Composition conserving defects and their influence on the electronic properties of thermoelectric TiNiSn

Formation of composition conserving defects is an inherent feature that appears in compounds for thermoelectric applications during the processes of their fabrication. Different types of such defects including exchange antisite defects, Schottky defects, and triple-, quatro- and penta-defects in TiNiSn are considered. Density functional theory calculations of the energy of formation of these defects are carried out. It is demonstrated that their formation may lead to a significant decrease of the band gap (E_g), simultaneously causing a transformation to p-type or semi-metal conductivity in this material. The role of nanopores is discussed. It is shown that preparing nanoporous compounds may be an efficient way to create p-type TiNiSn, simultaneously decreasing the thermal conductivity and improving its thermoelectric parameters.