[15a-B12-1~10] 22.1 Joint Session M "Phonon Engineering"

[15a-B12-5] Intermetallic charge transfer in Bi_1-xSb_xNiO₃, colossal negative thermal expansion material

〇Takumi Nishikubo¹, Kengo Oka², Yuki Sakai³, Masaki Azuma¹ (1.MSL, Tokyo Tech, 2.Chuo Univ., 3.KAST)

Keywords:negative thermal expansion

BiNiO₃ is a perovskite compound stabilized by high-pressure synthesis. It has an unusual Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ valence state and a triclinically distorted structure with distinct crystallographic sites for Bi³⁺ and Bi⁵⁺. Simultaneous melting of charge disproportionation and charge transfer between Bi⁵⁺ and Ni²⁺ accompanied with a triclinic to orthorhombic (Pnma) structural transition is induced by application of pressure. Lanthanide (Ln) substitution for Bi and Fe substitution for Ni result in the appearance of charge transfer transition by heating at ambient pressure. The shrinkage of Ni-O bond owing to the oxidation of Ni from 2+ to 3+ leads to about 3% decrease in the unit cell volume. Negative thermal expansion (NTE) is thus observed. High-temperature and low-temperature phases linearly change their fractions, but the temperature hysteresis because of the first nature of the transition is the problem.
In this study, we investigated NTE of Bi_1-xSb_xNiO₃. Sb has 5s⁰ and 5s² charge degree of freedom. Triclinic to orthorhombic phase transition with volume shrinkage essentially the same as in Bi_1-xLa_xNiO₃ was observed for the samples with x<0.075. On the other hand, the samples with x>0.075 showed orthorhombic to orthorhombic transition with the volume shrinkage. Interestingly, the unit cell volume of each orthorhombic phase shrunk on heating, suggesting that the transition changed to of second order.