Robust antiferromagnetism and structural disorder in Bi _xCa _1-xFeO ₃ perovskites

An investigation of Ca substitution in the multiferroic material BiFeO ₃ shows that a wide range of perovskites Bi _xCa _1-xFeO ₃ (0.4 ≤ × ≤ 1.0) can be prepared by sintering in air at 810-960 °C. 0.4 ≤ × ≤ 0.8 samples are cubic Pm3̄m, whereas x = 0.8 and 0.9 show a coexistence of the cubic and the rhombohedral R3c 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 _xCa _1-xFeO ₃ (0.4 ≤ × ≤ 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.