Structural and electrical properties of BKT rich Bi0.5K0.5TiO3-K0.5Na0.5NbO3 system

A new lead free (1−x) Bi0.5K0.5TiO3–(x) K0.5Na0.5NbO3 (BKT-KNN) system (0.0 ≤ x ≤ 0.15) with density of 94-97% is reported. Tetragonal structure of BKT-KNN is apparent for 0 ≤ x ≤ 0.06 with 1.02 ≤ (c/a) ≤ 1.012. Compositions with x ≥ 0.15 show orthorhombic structure (like KNN) and x = 0.08 & x = 0.10 are pseudocubic. Addition of KNN by a small amount (up to 4 mole %) is shown to improve piezoelectric properties, keeping the ferroelectric and dielectric values comparable to those of BKT. Optimized properties with d33 = 129 pC/N, kp = 29%, Pmax = 35 μC/cm2, Pr = 24 μC/cm2, ɛm = 4600, Tm = 370 °C and Td = 260 °C are observed for x = 0.03 sample. In the tetragonal range, a correlation between lattice distortion (c/a), piezoelectric coefficient (d33) and depolarization temperature Td is established. Decrease in c/a seems to result in easy poling by field and easy depoling by temperature, leading to an increase in d33 but a decrease in Td. A phase diagram for 1-xBKT-xKNN (0 ≤ x ≤ 0.10) is proposed. It is shown that addition of more than 6 mole % of KNN leads to the lowering of Td (where the structure becomes tetragonal to pseudocubic) considerably, resulting into a shift in tetragonal- pseudocubic phase boundary towards lower temperature.