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The lattice thermal conductivities of silicon clathrate frameworks II andVIII are investigated by using ab initio lattice dynamics and iterativesolution of the linearized Boltzmann transport equation(BTE) for phonons.Within the temperature range 100-350 K, the clathrate structures II and VIIIwere found to have lower lattice thermal conductivity values than silicondiamond structure (d-Si) by factors of 1/2 and 1/5, respectively. The mainreason for the lower lattice thermal conductivity of the clathrate structure IIin comparison to d-Si was found to be the harmonic phonon spectra, while in thecase of the clathrate structure VIII, the difference is mainly due to theharmonic phonon spectra and partly due to shorter relaxation times of phonons.In the studied clathrate frameworks, the anharmonic effects have larger impacton the lattice thermal conductivity than the size of the unit cell. For thestructure II, the predicted lattice thermal conductivity differs approximatelyby factor of 20 from the previous experimental results obtained for apolycrystalline sample at room temperature.

Ab initio studies on the lattice thermal conductivity of silicon clathrate frameworks II and VIII