Akihiko Nakatsuka

Cation distribution and bond lengths in CoAl2O4 spinel

Abstract It is widely known that CoAl2O4, IV(Co1−xAlx) VI[CoxAl2−x]O4, is a largely normal spinel (0

Symmetry change of majorite solid-solution in the system Mg3Al2Si3O12-MgSiO3

Abstract Six single crystals of Mg3(MgxSixAl2-2x)Si3O12 with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 °C with a “6-8” type uniaxial splitsphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors (Beq), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 ≤ x ≤ 0.24 show no birefringence, whereas those of x = 0.64 have a slight optical anisotropy. Moreover, the cell symmetry for x = 0.64 obtained using synchrotron X-ray radiation is tetragonal with a slight deviation from cubic. On the basis of site splitting expected from compositional dependence of Beq obtained by cubic refinement, the most probable space group in the range 0.38 ≤ x ≤ 0.64 is I41/acd (tetragonal), which is the maximal subgroup of the space group Ia3̅d (cubic). Given that the previous reports that crystals with 0.8 ≤ x ≤ 1.0 have the tetragonal space group I41/a, the majorite solid-solution in this system undergoes the series of symmetry changes, Ia3̅d →I41/acd→I41/a, with increasing MgSiO3 component. The symmetry changes from Ia3̅d to I41/acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg2+) and tetrahedral (Si4+) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I41/acd structure loses the center of symmetry with the symmetry reduction from Ia3̅d to I41/acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.

Reinvestigation of the MgSiO3 perovskite structure at high pressure

Abstract High-pressure single-crystal X-ray diffraction experiments of MgSiO3 perovskite have been carried out up to 15 GPa in a diamond-anvil cell using synchrotron radiation. Precise crystal structural parameters, including the anisotropic displacement parameters of every atom in MgSiO3, are determined under high pressure. In the pressure range up to 15 GPa, the most important responses of the structure are the compressions of SiO6 and MgO8 polyhedra and an increase in tilting of SiO6 octahedra represented by the decrease in angles between octahedra (both Si-O2-Si angle in the a-b plane and Si-O1-Si angle in the b-c plane decrease). The degree of the change in both angles in the a-b and b-c planes is the same. The amplitude of mean square displacement for the Mg atom has the largest value in the structures and its thermal vibration is significantly anisotropic at ambient pressure. Under high pressure, all atoms in the structure have obvious anisotropy of thermal vibration and the largest amplitudes of thermal vibration for Mg, Si, and O2 atoms are directed toward vacant space in the structure. Anisotropy of the structure increases with pressure.

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.

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.

Structure refinement of birefringent Cr-bearing majorite Mg3(Mg.34Si.34Al.18Cr.14)2Si3O12

Abstract A single crystal of a birefringent Cr-bearing majorite, Mg3Mg0.34Si0.34Al0.18Cr0.14)2Si3O12, was synthesized at 20 OPa and 2000 °C using “6-8” type uniaxial split-sphere apparatus. This garnet is tetragonal with the unit-cell parameters a ≈ c and deviates slightly from cubic symmetry. The structure refinements using single-crystal X-ray diffraction intensity data were carried out by assuming three space groups (one cubic and two tetragonal) to determine the most probable symmetry. The most probable space group is I41/a (tetragonal). The Cr ions show a disordered distribution between the two nonequivalent octahedral sites in the I41/a structure.

Crystal structure of single-crystal CaGeO3 tetragonal garnet synthesized at 3 GPa and 1000 °C

Abstract Single crystals of CaGeO3 garnet were synthesized at 3 GPa and 1000 °C using a cubic anvil type of high pressure apparatus and the crystal structure was refined from single crystal X-ray diffraction data. This garnet is tetragonal with lattice parameters of a = 12.535(2) Å, c = 12.370(2) Å, V = 1943.5(5) Å3 and belongs to space group I41/a. Two dodecahedral sites are occupied only by Ca with mean Ca- O bond lengths of 2.480(4) and 2.467(4) Å. The Ca and Ge cations are completely ordered at two octahedral sites with mean Ca-O = 2.301(3) Å and mean Ge-O = 1.910(3) Å. Three tetrahedral sites are occupied only by Ge, and their mean Ge-O bond lengths are 1.753(3), 1.787(4), and 1.764(4) Å. Furthermore, the present tetragonal garnet has an unusual feature in that the mean value [2.704(5) Å] of the shared edge lengths of GeO6 octahedron is larger than that [2.699(5) Å] of the unshared ones, as has also been observed for other tetragonal garnets with I41/a.

Elastic anisotropy of experimental analogues of perovskite and post-perovskite help to interpret D'' diversity

Recent studies show that the D′′ layer, just above the Earths core–mantle boundary, is composed of MgSiO3 post-perovskite and has significant lateral inhomogeneity. Here we consider the D′′ diversity as related to the single-crystal elasticity of the post-perovskite phase. We measure the single-crystal elasticity of the perovskite Pbnm-CaIrO3 and post-perovskite Cmcm-CaIrO3 using inelastic X-ray scattering. These materials are structural analogues to same phases of MgSiO3. Our results show that Cmcm-CaIrO3 is much more elastically anisotropic than Pbnm-CaIrO3, which offers an explanation for the enigmatic seismic wave velocity jump at the D′′ discontinuity. Considering the relation between lattice preferred orientation and seismic anisotropy in the D′′ layer, we suggest that the c axis of post-perovskite MgSiO3 aligns vertically beneath the Circum-Pacific rim, and the b axis vertically beneath the Central Pacific.

Oxygen-deficient strontium cobaltate, SrCoO2.64.

Single crystals of strontium cobaltate, SrCoO(3-x), have been grown by the floating-zone method in an oxygen flow. The compound crystallizes with the cubic perovskite structure, with Pm-3m symmetry, as determined by X-ray diffraction. Refinement of the O-atom site occupancy yields the chemical composition SrCoO(2.64) [x = 0.36 (3)]. The anisotropic displacement ellipsoids of the O atoms suggest that their positional disorder occurs in a direction perpendicular to that of the Co-O bonds.