First-principles calculations of structural and bonding properties of Li-doped tetrahedrite thermoelectrics

Tetrahedrite (copper antimony sulfosalt) is a promising p-type thermoelectric material due to its very low intrinsic thermal conductivity and moderately-high power factor, with one of the limitations being the lack of n-type variant to create a thermoelectric generator. In this paper, DFT calculations have been carried out to study tetrahedrite doped with Li into structural voids, LixCu12Sb4S13 (0≤x≤3). Enthalpies of formation show that the introduction of Li into both 6b and 24 g sites is energetically favorable. Dopants in those positions differently affect the rattling Cu(12e) behavior, as well as vary in the magnitude of induced local disorder. Topological analysis of charge density classifies tetrahedrite as a closed-shell, ionic system of interactions with some degree of covalency. The addition of Li increases bond strain and decreases structural stability. Electronic band structure shows that for x>2.0, material becomes n-type; however, results are not precisely conclusive on whether structure will be synthesizable, which should be determined experimentally.