Enhanced Negative Thermal Expansion Induced by Simultaneous Charge Transfer and Polar-Nonpolar Transitions

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar–nonpolar transitions, was observed for the first time in BiNi1–xFexO3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1–x)Ni2+1–xFe3+xO3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar–nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.