Single-crystal structure refinements and Debye temperatures of Ir2S3 kashinite and Rh2S3 bowieite.

Single crystals of Ir2S3 (diiridium trisulfide) and Rh2S3 (dirhodium trisulfide) were grown in evacuated silica-glass tubes using a chemical transport method and their crystal structures were determined by single-crystal X-ray diffraction analysis. These compounds have a unique sesquisulfide structure in which pairs of face-sharing octahedra are linked into a three-dimensional structure by further edge- and vertex-sharing. Ir2S3 and Rh2S3 had similar unit-cell parameters and bond distances. The atomic displacement parameter (MSD: mean-square displacement) of each atom in Ir2S3 was considerably smaller than that in Rh2S3. The Debye temperatures (ΘD) estimated from the observed MSDs for the Ir, S1 and S2 sites in Ir2S3 were 259, 576 and 546 K, respectively, and those for Rh, S1 and S2 in Rh2S3 were 337, 533 and 530 K, respectively. The bulk Debye temperature for Ir2S3 kashinite (576 K) was found to rank among the higher values reported for many known sulfides. The bulk Debye temperature for Rh2S3 bowieite (533 K) was lower than that for Ir2S3 kashinite, which crystallizes in the early sequences of mineral crystallization differentiation from the primitive magma in the Earth's mantle.

Aluminous hydrous magnesium silicate as a lower-mantle hydrogen reservoir: a role as an agent for material transport.

The potential for storage of a large quantity of water/hydrogen in the lower mantle has important implications for the dynamics and evolution of the Earth. A dense hydrous magnesium silicate called phase D is a potential candidate for such a hydrogen reservoir. Its MgO-SiO2-H2O form has been believed to be stable at lower-mantle pressures but only in low-temperature regimes such as subducting slabs because of decomposition below mantle geotherm. Meanwhile, the presence of Al was reported to be a key to enhancing the thermal stability of phase D; however, the detailed Al-incorporation effect on its stability remains unclear. Here we report on Al-bearing phase D (Al-phase D) synthesized from a bridgmanite composition, with Al content expected in bridgmanite formed from a representative mantle composition, under over-saturation of water. We find that the incorporation of Al, despite smaller amounts, into phase D increases its hydrogen content and moreover extends its stability field not only to higher temperatures but also presumably to higher pressures. This leads to that Al-phase D can be one of the most potential reservoirs for a large quantity of hydrogen in the lower mantle. Further, Al-phase D formed by reaction between bridgmanite and water could play an important role in material transport in the lower mantle.

Incorporation mechanism of Fe and Al into bridgmanite in a subducting mid-ocean ridge basalt and its crystal chemistry.

The compositional difference between subducting slabs and their surrounding lower-mantle can yield the difference in incorporation mechanism of Fe and Al into bridgmanite between both regions, which should cause heterogeneity in physical properties and rheology of the lower mantle. However, the precise cation-distribution has not been examined in bridgmanites with Fe- and Al-contents expected in a mid-ocean ridge basalt component of subducting slabs. Here we report on Mg0.662Fe0.338Si0.662Al0.338O3 bridgmanite single-crystal characterized by a combination of single-crystal X-ray diffraction, synchrotron 57Fe-Mössbauer spectroscopy and electron probe microanalysis. We find that the charge-coupled substitution AMg2+ + BSi4+ ↔ AFe3+(high-spin) + BAl3+ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO6 octahedra, yielding the unusual expansion of mean bond-length due to flexibility of A-O bonds for the structural distortion, in contrast to mean bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase-transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO12 (practically AO8) and BO6 polyhedra.

Crystal structure refinements of stoichiometric Ni3Se2 and NiSe.

Single crystals of Ni3Se2 (trinickel diselenide) and NiSe (nickel selenide) with stoichiometric chemical compositions were grown in evacuated silica-glass tubes. The chemical compositions of the single crystals of Ni3Se2 and NiSe were determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). The crystal structures of Ni3Se2 [rhombohedral, space group R32, a = 6.02813 (13), c = 7.24883 (16) Å, Z = 3] and NiSe [hexagonal, space group P63/mmc, a = 3.66147 (10), c = 5.35766 (16) Å, Z = 2] were analyzed by single-crystal X-ray diffraction and refined to yield R values of 0.020 and 0.018 for 117 and 85 unique reflections, respectively, with Fo > 4σ(Fo). R32 is a Sohncke type of space group where enantiomeric structures can exist; the single-domain structure obtained by the refinement was confirmed to be correct by a Flack parameter of -0.05 (2). The existence of Ni-Ni bonds was confirmed in both compounds, in addition to the Ni-Se bonds. The value of the atomic displacement parameter (mean-square displacement) of each atom in NiSe was larger than that in Ni3Se2. The larger amplitude of the atoms in NiSe corresponds to longer Ni-Se and Ni-Ni bond lengths in NiSe than in Ni3Se2. The Debye temperatures, θD, estimated from observed mean-square displacements for Ni and Se in Ni3Se2, were 322 and 298 K, respectively, while those for Ni and Se in NiSe were 246 and 241 K, respectively. The existence of large cavities in the structure and the weak bonding force are likely responsible for the brittle and soft nature of the NiSe crystal.

Natural arsenic with a unique order structure: potential for new quantum materials.

Study of arsenic (As) provides guidelines for the development of next-generation materials. We clarify the unique structure of the third crystalline polymorph of natural As (Pnm21-As) by crystallographical experiment and the electronic structure by first-principles computational method. The crystal structure of Pnm21-As is a novel structure in which the basic portions of semi-metalic grey-As and semi-conductor black-As are alternately arranged at the atomic level. For both covalent and van der Waals bonding, the contributions of sd and pd hybridizations are important. Van der Waals bonding characteristics and d orbital contributions can be varied by control of layer stacking. Total charges are clearly divided into positive and negative in the same elements for the grey-As and black-As portions, respectively, is of importance. The sequence in which one-dimensional electron donor and acceptor portions alternate in the layer will be the first description.

Rutile- and anatase-type temperature-dependent pre-edge peak intensities in K-edge XANES spectra for AO (A†=†Mn), A2O3 (A†=†Sc, Cr and Mn) and AO2 (A†=†Ti and V).

Pre-edge peaks in 3d transition-metal element (Sc, Ti, V, Cr and Mn) K-edge XANES (X-ray absorption near-edge structure) spectra in AO2 (A = Ti and V), A2O3 (A = Sc, Cr and Mn) and AO (A = Mn) are measured at various temperatures. Quantitative comparisons for the XANES spectra were investigated by using absorption intensity invariant point normalization. The energy position of the difference peak (D peak) is obtained from the difference between the low- and high-temperature XANES spectra. There are two kinds of temperature dependence for pre-edge peak intensity: rutile- and anatase-type. The true temperature dependence of a transition to each orbital is obtained from the difference spectrum. In both anatase and rutile, the pre-edge peak positions of A2 and A3 are clearly different from the D1- and D2-peak positions. The A1 peak-top energies in both phases of VO2 differ from the D1 peak-top energies. The D-peak energy position determined by the difference spectrum should represent one of the true energies for the transition to an independent orbital. The peak-top positions for pre-edge peaks in XANES do not always represent the true energy for independent transitions to orbitals because several orbital transitions overlap with similar energies. This work suggests that deformation vibration (bending mode) is effective in determining the temperature dependence for the D-peak intensity.

The vanadate garnet Ca2NaCd2V3O12: a single-crystal X-ray diffraction study.

Single crystals of the vanadate garnet Ca2NaCd2V3O12 (dicalcium sodium dicadmium trivanadate) were synthesized using the floating-zone method and the crystal structure was investigated using single-crystal X-ray diffraction. We considered the effectiveness of substitution of the Y-site cation with reference to previous structural studies of vanadate garnets. The structures of vanadate garnets are subject to geometric constraints similar to those of silicate garnets. These constraints force the tetrahedral-dodecahedral shared edge length in vanadate garnets to become shorter than the unshared dodecahedral edge length, as in ugrandite (uvarovite, grossular and andradite) garnets. However, the vanadate garnet Ca2NaCd2V3O12 exhibits the normal structural feature, similar to pyralspite (pyrope, almandine and spessartine) garnets, namely that the dodecahedral-dodecahedral shared edge length is shorter than the unshared dodecahedral edge length. With increasing ionic radius of the Y-site cation, the atomic coordinates x, y and z of oxygen adopt values which satisfy Pauling's third rule.

A new high-pressure strontium germanate, SrGe2O5.

The Sr-Ge-O system has an earth-scientific importance as a potentially good low-pressure analog of the Ca-Si-O system, one of the major components in the constituent minerals of the Earth's crust and mantle. However, it is one of the germanate systems that has not yet been fully examined in the phase relations and structural properties. The recent findings that the SrGeO3 high-pressure perovskite phase is the first Ge-based transparent electronic conductor make the Sr-Ge-O system interesting in the field of materials science. In the present study, we have revealed the existence of a new high-pressure strontium germanate, SrGe2O5. Single crystals of this compound crystallized as a co-existent phase with SrGeO3 perovskite single crystals in the sample recovered in the compression experiment of SrGeO3 pseudowollastonite conducted at 6 GPa and 1223 K. The crystal structure consists of germanium-oxygen framework layers stacked along [001], with Sr atoms located at the 12-coordinated cuboctahedral site; the layers are formed by the corner linkages between GeO6 octahedra and between GeO6 octahedra and GeO4 tetrahedra. The present SrGe2O5 is thus isostructural with the high-pressure phases of SrSi2O5 and BaGe2O5. Comparison of these three compounds leads to the conclusion that the structural responses of the GeO6 and GeO4 polyhedra to cation substitution at the Sr site are much less than that of the SrO12 cuboctahedron to cation substitution at the Ge sites. Such a difference in the structural response is closely related to the bonding nature.

Effect of cation substitution on bridgmanite elasticity: A key to interpret seismic anomalies in the lower mantle.

Seismological observations show that, in some regions of the lower mantle, an increase in bulk sound velocity, interestingly, occurs in the same volume where there is a decrease in shear velocity. We show that this anti-correlated behavior occurs on cation substitution in bridgmanite by making single crystal elasticity measurements of MgSiO3 and (Mg,Fe,Al)(Si,Al)O3 using inelastic x-ray scattering in the ambient conditions. Cation substitution of ferrous iron and aluminum may explain large low shear velocity provinces in the lower mantle.

High-temperature single-crystal X-ray diffraction study of tetragonal and cubic perovskite-type PbTiO3 phases.

A high-temperature single-crystal X-ray diffraction study of a synthetic PbTiO3 perovskite was carried out over the wide temperature range 298-928 K. A transition from a tetragonal (P4mm) to a cubic (Pm \bar 3 m) phase has been revealed near 753 K. In the non-centrosymmetric P4mm symmetry group, the difference in relative displacement between Pb and O along the c-axis is much larger than that between Ti and O. The Pb and Ti cations contribute sufficiently to polarization being shifted in the opposite direction compared with the shift of O atoms. Deviation from the linear changes in Debye-Waller factors and bonding distances in the tetragonal phases can be interpreted as a precursor phenomenon before the phase transition. Disturbance of the temperature factor Ueq for O is observed in the vicinity of the transition point, while Ueq values for Pb and Ti are continuously changing with increasing temperature. The O site includes the clear configurational disorder in the cubic phase. The polar local positional distortions remain in the cubic phase and are regarded as the cause of the paraelectricity. Estimated values of the Debye temperature ΘD for Pb and Ti are 154 and 467 K in the tetragonal phase and decrease 22% in the high-temperature phase. Effective potentials for Pb and Ti change significantly and become soft after the phase transition.

Variable-temperature single-crystal X-ray diffraction study of tetragonal and cubic perovskite-type barium titanate phases.

A variable-temperature single-crystal X-ray diffraction study of a synthetic BaTiO3 perovskite has been performed over the temperature range 298-778 K. A transition from a tetragonal (P4mm) to a cubic (Pm3m) phase has been revealed near 413 K. In the non-centrosymmetric P4mm symmetry group, both Ti and O atoms are displaced along the c-axis in opposite directions with regard to the Ba position fixed at the origin, so that Ti(4+) and Ba(2+) cations occupy off-center positions in the TiO6 and BaO12 polyhedra, respectively. Smooth temperature-dependent changes of the atomic coordinates become discontinuous with the phase transition. Our observations imply that the cations remain off-center even in the high-temperature cubic phase. The temperature dependence of the mean-square displacements of Ti in the cubic phase includes a significant static component which means that Ti atoms are statistically distributed in the off-center positions.

Crystal structure of post-perovskite-type CaIrO3 reinvestigated: new insights into atomic thermal vibration behaviors.

Single crystals of the title compound, the post-perovskite-type CaIrO3 [calcium iridium(IV) trioxide], have been grown from a CaCl2 flux at atmospheric pressure. The crystal structure consists of an alternate stacking of IrO6 octa-hedral layers and CaO8 hendeca-hedral layers along [010]. Chains formed by edge-sharing of IrO6 octa-hedra (point-group symmetry 2/m..) run along [100] and are inter-connected along [001] by sharing apical O atoms to build up the IrO6 octa-hedral layers. Chains formed by face-sharing of CaO8 hendeca-hedra (point-group symmetry m2m) run along [100] and are inter-connected along [001] by edge-sharing to build up the CaO8 hendeca-hedral layers. The IrO6 octa-hedral layers and CaO8 hendeca-hedral layers are inter-connected by sharing edges. The present structure refinement using a high-power X-ray source confirms the atomic positions determined by Hirai et al. (2009 ▸) [Z. Kristallogr. 224, 345-350], who had revised our previous report [Sugahara et al. (2008 ▸). Am. Mineral. 93, 1148-1152]. However, the displacement ellipsoids of the Ir and Ca atoms based on the present refinement can be approximated as uniaxial ellipsoids elongating along [100], unlike those reported by Hirai et al. (2009 ▸). This suggests that the thermal vibrations of the Ir and Ca atoms are mutually suppressed towards the Ir⋯Ca direction across the shared edge because of the dominant repulsion between the two atoms.

Crystal structure of SrGeO3 in the high-pressure perovskite-type phase.

Single crystals of the SrGeO3 (strontium germanium trioxide) high-pressure phase have been synthesized successfully at 6 GPa and 1223 K. The compound crystallizes with the ideal cubic perovskite-type structure (space group Pm-3m), which consists of a network of corner-linked regular GeO6 octa-hedra (point-group symmetry m-3m), with the larger Sr atoms located at the centers of cavities in the form of SrO12 cubocta-hedra (point-group symmetry m-3m) in the network. The degrees of covalencies included in the Sr-O and the Ge-O bonds calculated from bond valences are 20.4 and 48.9%, respectively. Thus, the Ge-O bond of the GeO6 octa-hedron in the SrGeO3 perovskite has a strong covalency, comparable to those of the Si-O bonds of the SiO4 tetra-hedra in silicates with about 50% covalency. The thermal vibrations of the O atoms in the title compound are remarkably suppressed in the directions of the Ge-O bonds. This anisotropy ranks among the largest observed in stoichiometric cubic perovskites.

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 Earth's 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.

Single-crystal metastable high-temperature C2/c clinoenstatite quenched rapidly from high temperature and high pressure.

The high-temperature clinoenstatite (HT-CEn) is one of the most important MgSiO3 pyroxene polymorphs, but the details of its structure and stability have been uncertain. The single crystal of the C2/c HT-CEn end-member is firstly synthesized by rapid pressure-temperature quenching from 15-16 GPa and 1173-2173 K. The single-crystal X-ray diffraction analysis shows unusual bonding distances and static disorder of the atoms frozen in this metastable structure. The degree of kinking of the silicate tetrahedral chains is 175° for HT-CEn. The chain angle for HP-CEn is substantially smaller (135°), but the angle for L-CEn is in the opposite direction at -160° (= 200°). The degree of kinking increases by being curved by more than 180° for the transition from HT-CEn to L-CEn. As for the reverse change from the expansion to the stretch, a potential barrier exists at the point of the continuity. It is suggested that the reason why a structure can be quenched under ambient conditions is as follows: the present HT-CEn single crystal has been formed by the isosymmetric phase transition from the high-pressure C2/c clinoenstatite (HP-CEn). The presence of HT-CEn from HP-CEn in natural rocks is an indicator of quenching history, which leads to the possibility that it exists in shocked meteorites and impact materials.

Temperature dependence of pre-edge features in Ti K-edge XANES spectra for ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase.

XANES (X-ray absorption near-edge structure) spectra of the Ti K-edges of ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase were measured in the temperature range 20-900 K. Ti atoms for all samples were located in TiO6 octahedral sites. The absorption intensity invariant point (AIIP) was found to be between the pre-edge and post-edge. After the AIIP, amplitudes damped due to Debye-Waller factor effects with temperature. Amplitudes in the pre-edge region increased with temperature normally by thermal vibration. Use of the AIIP peak intensity as a standard point enables a quantitative comparison of the intensity of the pre-edge peaks in various titanium compounds over a wide temperature range.

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.

Vanadate garnet, Ca2NaMg2V3O12.

The vanadate garnet Ca(2)NaMg(2)V(3)O(12) (dicalcium sodium dimagnesium trivanadium dodecaoxide), synthesized by a floating zone method, has a notable structural feature in that the dodecahedral-dodecahedral shared edge length is longer than the unshared dodecahedral edge length. It is also noteworthy that the octahedral-dodecahedral shared edge length is as long as the unshared octahedral edge length. These unusual structural features are closely related to the weak repulsions between dodecahedral cations and between dodecahedral and octahedral cations.