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

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