Theoretical study of the lattice thermal conductivity in Ge framework semiconductors

The lattice thermal conductivity of Ge clathrates is investigated by evalua ting the linear response theory heat current correlation functions using mo lecular dynamics. Clathrate crystals with and without guest atoms in their fullerane cages are studied. In comparison with that of diamond-phase Ge, t he clathrate conductivity is reduced by approximately 1 order of magnitude due to the open framework itself. The addition of an encapsulated (rattling ) Sr guest atom produces a further order of magnitude reduction in the cond uctivity, making it comparable to that of amorphous Ge. Our results are con sistent with experiments, and have impact on the search for improved thermo electric materials.