Robust Antiferromagnetism and Structural Disorder in BixCa1−xFeO3Perovskites.

An investigation of Ca substitution in the multiferroic material BiFeO3shows that a wide range of perovskites BixCa1−xFeO3(0.4 ≤ x≤ 1.0) can be prepared by sintering in air at 810−960 °C. 0.4 ≤ x< 0.8 samples are cubic Pm3̅m, whereas x= 0.8 and 0.9 show a coexistence of the cubic and the rhombohedral R3cBiFeO3-type structure. Considerable disorder arising from Bi3+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 BixCa1−xFeO3(0.4 ≤ x≤ 1.0) materials show a remarkably robust antiferromagnetic order with TN= 623−643 K and ordered moments of 3.6−4.1 μB. 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 BiFeO3-based multiferroics. [ABSTRACT FROM AUTHOR]