First principles calculations of the structural and electronic properties of the type-I semiconductor clathrate alloys Ba8Ga16SiGe30-x and Sr8Ga16SiGe30-x

We studied the structural and electronic properties of some of the Ba and Sr guest-containing type-I semiconductor clathrate alloys Ba8Ga16SixGe30-x and Sr8Ga16SixGe30-x for three values of the Si composition x (x=0,5,15). Our calculations are based on the generalized gradient approximation to density functional theory. Starting with the stable structures of the clathrate semiconductors Ba8Ga16Ge30 and Sr8Ga16Ge30 containing no Ga-Ga bonds, we constructed unit cells of Ba8Ga16SixGe30-x and Sr8Ga16SixGe30-x by replacing appropriate numbers of the framework Ge atoms with Si. For the values of the Si composition x that we considered, we found that the fundamental band gap of Ba8Ga16SixGe30-x decreases with increasing x. However, we found that the band gap of Sr8Ga16SixGe30-x increases with increasing x. Our results also show that several electronic states near the top of the valence band and near the bottom of the conduction band in both materials are modified by the Si p states. The trends in the structural and electronic properties of these materials as x is varied are discussed, and our results are compared to experiment where possible.