New PbTiO₃‑Type Giant Tetragonal Compound Bi₂ZnVO₆ and Its Stability under Pressure

A new PbTiO₃-type compound, Bi₂ZnVO₆, with a giant tetragonal distortion of <i>c/a</i> = 1.26 (<i>a</i> = 3.7869(3) Å, <i>c</i> = 4.7660(7) Å) was synthesized under high pressure–high temperature conditions (9 GPa and 1373 K). A point charge model calculation based on the atomic positions refined by Rietveld analysis of synchrotron X-ray diffraction (SXRD) data gave an electrical ionic polarization of 126 μC/cm², the largest value among PbTiO₃-type perovskite compounds. The tetragonality (<i>c/a</i>) decreased with increasing temperature from 100 to 570 K without any trace of a phase transition. Instead, a pressure-induced transition from a polar tetragonal structure to a paraelectric GdFeO₃ one accompanied by a 2.4% volume collapse was observed at 6.01 GPa. Bi₂ZnVO₆ showed paramagnetic behavior with <i>S</i> = 1/2 because of the random distribution of nonmagnetic Zn²⁺ and magnetic V⁴⁺ ions. Transport measurements indicated semiconductivity with an activation energy of 0.43 eV

Melting of Pb Charge Glass and Simultaneous Pb–Cr Charge Transfer in PbCrO₃ as the Origin of Volume Collapse

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO₃, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO₃ has a valence state of Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ with Pb²⁺–Pb⁴⁺ correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb–Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse

A‑Site and B‑Site Charge Orderings in an <i>s–d</i> Level Controlled Perovskite Oxide PbCoO₃

Perovskite PbCoO₃ synthesized at 12 GPa was found to have an unusual charge distribution of Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with charge orderings in both the A and B sites of perovskite ABO₃. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ quadruple perovskite structure. It is shown that the average valence distribution of Pb^3.5+Co^2.5+O₃ between Pb³⁺Cr³⁺O₃ and Pb⁴⁺Ni²⁺O₃ can be stabilized by tuning the energy levels of Pb 6<i>s</i> and transition metal 3<i>d</i> orbitals