Polar–Nonpolar Transition-Type Negative Thermal Expansion with 11.1% Volume Shrinkage by Design

Chemical substitution for the tuning of the working temperature of phase-transition-type negative thermal expansion (NTE) materials generally reduces the volume shrinkage during the transition. We have investigated the effects of electron doping and reduction of 6s2 lone-pair activity in PbVO3 with a large polar distortion (c/a = 1.23) and found that the combination of Bi and Sr substitutions for Pb enables a temperature-induced polar to non-polar transition with 11% volume shrinkage, even larger than the pressure-induced volume collapse of PbVO3 (∼10.6%), and is the largest value among the NTE materials reported so far. The domain structure of the coexisting cubic and tetragonal phases with such a huge volume difference was successfully observed by high-angle annular dark-field scanning transmission electron microscopy and the spatial distribution of domains by Bragg coherent X-ray diffraction imaging. The temperature hysteresis is reduced by repeated heating/cooling cycles, suggesting that the changes in the domain structure dominate the NTE properties

Glassy Distribution of Bi³⁺/Bi⁵⁺ in Bi_1–<i>x</i>Pb_<i>x</i>NiO₃ and Negative Thermal Expansion Induced by Intermetallic Charge Transfer

The valence distribution and local structure of Bi_1–<i>x</i>Pb_<i>x</i>NiO₃ (<i>x</i> ≤ 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi³⁺ and Bi⁵⁺ was observed for all the samples, but it was a long-ranged one with distinct crystallographic sites in the <i>P</i>1̅ triclinic structure for <i>x</i> ≤ 0.15, while the ordering was short-ranged for <i>x</i> = 0.20 and 0.25. An intermetallic charge transfer between Bi⁵⁺ and Ni²⁺, leading to large volume shrinkage, was observed for all the samples upon heating at ∼500 K

Pressure Induced Amorphization of Pb²⁺ and Pb⁴⁺ in Perovskite PbFeO₃

Perovskite-type oxides have been the subject of intense research due to their various fascinating physical properties stemming from their charge degree of freedom. PbFeO3 has an unusual Pb2+0.5Pb4+0.5Fe3+O3 charge distribution with a long-ranged ordering of Pb2+ and Pb4+ and two inequivalent Fe3+ sites in a perovskite structure. Combined synchrotron X-ray diffraction and Mössbauer spectroscopy revealed a change to an orthorhombic GdFeO3 structure with a unique Fe3+ site and randomly distributed Pb2+ and Pb4+ at 29.0 GPa, namely, pressure-induced amorphization of Pb2+ and Pb4+. The absence of a charge transfer transition to the Pb2+Fe4+O3 phase, which was expected from the comparison with PbCrO3 and PbCoO3, was verified using ab initio density functional theory calculations in the range of 0–70 GPa

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