Phase Formation and Phase Transitions in Nonstoichiometric Sodium Bismuth Titanate Ceramics

We have studied the phase formation, microstructure, and dielectric and ferroelectric properties of (Na_0.5–xBi_0.5)TiO₃ and (Na_0.5Bi_{0.5 + x})TiO₃ nonstoichiometric ceramics with Na/Bi < 1 and x = 0–0.1. The grain size of the ceramics has been shown to decrease with increasing x. The temperature dependences of dielectric permittivity for the samples studied have anomalies near ~400 K and peaks at ~600 K, corresponding to ferroelectric phase transitions. The phase transitions near 400 K demonstrate relaxor behavior, indicative of the presence of polar regions in a nonpolar matrix, as supported by laser second harmonic generation measurements. In addition, the (Na_0.5Bi_{0.5 + x})TiO₃ samples with x > 0.05 have anomalies near 900 K, confirming the presence of Bi₄Ti₃O₁₂ as an impurity phase, which is accompanied by an increase in the spontaneous polarization of these samples.