The insulator-to-metal transition in Si-Na ‘clathrate’ compounds: A search for superconductivity

Abstract

Abstract The Si-Na ‘clathrate’ phases are cage structures that undergo an insulator-to-metal transition when doped with alkali metals and so have some structural and chemical resemblance to the C60 fullerenes. In the fullerenes the alkali-metal atoms lie interstitially between the C60 polyhedra, whereas in the clathrates they are contained within the Si polyhedra. Consequently, the latter are much more stable chemically than the former. The compounds Si136Nax (1.5< x < 22) and Si46Na8 have been checked for superconductivity down to 2 K, but with negative results.