Near band edge optical properties of the TlInS2 and TlGaS2 incommensurate ferroelectrics

Near band edge optical absorption is investigated in T1InS(2) and T1GaS(2) incommensurate ferroelectricsis over the temperature region 20-300K. Both materials are found to be remarkable in the fact of the existence of an exciton spectrum. In T1InS(2) the temperature dependence of the exciton energy is certainly mirrors the successive phase transitions taking place in the material and the description in terms of incommensurate phase transition is quite relevant to this case. In T1GaS(2) the temperature dependence of the exciton energy is not seemingly affected by phase transitions and the commonly used description in terms of the lattice deformation and exciton-phonon interaction effect on exciton levels is still sufficient. The deformation potential is estimated to be similar to 4eV.