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

摘要

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar–nonpolar transitions, was observed for the first time in BiNi_(1–x )Fe_(x )O_(3) (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R 3c ) with a Bi~(3+)_(0.5(1+x ))Bi~(5+)_(0.5(1–x ))Ni~(2+)_(1–x )Fe~(3+)_(x )O_(3) charge distribution and short-range ordering of Bi~(3+) and Bi~(5+). The volume reduction upon heating that was induced by charge transfer between Bi~(5+) and Ni~(2+) decreased with increasing x because of the reduction in the amount of Ni~(2+). Simultaneous polar–nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.