Intrinsic defects in sulvanite compounds: The case of transparent Cu3TaS4 and absorbing Cu3VSe4

Sulvanites are semiconducting compounds with the chemical formula Cu3TMX4 where TM = V, Nb, Ta; X = S, Se, Te. Semiconductor electronic and optical properties are highly influenced by intrinsic defects such as vacancies, antisites, and atoms residing in interstitial positions inside the crystal structure. Even though intrinsic defects are extremely important, very little is known about defects in sulvanites. Here we report the properties of all intrinsic defects in two representative sulvanite compounds (Cu3TaS4 and Cu3VSe4) by using computational quantum mechanical methods. Our results indicate that Cu vacancies are the most frequent defects in these compounds, also responsible for limiting the possibility of their n-type doping and setting the pinning of the Fermi energy to positions close to the valence band. These results explain why as-grown sulvanites are usually p-type and open a path to search for ways to design and tune the properties of these compounds.