  The temperature dependent diffuse reflectance spectroscopy measurements were
carried out on the polycrystalline samples of BaTiO3 across the tetragonal to
cubic structural phase transition temperature. The values of various optical
parameters such as band gap (Eg), Urbach energy (EU) and Urbach focus (E0) are
estimated in the range of 300 K to 470K. It is observed that near structural
phase transition temperatures there exists two value of E0, suggesting presence
of electronic heterogeneity over wide temperature range. Further near
transition temperature EU shows metastability i.e. value of EU at temperature T
is not constant but is a function of time (t). Interestingly it is observed
that the ratio of EU(t=0)/ EU(t = tm), is almost remains constant at 295 K
(pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio
shows decreasing behavior close to structural phase transition temperature,
which confirms the presence of electronic metastibility in the pure BaTiO3. The
observed metastibility can be fitted with the stretch exponents relaxation
behavior, suggesting the presence of dynamic heterogeneous electronic disorder
present in the sample across the transition. Further it appears that these
metastable Urbach tail states (electronic disorder) may couple with the soft
phonon modes and responsible for the observed terahertz dielectric relaxation
(Phys. Rev. Lett. 101, 167402 (2008)). Further; present studies suggest that
the optical studies appear to be more sensitive to probe the
disorder/heterogeneity present in the sample.
