Localized Mg-vacancy states in the thermoelectric material Mg2-δSi0.4Sn0.6

Abstract

Mg2SixSn1-x has been widely studied as a thermoelectric material owing to its high figure-of-merit, low cost, and non-toxicity. However, its electronic structure, particularly when the material contains Mg vacancies, has not been adequately described. The n-type nature of Mg2-δSi0.4Sn0.6 has been a puzzle. Mg deficiency can be present in Mg2SixSn1-x due to Mg evaporation and oxidation. Therefore, an investigation of the role of Mg vacancies is of great interest. In this work, we have prepared a series of samples with various Mg deficiency and Sb doping levels and measured their transport properties. The Seebeck coefficient of these samples all reveals n-type conduction. We propose that Mg vacancies in Mg2-δSixSn1-x create localized hole states inside the band gap instead of simply moving the Fermi-level into the valence band as would be predicted by a rigid band model. Our hypothesis is further confirmed by density-functional theory calculations, which show that the hole states are trapped at Mg vacancies above the valence band. Moreover, this localized hole-states model is used to interpret electrical transport properties. Both the Seebeck coefficient and resistivity of Mg2-δSi0.4Sn0.6 indicate an electron-hopping transport mechanism. In addition, the data suggest that localized band-tail states may exist in the conduction-band edge of Mg2SixSn1-x.