Abstract
A new model describing the Urbach tail of exciton absorption spectra in ferroelectric materials with an order-disorder-type phase transition is proposed. In addition to the thermal effect of the lattice vibration, the internal Stark effect for excitons by the local electric field due to the displacement of protons is taken into account. It turns out that the randomness of the orientation of the polarization associated with the proton ordering decreases the steepness of the exponential tail in absorption spectra. By the present model, the anomalous temperature dependence of the steepness parameter for the Urbach tail observed in PbHPO 4 is reproduced.
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