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 <i>b</i>-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

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 <i>R</i>3<i>c</i> 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/cm², 88 μC/cm²) 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/cm², 65 μC/cm²), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the <i>c</i>-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/cm², 76 μC/cm²), 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 d⁰ ions such as Ti⁴⁺

High-Pressure Synthesis, Crystal Structure, and Electromagnetic Properties of CdRh₂O₄: an Analogous Oxide of the Postspinel Mineral MgAl₂O₄

The postspinel mineral MgAl₂O₄ exists only under the severe pressure conditions in the subducted oceanic lithosphere in the Earth’s deep interior. Here we report that its analogous oxide CdRh₂O₄ exhibits a structural transition to a quenchable postspinel phase under a high pressure of 6 GPa at 1400 °C, which is within the general pressure range of a conventional single-stage multianvil system. In addition, the complex magnetic contributions to the lattice and metal nonstoichiometry that often complicate investigations of other analogues of MgAl₂O₄ are absent in CdRh₂O₄. X-ray crystallography revealed that this postspinel phase has an orthorhombic CaFe₂O₄ structure, thus making it a practical analogue for investigations into the geophysical role of postspinel MgAl₂O₄. Replacement of Mg²⁺ with Cd²⁺ appears to be effective in lowering the pressure required for transition, as was suggested for CdGeO₃. In addition, Rh³⁺ could also contribute to this reduction, as many analogous Rh oxides of aluminous and silicic minerals have been quenched from lower-pressure conditions

High-Pressure Synthesis of 5d Cubic Perovskite BaOsO₃ at 17 GPa: Ferromagnetic Evolution over 3d to 5d Series

In continuation of the series of perovskite oxides that includes 3d⁴ cubic BaFeO₃ and 4d⁴ cubic BaRuO₃, 5d⁴ cubic BaOsO₃ was synthesized by a solid-state reaction at a pressure of 17 GPa, and its crystal structure was investigated by synchrotron powder X-ray diffraction measurements. In addition, its magnetic susceptibility, electrical resistivity, and specific heat were measured over temperatures ranging from 2 to 400 K. The results establish a series of d⁴ cubic perovskite oxides, which can help in the mapping of the itinerant ferromagnetism that is free from any complication from local lattice distortions for transitions from the 3d orbital to the 5d orbital. Such a perovskite series has never been synthesized at any d configuration to date. Although cubic BaOsO₃ did not exhibit long-range ferromagnetic order unlike cubic BaFeO₃ and BaRuO₃, enhanced feature of paramagnetism was detected with weak temperature dependence. Orthorhombic CaOsO₃ and SrOsO₃ show similar magnetic behaviors. CaOsO₃ is not as conducting as SrOsO₃ and BaOsO₃, presumably due to impact of tilting of octahedra on the width of the <i>t</i>_2g band. These results elucidate the evolution of the magnetism of perovskite oxides not only in the 5d system but also in group 8 of the periodic table

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, M⁴⁺O^2–₂, rather than the pyrite-type structure, M^<i>n</i>+(O₂)^<i>n</i>− (<i>n</i> < 4). The MO₂ compounds (M = Ru, Rh, Os, Ir) exhibited metallic conduction, while PtO₂ was highly insulating, probably because of the fully occupied t_2g 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