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Temperature variation of optical absorption edge in Sn2P2S6 and SnP2S6 crystals

Abstract 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 ferroelectric phase transition. In Sn2P2S6 crystals the exponential parts of the absorption edge plots 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 SnP2S6 crystals is analyzed.