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The temperature dependent diffuse reflectance spectroscopy measurements werecarried out on the polycrystalline samples of BaTiO3 across the tetragonal tocubic structural phase transition temperature. The values of various opticalparameters such as band gap (Eg), Urbach energy (EU) and Urbach focus (E0) areestimated in the range of 300 K to 470K. It is observed that near structuralphase transition temperatures there exists two value of E0, suggesting presenceof electronic heterogeneity over wide temperature range. Further neartransition temperature EU shows metastability i.e. value of EU at temperature Tis not constant but is a function of time (t). Interestingly it is observedthat 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 ratioshows decreasing behavior close to structural phase transition temperature,which confirms the presence of electronic metastibility in the pure BaTiO3. Theobserved metastibility can be fitted with the stretch exponents relaxationbehavior, suggesting the presence of dynamic heterogeneous electronic disorderpresent in the sample across the transition. Further it appears that thesemetastable Urbach tail states (electronic disorder) may couple with the softphonon modes and responsible for the observed terahertz dielectric relaxation(Phys. Rev. Lett. 101, 167402 (2008)). Further; present studies suggest thatthe optical studies appear to be more sensitive to probe thedisorder/heterogeneity present in the sample.

Metastable behavior of Urbach tail states in BaTiO3 across phase transition