5 research outputs found
Postperovskite Phase Transition of ZnGeO<sub>3</sub>: Comparative Crystal Chemistry of Postperovskite Phase Transition from Germanate Perovskites
The postperovskite phase of ZnGeO<sub>3</sub> 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 MnGeO<sub>3</sub> and MgGeO<sub>3</sub>. 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 GdFeO<sub>3</sub>-type
perovskites that transform to the CaIrO<sub>3</sub>-type postperovskite
phase, ZnGeO<sub>3</sub> perovskite eventually transformed to the
CaIrO<sub>3</sub>-type postperovskite phase at a critical rotational
angle of the GeO<sub>6</sub> octahedron. The formation of the postperovskite
structure at a very low critical rotational angle for MnGeO<sub>3</sub> suggests that relatively large divalent cations likely break down
the corner-sharing GeO<sub>6</sub> frameworks without a large rotation
of GeO<sub>6</sub> to form the postperovskite phase
High-Pressure Synthesis, Crystal Structure, and Phase Stability Relations of a LiNbO<sub>3</sub>‑Type Polar Titanate ZnTiO<sub>3</sub> and Its Reinforced Polarity by the Second-Order Jahn–Teller Effect
A polar
LiNbO<sub>3</sub>-type (LN-type) titanate ZnTiO<sub>3</sub> has been
successfully synthesized using ilmenite-type (IL-type)
ZnTiO<sub>3</sub> under high pressure and high temperature. The first
principles calculation indicates that LN-type ZnTiO<sub>3</sub> 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 ZnTiO<sub>3</sub> crystallizes into a hexagonal structure with a polar space group <i>R</i>3<i>c</i> and exhibits greater intradistortion
of the TiO<sub>6</sub> octahedron in LN-type ZnTiO<sub>3</sub> than
that of the SnO<sub>6</sub> octahedron in LN-type ZnSnO<sub>3</sub>. The estimated spontaneous polarization (75 μC/cm<sup>2</sup>, 88 μC/cm<sup>2</sup>) using the nominal charge and the Born
effective charge (BEC) derived from density functional perturbation
theory, respectively, are greater than those of ZnSnO<sub>3</sub> (59
μC/cm<sup>2</sup>, 65 μC/cm<sup>2</sup>), 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 ZnTiO<sub>3</sub> is greater than that of
LiNbO<sub>3</sub> (62 μC/cm<sup>2</sup>, 76 μC/cm<sup>2</sup>), indicating that LN-type ZnTiO<sub>3</sub>, like LiNbO<sub>3</sub>, is a candidate ferroelectric material with high performance.
The second harmonic generation (SHG) response of LN-type ZnTiO<sub>3</sub> is 24 times greater than that of LN-type ZnSnO<sub>3</sub>. The findings indicate that the intraoctahedral distortion, spontaneous
polarization, and the accompanying SHG response are caused by the
stabilization of the polar LiNbO<sub>3</sub>-type structure and reinforced
by the second-order Jahn–Teller effect attributable to the
orbital interaction between oxygen ions and d<sup>0</sup> ions such
as Ti<sup>4+</sup>
High-Pressure Synthesis, Crystal Structure, and Electromagnetic Properties of CdRh<sub>2</sub>O<sub>4</sub>: an Analogous Oxide of the Postspinel Mineral MgAl<sub>2</sub>O<sub>4</sub>
The postspinel mineral MgAl<sub>2</sub>O<sub>4</sub> 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 CdRh<sub>2</sub>O<sub>4</sub> 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 MgAl<sub>2</sub>O<sub>4</sub> are absent in CdRh<sub>2</sub>O<sub>4</sub>. X-ray crystallography
revealed that this postspinel phase has an orthorhombic CaFe<sub>2</sub>O<sub>4</sub> structure, thus making it a practical analogue for
investigations into the geophysical role of postspinel MgAl<sub>2</sub>O<sub>4</sub>. Replacement of Mg<sup>2+</sup> with Cd<sup>2+</sup> appears to be effective in lowering the pressure required for transition,
as was suggested for CdGeO<sub>3</sub>. In addition, Rh<sup>3+</sup> 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 BaOsO<sub>3</sub> at 17 GPa: Ferromagnetic Evolution over 3d to 5d Series
In continuation of the series of
perovskite oxides that includes
3d<sup>4</sup> cubic BaFeO<sub>3</sub> and 4d<sup>4</sup> cubic BaRuO<sub>3</sub>, 5d<sup>4</sup> cubic BaOsO<sub>3</sub> 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 d<sup>4</sup> 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 BaOsO<sub>3</sub> did not exhibit long-range ferromagnetic
order unlike cubic BaFeO<sub>3</sub> and BaRuO<sub>3</sub>, enhanced
feature of paramagnetism was detected with weak temperature dependence.
Orthorhombic CaOsO<sub>3</sub> and SrOsO<sub>3</sub> show similar
magnetic behaviors. CaOsO<sub>3</sub> is not as conducting as SrOsO<sub>3</sub> and BaOsO<sub>3</sub>, presumably due to impact of tilting
of octahedra on the width of the <i>t</i><sub>2g</sub> 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 PdF<sub>2</sub>‑Type Oxides MO<sub>2</sub> (M = Ru, Rh, Os, Ir, Pt)
The polycrystalline MO<sub>2</sub>’s (HP-PdF<sub>2</sub>-type MO<sub>2</sub>, M = Rh, Os, Pt)
with high-pressure PdF<sub>2</sub> 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-PdF<sub>2</sub>-type RuO<sub>2</sub> and IrO<sub>2</sub> were also
studied. The refined structures clearly indicated that all compounds
crystallized into the HP-PdF<sub>2</sub>-type structure, M<sup>4+</sup>O<sup>2–</sup><sub>2</sub>, rather than the pyrite-type structure,
M<sup><i>n</i>+</sup>(O<sub>2</sub>)<sup><i>n</i>−</sup> (<i>n</i> < 4). The MO<sub>2</sub> compounds
(M = Ru, Rh, Os, Ir) exhibited metallic conduction, while PtO<sub>2</sub> was highly insulating, probably because of the fully occupied
t<sub>2g</sub> 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