Domain structure of La-doped BiNiO₃ with colossal negative thermal expansion

Abstract

Although BiNiO₃ is an insulator with Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ oxidation state at ambient pressure, it undergoes a pressure-induced phase transition to metallic Bi³⁺Ni³⁺O₃ at 4GPa. In the case of Bi0.95La0.05NiO3, this charge-transfer transition occurs at ambient pressure on heating, and the volume shrinks due to the Ni-O bond shrinkage. This transition is essentially first order, however, changing proportions of coexisting low- and high-temperature phase results in colossal negative thermal expansion. In this study, in order to understand the mechanism of this unusual transition, the microstructure of Bi_0.95La_0.05NiO₃ was studied by transmission electron microscopy.