The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical $\mathrm{Na}{}_x\mathrm{Si}{}_{46}$ system are studied in detail here. It is shown that, despite the fact that the $\mathrm{Na}/\mathrm{Si}$ clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.

• Received 20 January 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.114