1 Chemistry at the Earth’s surface at 100 kPa Chemistry in the Universe at hight pressures and temperatures - deep within the planets and stars Laboratory: Pressures up to 770 GPa (static), 2 TPa (ns), 100 TPa (shock wave) High temperatures ~7000 C 1 bar = 100 kPa 1 Mbar = 100 GPa p-V work during compression to 1 Mbar equivalent to approx. 1 eV  chemical bond energy In-situ observations by diffraction, spectroscopy to probe chemical reactions, structural transformations, crystallization, amorphization, phase transitions Methods of obtaining high pressures  Diamond anvils, tetrahedral and octahedral  Shock waves (km s-1)  Laser compression (10 ns)  Explosions, projectiles  Go to another planet: Jupiter (H2 is metallic at 100 Gbar) Dry High-Pressure Methods Pressure Scale 2 Pressure bar System 1 Mbar = 100 GPa 10-12 high vacuum chamber 1 atmospheric pressure 1.5 kitchen pressure cooker 2.0 car tire 50 a lady in stilleto heels 60 breakdown of human nervous system - divers 73.8 critical pressure of CO2 150 autoclave (safety burst disc) 221.2 critical pressure of H2O 103 pressure at the bottom of the ocean (11 km) 2.103 LDPE 104 Earth crust (30 km) 105 synthetic diamond production 3.4 106 pressure at the center of the Earth (6378 km) 107 Saturn, Jupiter, metallic hydrogen 108 neutron stars 50 orders of magnitude 3 Earth Core 3.4 Mbar = 340 GPa, 6000 K ε-Fe hcp MgSiO3 most abundant silicate mineral within our planet ! Olivine Mg2SiO4 (orthosilicate = nesosilicate = isolated SiO4) pyroxene (silicate chains) > spinel Mg2SiO4 ilmenite > garnet (HT) > perovskite MgSiO3 - Si CN = 6 4 Dry High-Pressure Methods Pressure techniques useful for synthesis of unusual structures A redistribution of the electronic density TD metastable yet kinetically stable when pressure released = pressure and temperature quenching Reconstructive transformation hindered at low temperature insufficient thermal energy for bond-breaking • high pressure phases • higher density • enhanced intermolecular interactions • bonds shorten with increasing p • higher coodination number (longer bonds) • higher symmetry • band broadening and mixing • transition to from nonmetal to metal CN = 4 CN = 6 5 Bond Distances at High Pressure Gray Sn (diamond type, stable below 13 C, semiconductor) Coordination number 4, Sn-Sn bond length 281 pm, a = 6.4892 Å, dens = 5.57 g/cm3 White Sn (metallic) Coordination number 6, Sn-Sn bond lengths 302 and 318 pm, a = 5.8316 Å, c = 3.1815 Å, dens = 7.31 g/cm3 Decrease of bond lengths with increasing pressure Pressure/Coordination Number Rule: increasing pressure  transition to higher CN Pressure/Distance Paradox: increasing pressure – longer bonds CN = 4 CN = 6 6 Dry High-Pressure Methods Examples of high pressure polymorphism for some simple solids Solid Normal structure Transf. P (kbar) Transf. T (oC) High pressure structure C Graphite 3 130 3000 Diamond 4 CdS Wurtzite 4:4 30 20 Rock salt 6:6 KCl Rock salt 6:6 20 20 CsCl 8:8 SiO2 Quartz 4:2 120 1200 Rutile 6:3 (stishovite) Li2MoO4 Phenacite 4:4:3 10 400 Spinel 6:4:4 NaAlO2 Wurtzite 4:4:4 40 400 Rock salt 6:6:6 CN = 4 CN = 6 7 p-T Phase Diagrams Gibbs Rule: F + P = C + 2 F = degree of freedom P = number of phases C = number of components C = 1 F = 0 P = 3 F = 1 P = 2 F = 2 P = 1 8 p-T Phase Diagram of SiO2 CN = 4 CN = 6 Gibbs Rule: F + P = C + 2 p-T Phase Diagram of Ice 9 Hexagonal Ice Ice 17 solid phases High-Pressure Phases 10 Water 17 phases of ice Ice-VII m.p. 100 C Ice-X fluorite, ionically conductive above 10 GPa Equalization of O-H covalent and hydrogen bonds above 60 GPa Max. pressure attained for water 210 GPa Ca ccp at ambient pressure bcc (!) above 20 GPa 4s-3d mixing, Ca become a transition metal 1 GPa floats sinks Condensed Gases 11 NO2 + N2O NO+ NO3 - calcite N2 semiconducting oligomers (-N-)x at 100-240 GPa cubic diamond 110 GPa, 2000 K Heating: 1-μm B plate (absorber of laser radiation rests on c-BN pieces that thermally insulate the plate from the bottom anvil. The sample squeezed between the anvils is surrounded by the c-BN/epoxy gasket followed by the metallic (Re) supporting ring 840 cm–1 2410 cm–1 Black Nitrogen 12 A transparent and crystalline allotrope = black phosphorus a diamond anvil cell, 140 GPa, heating with a laser to 4000 K 1 D Laniel et al, Phys. Rev. Lett, 2020, DOI: 10.1103/PhysRevLett.124.216001 2 C Ji et al, Sci. Adv, 2020, DOI: 10.1126/sciadv.aba9206 13 Phase Diagram of Hydrogen H–H bond 0.74 Å, bond dissociation energy of 4.52 eV Prediction: at 25 GPa - non-molecular (atomic and metallic) Solid H2 at 5.5 GPa or at 14 K - phase I = disordered orientationally freely rotating H2 molecules in hcp H2 metallic conductivity in dense fluid hydrogen H2 + H2 - 14 Phase Diagram of CO2 I III CO2-V Quartz superhard CO2 heating at 10-20 GPa sp3 bonded CO4 cristobalite, tridymite 40 GPa quartz (noncentrosymmetric) fcc 15 Reaction Equlibrium and Pressure The reaction volume V0 = the volume difference between the products (C) and the reactants (A) A ⇄ C Associative type = negative V0 (VC  VA) K increases with increasing pressure A  C Dissociative type = positive V0 (VC  VA) K decreases with increasing pressure A  C    eq eq A C K  A C TS 16 Reaction Kinetics and Pressure The activation volume V≠ = the volume difference between the transition state complex (TS) and the reactants (A) Associative type = the rate determining step involves the formation of a covalent bond negative V ≠ (VTS  VA)  reaction rate increases with increasing pressure Dissociative type = the breaking of a covalent bond positive V ≠ (VTS  VA)  reaction rate decreases with increasing pressure Room-temperature pressure dependence of the rate constant for different activation volume V≠ values (in cm3 mol1) A ⇄ TS  C 17 Diamond Anvil Cell Percy Williams Bridgman (1882 – 1961, NP in Physics 1946) • hydraulic press • anvils • sample assembly 18 Diamond anvil cell (DAC) p = F/A p = 40 GPa Atable / Aculet = 10 : 1 Aculet = 100-200 μm Nanocrystalline anvils Re or steel gasket Pressure transmitting medium: BN, MgO, NaCl, AgCl, He, Ne, N2, Ar, methanol:ethanol 4:1 Diamond transparent to radiation from IR to X-ray Laser heating T > 2500 C Resistive heating Diamond Anvil Cell 19 Diamond Anvil Cell Calibrating a high pressure diamond anvil cell • Ruby (0.5 wt %Cr doped corundum) - fluorescence transition, up to 400 C, Sm:YAG, SrBO4 • Bi, Pb, Tl, Ba pressure induced phase transition • Cu, Ag, Au, Pt, and NaCl - the unit cell size from the equation of state Temperature measurement Raman - the Stokes and anti-Stokes vibrational excitations Measurements in DAC X-ray diffraction (synchrotron) Optical, UV, IR and Raman spectroscopies Magnetic measurements Electric conductivity Ultrasonic interferometry 20 Rb2KCrF6 Optical, UV, IR and Raman spectroscopies Cubic Anvil Cell 21 22 pressure - transmitting medium pressure - transmitting medium Fe ring (electrical connection) Corundum disc (thermal insulation) Mo disc (electrical connection) sample container + lids graphite heating mantle High Pressure Two-Die Belt-Type Apparatus Belt (gasket) Sample assembly 23 High Pressure Synthesis High pressures required to access new compounds or highpressure phases Some remain stable (metastable) after decompression back to ambient conditions - intact after the pressure and temperature are quenched, a subset of these compounds can persist indefinitely – e.g., diamond  Delocalization of d-electrons  Stabilization of high oxidation states (perovskite CaFeO3, BaNiO3)  Suppression of ferroelectric displacement  Change in site preferences (Zn: 4  6) MnO2 + ZnO  ZnMnO3 Ilmenite at high pressure, CN = 6:6 Zn[ZnMn]O4 spinel at normal pressure 24 High Pressure Synthesis  Suppression of 6s2 core polarization in Tl+, Pb2+ , Bi3+ SnO2 + Pb2SnO4  2 PbSnO3 perovskite at 7 GPa, 400 C At ambient pressure, the reaction provides only SnO2 and PbO  Faster kinetics - LnFeO3, LnRhO3, LnNiO3 – few hours at h.p., at normal pressure need heating for days, LnFeO3, LnRhO3 not formed  Stabilization of new bonds Fe-Bi - no stable intermetallic compounds at ambient pressure No Fe−Bi bonds known (perovskite BiFeO3: Bi-Fe nonbonding) Phase diagram - complete immiscibility, even as molten liquids Fe + Bi  FeBi2 at 1400 K, 32 GPa d(Fe−Bi) = 2.72 Å at 30 GPa 25 Unusual Stoichiometries under High-Pressure + Cl2 + Na NaCl NaCl Laser heating 60 GPa, 2000 K 26 a – shock wave production of diamond b – high-temperature, high-pressure synthesis of diamond c – catalytic region for diamond formation d – CVD diamond e – transformation of C60 into diamond p, T - Diagram of Carbon Diamonds 27 • Exceptional hardness - the hardest known substance • Wide spectral range transparency • Chemical inertness, very resistant to chemical corrosion • The highest thermal conductivity • The highest atomic number density • The highest elastic modulus, very low coefficient of expansion • Low coefficient of friction, comparable to Teflon • Biological compatibility • Good electrical insulator, on doping becomes semiconducting • Negative electron affinity of H-terminated surface - no energy barrier prevents electrons at the conduction band minimum to exit into the vacuum - photocathodes and cold-cathode emitters 28 High Pressure High Temperature (HPHT) Synthesis of Diamonds Difficult to transform graphite into diamond The activation energy required for a sp2 3-coordinate to a sp3 4-coordinate structural transformation is very high, so requires extreme conditions Industrial diamonds (GE, 1954) made from graphite at 3000 oC and 13 GPa H. Tracy Hall (1919 – 2008) Graphite  Diamond G = + 2.9 kJ  Hr = + 1.86 kJ/mol S = 3.36 J/K 29 HPHT Synthesis of Diamonds G P (GPa)1 2 T = 300 K G T (K)800 1300 2.9 kJ 300 P = 1 bar 2.9 kJ Sg = 5.74 J/K Sd = 2.38 J/K S = 3.36 J/K The molar volume Vg = 5.31 106 m3 Vd = 3.42 106 m3 integrate Pgd = 1.534 109 Pa G = + 2.9 kJ  Hr = 1.86 kJ/mol G =  H  T S V±  0 Higher pressure slows down the reaction Higher T makes g more stable than d Higher temperature speeds up the reaction 30 Ways of getting round the difficulty  Catalyst: transition metals (graphite is dissolved in molten metal: Fe, Ni, Co, 6 GPa, 1000 C), alloys (Nb-Cu), CaCO3, hydroxides, sulfates, P (7.7 GPa, 2200 C, 10 min)  Squeezing (uniaxial not hydrostatic pressure), no heating, buckyball carbons are already intermediate between sp2-3 C60, diamond anvil, 25 GPa instantaneous transformation to bulk crystalline diamond, highly efficient process, fast kinetics  Carbon onions, electron irradiation of graphite, concentric spherical graphite layers, spacing decreases from 3.4 Å to 2.2 Å in the onion center, 100 GPa, 200 keV beam, in several hours, pressureless conversion to diamond  Using CH4/H2 microwave discharges to create reactive atomic carbon whose valencies are more-or-less free to form sp3 diamond, atomic hydrogen saturates the dangling bonds, dissolves soot faster than diamond, a route for making diamond films, 50 mm Synthesis of Diamonds 31 Carbon Onions 32 Topochemical 3D Polymerization of C60 under High P and T Micro-Vickershardness(MVH) Lonsdaleite - Hexagonal Diamond 33 Discovered in the Canyon Diablo meteorite (AZ, 50 ky, 30 t) Found also in some rocks May be stronger and stiffer than diamond Synthesized in the laboratory at static pressure of 130 kbar and temperature over 1000 °C from well-crystallized graphite in which the c axes of the crystallites are parallel to each other and to the direction of compression The crystal structure is hexagonal with a = 2.52 Å and c = 4.12 Å. density is 3.51 g/cm3, same as cubic diamond Prepared also from crystalline graphite by a method involving intense shock compression and strong thermal quenching Pressure and Electrical Conductivity 34 • Electrical conductivity of semiconductors increases with T • The change of conductivity with T is one way of measuring the band gap • Conductivity also increases with p, because atoms are pushed closer together • All elements eventually adopt metallic structures at high p • The interior of Jupiter is thought to contain metallic hydrogen! • Room-temperature superconductivity in a carbonaceous sulfur hydride - (H2S)(CH4)H2 - achieved at 287 K and 270 GPa Pressure and Electrical Conductivity 35 Sodium - BCC structure at ambient conditions, FCC at 65 GPa, further transformations Prediction: will transform under pressure into insulating states, owing to pairing of alkali atoms Pressure-induced transformation of Na into an optically transparent phase at 200 GPa Core electrons overlap, p–d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices, ionic cores and localized interstitial electron pairs, in analogy to electrides Hexagonal Na1 octahedral Na2 triangular prismatic