We have investigated the crystal structure of the CaThF6 compound that was prepared in our laboratory. In that regard we performed a Rietveld analysis on X-ray diffraction data. CaThF6 is isostructural to LaF3. However several controversial structures have been reported for LaF3. The currently accepted structure is the one determined from a combination of X-ray and neutron diffraction experiments on single crystal. The space group is reported to be P-3c1 (N°= 165) and Z=4. However several authors have indexed the X-ray pattern of LaF3 in the hexagonal system with the space group P63cm (N°185). In our refinement both models were tested but X-ray cannot tell the difference from a polycrystalline material. A simulation with neutron diffraction data showed that such experiment would unambiguously identify the correct space group. Because thorium is a heavy element, the sensitivity on the Fluoride positions could be greatly enhanced by the use of neutrons. We thus propose to combine X-ray and neutron diffraction data and to perform a simultaneous X-ray and neutron Rietveld analysis. Besides, neutron diffraction will unambiguously identify the correct space group