This proposal concerns structural investigations on thermoelectric type-I clathrate compounds Ba8Ge43 and Cs8Sn44. Both compounds possess vacancies in their crystal structure whose concentration can be tuned by substituting Ge and Sn by In. The key crystallographic property of these clathrates lie in the presence of encaged atoms (Ba and Cs). The large size of the cages results in higher isotropic displacement paremeters (ADP) that can be properly determined by neutron diffraction. Our goal is to characterize both the crystal structure and the ADP values when the vacancy concentration is progressively decreased. We thus envisage to measure the two binary compounds as well as some In-containing samples both in the 4-300 K temperature range using POLARIS.