Structural, optical and frequency dependent dielectric studies of nanoscale Na0.5Bi0.5TiO3 processed via non-aqueous route

Abstract

The present work reports on structural, optical, and dielectric properties of nanoscale Na0.5Bi0.5TiO3 (NBT) derived through a non-aqueous, solid-state cum sintering route. Whereas transmission electron microscopy (TEM) has revealed the formation of nearly spherical particles, x-ray diffraction (XRD) analyses suggest particles crystallize into rhombohedral crystal structure. The band gap of NBT is of direct type, as predicted through first principle calculations. The optical band gaps are also determined from the UV–visible absorption spectra, which showed a declining trend with increase in sintering temperature. The Urbach energy, which accounts for the transitional events between the extended and localized states and featuring long tailing in the absorption spectra is found to be smaller than the Tauc gap by several orders. Moreover, a broad-luminescence response as acquired on Photoluminescence spectroscopy, is discussed in the light of quantum confinement effect, near band-edge features, defect states, and self-trapped excitons. Furthermore, dielectric studies have been carried out in the frequency range, 100 Hz-5 MHz, in which one could observe a decreasing trend of capacitance with increasing frequency. However, the real part of the impedance experiences a slow declining trend initially and abrupt fall eventually beyond 1 MHz thereby suggesting a rise in conductivity at higher frequencies as a greater number of carriers are now able to follow the frequency imposed. At the same time, the single semi-circular peak of the Cole-Cole plot suggests a simple equivalent circuit of the device comprising parallel resistance (grain boundary resistance, Rp) and capacitance (grain boundary capacitance, Cp) network in series with a resistance Rs due to the grain.