Thermal conductivity of elemental crystalline silicon clathrate Si136

The thermal conductivity and heat capacity of a guest-free polycrystalline silicon clathrate with the type-II hydrate crystal structure is reported. The magnitude of the thermal conductivity at room temperature is only slightly larger than that of vitreous silica, and is thirty times lower than that of diamond structured Si. The temperature dependence of the thermal conductivity of ${\mathrm{Si}}_{136}$ follows the well-known Debye form, and is dissimilar to that of clathrates with “guest” atoms inside their polyhedra. The Debye temperature of ${\mathrm{Si}}_{136},$ estimated from low temperature heat capacity measurements, is 470 K. The potential of guest-free clathrates for thermoelectric applications is discussed.