Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: An experimental and theoretical investigation

Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (T-P). The values of various optical parameters such as band gap (E-g), Urbach energy (E-u), and Urbach focus (E-0) were estimated in the temperature range of 300K to 480 K. It was observed that with increasing temperature, E-g decreases and shows a sharp anomaly at T-P. First principle studies were employed in order to understand the observed change in E-g due to the structural phase transition. Near T-P, there exist two values of E-0, suggesting the presence of electronic heterogeneity. Further, near T-P, E-u shows metastability, i.e., the value of E-u at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of E-u ((t = 0))/E-u ((t = tm)), almost remains constant at 300K (pure tetragonal phase) and at 450K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of E-u, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across T-P. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample. Published by AIP Publishing.