The electronic structure of metallo-silicon clathrates NaxSi136 (x = 0, 4, 8, 16 and 24)

The electronic band structures of sodium-containing silicon clathrates (NaxSi136) have been studied with the first principles all electron full potential linearized plane wave method. It is shown that at low dopant concentration (x ⩽ 8), the Si clathrates behave either as an insulator or a semi-metal with very small electrical conductivity. However, the metallic character becomes more prominent with increasing sodium concentration. The theoretical results help to elucidate the mechanism for the observed insulator to metal transition in these compounds when the concentration of the Na metal exceeds x > 8.