We studied the structural and electronic properties of some of the Ba and Sr guest-containing type-I semiconductor clathrate alloys Ba8 Ga16 Six Ge30-x and Sr8 Ga16 Six Ge30-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 Ba8 Ga16 Ge30 and Sr8 Ga16 Ge30 containing no Ga-Ga bonds, we constructed unit cells of Ba8 Ga16 Six Ge30-x and Sr8 Ga16 Six Ge30-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 Ba8 Ga16 Six Ge30-x decreases with increasing x. However, we found that the band gap of Sr8 Ga16 Six Ge30-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 Sip 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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - May 9 2008|