Role of K/Bi disorder in the electronic structure of β -K2 Bi8 Se13

Abstract

We have carried out tunneling spectroscopy and first-principles studies for β -K2 Bi8 Se13, a promising thermoelectric material with partially disordered mixed K/Bi sites. The tunneling data, obtained with a scanning tunneling microscope (STM), show that the system is a semiconductor with a band gap of □0.4 eV and band-tail states near the valence-band top and the conduction-band bottom. First-principles calculations, on the other hand, show that β -K2 Bi8 Se13 can be semimetallic or semiconducting depending on the arrangements of the K and Bi atoms in the mixed sites. The electronic structure of β -K2 Bi8 Se13 near the band-gap region is largely determined by unbonded Sep states and states associated with strained bonds which are present due to K/Bi disorder and by the Bip-Sep hybridization which tends to drive the system toward metallicity. Among the different K/Bi arrangements investigated, we have identified a structural model (quasidisordered structure) that is able to satisfactorily reproduce the atomic and electronic structures of β -K2 Bi8 Se13

i.e., the local composition in the mixed channels as observed experimentally and the band gap and tails as seen in the STM measurements. We argue that transport properties of β -K2 Bi8 Se13 can be qualitatively understood in terms of the electronic structure obtained in calculations using the above structural model. © 2009 The American Physical Society.