Structure of fluorides. Part XII. Single-crystal neutron diffraction study of uranium hexafluoride at 293 K
A single-crystal neutron diffraction study of UF6 has been carried out at 293 K. UF6 is orthorhombic, space group Pnma with a= 9.900(2), b= 8.962(2), c= 5.207(2)Å, Z= 4. 475 independent Fo2(hkl) values were obtained, the three-dimensional intensity data being collected on an automatic neutron diffractometer. Least-squares refinement on F2 led to R 0.090 (R′ 0.094). Analysis of the thermal parameters indicated almost perfect rigid-body motion of the UF6 octahedron, the translational and librational tensors being nearly isotropic with root-mean-square principal axis translations and librations of ca. 0.19 Å and 4.5°. The R factor for the agreement between the measured and calculated Uij for the rigid-body hypothesis was 0.041. Corrections to measured bond lengths arising from the librational motions of the fluorine atoms were +0.015 Å for U–F and +0.020 Å for F ⋯ F distances. Corrected distances are U–F 1.992(3)–2.004(4)Å, F ⋯ F 2.804(6)–2.826(3)Å, and F–U–F angles 89.42(17)–90.20(11)°. In the crystal the molecules are slightly distorted from perfect octahedral symmetry.