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 d(33) = 129 pC/N, k(p) = 29%, P-max = 35 mu C/cm(2), P-r = 24 mu C/cm(2), epsilon(m) = 4600, T-m = 370 degrees C and T-d = 260 degrees C are observed for x = 0.03 sample. In the tetragonal range, a correlation between lattice distortion (c/a), piezoelectric coefficient (d(33)) and depolarization temperature T-d is established. Decrease in c/a seems to result in easy poling by field and easy depoling by temperature, leading to an increase in d(33) but a decrease in T-d. 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 T-d (where the structure becomes tetragonal to pseudocubic) considerably, resulting into a shift in tetragonal-pseudocubic phase boundary towards lower temperature. Copyright 2013 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4796166]