Chemical heterogeneity and approaches to its control in BiFeO₃- BaTiO₃ lead-free ferroelectrics

1 mol% MnO ₂ was used to improve electrical resistivity of lead-free 0.75BiFeO ₃-0.25BaTiO ₃ (75BFBT) ferroelectric ceramics; the materials were perovskite structured with major rhombohedral (R3c) phase. The method of incorporation of MnO ₂ was found to exert a significant influence on the structure, microstructure and electrical properties. Chemical heterogeneity in the form of core-shell grain microstructures was observed when MnO ₂ was added into the undoped calcined powder, in contrast to the relatively homogeneous materials that resulted from adding MnO ₂ into the precursor oxide mixture prior to calcination. Compositionally graded regions were detected across the grains consisting of a BF-rich core and BF-depleted shell. The occurrence of core-shell type microstructures led to various characteristic features including a high cubic phase fraction, contrast between ordered ferroelectric domain configurations in the rhombohedral core and the relatively featureless pseudo-cubic shell, constrained ferroelectric domain switching, and two distinct anomalies in dielectric permittivity at temperatures of 485 and 635 °C. The latter features are attributed to separate phase transitions in the relaxor ferroelectric shell and normal ferroelectric core regions respectively. The application of a thermal quenching procedure caused the formation of ferroelectric domain structures throughout the microstructure and resulted in dramatically enhanced ferroelectric switching behaviour. For example, the remnant polarisation of the as-sintered 75BFBT ceramic increased from 0.06 to 0.31 C m ^-2 after air-quenching. These effects are tentatively attributed to nanoscale phase segregation in the shell region of the as-sintered ceramics, resulting from thermodynamic immiscibility between the BF and BT solid solutions.