Osamu Ohtaka

Noble gas partitioning between metal and silicate under high pressures

Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earths core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.

Detailed structures of hexagonal diamond (lonsdaleite) and Wurtzite-type BN

Hexagonal diamond (hDIA) and wurtzite-type BN (wBN) powders were synthesized using a Kawai-type high-pressure apparatus and the essential details of their structures were examined by Reitveld refinements in order to investigate their thermodynamic stability and transition mechanism. X-ray diffraction profiles of the products were well explained by a mixture of hDIA and cubic diamond (cDIA) with stacking faults. The mass fraction of hDIA and cDIA was 50:50 for the products annealed between 800 and 1400°C and it became 20:80 for the product annealed at 1600°C. Temperatures higher than 1600°C seem to favor the formation of cDIA or to induce the conversion from hDIA to cDIA. Structure refinement revealed that a decrease and an increase in the basal and apical distances of C–C and B–N bonds in hDIA and wBN, respectively, are introduced by lowering the symmetry from cubic to hexagonal. Since the relative stability of wurtzite-type compounds largely depends on the distortion of the tetrahedral bond angle, the deviation from the ideal tetrahedron in both hDIA and wBN was refined to discuss their stability. The transition mechanism from graphite and graphite-like BN to hDIA and wBN is discussed by comparing the present results and those of previous simulation studies. Based on analogous features observed in the synthetic hDIA and lonsdaleite (natural hDIA found in meteorites), the formation mechanism of hDIA in meteorites is proposed.

Single-crystal X-ray diffraction study of CaIrO3

Abstract Single crystals of CaIrO3 were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b = 9.866(6), and c = 7.302(5) Å. The structure is a three-dimensional dense structure with small vacant spaces. The CaIrO3 structure can be described as a pseudo-one-dimensional oxide and is compared with Ca4IrO6 structure. The IrO6 octahedra are significantly distorted, in contrast to other octahedral Ir4+ compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling’s rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIrO3 crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIrO3-type post perovskite MgSiO3 crystals may develop under the shear flow in the Earth’s mantle.

Cubic phase of single-crystal LaAlO3 perovskite synthesized at 4.5 GPa and 1273 K

Single crystals of LaAlO3 (lanthanum aluminium trioxide) have been synthesized at 4.5 GPa and 1273 K, in the presence of an NaCl + KCl flux. The compound crystallizes with the cubic perovskite structure (space group Pm\overline{3}m). The thermal vibration of the O atom is remarkably suppressed in the directions of the Al—O bonds, and this anisotropy ranks among the largest observed in stoichiometric cubic perovskites.

Electric conductivity of olivine under pressure investigated using impedance spectroscopy

We have set up an electrical conductivity measurement system under high-pressure and high-temperature conditions with a multi-anvil high-pressure apparatus based on an AC complex impedance method. With this system, we have successfully measured the electrical conductivity of San Carlos olivine under pressure up to 5 GPa. We explain the physical meaning of the activation enthalpies and activation volumes and give values for these data, which will be a great help for understanding the conduction mechanism for olivine.

Static disorders of atoms and experimental determination of Debye temperature in pyrope: Low- and high-temperature single-crystal X-ray diffraction study

Abstract Low- and high-temperature single-crystal X‑ray diffraction studies of synthetic pyrope garnet have been conducted at 20 temperature-points over a wide temperature range from 96.7 to 972.9 K. From precise structure refinements, the possibility of static disorder and anharmonic thermal vibration of Mg in the dodecahedral site, which has long been under debate, has been assessed together with the thermal expansion behavior. Application of the Debye model to the temperature dependence of the resulting mean square displacements (MSDs) of atoms shows that they clearly include significant static disorder components in all the constituent atoms, of which Mg has the most dominant static disorder component. The residual electron density analyses and the structure refinements based on the split-atom model at a low temperature of 96.7 K provide direct proof of the Mg static disorder. Anharmonic structure refinements applying the Gram-Charlier expansion up to the fourth-rank tensor show that the anharmonic contribution to atomic thermal vibrations begins to appear in all atoms except Si at a high temperature of T > 800 K. However, at lower temperatures, anharmonic refinements show no sign of anharmonic thermal vibrations on any atom and instead provide an indication of their static disorder.

Phase Relations and EOS of ZrO 2 and HfO 2 Under High-temperature and High-pressure

The phase relations and equations of state of ZrO 2 and HfO 2 high-pressure polymorphs have been investigated by means of in situ observation using multi-anvil type high-pressure devices and synchrotron radiation. Baddeleyite (monoclinic ZrO 2 ) transforms to two distorted fluorite (CaF 2 )-type phases at 3-4 GPa depending on temperature: an orthorhombic phase, orthoI, below 600 °C and a tetragonal phase, which is one of the high-temperature forms of ZrO 2 , above 600 °C. Both orthoI and tetragonal phases then transform into another orthorhombic phase, orthoII, with a cotunnite (PbCl 2 )-type structure above 12.5 GPa and the phase boundary is almost independent of temperature. OrthoII is stable up to 1800 °C and 24 GPa. In case of HfO 2 , orthoI is stable from 4 to 14.5 GPa below 1250-1400 °C and transforms to the tetragonal phase above these temperatures. OrthoII of HfO 2 appears above 14.5 GPa and is stable up to 1800 °C at 21 GPa. The unit cell parameters and the volumes of these high-pressure phases have been determined as functions of pressure and temperature. The orthoI/tetragonal-to-orthoII transition of both ZrO 2 and HfO 2 is accompanied by about 9% volume decrease. The bulk moduli of orthoII calculated using Birch-Murnaghans equations of state are 296 GPa and 312 GPa for ZrO 2 and HfO 2 , respectively. Since orthoII of both ZrO 2 and HfO 2 are quenchable to ambient conditions, these are candidates for super-hard materials.

Structural change of GeO2 under pressure

Pressure-induced amorphization of α-quartz type GeO2 was studied with a newly developed X-ray diffraction system which consists of a 4-circle goniometer and a curved position sensitive detector. Single-crystal diffraction was measured under pressurs up to 7.3 GPa at room temperature in order to investigate pretransitional phenomena. Diffraction intensity and line width of the diffraction profiles showed no remarkable change up to 5.9 GPa. However, no sharp diffraction line was observed at pressures over 6.5 GPa. The bulk modulus at 0.1 MPa and its pressure derivative of α-quartz type GeO2 were determined to be KT=32.8(3.3) GPa and K′T=6.0(2.0), respectively. In situ microscopic observations of the amorphization transformation was also performed. The large volume change due to amorphization was observed and estimated to be about 10%.

Thermo-elastic property of Ca(OH)2 portlandite

An equation of state (EoS) for Ca(OH)2 portlandite has been obtained through measurements of pressure and temperature dependence of volume by means of in-situ X-ray observation. The bulk modulus and its pressure derivative at zero pressure calculated using third-order Birch-Murnaghans equation of state is 33.1 GPa and 4.2 at 300 K, respectively. The unit cell parameters and the volumes have been also determined at 573 K and 673 K. Temperature derivatives of the bulk modulus and its pressure derivative have been calculated to be −0.022 GPa/K and 0.0072 K−1, respectively. Thermal expansion coefficient of portlandite has been calculated from the EoS. The pressure dependence of entropy has been obtained from the present thermo-elastic parameters.

XAFS study of GeO2 glass under pressure

Using a large-volume high-pressure apparatus, Li2O–4GeO2 glass and pure GeO2 gel have been compressed to 14 GPa at room temperature and their local structural changes have been investigated by an in situ XAFS (x-ray absorption fine-structure) method. On compression of Li2O–4GeO2 glass, the Ge–O distance gradually becomes short below 7 GPa, showing the conventional compression of the GeO4 tetrahedron. Abrupt increase in the Ge–O distance occurs between 8 and 10 GPa, which corresponds to the coordination number (CN) changing from 4 to 6. The CN change is completed at 10 GPa. On decompression, the reverse transition occurs gradually below 10 GPa. In contrast to the case for Li2O–4GeO2 glass, the Ge–O distance in GeO2 gel gradually increases over a pressure range from 2 to 12 GPa, indicating that continuous change in CN occurs. The Ge–O distance at 12 GPa is shorter than that of Li–4GeO2 indicating that the change in CN is not completed even at this pressure. On complete release of pressure, the Ge–O distance reverts to that of the starting gel.