Temperature variation of the structural parameters in actinide tetrafluorides

We have used neutron diffraction on polycrystalline samples of AnF₄ (An=Th, U, and Np) to examine the structures. All are isostructural with monoclinic space group C2/c. The structure of UF₄ has been examined in some detail as a function of temperature. The volume of the unit cell has a minimum not at lowest temperature (15 K) examined, but rather at ∼350 K. ThF₄ also has a larger volume at low rather than room temperature; suggesting that this effect is caused by structural rather than electronic (as ThF₄ has no 5f electrons) interactions. An examination of the coordination around the two independent An sites in UF₄ shows that the fluorine configuration changes appreciably around An2 as a function of temperature. At low temperatures the distorted cubic antiprism structure is considerably more ‘‘puckered’’ than at higher temperatures. This puckering requires more volume for the molecule, thus causing the volume to expand as the temperature is lowered. The temperature factors and the interatomic distances vary smoothly with temperature and show no unusual behavior.