Dry High-Pressure Methods 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 250 GPa, 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 Anvils, diamond, tetrahedral and octahedral $C Shock waves (km s-1) Explosions, projectiles $C Go to another planet: Jupiter (hydrogen is metallic at 100 Gbar) PRESSURE SCALE Pressure, bar System 1 Mbar = 100 GPa 1012 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 2 Dry High-Pressure Methods Pressure techniques useful for synthesis of unusual structures 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 •higher coodination number •higher symmetry •transition to from nonmetal to metal •band mixing Pressure/Coordination Number Rule: increasing pressure - higher CN Pressure/Distance Paradox: increasing pressure - longer bonds 3 Dry High-Pressure Methods Gray Sn (diamond type) stable below 13 °C Coordination number 4, Sn-Sn bond length 281 pm White Sn Coordination number 6, Sn-Sn bond lengths 302 and 318 pm 4 Dry High-Pressure Methods Examples of high pressure polymorphism for some simple solids Solid Normal structure and coordination number Typical transformation conditions P(kbar) Typical transformation conditions T(oC) High pressure structure and coordination number 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 Li2MoO4 Phenacite 4:4:3 10 400 Spinel 6:4:4 NaAlO2 Wurtzite 4:4:4 40 400 Rock salt 6:6:6 5 High-Pressure Phase Transformations 6 Phase Diagrams 9 Water 12 phases of ice up to 8 GPa 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 most abundant silicate mineral within our planet ! pyroxene (silicate chains) ilmenite > garnet > perovskite Si CN = 6 10 Condensed gases H2 metallic conductivity in dense fluid hydrogen H2+ H2- NO2 + N2O NO+ NO3- calcite CO2 heating at 10-20 GPa sp3 bonded CO4 cristobalite, tridymite 40 GPa quartz (noncentrosymmetric) N2 semiconducting oligomers (-N-)x at 100-240 GPa 11 Earth's Core 3.4 Mbar = 340 GPa, 6000 K s-Fe hcp 12 Reaction Equlibrium and Kinetics -1-r-1-1-1-T-'-1-1-T-1-1 0 50 100 150 200 Pressure {MPa} Room-temperature pressure dependence of the rate constant for different activation volume values (in cm3 mol-1) 13 Reaction Kinetics The activation volume AV* the volume difference between the transition state complex and the reactants associative type = the rate determining step involves the formation of a covalent bond negative AV* dissociative type = the breaking of a covalent bond positive AV* 14 Diamond Anvil Cell Diamond anvil cell p = F/A p = 40 GPa Atable / Aculet 10 : 1 Aculet = 100-200 laser heating T > 2500 °C Re, steel gasket Diamond transparent to radiation from IR to X-ray pressure transmitting medium: solid Ar, N2, O2, 15 Diamond Anvil Cell 16 Diamond Anvil Cell Load Load 17 Dry High-Pressure Methods Calibrating a high pressure diamond anvil • Ruby - fluorescence transition • Bi, Tl, Ba pressure induced phase transition Ruby = Cr doped corundum High pressure synthesis SnO2 + Pb2SnO4 ^ 2 PbSnO3 perovskite 7 GPa, 400 °C At ambient pressure only SnO2 and PbO products 1S CrF63- a,g (breathing) vibrational mode v r- Rb2KCrF6 ■— a -a Cfl C Si 10000 11000 12000 13000 14000 15000 (cm1) -1-1-1-1-n-£850 800 750 700 650 -1 X (nm) r T T I J & o.s .1 _ "J II I. P = 0.2 GPa T = 290 K 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Photon energy (eV) High Pressure Two-Die Belt-Type Apparatus h f pressure - transmitting medium Fe ring (electrical connection) Corundum disc (thermal insulation) Mo disc (electrical connection) sample container + lids graphite heating mantle pressure - transmitting medium 20 Synthesis of Diamonds The hardest known substance, the highest thermal conductivity Difficult to transform graphite into diamond Industrial diamonds (GE) made from graphite around 3000 oC and 13 GPa 21 p, T Diagram of Carbon T/K 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 22 98 The activation energy required for a sp2 3-coordinate to a sp3 4-coordinate structural transformation is very high, so requires extreme conditions 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 A to 2.2 A 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 |o,m 23 24 26 Organic molecule theory of diamond cleavage The jeweler's chisel if placed correctly on a diamond, with a well oriented blow, always cause cleavage along {111} greater than 90% of the time, imagine the cost of a mistake with a large crystal The number of bonds broken per unit area (that is, surface energies) for different planes does not explain the observations of preferential {111} cleavage!!! Diamond viewed in terms of layers of polycondensed cyclohexane rings with axial bonds between layers and equatorial bonds within layers Unfavorable axial-axial C-C bond interactions at 2.51 A versus equatorial-equatorial at 2.96 A 27 Model compounds like cis-decalin versus trans-decaline cannot be relieved by bond rotation as in cyclohexane itself, cis can only isomerize to trans by bond cleavage followed by recombination, hence origin of the high activation energy for the cis-to-trans isomerization of decalin. A breaking molecule theory: axial-axial unfavorable interactions cause the mechanical energy of the jeweler's chisel to be funneled into preferential breakage of an axial C-C bond This then induces a kind of domino effect whereby the adjacent axial C-C bonds break and C-C bonds throughout the entire {111} plane are severed 28 Topochemical 3D Polymerization of C60 under High P and T Density, g/cm 29 Polymerization of C60 Caa phase no. of covalently bonded neighbors MVH, kg/mm2 4*1« g^m3 dobb g/crn3 monomer 0 15 1.684 1.68 2D Immm 4 SO 1.936 1.93 2D Rim i: 100 2.004 1.98 3D Immm 8 3,500 2.78 2.65 12 4,500 2.81 2.61 Diamond3 10,000 3.52 c-BN* 5,000 30 • 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! 31