Refractive index and domain structure of undoped and Nb doped (Na0.41K0.09Bi0.50)TiO3 (NBT-KBT) single crystal grown at MPB composition

Single crystals of undoped and Nb doped NBT-KBT with morphotropic phase boundary composition [(Na0.41K0.09Bi0.50)TiO3] were grown by spontaneous nucleation technique. The lattice parameter was found to be unaffected on Nb doping. Further the temperature dependent refractive indices were measured for four different wavelengths and the Sellmeier coefficients were calculated in the range of 30-160 degrees C. Refractive index was found to increase linearly with temperature (thermal coefficient of refractive index similar to 10(-4)-10(-5)/ degrees C). Further, the thermal coefficient was found to increase with the Nb concentration. A set of relations of the thermal coefficient of refractive index as a function of wavelength was established. In addition, the crystal was investigated using polarized light optical microscope. Regions with both 'a' and 'c' domains, typical of tetragonal structure, were observed in the samples. Further, the conoscopic study revealed that the material is of negative uniaxial type. The presence of domains related to the tetragonal and rhombohedral structures hints the possible coexistence of both the structures as expected for the morphotropic phase boundary composition. The outcome of the investigation would provide valuable inputs about NBT-KBT single crystals for photonic or optoelectronic applications.