High Pressure Synthesis and Physical Properties of Sr_{0.5−<i>x</i>}Ca_<i>x</i>Bi_0.5FeO₃

Abstract

Sr 0.5−x Ca x Bi 0.5 FeO 3 (0.0 ≤ x ≤ 0.5) perovskite compounds, which are solid solutions between Sr 0.5 Bi 0.5 FeO 3 (x = 0.0) that shows a single charge disproportionation (CD) transition and Ca 0.5 Bi 0.5 FeO 3 (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+ → 3/4Fe 3+ + 1/4Fe 5+), and Bi 3+ ion does not act as counter cations accepting charges as in Ca 0.5 Bi 0.5 FeO 3. 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.