Phase relations of garnet with variable compositions were studied using different types of high pressure apparatus and synchrotron radiation. The results of in situ X-ray observations and transmitting electron microscope (TEM) analysis of the recovered samples revealed that garnet first breakdown into an assemblage consisted of aluminous perovskite and other phase (s) under the lower mantle condition. The solubility of aluminum into the perovskite phase is likely to be enhanced by pressure and an existence of iron and calcium.

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The subsolidus phase boundaries and melting curves in the system CaO-MgO-SiO₂ produce a grid of sufficient density and coverage at temperatures up to 2600°C and pressures up to 25 GPa to be suitable as a calibration grid for multianvil experiments. The relative positions of these phase boundaries have been determined with high precision in high-pressures experiments using the same multianvil apparatus, sample assembly, experimental procedures and calibrations to maximize the internal consistency of the results. The grid can be easily adjusted in the future to reflect a consensus based on new advances in our understanding of the absolute temperatures and pressures for selected calibration points.

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The Dabie high- and ultrahigh-pressure metamorphic belt is a collisional orogen where coesite and microdiamond inclusions were found. Ten eclogite samples, five from the high-pressure (HP) belt and five from the ultrahigh-pressure (UHP) belt, were selected and measured the compressional wave velocities (V_p) in three mutually orthogonal directions. The UHF eclogites seem to have higher V_p than the HP samples at the identical pressures. The anisotropy in samples from UHP belt (4. 16 %) is less than that from HP belt (6. 37 %). Eclogite, with a possible content from 7. 8 to 16. 5 % is distributed in the crust beneath the central Dabieshan.

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We report recent experimental results on the electrical conductivity of Earth's mantle materials synthesised and measured in a laser heated diamond anvil cell. This results show an effect of oxygen partial pressure on the conductivity and on the chemistry of the high pressure phases synthesised in the DAC. Previous treatments under controlled fO2 conditions have a significant effect for natural olivines, attributed to the presence of a small amount of nickel. No effect was observed for pure synthetic olivines. The gas trapped in the sample during the closure of the cell has an influence on the produced material. Our results on the electrical conductivity of perovskite and magnesiowustite are in good agreement with recent geomagnetic data.

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The method is described which utilizes slow (10-100 mHz) AC current to determine the electrical conductivity of ferromagnesian minerals at pressures up to 23 GPa and temperatures of 300-2000 K in the 6-8 type multi-anvil high-pressure apparatus. The oxidation state of the sample is controlled just below IW buffer. The range of electrical conductivity which can be measured using the present method is 100Ω-5MΩ. An example of measurement was conducted for Mg_{0. 93}Fe_{0. 07}SiO₃ ilmenite.

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Electrical conductivities for olivine at 10 GPa, wadsleyite at 15 GPa and ringwoodite at 20 GPa, all with San Carlos (Mg_{0. 9}Fe_{0. 1}) ₂SiO₄ olivine starting composition were determined by complex impedance spectroscopy in a multianvil apparatus. Results for olivine are in good agreement with previous studies, both at elevated pressure and ambient pressure, suggesting that its conductivity is only weakly dependent on pressure. Conductivities for wadsleyite and ringwoodite are significantly higher than olivine (about 2 orders of magnitude), but with slightly lower activation energies. These differences can be attributed to the likely presence of ferric iron in the wadsleyite and ringwoodite.

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We have conducted an in situ synchrotron x-ray diffraction study on a monoclinic pyroxene: CaMgSi₂O₆ diopside at simultaneous high pressures and high temperatures up to 8. 2 GPa and 1280 K. A modified Rietveld profile refinement program has been applied to refine the diffraction spectra of low symmetry and multiple phases observed in energy dispersive mode. Thermoelastic parameters for diopside CaMgSi₂O₆ are derived by fitting the P-V-T data to the high-T Birch-Murnaghan equation of state. We obtained isothermal bulk modulus K_TO =109. 1 GPa with a pressure derivative of bulk modulus K'=∂K/∂P=4. 84, temperature derivative of bulk modulus K=∂K/∂T=-2. 05×10^-2 GPa/K, volumetric thermal expansivity α=a+ bT with values of a=2. 32×10^-5K^-1 and b=1. 88×10^-8K^-2.

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During the transformation of olivine (Fo₉₀) to ringwoodite and wadsleyite at 18-20 GPa three transformation microstructures develop: (i) Reaction rims form around olivine by incoherent grain boundary nucleation; the growth rates of these rims decrease strongly with reaction time because the volume decrease causes elastic strain to develop. (ii) Intracrystalline, coherent ringwoodite lamellae nucleate on previously formed (100) _α stacking faults. (iii) Polycrystalline ringwoodite and/or wadsleyite lenses develop in the interior of relict olivine crystals by nucleation of incoherent grains on the coherent ringwoodite lamellae. The observed intracrystalline transformation mechanism and the time-dependence of growth have important implications for predicting the depth of the olivine-spinel transformation in subducting lithosphere.

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Phase transformations in earth interiors is discussed on the basis of the lattice instability under the condition of homogeneous or inhomogeneous stress field. X-ray diffractometry and molecular dynamics calculation elucidate. the atomic positional displacement and fluctuation of crystallographic translation operation resulting in the lattice deformations. The lattice instability is clarified by diffraction profile analysis. Reversible pressure-induced transformations come out from the elastic deformation. On the other hand irreversible transformation are caused by the plastic deformation.

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A number of ABO₃ compounds having the ilmenite structure at low pressures have been shown to transform to the orthorhombic perovskite structure at high pressures and temperatures. The compounds that exhibit this behavior contain cations having a relatively restricted range of radii and radius ratios, and include MnTiO₃, FeTi_O3, MgTiO₃, MgGeO₃, and MnSnO₃. In addition, two silicates with mixed cation content show similar behavior. When these materials are quenched to ambient conditions, the products often are twinned crystals having the LiNbO₃ structure. It was thought originally that twinning was required in this transition, but examination of TEM photographs indicates the presence of untwinned regions of the LiNbO₃-type phase. However, this structure appears to be metastable with respect to ilmenite. A₂O₃ compounds such as Fe₂O₃ also transform similarly to the perovskite structure at high pressures and temperatures, but the intermediate metastable form is different from the ABO₃ type. It has been predicted that this could be the Rh₂O₃ II structure.

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High pressure high temperature transition of almandine garnet (Fe₃Al₂Si₃O₁₂) was examined at 20-23. 5 GPa at 1200-1600°C in the oxygen fugacity of iron-wustite buffer, using a multi-anvil apparatus. It was found that almandine dissociates into a mixture of wustite, corundum and stishovite. The dissociation boundary is placed at 20-21 GPa at 1200-1600°C with a slightly negative slope. Enthalpy of formation of almandine was measured by high temperature solution calorimetry and thus enthalpy of the almandine dissociation was obtained. The thermodynamically calculated dissociation boundary agrees with that determined by the phase equilibrium experiments. This dissociation boundary may be used as a pressure standard at around 20 GPa at high temperature. The present results in combination with those on pyrope suggest that almandine-pyrope garnet is stable in a depth range down to the upper part of the Earth's lower mantle.

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The present study deals with single crystal structure investigations of Mg _{0. 4}Al_{2. 4}O₄ and ZnCr₂S₄ by means of X ray scattering. The compressibility of the oxide substance is very small, whereas the structural changes are surprisingly high. In contrast, the compressibility of the thiospinel is 3 times higher, but the structural changes are very small. The transition to a non-detectable high pressure phase of Mg_{0. 4}Al_{2. 4}O₄ turns out to be reversible considering the existence of a spinel single crystal, but not reversible concerning the values of the lattice constant and the positional parameter of the oxygen anion. This can be explained by pressure induced disordering of the cations, which is preserved on abrupt pressure decrease.

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Powder x-ray diffraction data for FeS polymorphs were collected at high pressure and temperature, using monochromatic synchrotron x-radiation and image plate. The diffraction data collected at 10. 95 GPa and 300 K revealed a monoclinic high-pressure phase (FeS III) with cell parameters, a = 8. 025 (4) Å, b = 5. 614 (2) Å, c = 6. 414 (4) Å, and β=93. 01 (4) º. The space group of this monoclinic phase is P2₁/a . We also collected diffraction data for the high PT phase (FeS IV) at 5. 2 GPa and 9. 0 GPa and at 530 K and for the high-T phase (FeS V) at 3. 4 GPa and 650 K. The FeS IV phase has a hexagonal (2a, c) unit cell (space group P6₃mc) and the FeS V phase has a simple NiAs-type structure (space group P6₃/mmc). The structure parameters were determined by Rietveld refinements.

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Breakdown reaction of natural garnet was studied by an in situ x-ray observation using an MA8-type multi-anvil apparatus up to about 32 GPa. Natural garnet with a composition (Mg_{0. 72}Fe_{0. l7}Ca_{0. 11}) ₃Al₂Si₃O₁₂ breaks down at a pressure between 24 and 27 GPa to a mixture of sub-aluminous orthorhombic silicate perovskite, stishovite, Al-rich phase with unknown structure, and possibly CaSiO₃ perovskite. The unreacted garnet was observed even after heating the sample at 31. 7 GPa and 2000 K for 6000 sec. This result implies that the transition rate of the garnet is lower than that of Al-free system such as olivine and pyroxene.

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Experiments in the system MgO-FeO-SiO₂ have been the foundation of current beliefs that iron partitions preferentially into (Mg, Fe) O relative to the Mg-perovskite phase and occurs nearly all as Fe²⁺. Recent experiments in the system MgO-FeO-SiO₂-Al₂O₃, however, show that (1) the presence of small amounts of Al stabilises Fe³⁺ in (Mg, Fe) SiO₃ perovskite to values exceeding 50% Fe³⁺/·Fe; and (2) the solubility of Fe³⁺ in (Mg, Fe) O is extremely low at high P, T, regardless of oxygen fugacity. These results have important implications for physical and chemical properties of the lower mantle.

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The influence of melt composition and temperature on the partitioning of siderophile elements between magnesiowustite (Mw) and silicate melt (Sil) has been studied systematically at 9 GPa. Additional experiments at pressures above 20 GPa were performed to determine the effect of pressure. The determined partition coefficients (K_D (M/Mg) ^Mw/Sil) of M = Fe, Ni, Co, Mn, Cr and V show systematic variation as a function of temperature, pressure and melt composition. Comparison to literature data indicates that variables other than those investigated, for example magnesiowüstite composition may be responsible for the large differences reported for Mw-silicate liquid partitioning. According to the present results magnesiowüstite fractionation can bc cxcluded to contribute significantly to the depletion of the observed elements in the mantle.

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Magnetic susceptibility and its anisotropy of rocks from depths down to 8081 m were investigated in dependence of directional stress. Dominant ferrimegnetic minerals were determined using thermomagnetic analysis of magnetic susceptibility. Depth layers, exhibiting unstable behaviour of the studied magnetic parameters under stresses corresponding to actual in situ conditions, were determined and discussed. Rock samples from these strata contain primarily magnetite or mixture of two pyrrhotite phases.

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Thermal analysis of the Hugoniot data of quartz was performed to discuss the equation of state (EOS) and phase transitions of stishovite, considering the changes in Grüneiisen parameter and internal energy due to phase transition. It was found that the effect of internal energy change was small, but the shock temperature and thermal pressure rather depended on the Grüneisen parameter. The derived isothermal compression curve of the high pressure phase was situated in the lower density region than those derived from the ultrasonic and Brillouin scattering data, and, at the pressures higher than 80-100 GPa, it was off towards the lower density region than the static X-ray data (room temperature).

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Ultrasonic techniques for multi-anvil high-pressure apparatus are adapted for use in a DIA-type cubic-anvil apparatus (SAM 85) and installed on the superconducing wiggler beamline (X17B) at the National Synchrotron Light Source of the Brookhaven National Laboratory. Sound wave velocities and lattice parameters have been measured on samples of polycrystalline alumina (GE Lucalox and Coors 998) to simultaneous pressures of 9 GPa and 800 K

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I propose a new experimental method to measure acoustic properties of liquid and soft material. The resonance method utilizes eigenvibration modes to determine elastic properties of a sample with given density, size, and shape [1, 2, 3]; it has been used for many solid samples, even at high temperatures up to ∼1800 K [4]. A new of idea vibrating a liquid sample together with its solid container enables us to apply the resonance method even to liquid, or extremely soft material. For that experimental analysis, I have developed the xyzr algorithm which enables us to calculate eigenfrequency of bored and laminated objects. I have conducted preliminary experiments on soft plastic (Juracon) accommodated in an aluminum tube and silicone rubber in a copper tube. I succeeded in identifying several peaks on that composite systems, and recalculated v_s=950 m/s for Juracon and V_s=220 m/s for silicone rubber, respectively. The resonance method enables us even to determine Q-value from peak width. Those Q is 59 for Juracon and 580 for silicone rubber, respectively at around 70 kHz.

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The thermal conductivity of iron, k, under conditions of high temperatures and pressures is evaluated from the measured temperature distributions in laser-heating experiments in the diamond-anvil cell. The interpretation of the results is based on the assumption that k is essentially electronic, and is dominated by electron-phonon scattering, normal and Umklapp processes being involved. Reasonable agreement with k data for iron, deduced from shock wave measurement is found.

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Melting of the compressible alkali halides (KBr, KCl, CsI, NaCl, LiF) have been measured up to 1 Mbar. Their melting slopes approach values between 1 and 2 K/kbar at high pressures, a trend similar to almost all other materials measured thus far. Based on this trend and the close agreement in the melting temperatures between the diamond cell and shock wave experiments for the alkali halides and aluminum, and based on systematics in the magnitude of the change in sound velocity upon melting, we reinterpret the shock sound velocity measurements on iron as indicating onset of melting at 2 Mbar instead of 2. 4 Mbar as previously reported. This may reconcile the assumed discrepancies between static and shock experiments for iron. A new method for analyzing samples from laser heated diamond cells using an ion probe is presented. Chemical diffusion profiles were measured on samples with dimensions of order 10 μm by depth profiling with a radial resolution of about 2μm and a depth resolution of about 100 nm. We present first results on the partitioning and diffusion of nickel and cobalt in Mg-Si-perovskite at pressures up to 800 kbar and temperatures up to 2500 K.

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Partitioning of REE between garnet and basaltic melt was performed at 7. 5GPa and 1900∼2090°C, considering the possible influence of Cr. It was confirmed that compatibility of light REE in Cr-bearing garnet increases significantly as comparing with that of Cr-free system, but no evident variation was detected for heavy REE. Based on the newly established partition coefficients, it is concluded that the light REE-enriched garnet inclusions in diamonds have not achieved equilibrium with metasomatic melts. Bulk mixing and subsequent recrystallization could be the main processes for the mineral inclusions in diamonds.

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The exchange reaction of Fe²⁺ and Mg²⁺ between olivine and clinopyroxene has been experimentally determined at 1150°C, and at pressures 1 and 1. 5 GPa in the Ca-poor and Ca-enriched systems, the bulk ratio Fe/ (Fe+Mg) which was fixed as about 0. 5. The CaO contents of olivine and clinopyroxene seriously affect on the Fe-Mg partition coefficient K_D [= (X^Cpx_Mg X^Ol_Fe) / (X^Cpx_Fe X^Ol_Mg) ], which increases with increasing X^Ol_Ca. The K_D was found as 1. 50 at 1 GPa for the assemblage of olivine (X^Ol_Ca = 0. 006) and subcalcic clinopyroxene (X^Cpx_Ca = 0. 2898) ; 2. 90 at 1. 5 GPa for the assemblage of diopside-hedenbergite clinopyroxene and olivine (X^Ol_Ca = 0. 1048), in contrast to 3. 00 at 1 GPa for the assemblage of diopside-hedenbergite clinopyroxene, olivine with X^Ol_Ca = 0. 1464 and ferroan monticellite. On the other hand, the distribution coefficient K_D was found to be insensitive to pressure comparing with the previous data. The olivine-ferroan monticellite solvus becomes narrow with increasing pressure, but this pressure effect on the solvus is very small.

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Element partitioning between mantle minerals (Mg-perovskite, Ca-perovskite, majorite, olivine) and coexisting silicate melt has been investigated at pressures 14 - 23 GPa and 2400 - 2600°C, by using 6-8 type multianvil high-pressure apparatus. The observed partition coefficients, D, were plotted on partition coefficient-ionic radius (PC-IR) diagrams. A peak was found at around 70 pm in the D-profile of olivine. It was difficult to identify peaks of D-profile in majorite/liquid and Mg-perovskite/liquid, but the profile of Mg-perovskite/liquid suggests a peak at around 70 pm, which may correspond to the 8-fold site of distorted perovskite structure. There was a bottom of the D-profile of Caperovskite/liquid at around 70 - 80 pm, and the existence of peaks at around 50 and 100 pm is expected from the observed D-profile. Partition coefficients of large ions, such as Ca and La, were large in Ca-perovskite/liquid.

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In-situ IR measurements were conducted with varying heating duration to study a time-dependent changes (i) an increase of the 3379cm^-1 absorption peak at room temperature (RT) assigned to OH associated with Al³⁺ substitution for Si (OH (Al) ) and (ii) a decrease of the 3482cm^-1 peak (RT) assigned to OH (Al) including a influence of Li⁺ interstitial defects or Li-O-Al as acharge compensating role. These (i) and (ii) are consideredto be dueto an exchangeof Li⁺ with H⁺, since theseions become more diffusive at higher temperatures.

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Water in the earth's crust has been found to be distributed heterogeneously in rocks by means of infrared microspectroscopy. Water is present as different chemical species in polycrystalline quartz aggregates: OH in crystal defects, fluid inclusions, water molecule layers at the grain boundaries and pore waters. Recent in-situ IR microspectrosopy revealed that the real physicochemical nature of water in silicates at high temperatures is different from that at ambiant conditions. In-situ dehydration kinetics of some hydrous minerals and glasses were investigated in order to provide quantitative bases for dynamic processes such as metamorphism, deformation and volcanism.

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High pressure in situ x-ray observations of silica (cristobalite) clarified very large effect of non-hydrostaticity on the crystal structure of high pressure polymorph and the apparent compressibility. Under quasi-hydrostatic compression, cristobalite transformed to a new polymorph, which gives very similar diffraction profile to that of stishovite, while under strongly non-hydrostatic compression, a completely different phase named "X-I" was formed. Very different compression curves of cristobalite were observed, depending on the hydrostaticity of applied pressure, which is well explained using the elastic theory.

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A one-dimensional transient (pulse) method for thermophysical property measurements has been applied to small samples of mantle materials in a uniaxial split-sphere high-pressure apparatus. Thermal diffusivity and thermal conductivity of fused silica and garnet have been measured. At pressures up to 8. 3 GPa the thermal conductivity and the thermal diffusivity of fused silica decrease with increasing pressure. For a principal mantle mineral, garnet, the pressure derivatives of thermal conductivity and the thermal diffusivity are positive: they are 2 per cent of increase a GPa at room temperature over 5 GPa.

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A four-parameter model for calculating P-V-T equation of state and adiabatic Lamé constants of MgSiO₃ and CaSiO₃ perovskites is presented. The resulting equations of state are in good agreement with available experimental data. The calculated density and sound velocities, v_p and v_s, of these silicate perovskites under lower-mantle conditions are compared with PREM.

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(Mg_1-x, Fe_x) SiO₃ majorites with starting composition of X=0. 2 synthesized at 20 GPa and ∼2000-2200°C, and natural majorite in the shocked Tenham meteorite were examined by analytical electron microscopy. The diffraction patterns of synthetic majorites in all the recovered specimens showed a tetragonal symmetry and {101} twins. Tweed structures were also observed in the specimens quenched only from temperatures ≥ 2200°C. On the other hand, natural cubic majorite shows neither tweed structure nor twinning. These contrasting observations suggest that {101} twin lamellae were formed by the cubic-tetragonal phase transition during quenching. Cubic (Ia 3 d) majorite seems to have a wide stability field at high pressures and high temperatures.

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In situ X-ray observation has been made on Mg₅SiO₄ forsterite in the pressure ranges over 25-30 GPa and 1250°C using double stage multianvil system with sintered diamond anvil window and intense X-rays from synchrotm radiation. Forsterite (α-phase) transfomed into spinel (γ-phase) at 28 GPa and 800°C in the fist heating cycles of the two runs. Spinel transformed into the assemblage of perovskite and periclase in the second heating cycless at 26. 4 GPa and 1200°C and at 28. 5 GPa and 950°C. The results show that α-γ transition can proceed outside the stability field of spinel, and that the blocking temperature, above which the stable assmblage of perovskite and periclase can be formed, has a pressure dependence.

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Phase relations of the MgSiO₃-Al₂O₃ system were examined with special interest of post garnet transition. High pressure experiments were performed using 6-8 type multianvil apparatus based on conventional quenching method at pressures of 26 to 37 GPa and 1600 to 1800°C. It was observed that pyrope dissociates into perovskite solid solution plus corundum solid solution above about 26. 5 GPa, and it was suggested that this phase boundary has a very small negative slope. The stability field of single phase perovskite solid solution in the system MgSiO₃-Al₂O₃ extends as pressure increases, and perovskite solid solution isochemical with pyrope stabilizes at about 37 GPa and 1600°C.

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The kinetics of the α-quartz-coesite transformation was investigated at 4 GPa using both powder and single crystal samples. The results could be analyzed by the Avrami rate equation. Determined exponent n gave values close to 1 for both powder and single crystal experiments. This fact suggests that the growth process is able to control the kinetics. Observations by an optical microscopy in partly transformed single crystals indicate that the coesite phase homogeneously nucleates on the surface of the crystal and grows into the interior. In the case of powder samples, the empirically activation energy of the transformation was determined as 200 (±30) kJ/mol.

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High pressure structural change of an obsidian from Iceland has been investigated using X-ray radial distribution function (RDF) analysis and Raman spectroscopy. Obsidians have been compressed under 4 GPa at 20°C and 500°C by a cubic anvil apparatus. The obsidian treated under 4 GPa-20°C condition shows almost the same density and RDF curve as the original obsidian. However, the permanent densification of about 9% was observed by the compression under 4 GPa-500°C Based on RDF and Raman analyses, the structure of obsidian may change in the dominant TO₄ (T=Si and Al) from 5 and 6 to 4 and 3 membered rings by the compression at 500°C.

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Crystal structure of a-CaSi₂O₅ synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P1. The structure is closely related to that of titanite, CaTiSiO₅ and consists the square pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0. 33 Å.

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A single crystal ofphase G, Mg_{1. 24}Si_{1. 76}H_{2. 48}O₆was analized with synchrotron radiation. The structure ofphase G has close structural similarity to that ofstishovite, SiO₂. The valence sum ofthe O site is 1. 52, indicating the existenceofhydrogen bonding as anion chemistry for oxygen being OH_1/2. The possible sites for hydrogen are along the O-O edge of vacant tetrahedrabetween corners of MgO₆ octahedrawhich are separated by a distance of 2. 654 (6) Å.

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Hydrostatic pressure-dependent Raman scattering studies in the range 0. 0 - 9. 66 Gpa were carried out in LiNaSO₄ at room temperature in order to investigate its phonon spectra in relation to the phase transition in the crystal. From the behavior of v₁ (SO₄) modes for A₁ species, we observe that the LiNaSO₄ undergoes two phase transitions: one at ∼ 2 GPa and the other for P>5. 5 GPa.

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The compressibilities of ethylenglycol and ethanolamin silica sodalite (EGS-SOD and EAS-SOD) have been measured up to 3. 1 and 3. 2 GPa, respectively. Both compounds show extraordinarily high compressibilities, from a fit to a Birch Murnaghan equation of state the bulk moduli and pressure derivatives calculate to K=8. 3 (5) GPa, K'=8 (1) and K=9. 7 (7) GPa, K'=3. 8 (7). EGS-SOD shows three phase transitions to a monoclinic and two triclinic high pressure phases in the pressure range investigated. These phase transitions are completely reversible and no twinning has been observed while passing the transition pressures. The structures of the monoclinic and of the 1st triclinic high pressure phase have been solved on the basis of intensity data collected with synchrotron radiation. Whereas the ethylenglycol guest molecules are dynamically disordered under room conditions, they adopt an ordered arrangement in the 2nd high pressure phase. Structure refmements of EASSOD again reveal a triclinic high pressure phase at 1. 1 GPa.

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Advances in the generation and transmission of very high frequency ultrasonic signals make it practical to measure elastic properties in single crystals small enough to be subjected to high pressures and temperatures in a new diamond anvil cell designed for simultaneous ultrasonic and x-ray diffraction measurements up to 5 GPa and 400°C. In addition to providing a new accurate method for measuring elastic properties this technique can provide simultaneous equations of state which can be used to produce a new primary pressure scale. Preliminary measurements on MgO show the feasibility of this new technique.

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The pressure range of single crystal structure determination using a sealed source was extended to over 30 GPa with the improved techniques in this study. Rare gas solid helium was found to be the only pressure medium that can secure hydrostatic environment to very high pressure. The solid argon or neon induces significant divitoric stresses in crystals at relatively low pressure. The crystal volume was doubled at certain targeting pressure by using a new gasket material. The intrinsic background of a pressure cell was significantly reduced by a special collimator. These procedures improved the signal/noise ratio to 3 - 4 times. The quality of the structure refinement and the accuracy of atomic coordinates at 33 GPa are comparable with measurements at ambient conditions in a diamond cell.

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Three phase transitions in MnO are newly found through in situ X-ray observation up to about 140 GPa using diamond anvil cell combined with synchrotron radiation. The rhombohedral distortion from B1 structure at 30 GPa continues to the next phase transition at 90 GPa. A change of the X-ray pattern is also observed at about 120 GPa and its crystal structure is successfully explained by B8 (NiAs) structure. The phase above 90 GPa is expected to be metallic from the highly reflective nature.

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The First International Pressure Calibration Workshop was held at Misasa, Japan on August 22-23, 1997. Sixty seven scientists, representing the diamond anvil and multianvil communities from six countries, attended. Thirty seven papers were presented in three oral sessions titled, "Equations of state and the absolute pressue scale", "Multianvil calibration", and "Diamond anvil and other studies" and eight papers were presented as posters. In a final discussion period, it was decided too publish a multi-authored paper describing the Preliminary International Pressure Scale (PIPS-97). It was decided that this scale should contain consensus pressure calibration phase transitions at room temperature, equations for well-characterized phase transitions at high temperatures and pressures, and the equations of state for NaCl, Au, and MgO.

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We investigate the two-dimensional Hubbard model by the variational Monte Carlo method. We consider off-diagonal wave functions where we improve the Gutzwiller wave function by multiplying off-diagonal factors. In the 4×4 system, we compare our results with the exact diagonalization. The variational energies are fairly close to the exact values. We show the results for the 10×10 Hubbard model.

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Using the full-potential linear muffin-tin-orbital method, we study hyperfine parameters of solid iodine under high pressure theoretically. Above 15 GPa, the direction of principal component of the electric field gradient (EFG) suddenly changes as reported by Mössbauer experiments. In the monatomic phase, the large asymmetry parameter of EFG is obtained as was observed by Pasternak et al. At the molecular dissociation pressure, the change of the contact density is very small. We conclude that for the solid iodine under pressure the molecular dissociation picture which was proposed by X-ray experiments is consistent with that of the Mössbauer experiments reported by Pasternak et al.

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The first non-empirical prediction of lattice thermal conductivity of MgO has been determined using molecular dynamics (MD), a non-empirical ionic model (the Variationally Induced Breathing (VIB) model), and Green-Kubo theory. The computation is first-principles is the sense that no parameters are fit to experiment. Results are presented at low pressure as a function of temperature, and for 2500K for pressures to 290 GPa. We find an unexpectedly small pressure effect at small compressions, perhaps due to saturation of thermal conductivity at the high temperatures due to the small mean free path. At higher pressure expected behavior is found.

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The electronic structures of two K-Ag alloys (K₂Ag and K₃Ag) formed under high pressure have been investigated with First Principles calculations. Although the participation of the K 3d orbitals is crucial in the bonding of these compounds, unlike elemental K, the s→d hybridization is found to be relatively small. The stablity of the K-Ag alloys and the short K-K distances (ca. 3. 2-3. 4Å) are a consquence of the reduction of K s - K s antibonding interactions and the enhancement of the K-Ag bonding interactions via the transfer of the electrons from the valence K 4s-bands to the Ag 5s bands.

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The high-pressure electronic structures and the phase stability of MnO and FeO were studied with the first-principles plane-wave basis pseudopotential calculations based on generalized gradient approximation (GGA) supplemented by the LDA+U method. The present calculations account well for the properties of MnO and FeO especially in the high-pressure region. Our results predict that the high-pressure phase of MnO should take the normal NiAs (nB8) structure rather than the CsCl (B2) structure. A very unique antiferromagnetic (AF) inverse B8 (iB8) structure rather than the nB8 structure is predicted as the high-pressure phase of FeO. Moreover, this AF iB8 structure with the stoichiometric composition should be a band insulator even under high-pressure.

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The stability of the B1 structure of the binary ionic compounds under high pressure has been studied theoretically, applying the density-functional formalism to AgCl, which exhibits the phase transition from the B1 to KOH-type structure. The result is that the ground state properties and the phase transition are described quite accurately. The difference in the stability of the B1 structure of AgCl and NaCl was discussed based on the atomic structure of cations.

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