Neutron powder structural studies of UF 6, MoF 6 and WF 6 at 77 K

Abstract

The crystal structures of the hexafluorides UF 6, MoF 6 and WF 6 have been studied at 77 K by powder neutron diffraction. Profile refinements gave R = ∑(ϵy oϵϵy cϵ)/∑y o values of 6.9, 7.6 and 5.8% respectively, where Y o and Y c are the background — corrected pattern intensities. The structures are compared with the room-temperature structure of UF 6, the structures of MoF 6, WF 6 at 193K and with the plastic cubic structures of MoF 6, WF 6 and SF 6. The metal-fluorine bond lengths do not decrease significantly on cooling to 77 K but the octahedra pack more efficiently, and the atomic coordinates more closely approach the ideal coordinates based on perfect hexagonal close-packing of the fluorine atoms. Because of their shorter metal-fluorine bonds, the MoF 6 and WF 6 structures are more distorted from the ideal than UF 6. MoF 6, WF 6 and especially SF 6 are more compact and spherically-shaped molecules than UF 6; this explains the absence of a plastic cubic phase for UF 6, and the greater stability range of the SF 6 cubic phase. The existence of the orthorhombic Pnma phase for UF 6, MoF 6 and WF 6 down to 77 K is confirmed.