Low-temperature structure and lattice dynamics of the thermoelectric clathrate Sn24P19.3I8

[Display omitted] ► Sn24P19.3I8 retains the crystal structure of the type-I clathrate even at 10K. ► Two Einstein and one Debye terms describe the lattice dynamics of Sn24P19.3I8. ► The thermal conductivity for Sn24P19.3I8 is almost entirely phononic. The crystal structure of the thermoelectric clathrate Sn24P19.3I8 was determined down to 10K showing no variations with the temperature. Even at 10K Sn24P19.3I8 crystallizes in the type-I clathrate structure, space group Pm3-n, with the cubic unit cell parameter ranging from 10.9173Å at 10K to 10.9554Å at room temperature. In its crystal structure, tin and phosphorus atoms form the framework that traps the guest iodine atoms in the polyhedral cavities of two different shapes. The temperature-dependent crystal structure data and the results of the heat capacity measurements revealed the localized vibrations of the guest atoms inside the oversized cavities with the characteristic Einstein temperatures of θE1=60K and θE2=78K, whereas the characteristic Debye temperature for Sn24P19.3I8 is 265K. The room temperature lattice thermal conductivity was calculated using the Debye model to be κL=0.85Wm−1K−1, which is in excellent agreement with the experimentally measured data and demonstrates that the thermal conductivity is almost entirely phononic.