Ab initiostudy of silicon in theR8phase

Abstract

We present a detailed ab initio study of the electronic and structural properties of the recently discovered $R8$ phase of silicon. Within the framework of density-functional theory in the local-density approximation and using pseudopotentials with a plane-wave basis, we study the energetics of the $R8$ phase compared to the other tetrahedrally bonded diamond, \ensuremath{\beta}-Sn, and BC8 phases. The bonding properties and the pressure dependence of the bond lengths of the BC8 and $R8$ phases are investigated. An analysis of the band structure reveals that $R8$ silicon could be a semimetal or semiconductor with a small, indirect band gap. Our computed density of states of $R8$ silicon shows a sharpening of the valence-band edge similar to the one observed for amorphous silicon.