High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV2O4, FeV2O4, and MnCr2O4: Crystal Chemistry of AB2O4 Postspinel Compounds.

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2O4, FeV2O4, and MnCr2O4. At 1200-1800 °C, MgV2O4 Sp decomposes at 4-7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V2O3, and they react at 10-15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV2O4 Sp transforms to CT-type FeV2O4 at 12 GPa via decomposition phases of FeO wüstite + Cor-type V2O3. MnCr2O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 °C. Rietveld refinements of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 confirm that both the CT- and CF-type structures have frameworks formed by double chains of edge-shared B3+O6 octahedra (B3+ = V3+ and Cr3+) running parallel to one of orthorhombic cell axes. A relatively large A2+ cation (A2+ = Mg2+, Fe2+, and Mn2+) occupies a tunnel-shaped space formed by corner-sharing of four double chains. Effective coordination numbers calculated from eight neighboring oxygen-A2+ cation distances of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 are 5.50, 5.16, and 7.52, respectively. This implies that the CT- and CF-type structures practically have trigonal prism (six-coordinated) and bicapped trigonal prism (eight-coordinated) sites for the A2+ cations, respectively. A relationship between cation sizes of VIIIA2+ and VIB3+ and crystal structures (CF- and CT-types) of A2+B23+O4 is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A2+B23+O4 crystallize in wide ionic radius ranges of 0.9-1.4 Å for VIIIA2+ and 0.55-1.1 Å for VIB3+, whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for VIIIA2+ and 0.6-0.65 Å for VIB3+. This would be attributed to the fact that the tunnel space of CT-type structure is geometrically less flexible due to the smaller coordination number for A2+ cation than that of CF-type.

Postperovskite phase transition of ZnGeO3: comparative crystal chemistry of postperovskite phase transition from germanate perovskites.

The postperovskite phase of ZnGeO3 was confirmed by laser heating experiments of the perovskite phase under 110-130 GPa at high temperature. Ab initio calculations indicated that the phase transition occurs at 133 GPa at 0 K. This postperovskite transition pressure is significantly higher than those reported for other germanates, such as MnGeO3 and MgGeO3. The comparative crystal chemistry of the perovskite-to-postperovskite transition suggests that a relatively elongated b-axis in the low-pressure range resulted in the delay in the transition to the postperovskite phase. Similar to most GdFeO3-type perovskites that transform to the CaIrO3-type postperovskite phase, ZnGeO3 perovskite eventually transformed to the CaIrO3-type postperovskite phase at a critical rotational angle of the GeO6 octahedron. The formation of the postperovskite structure at a very low critical rotational angle for MnGeO3 suggests that relatively large divalent cations likely break down the corner-sharing GeO6 frameworks without a large rotation of GeO6 to form the postperovskite phase.

Synthesis, crystal structure, and electronic properties of high-pressure PdF2-type oxides MO2 (M = Ru, Rh, Os, Ir, Pt).

The polycrystalline MO2's (HP-PdF2-type MO2, M = Rh, Os, Pt) with high-pressure PdF2 compounds were successfully synthesized under high-pressure conditions for the first time, to the best of our knowledge. The crystal structures and electromagnetic properties were studied. Previously unreported electronic properties of the polycrystalline HP-PdF2-type RuO2 and IrO2 were also studied. The refined structures clearly indicated that all compounds crystallized into the HP-PdF2-type structure, M(4+)O(2-)2, rather than the pyrite-type structure, M(n+)(O2)(n-) (n < 4). The MO2 compounds (M = Ru, Rh, Os, Ir) exhibited metallic conduction, while PtO2 was highly insulating, probably because of the fully occupied t2g band. Neither superconductivity nor a magnetic transition was detected down to a temperature of 2 K, unlike the case of 3d transition metal chalcogenide pyrites.

High-pressure synthesis, crystal structure, and phase stability relations of a LiNbO3-type polar titanate ZnTiO3 and its reinforced polarity by the second-order Jahn-Teller effect.

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 µC/cm(2), 88 µC/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 µC/cm(2), 65 µC/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 µC/cm(2), 76 µC/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti(4+).

Perovskite-to-postperovskite transitions in NaNiF3 and NaCoF3 and disproportionation of NaCoF3 postperovskite under high pressure and high temperature.

High-pressure structural phase transitions in NaNiF(3) and NaCoF(3) were investigated by conducting in situ synchrotron powder X-ray diffraction experiments using a diamond anvil cell. The perovskite phases (GdFeO(3) type) started to transform into postperovskite phases (CaIrO(3) type) at about 11-14 GPa, even at room temperature. The transition pressure is much lower than those of oxide perovskites. The anisotropic compression behavior led to heavily tilted octahedra that triggered the transition. Unlike oxide postperovskites, fluoropostperovskites remained after decompression to 1 atm. The postperovskite phase in NaCoF(3) broke down into a mixture of unknown phases after laser heating above 26 GPa, and the phases changed into amorphous ones when the pressure was released. High-pressure and high-temperature experiments using a multianvil apparatus were also conducted to elucidate the phase relations in NaCoF(3). Elemental analysis of the recovered amorphous samples indicated that the NaCoF(3) postperovskite disproportionated into two phases. This kind of disproportionation was not evident in NaNiF(3) even after laser heating at 54 GPa. In contrast to the single postpostperovskite phase reported in NaMgF(3), such a postpostperovskite phase was not found in the present compounds.

Synthesis and magnetic and charge-transport properties of the correlated 4d post-perovskite CaRhO3.

A high-quality polycrystalline sample of the correlated 4d post-perovskite CaRhO(3) (Rh(4+): 4d(5), S(el) = 1/2) was attained under a moderate pressure of 6 GPa. Since the post-perovskite is quenchable at ambient pressure/temperature, it can be a valuable analogue of the post-perovskite MgSiO(3) (stable higher than 120 GPa and unstable at ambient pressure), which is a significant key material in earth science. The sample was subjected for measurements of charge-transport and magnetic properties. The data clearly indicate it goes into an antiferromagnetically ordered state below approximately 90 K in an unusual way, in striking contrast to what was observed for the perovskite phase. The post-perovskite CaRhO(3) offers future opportunities for correlated electrons science as well as earth science.

Pressure-induced transformation of 6H hexagonal to 3C Perovskite structure in PbMnO3.

A tetragonal perovskite PbMnO(3) was obtained by treating the 6H hexagonal perovskite phase at 15 GPa and 1273 K. Structural analysis using synchrotron X-ray diffraction suggested that PbMnO(3) crystallizes in the centrosymmetric space group P4/mmm, unlike PbTiO(3) and PbVO(3) which have a polar structure in space group P4mm. Iodometric titration revealed the presence of the oxygen deficiency of x = 0.06 for PbMnO(3-x). The hexagonal 6H and the 3C perovskite phases exhibited antiferromagnetic ordering at 155 and 20 K, respectively.