Abstract
Temperature dependent polarized transmission intensity studies in strongly converging and parallel light beams had been accomplished for off-zone-center incommensurate semiconductors-ferroelectrics TlInS2, TlGaSe2 and TlGaS2 having a layered structure and reportedly a C2h6 space group of symmetry at room temperature. All above materials were biaxial. TlInS2, as compared to TlGaS2 and TlGaSe2, displayed a stronger wavelength dispersion of the normal-to-layer-plane optical anisotropy due to the strong polarization of the excitonic transitions near the absorption edge. Temperature dispersion of the dielectric axes of TlInS2 and TlGaSe2 was shown to include forbidden rotation that might be associated with a lower than monoclinic symmetry or spatially dispersed microscopic domains. Macroscopic, spatially ordered domain structure was found in TlGaS2, for which, however, above-like rotation was not observed. Unusual effect with temperature behavior encountered in soliton-like incommensurate phases was observed for TlInS2. The optical anisotropy of the incommensurate phase was estimated at ∼ 10-3 with the background value of ∼ 10-4 for basic lattice.
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