Manifestation of Ferroelastoelectric Phase Transition in Temperature Changes of the Optical Absorption Edge in (nh4)2cucl4·2h2o Crystal

This work investigates temperature changes of the optical absorption edge of (NH4)(2)CuCl4 center dot 2H(2)O crystals in the region of the ferroelastoelectric (FEE) phase transition. The low-energy tail of the absorption edge in these materials shows an exponential shape. The temperature range above 200.5 K and below 100 K follows the empirical Urbach's rule. The obtained experimental data confirmed the existence of the FEE phase transition at 200.5 K. Nonfulfillment of Urbach's rule just below the phase transition temperature is explained by the scattering of light on the FEE domains.