Manifestations of the polytype structure of β-TlInS2 crystals in their optical anisotropy parameters

We present the results of experimental studies of the temperature dependences of optical birefringence and optical indicatrix (OI) orientation increments for beta-TlInS2 crystals. Anomalous behaviours of the above characteristics are revealed at the two temperature points, T-C1 approximate to 228 K and T-C2 approximate to 197 K, which correspond to ferroelectric phase transitions in the polytypes with c = 16c* and c = c*. The maps of increments of OI orientation angle and phase difference are obtained in the range from the room temperature to 100 K. It is found that the spatial distribution of increment of the OI orientation angle in ab plane is homogeneous in the whole temperature range under study. The increment of the phase difference is homogeneously distributed in the ab plane only at 197 K < T < 290 K. Below T-C2 approximate to 197 K, two regions are revealed in our sample, which differ by their phase difference. Most probably, these regions correspond to two different polytypes of beta-TlInS2.