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Electronic Structure of K 2Bi8 Se13 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 Naval Research Laboratory, Washington D.C., U.S.A. 2 Michigan State University, Department of Chemistry, East Lansing, MI 48824, U.S.A. ABSTRACT K 2Bi8 Se13 belongs to a class of complex Bi-Te-Se systems which show great potential for thermoelectric performance. This compound forms in two distinct phases α-K 2Bi8 Se13 (triclinic with space group P-1) and β-K 2Bi8 Se13 (monoclinic with space group P 21/m). In the β-phase, there is substantial disorder at four sites in the unit cell, occupied by two K and two Bi atoms. To understand the electronic properties of these two different phases we have carried out band structure calculations within ab initio density functional theory (DFT) using full potential linearized augmented plane wave (LAPW) method. The generalized gradient approximation (GGA) 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. For the β-phase we have chosen two different ordered structures. The system is a semi-metal for one of the structures whereas for the other, it is a narrow gap semiconductor with a gap of 0.38eV in the absence of SOI, but the gap collapses and the system becomes a semimetal with low density of states at the Fermi energy when SOI is included.

INTRODUCTION The best known bulk materials for room and high temperature thermoelectric applications are simple chalcogenides, specifically PbTe and Bi2Te3 alloys. These are doped narrow-gap semiconductors. During past several years, complex chalcogenides have provided a promising avenue for searching for new thermoelectric materials. These chalcogenides with their large unit cells containing weakly bonded atoms called "rattlers", have been studied to reduce the phonon thermal conductivity without affecting the thermopower S and electrical conductivity σ [1]. An important example is CsBi4Te6 [2], which has the best thermoelectric performance below 250K. Recently, a new promising complex chalcogenide K 2Bi8 Se13 has been synthesized and its thermoelectric properties have been investigated.[3] This compound occurs in two distinct phases denoted as α-K 2Bi8 Se13 and β-K 2Bi8 Se13 . These two phases represent an example where similar buildings blocks combine to give compounds with the same stoichiometry but different architectures. α-K 2Bi8 Se13 consist of Bi2Te3-,CdI2- and Sb2Se3-type rod fragments parallel to the c-axis (Figure 1a) whereas β-K 2Bi8 Se13 possesses an architecture made up of Bi2Te3-, CdI2-, and NaCl-type rod fragments (Figure 1b). The CdI2-type and Bi2Te3-type rods in α− and βphase are arranged side by side to form layers perpendicular to the b-axis with tunnels filled with K+ cations along the c-axis. Whereas in the β−phase the NaCl-type rod fragments connect the G1