High Pressure Synthesis and Physical Properties of Sr_0.5−xCa_xBi_0.5FeO₃

抄録

Sr_0.5−xCa_xBi_0.5FeO₃ (0.0 ≤ x ≤ 0.5) perovskite compounds, which are solid solutions between Sr_0.5Bi_0.5FeO₃ (x = 0.0) that shows a single charge disproportionation (CD) transition and Ca_0.5Bi_0.5FeO₃ (x = 0.5) that shows successive CD and intermetallic-charge-transfer (CT) transitions, were obtained by high pressure syntheses. The crystal structure at room temperature changes from rhombohedral (0.0 ≤ x ≤ 0.1) to orthorhombic (0.2 ≤ x ≤ 0.5). Unusual-high-valence Fe^3.5+ ion is stabilized in the entire range of the solid solution at room temperature, and undergoes a charge transition to relieve the electronic instability. Fe^3.5+ in the range 0.0 ≤ x ≤ 0.4 shows a single 3:1 CD transition (Fe^3.5+ → 3/4Fe³⁺ + 1/4Fe⁵⁺), and Bi³⁺ ion does not act as counter cations accepting charges as in Ca_0.5Bi_0.5FeO₃. The difference in the crystal structure symmetry does not influence the 3:1 CD transition, and the transition temperature decreases monotonously with increasing x in the region 0.0 ≤ x ≤ 0.4.