Role of K/Bi disorder in the electronic structure of β-K₂Bi₈Se₁₃

We have carried out tunneling spectroscopy and first-principles studies for β-K₂Bi₈Se₁₃, 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-K₂Bi₈Se₁₃ can be semimetallic or semiconducting depending on the arrangements of the K and Bi atoms in the mixed sites. The electronic structure of β-K₂Bi₈Se₁₃ near the band-gap region is largely determined by unbonded Se p states and states associated with strained bonds which are present due to K/Bi disorder and by the Bi p-Se p 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-K₂Bi₈Se₁₃; 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-K₂Bi₈Se₁₃ can be qualitatively understood in terms of the electronic structure obtained in calculations using the above structural model.