The lattice thermal conductivity of silicon clathrate framework Si and two Zintl clathrates, [SiP]Cl and Na[AlSi], is investigated by using an iterative solution of the linearized Boltzmann transport equation (BTE) in conjuntion with ab initio lattice dynamical techniques. At 300 K, the lattice thermal conductivities for Si, [SiP]Cl, and Na[AlSi] were found to be 43 W/(m K), 25 W/(m K), and 2 W/(m K), respectively. In the case of Na[AlSi], the order-of-magnitude reduction in the lattice thermal conductivity was found to be mostly due to relaxation times and group velocities differing from Si and [SiP]Cl. The difference in the relaxation times and group velocities arises primarily due to the phonon spectrum at low frequencies, resulting eventually from the differences in the second-order interatomic force constants (IFCs). The obtained third-order IFCs were rather similar for all materials considered here.

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