Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates

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 NaxSi46 system are studied in detail here. It is shown that, despite the fact that the Na/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.