Diffuse dielectric behaviour in Na0.5K0.5NbO3–LiTaO3–BiScO3 lead-free ceramics

Dielectric and piezoelectric properties are presented for lead-free piezoelectric compositions in the Na0.5K0.5NbO3–LiTaO3–BiScO3 ternary system, focusing on the compositional join 0.98[(1 − x)(Na0.5K0.5NbO3)–xLiTaO3]–0.02[BiScO3]; 0 ≤ x ≤ 0.1. The end-member composition, 0.98Na0.5K0.5NbO3–0.02BiScO3 has previously been reported to exhibit relatively high piezoelectric coefficients; the present paper examines changes in properties resulting from incorporation of LiTaO3. Measurements of relative permittivity versus temperature showed a Curie peak at ∼350 °C in x ≤ 0.02 samples. This peak broadened as the LiTaO3 content increased from x = 0.02 to 0.03. At x = 0.05 and 0.07, the relative permittivity–temperature curve became even more diffuse, and a second peak occurred at ∼470 °C; transmission electron microscopy revealed this was the result of elemental segregation and the formation of a core–shell grain structure. Maximum values of d33 piezoelectric charge coefficients were ∼200 pC/N in samples x = 0.01 and 0.02, for which dielectric and X-ray diffraction data inferred an orthorhombic–tetragonal phase transition close to room-temperature.