The optical absorption edge of CuInP 2 S 6 crystals is studied between 77 and 573 K. The nature of optical transitions, resulting in absorption edge formation in ferrielectric and paraelectric phases, is elucidated. The temperature variation of the energy gap in the ferrielectric phase transition range and the temperature behavior of the Urbach absorption edge in the paraelectric phase are studied. The effect of temperature and structural disordering on the absorption edge energy position and shape is analyzed.
Temperature variation of the optical absorption edge of sn2P2s6 ferroelectric with three-dimensional crystalline structure and SnP2S6 layered crystals with 2D structure is studied. In both cases the Urbach shape of the absorption edge is revealed, its temperature behaviour in Sn2P2S6 crystal being determined by the presence of a ferroelechic phase transition. In Sn2P2S6 crystals the exponential parts of the absorption edge plob both in ferroelectric and paraelectric phases have a common convergence point. The anisotropy of the Urbach absorption edge parameters and their temperature variation in the range of the phase transition in Sn2P2S6 is studied. The influence of different types of disordering, including 3D+2D transition, on the absorption edge energy position and shape in Sn2P2S6 and SnP& crystals is analyzed.
The temperature behaviour of the optical absorption edge in the range of phase transitions for CuCrP 2 S 6 crystals is studied. The absorption edge is shown to be described by the Urbach rule with different convergence point coordinates in antiferroelectric and paraelectric phases. In the intermediate phase there is no convergence point and a parallel shift of the exponential absorption edge is observed. The specific features of the temperature behaviour of the absorption edge are treated assuming the intermediate phase in CuCrP 2 S 6 crystals to be an incommensurate one.
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