Specific features of the optical absorption edge anisotropy in In4(P2S6)3 layered crystals

The optical absorption edge of In4(P2S6)3 layered crystals is studied in the temperature range 77–355 K. In the spectral range of direct optical transitions, the absorption edge is of Urbach shape. Electron–phonon interaction result in the smearing of energy bands and, consequently, in appearing of Urbach absorption tails. The temperature variation of the optical pseudogap Eg∗ and the absorption edge energy width w are well described in the Einstein model. Strong anisotropy of the absorption edge spectral position and shape, as well as of electron–phonon interaction, is revealed.