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S6.5.1

Effect of K/Bi ordering on the electronic structure of K2Bi8Se13 Daniel I Bilc, Paul Larson1, S.D. Mahanti and M.G. Kanatzidis2 Michigan State University, Department of Physics and Astronomy, East Lansing, MI 48824, U.S.A. 1 Case Western Reserve University, Department of Physics, Cleveland, OH 44106, U.S.A. 2 Michigan State University, Department of Chemistry, East Lansing, MI 48824, U.S.A.

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 βphase, 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 1x1x2 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.

INTRODUCTION

Since the best bulk materials for thermoelectric applications are simple chalcogenides, specifically PbTe and Bi2Te3 alloys, complex chalcogenides provide a promising avenue for searching for new thermoelectric materials. Complex chalcogenides with large unit cells containing weakly bonded atoms or molecules, called "rattlers", have been studied to reduce the phonon thermal conductivity without affecting the thermopower S and electrical conductivity σ, thereby enhancing their thermoelectric efficiency. α-K2Bi8Se13 and β-K2Bi8Se13 represent an example where similar buildings blocks combine to give compounds with the same stoichiometry but different architecture at the atomic level. α-K2Bi8Se13 consist of Bi2Te3-,CdI2- and Sb2Se3-type rod fragments parallel to the z-axis (Figure 1) whereas β-K2Bi8Se13 possesses an architecture made up of Bi2Te3-, CdI2-, NaCl-type of rod fragments (Figure 2). These different types of fragments are common in other bismuth

S6.5.2

chalcogenides such as Cs3Bi7Se12 [1] and KBi6.33S10 [2]. The CdI2-type and Bi2Te3-type rods in α and β-phase are arranged side by side to form layers perpendicular to the y-axis with tunnels filled with K+ cations along the c-axis. Whereas in the β