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 Si₁₃₆Na_x (1.5< x < 22) and Si₄₆Na₈ have been checked for superconductivity down to 2 K, but with negative results.