Optical crystallographic study of piezoelectric KxNa1−xNbO3 (x = 0.4, 0.5 and 0.6) single crystals using linear birefringence

In this study, we report an optical crystallographic study of KxNa1−xNbO3 (x = 0.4, 0.5 and 0.6) single crystals in the temperature range of 30–600 °C. At room temperature, these three compositions were found to exhibit monoclinic MC, monoclinic MA/MB and orthorhombic symmetries respectively. All the compositions showed a transformation to tetragonal and to cubic symmetries around 200 °C and 400 °C respectively. In the tetragonal phase field (200–400 °C), coexistence of a lower symmetry phase was also evident. In the paraelectric regime (400–600 °C), all crystals exhibited non-zero birefringence, suggestive of deviation from ideal cubic symmetry mm. Furthermore, crystals of the morphotropic phase boundary composition (x = 0.5) were found to have very small and organized domains, as compared to the other two compositions.