Electronic structure and magnetism in Bi2Te3, Bi2Se3, and Sb2Te3 doped with transition metals (Ti-Zn)

A less studied class of magnetic semiconductors based on the tetradymite structure compounds, which include Bi2Te3, Bi2Se3, and Sb2Te3, doped with 3d transition-metal (T) elements is investigated using the full-potential linearized muffin-tin orbital method in the local spin-density approximation (LSDA). The small size of the T atoms (Ti-Zn) relative to the larger Bi/Sb atom site leads to a strong lattice relaxation, which primarily affects the atoms close to the defect in neighboring layers but has less impact within the layer containing the T ion. Even with the relaxation, the size difference leads to quite localized 3d states and hence a high-spin state with large magnetic moments. The valence is 3+ for the first half of the series but reduces closer to 2+ (Co) or 1+ (Ni) for the latter half of the series. This reduction is due to hybridization in the transition-metal 3d state with the Se/Te p state of the atom in the layer closest to the T site which reduces the amount of charge transfer between these two atoms. It corresponds to a competition between the stabilization by the exchange energy dominating for the early T ions and the covalent bonding effects for the later ones. For Cu and Zn the trend reverses and the valency is between 2+ and 3+ with a small magnetic moment remaining for Cu but a negligible moment for Zn. LSDA+U calculations for the Fe-Ni atoms slightly change this balance of the two effects and promote higher valency and larger magnetic moment.