Non-phonon low-temperature thermal conductivity in clathrate semiconductors

Abstract In a recent paper Cohn et al. (Phys. Rev. Lett. 82 (1999) 779) report an observation of low-temperature glasslike (∼ T 2 ) heat conduction in semiconductors Sr 8 Ga 16 Ge 30 with type-I clathrate hydrate crystal structure. This behavior was attributed to phonon scattering by a broad distribution of two-level systems (TLS). The experiment was carried out on polycrystalline samples with typical grain size around 10 μm . Our simple estimates however show that in such a case at T K the mean free path of phonons should be limited by boundary scattering inside single grains and not by resonant scattering on TLS. Therefore we conclude that the observed low-temperature thermal conductivity might be of non-phonon origin and rather due to the long-range interaction between dopant ions of Sr in big 24-vertex polyhedral cages.