Effect of K/Bi ordering on the electronic structure of K₂Bi₈Se₁₃

Abstract

K₂Bi₈Se₁₃ belongs to a class of complex chalcogenides which show potential for superior thermoelectric performance. This compound forms in two distinct phases, α and β. The β-phase, which has several sites with mixed K/Bi occupancy is a better thermoelectric. To understand the origin of this difference we have carried out electronic structure calculations within ab initio density functional theory using full potential linearized augmented plane wave (FLAPW) method. The generalized gradient approximation was used to treat the exchange and correlation potential. Spin-orbit interaction (SOI) was incorporated using a second variational procedure. The α-phase is found to be a semiconductor with an indirect band gap of 0.47eV compared to 0.76eV for the observed direct optical gap. For the β-phase we have chosen two different ordered structures with full occupancies of K and Bi atoms at the “mixed sites”. The system is a semi-metal for both the ordered structures. To incorporate the effect of mixed occupancy we have chosen a 1×1×2 supercell with an alternative K/Bi occupancy at the “mixed sites”. The superlattice ordering gives a semiconductor with an indirect gap of 0.38eV. Mixed occupancy is crucial for the system to be a semiconductor because the Bi atoms at the “mixed sites” stabilize the p orbitals of the neighboring Se atoms by lowering their energy, and opening up a gap at the chemical potential.