Polar-Nonpolar Phase Transition Accompanied by Negative Thermal Expansion in Perovskite-Type Bi_{1- x}Pb_xNiO₃

Perovskite-oxide Bi_1-xPb_xNiO₃ for 0.60 ≤ x ≤ 0.80 was found to show a polar orthorhombic-to-nonpolar orthorhombic phase transition accompanied by negative thermal expansion. Bi_1-xPb_xNiO₃ showed successive crystal structure changes depending on the amount of Pb. As the amount of Pb increased, the crystal structure changed from a triclinic one with Bi³⁺/Bi⁵⁺ long-range ordering to an orthorhombic one with Bi³⁺/Bi⁵⁺ short-range ordering; then, it changed into a polar orthorhombic structure without Bi³⁺/Bi⁵⁺ ordering and finally to a polar LiNbO₃-type one. The key to the inversion symmetry breaking in PbNiO₃, where both 6s² lone-pair and Jahn-Teller active cations are absent, is the high-valency state of Pb⁴⁺. Our results suggest that the polar orthorhombic phase can be realized by using high-valence A-site cations in addition to controlling the tolerance factor.