Phase transition and electrical properties of lanthanum-modified sodium bismuth titanate

Structural, microstructural and electrical properties of lanthanum-modified sodium bismuth titanate (Na0.5Bi0.5TiO3) (NBT) ceramics were investigated. X-ray diffraction (XRD) analysis of the prepared materials confirmed the formation of the compounds with rhombohedral crystal system. The nature of scanning electron micrographs of the compounds showed (i) the uniform distribution of grains on the sample surface with high density and (ii) reduction of grain size on La substitution at the (NaBi) sites of NBT. Detailed studies of dielectric and impedance properties of the materials, carried out in the frequency range of 102–106 Hz at different temperatures (room temperature to 500 °C), have provided many interesting properties. The dielectric constant at transition temperature was found to be decreased with increase of broadening of the dielectric peak on increasing La content in the sample. The transition temperature (Tm) shifted to higher temperature side on addition of La (up to 6%), whereas the reverse trend was observed for higher concentration of La (i.e. 8%). The depolarization temperature (Td) of La-modified NBT compounds was found to be smaller than that of pure NBT. The tangent loss was also found to be decreased on the incorporation of La into NBT. The ac conductivity of the La-modified NBT obeyed the double power law behavior. Complex impedance spectroscopy has been carried out for better understanding of relaxation process and correlations between the microstructure–electrical properties of the materials.