Robust Antiferromagnetism and Structural Disorder in Bi_xCa_1−xFeO₃ Perovskites

An investigation of Ca substitution in the multiferroic material BiFeO 3 shows that a wide range of perovskites Bi x Ca 1− x FeO 3 (0.4 ≤ x ≤ 1.0) can be prepared by sintering in air at 810−960 °C. 0.4 ≤ x < 0.8 samples are cubic Pm 3̅ m, whereas x = 0.8 and 0.9 show a coexistence of the cubic and the rhombohedral R 3 c BiFeO 3 -type structure. Considerable disorder arising from Bi 3+ lone-pair distortions is evidenced by synchrotron X-ray and neutron studies of the average structures of the cubic phases, and electron microscopy reveals commensurate and incommensurate local superstructures. The Bi x Ca 1− x FeO 3 (0.4 ≤ x ≤ 1.0) materials show a remarkably robust antiferromagnetic order with T N = 623−643 K and ordered moments of 3.6−4.1 μ B . They are “leaky” dielectrics with relative permittivities of ∼30−100 and bulk resistivities ∼50−500 kΩ cm at room temperature. The activation energy for bulk conduction increases from 0.27 eV for x = 0.4 to 0.5 eV for x = 1, but with a discontinuity at the cubic-rhombohedral boundary. Further processing of the x = 0.8 and 0.9 compositions to reduce conductivity through control of oxygen content could lead to improved BiFeO 3 -based multiferroics.