Structural and electronic properties of p-doped silicon clathrates

We present an ab initio study of the structural and electronic properties of type-I and type-II silicon clathrates doped by elements chosen to be more electronegative than silicon. Depending on the intercalated element, we show that the electronic properties of doped silicon clathrates can exhibit metallic, semiconducting, or insulating behavior. It is found in particular that doping can lead to silicon-based materials with a band gap in the visible range and that, in type-II clathrates, the gap can be direct. However, the analysis of the selection rules show that the optical transitions are forbidden in type-I and type-II clathrates. Concerning the structural properties, the bonding between the dopant atom and silicon can significantly decrease the compressibility of the host network to values equivalent to the one of the much denser diamond phase. The present results are complemented and rationalized by the study of endohedrally doped SinHn (n=20,24,28) silicon clusters.