20 MnC03 + 2 HCOOH -► Mn(HCOO)2 + C02 + H20 added to citric acid, water removal, calcination @ 690 °C gives ScMn03 without citric acid only mixture of Sc203 and Mn203 is formed 8 Double Salt Precursors Double salts of known and controlled stoichiometry such as: Ni3Fe6(CH3COO)1703(OH).12Py Burn off organics 200-300 °C, then 1000 °C in air for 2-3 days Product highly crystalline phase pure NiFe204 spinel Good way to make chromite spinels, important tunable magnetic materials Juggling the electronic-magnetic properties of the Oh and Td ions in the spinel lattice Chromite spinel Precursor Ignition T, °C MgCr204 (NH4)2Mg(Cr04)2.6H20 1100-1200 NiCr204 (NH4)2Ni(Cr04)2.6H20 1100 MnCr204 MnCr207.4C5H5N 1100 CoCr204 CoCr207.4C5H5N 1200 CuCr204 (NH4)2Cu(Cr04)2.2NH3 700-800 ZnCr204 (NH4)2Zn(Cr04)2. 2NH3 1400 FeCr204 (NH4)2Fe(CrQ4)2 1150 Single Source Precursor Compounds containing desired elements in a proper stoichiometric ratio Easy chemical pathway for ligand removal ^O-Si-O^ M ^ H20 + (M-0-Si03)x 7V 10 Vegard's Law Vegard law behavior: Any property P of a solid-solution member is the atom fraction weighted average of the end-members The composition of the A1-XBX alloy can be calculated from Vegard's law The lattice parameter of a solid solution alloy will be given by a linear dependence of lattice parameter on composition: aCA^A) = x a(B) + (1-x) a(A) 11 Vegard's Law c(CdSe,.xSx) = x c(CdS) + (1-x) c(CdSe) Vegard's Law P (YxCe!xRhIn5) = xP (YxCe!xRhIn5) + (1-x) P (YxCe!xRhIn5) Any property P of a solid-solution member is the atom fraction weighted average of the end-members Tetragonal lattice constant a as a function of Y concentration x for the Ce1_xYxRhIn5 system Y concentration (mol) Vegard's Law A linear relationship exists between the concentration of the substitute element and the size of the lattice parameters The direction of the linear relationship, increasing or decreasing, depends upon the system being analyzed As the concentration of Y is increased, lattice constant a decreases, implying the cell is contracting along the a axis 14 Vegard's Law Vegard law behavior: P (YxGd3xFe5012) = x/3 P (Y3Fe5012) + (3-x)/3 P (Gd3Fe5012) Any property P of a solid-solution member is the atom fraction weighted average of the end-members 15 Vegard's Law Tunable magnetic properties by tuning the x value in the binary garnet YxGd3.xFe5012 3 Td Fe3+ sites, 5 UPEs 2 Oh Fe3+ sites, 5UPEs Ferrimagnetically coupled material, oppositely aligned electron spins on the Td and Oh Fe3+ magnetic sublattices Counting spins Y3Fe5012 ferrimagnetic at low T: 3x5-2x5 = 5UPEs Counting spins Gd3Fe5012 ferrimagnetic at low T:3x7-3x5 + 2x5 = 16 UPEs YxGd3xFe5012 creates a tunable magnetic garnet that is strongly temperature and composition dependent, applications in permanent magnets, magnetic recording media, magnetic bubble memories and so forth, similar concepts apply to magnetic spinels 16 LalxCexCr03 Flux Method Molten salts (inert or reactive), oxides, metals MN03, MOH, (M = alkali metal) FLINAK: LiF-NaF-KF M2QX (M = alkali metal, Q = S, Se, Te) molten salts ionic, low mp, eutectics, completely ionized act as solvents or reactants, T = 250-550 °C enhanced diffusion, reduced reaction temperatures in comparison with powder method products finely divided solids, high surface area (SA) slow cooling to grow crystals separation of water insoluble product from a water soluble flux incorporation of the molten salt ions in product prevented by using salts with ions of much different sizes than the ones in the product (PbZr03 in a B203 flux) Flux Method Lux-Flood formalism oxide = strong base acid = oxide acceptor A + OB —► AO + B base = oxide donor 700 K Zr(S04)2 + eut. (Li/K)N03 -► Zr02 540 K Zr(S04)2 + eut. (Li/K)N02 -► Zr02 520 K ZrOCl2 + eut. (Na/K)N03-► Zr02 amorph. —► t- Zr02 720 K ZrOCl2 + YC13 + eut. (Na/K)N03-► Zr02 BaC03 + SrC03 + Ti02 + eut. (Na/K)OH 570 K _^ cubic-Ba075 Sr025TiO3 19 Flux Method fly ash (aluminosilicates) NaOH, NH4F, NaN03 ^ zeolites (sodalite, cancrinite) NH4H2P04 + (Na/K)N03 + M(N03)2 -► (Na/K)MP04 4 SrC03 + A1203 + Ta2Os -► Sr2AlTa06 900 °C in SrCl2flux 1400 °C required for a direct reaction K2Tex + Cu -► K2Cu5Te5 K2Tex reactive flux, 350 °C 20 Flux Method Electrolysis in molten salts Reduction of Ti02 pellets to Ti sponge in a CaCl2 melt at 950 °C O2- dissolves in CaCl2, diffuses to the graphite anode insulating Ti02 —> Ti02x conductive graphite anode anodic oxidation 2 O2- —> 02 + 4 e- cathode Ti02 pellet cathodic reduction Ti4+ + 4 e' —» Ti 21 Ionic Liquids Organic cations (containing N, P) Inorganic anions: CI", A1C14, A12C17, Al3Cli0, PF6, SnCl3, BC13, BF4, N03, OS02CF3 (triflate), CH3C6H4S03, N(S02CF3)2, P043 Ionic Liquids Oldest known (1914): EtNH3+N03" mp 12 °C ■^Liquids at room temperature or low mp Thermal operating range from -40 °C to 400 °C ■^Higly polar, noncoordinating, completely ionized ■^Nonvolatile - no detectable vapor pressure ■^Nonflamable, nonexplosive, nonoxidizing, high thermal stability ■^Electrochemical window > 4V (not oxidized or reduced) ■^Immiscible with organic solvents ■^Hydrophobic IL immiscible with water Ionic Liquids m 11 24 Synthesis of Ionic Liquids NR3 + RC1 -» [NR4]+ Cl-Aluminates [NR4]+ CI" + AICI3 -> [NR4]+ [AICIJ- Metal halide elimination [NR4]+ CI" + MA -» MCI + [NR4]+ A" Reaction with an acid [NR4]+ CI" + HA -» HC1 + [NR4]+ A" Ion exchange [NR4]+ CI" + Ion exchanger A -» [NR4]+ A" 25 Halogenoaluminate(III) Ionic Liquids The most widely studied class of IL High sensitivity to moisture - handling under vacuum or inert atmosphere in glass/teflon RC1 + A1C13±+ R+[A1C14]- 2 [AICIJ-1; [A12C17]- + Cl- autosolvolysis Keq = 1016 to 1017 at 40 °C 2 [A12C17]-1; [A13C110]-+ [A1C1J- Acidic: excess of A1C13 as [A12C17]- x(A1C13) > 0.5 Basic: excess of CI" x(A1C13) < 0.5 Neutral: [AICIJ- x(A1C13) = 0.5 Equilibria in Halogenoaluminate(III) IL Halogenoaluminate(III) Ionic Liquids 2 [AICIJ- í+ [A12C17]- + Cl- autosolvolysis Keq = 1016 to 1017 at 40 °C Acidic IL with an excess of A1C13 HCl + [A12C17]- ±ř H+ + 2 [A1C14]- Proton extremely poorly solvated = high reactivity Superacid [EMIM]C1/A1C13/HC1 H0 = -19 (HS03F: H0 = -15) Latent acidity MCI + [A12C17]- ±4 M+ + 2 [AICI4]- buffered IL B + M+ + [AICI4] *5 MCI + B-A1C13 28 Superacidity Liquid hf-£tF3 FS03h^SbF5 IB 24 Solid I !ri|Eli LL FSOjH ioim h2so4 10 1* 12 29 Superacidic [EMIM]C1/A1C13/HC1 GhrvBepe J-MBLh¥lnrehlhileag-Si02 + 2 MeOH + 2HCOOMe Natural gas sweetening (H2S, C02 removal) Electrolytes in batteries or solar cells Dissolving spent nuclear fuel (U4+ oxidized to U6+) Extraction Enyzme activity 39 Applications of Ionic Liquids Olefin polymerization Ethene + TiCl4 + AlEtCl2 in acidic IL Ethene + Cp2TiCl2 + Al2Me3Cl3 in acidic IL Cp2TiCl2 + [cation]+[Al2Cl7]- t; [Cp2TiCl] + + [cation]+ + 2 [AICIJ- Olefin hydrogenation Cyclohexene + H2 + [RhCl(PPh3)3] (Wilkinson's catalyst) 40