Understanding the mechanism of large dielectric response in Pb-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ferroelectric ceramics

(1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) ceramics have been reported to exhibit large dielectric response in the vicinity of the multi-phase-coexisting point (i.e. triple point). However, the reason for large dielectric response in such a material system is still unclear and thus awaits explanation. In this paper, we investigate the reason for large dielectric response by studying the phase transition behavior around the triple point of BZT-xBCT ceramics. Our results show that the transition enthalpy nearly vanishes and the associated specific heat shows discontinuity on the triple point, which suggest tricritical behavior (i.e. crossover point from first to second order phase transition) for such a triple point. Further Rayleigh analysis indicates that strong dielectric response is due to large reversible contribution which may be caused by phase transition. Moreover, TEM study shows a mottled domain structure with numerous nanodomains close to tricritical triple point, which reveals a polarization isotropic state. In addition, a six-order Landau free energy modeling demonstrates that the energy barrier between paraelectric and ferroelectric phases nearly vanishes on the tricritical triple point, which facilitates large polarizability in the presence of external electric field and is thus responsible for large dielectric permittivity in BZT-xBCT. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.