Polar-nonpolar phase transition accompanied by negative thermal expansion in perovskite-type Bi1−xPbxNiO3

Perovskite-oxide Bi1–xPbxNiO3 for 0.60 ≤ x ≤ 0.80 was found to show a polar orthorhombic-to-nonpolar orthorhombic phase transition accompanied by negative thermal expansion. Bi1–xPbxNiO3 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.