Effect of the "negative chemical" pressure on the temperatures of phase transitions in the TlInS₂ layered crystal

The temperature dependences of the permittivity of TlInS_{2(1 -x)}Se_2x solid solutions have been experimentally investigated in the temperature range including the points of structural phase transitions in the solid solutions. It has been established that the isovalent substitution of selenium for sulfur in the anion sublattice of the TlInS_{2(1 - x)}Se_2x solid solutions shifts the phase transition temperatures T_i and T_c toward the low-temperature range with a simultaneous decrease in the temperature interval of the existence of the incommensurate phase. The T-x phase diagram is constructed for the solid solutions under study and the coordinates are determined for a critical point (of the Lifshitz type) at which the lines T_i(x) and T_c(x) converge in the phase diagram. The pattern of the T-x phase diagram for the TlInS_{2(1 - x)}Se_2x solid solutions has been analyzed within the phenomenological model of a virtual crystal.