Electric field-temperature phase diagram of Bi1/2(Na0.8K0.2)1/2TiO3 relaxor ferroelectrics with Fe doping

Electrically and thermally induced transitions between ferroelectric and relaxor states are of great importance for Bi1/2Na1/2TiO3 (BNT)-based materials because of their close relevance to electromechanical properties. In this study, the electric field-temperature (E-T) phase diagrams of Fe doped Bi1/2(Na0.8K0.2)1/2TiO3 (BNKT) ceramics are constructed via an experiment and theory combined approach. A novel phenomenological model based on the Landau-Devonshire theory and the Preisach model is proposed to describe the electric field induced phase transitions. Using this model, an approximate free-energy landscape is obtained by fitting the experimental double polarization-electric field loop, and then the electric field representing ferroelectric and relaxor two-phase equilibrium is calculated from the free-energy landscape for the construction of an E-T phase diagram. The constructed E-T phase diagrams meet basic thermodynamics requirements, for example, the Clausius-Clapeyron relation, and, therefore, overcome some of the shortcomings of previously reported E-T phase diagrams for BNT-based ceramics. The relationship between the E-T phase diagram and electromechanical properties is also established. From the E-T phase diagrams, it is predicted that Fe doping could lower the threshold field of triggering giant strains of BNKT ceramics at a high temperature. This prediction is successfully verified by experimental measurement of the electric field induced strain. At the optimized temperature for strain property, the threshold field of triggering giant strain is estimated to be about 2.6 kV/mm for a 3.0% Fe doped sample, significantly lower than 3.5 kV/mm for the undoped sample. This shows that the E-T phase diagram can provide valuable guidance for the improvement of electromechanical properties of BNT-based ceramics.