Anomalous phase transitions of lead-free piezoelectric xNa0.5Bi0.5TiO3-(1-x)BaTiO3 solid solutions with enhanced phase transition temperatures

Temperature-dependent structural phase transitions and dielectric properties for a series of xNa(0.5)Bi(0.5)TiO(3)-(1-x)BaTiO3 (NBT-BT) samples where x ranges from 0.05 to 0.30 have been investigated. Non-ambient x-ray diffraction together with dielectric data as a function of temperature have demonstrated an enhancement in the ferroelectric to paraelectric phase transition temperature with the increase in NBT content, including inhibition of two well-known low-temperature polymorphic phase transitions of BaTiO3. Anomalous dielectric response has been observed for samples with x >= 0.20 suggesting a crossover from a first-order to a second-order phase transition. The critical point lies between x=0.15 and x=0.20. Deviation from the classical Curie-Weiss law has been seen for samples having x >= 0.10. A modified Curie-Weiss law was employed to model the dielectric response in the paraelectric phase and the maximum value of the critical exponent gamma was found to be 1.45 for x=0.30. It is suggested that the dielectric properties are heavily influenced by the induced strain upon doping rather than chemical inhomogeneities and structural disorder, which are common occurrences in doped ferroelectrics.