[15p-502-1~8] Approaching the Crystal Properties of Metal Oxide

[15p-502-5] Systematic Charge Distribution Change in Bi, Pb Perovskite Oxides

〇Masaki AZUMA¹ (1.MSL, Tokyo Tech)

Keywords:Perovskite, Charge Transfer, Negative Thermal Expansion

Bi and Pb are main-group elements, but they have charge degree of freedom stemming from the possibility of having either the 6s² (Bi³⁺, Pb²⁺) or the 6s⁰ (Bi⁵⁺, Pb⁴⁺) electronic configurations. They are called valence-skipping (or negative-U) ions because the 6s¹ configurations (Bi⁴⁺, Pb³⁺) are prohibited. Since the 6s states of these elements are close to the d levels of transition metals and the oxygen 2p level, BiMO₃ and PbMO₃ exhibit systematic valence distribution changes with changes in the 3d transition metal M. From left to right in the periodic table, BiCrO₃ through BiCoO₃ are all Bi³⁺M³⁺O₃ but BiNiO₃ has the unusual valence state Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃. Temperature-induced intermetallic charge transfer accompanied by negative thermal expansion is observed for Bi_1–xLn_xNiO₃ (Ln: lanthanides) and BiNi_1–xFe_xO₃. Similar charge distribution change is observed three times in PbMO₃ depending on the depth of the 3d level of the transition metal: from Pb²⁺M⁴⁺O₃ (M = Ti or V) to Pb²⁺_0.5Pb⁴⁺_0.5M³⁺O₃ (M = Cr or Fe, with an average valence state of Pb³⁺M³⁺O₃) to Pb²⁺_0.25Pb⁴⁺_0.75M²⁺_0.5M³⁺_0.5O₃ (M = Co, Pb^3.5+M^2.5+O₃), and finally to Pb⁴⁺M²⁺O₃ (M = Ni).