Improper and proper ferroelectric phase transitions in TlInS2 layered crystal with incommensurate structure

The dielectric constant of TlInS2 crystal was measured in the temperature range including incommensurate (IC1 and IC2) and commensurate (C1 and C2) phase transitions under external bias electric field. It has been revealed that maxima in the temperature dependence of dielectric constant of TlInS2 at the C1 and C2 phase transition points shift to higher temperatures. The anomaly at IC2 transition point shifts to lower temperatures on applying the external electric fields. A new complete theoretical model including the presence of two order parameters and two polar sublattices in TlInS2 has been suggested.