Investigation of electronic,transport and thermoelectric properties of Ag8SnS6 and Ag8SnSe6 argyrodite compounds from first-principles

Thermoelectric systems are preferable systems with environmental friendly and renewable energy properties. Thermoelectric power applications with different advantages have the potential to expand in various fields such as cooling, heating, electricity generation and air conditioning. Thermoelectric materials must have high electrical conductivity and high Seebeck coefficient and low thermal conductivity. Few materials exists having all these features at the same time. Ag8SnS6 and Ag8SnSe6 materials are highly disordered semiconductors which are known to have low thermal conductivity and attract attention due to their electronic transport properties. In this study, structural, electronic and thermoelectric properties of Ag8SnS6 and Ag8SnSe6 semiconductors were examined from first principles calculations and the results are compared with the available experiments. The calculations were carried out with density functional theory and semi-classical Boltzmann transport theory. Lattice constants, bulk modulus, electronic band structures, total and partial density of states and thermoelectric coefficients were calculated without using any parameters from the experiments. The predicted ZT are compatible with the existing n-type doped experiments and it was found that the ZT values can be much higher when p-type doping was performed.