Phase transitions and optical absorption edge in CuInP2Se6 layered ferrielectrics

Temperature studies of optical absorption spectra in the range of the fundamental absorption edge for layered CuInP2Se6 single crystals obtained by the technique combining recrystallization and vapor-phase transport, are reported. The temperature behavior of the absorption-edge spectra is the evidence for the essential changes of the dynamical disordering of CuInP2Se6 crystal structure at second-order (T-0 approximate to 249 K) and first-order (T-c approximate to 236 K) phase transitions. The present anomalous temperature hysteresis and the glass-like behavior of the exponential absorption edge in the intermediate phase resemble those being observed in the incommensurate phases of ferroelectrics.