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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.

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Results are given of an investigation of the absorption processes at the absorption edge of the ternary system (GaxIn1−x)2Se3 in the In‐rich region. In E ∥ c polarization, the absorption coefficient for all investigated concentrations exhibits an exponential dependence on photon energy following Urbach's rule. The analysis of the results confirms that the absorption processes in the absorption edge of γ1‐(GaxIn1−x)2Se3 are governed by a superposition of at least two different mechanisms, one related to the crystal phonons and the other resulting from structural and compositional disorder. The latter mechanism seems to be fundamental, influencing the absorption processes equivalently in amorphous and monocrystal states. In both E ∥ c and E ⊥ c polarization an apparent dependence of absorption coefficient on sample thicknesses at constant photon energy is observed. The effect is assigned to a subsurface damage introduced by mechanical polishing.

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