The effect of impurities on the phase transitions in the ferroelectric semiconductors TlInS₂ and TIGaSe₂

The temperature dependences of the dielectric constants of the ferroelectric semiconductors TlInS₂ and TlGaSe₂ have been studied following their annealing within the incommensurate phase. Unusual memory effects accompanied by both a remarkable inflection of the temperature dependence curves in the incommensurate phase and various shifts of the incommensurate (T_i) and commensurate (T_c) phase transition temperatures have been revealed in both crystals. The observed effects are explained on the basis of a defect density wave model taking into account the interaction of modulation waves with charge carriers localized at impurity states. The thermally activated population of these states during the heating or cooling processes is responsible for the changes of the phase transition temperatures.