Nanostructured Clathrate Phonon Glasses: Beyond the Rattling Concept

Using first-principles calculations, we investigated the thermoelectric properties of a newly synthesized Si-based ternary clathrate K8Al8Si38, composed of similar to 1 nm hollow cages with a metal atom inside. This compound contains solely Earth abundant elements. We found that, similar to other nanostructured type I clathrates, this system is a semiconductor and has a low thermal conductivity (similar to 1 W/mK). It was long believed that the mere presence of rattling centers was responsible for the low lattice thermal conductivity of type I clathrates. We found instead that the cage structural disorder induced by atomic substitution plays a crucial role in determining the conductivity of these materials, in addition to the dynamics of the guest atoms. Our calculations showed that the latter is substantially affected by the charge transfer between the metal and the cages. Our results provide design rules for the search of new types of promising nanocage structured thermoelectric materials.