Nature of thermoelectric properties formation in defected Sn2P2S6 chalcogenide crystals

Detailed first principles calculations were performed to explain the physical nature of the thermoelectric properties occurring in the Sn2P2S6 (SPS) crystal with deviation from stoichiometry. In consequence, the computational model was developed to calculate the thermoelectric parameters of nonstoichiometric SPS crystal in paraelectric phase taking into account calculations of the cohesive energy Ecoh, crystal formation energy Ef and vacancies formation energy Evf using the DFT and DFT+U methods. Using the Boltzmann formula the relative concentration of different types of point defect in the bulk SPS system was predicted. These data were used to calculate temperature behaviour of the nonstoichiometric SPS crystal. Experimentally temperature dependence of conductivity was measured. It gave the possibility to estimate the magnitude of the concentration of charge carriers, their mobility and temperature behaviour of the Seebeck coefficient in the SPS crystal. Experimentally obtained dependencies of the thermoelectric effect demonstrate agreement with experimental measurements confirming the validity of proposed model of point defects formation in considered material and prove unexpected properties of the SPS crystal.