Temperature dependences of optical indicatrix and thermal expansion parameters of TlIn(S1�xSex)2 solid solutions (x = 0, 0.02 and 0.06)

. Temperature dependences of relative elongation, thermal expansion coefficient, birefringence increment and angle of optical indicatrix rotation are studied for TlIn(S 1–x Se x ) 2 crystals (x = 0, 0.02 and 0.06). We have found that all of these crystals reveal at least two diffused phase transitions (PTs), from normal phase to incommensurate phase and from incommensurate phase to ferroelectric phase. The temperature regions where these PTs take place are determined. A low-temperature PT represents a diffused PT of the first order. We have detected with our techniques that the solid solution with x = 0.02 is non-uniformly stressed. These internal stresses can be caused by a polytype structure. An additional anomaly in the temperature dependence of thermal expansion coefficient occurring at the temperature T 1 = 214 K in the compound with x = 0.06 also points to the existence of polytype structure.