Optical properties of AIV2BV2CVI6 ferroelectrics-semiconductors: the effect of temperature and hydrostatic pressure Review

In this review the results of experimental studies for the influence of temperature and high pressure on the fundamental absorption edge, optical birefringence and vibrational spectra of A(2)(IV)B(2)(V)C(6)(VI) -group crystals are presented. It is revealed that the high-energy part of the absorption edge is described by the Urbach rule, while its low-energy part is formed by indirect optical transitions. Violation of the Urbach rule is detected in the region of incommensurate phase. The temperature and baric coefficients of the forbidden gap in the ferroelectric, paraelectric and incommensurate phases are found to be negative. Electron-phonon interaction plays a major part in the temperature changes of the bandgap. According to the theory, the bandgap suffers characteristic anomalies that depend upon the phase transition order. The critical indices of the order parameter and heat capacity and the Landau-Ginzburg expansion coefficients derived from the baric studies of optical birefringence agree well with a presence of Lifshitz point at the p, T-diagrams of the crystals under test. Relative shift in the frequencies of the Raman spectra occurring under mechanical stresses testifies a notable nonequivalence of atomic bonds and a possibility of dividing the vibrations in Sn2P2S6 into the external and internal ones. The results of baric investigations of the Raman spectra show that the structural transformation in Sn2P2S6 is mainly linked to Sn-S bonds. The results analysed by us testify a possibility for practical applications of Sn2P2S6 ferroelectrics as a converter of coherent long-wave radiation into shorter-wave one.