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

► Sn24P19.3I8 retains the crystal structure of the type-I clathrate even at 10 K. ► Two Einstein and one Debye terms describe the lattice dynamics of Sn24P19.3I8. ► The thermal conductivity for Sn24P19.3I8 is almost entirely phononic.