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

Colossal Volume Contraction in Strong Polar Perovskites of Pb(Ti,V)O₃

The unique physical property of negative thermal expansion (NTE) is not only interesting for scientific research but also important for practical applications. Chemical modification generally tends to weaken NTE. It remains a challenge to obtain enhanced NTE from currently available materials. Herein, we successfully achieve enhanced NTE in Pb­(Ti_1–<i>x</i>V_<i>x</i>)­O₃ by improving its ferroelectricity. With the chemical substitution of vanadium, lattice tetragonality (<i>c</i>/<i>a</i>) is highly promoted, which is attributed to strong spontaneous polarization, evidenced by the enhanced covalent interaction in the V/Ti–O and Pb–O2 bonds from first-principles calculations. As a consequence, Pb­(Ti_0.9V_0.1)­O₃ exhibits a nonlinear and much stronger NTE over a wide temperature range with a volumetric coefficient of thermal expansion α_V = −3.76 × 10^–5/°C (25–550 °C). Interestingly, an intrinsic giant volume contraction (∼3.7%) was obtained at the composition of Pb­(Ti_0.7V_0.3)­O₃ during the ferroelectric-to-paraelectric phase transition, which represents the highest value ever reported. Such volume contraction is well correlated to the effect of spontaneous volume ferro­electro­striction. The present study extends the scope of the NTE family and provides an effective approach to explore new materials with large NTE, such as through adjusting the NTE-related ferroelectric property in the family of ferroelectrics

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