Crystal chemical studies of the 5f-series of elements. III. A study of the disorder in the crystal structure of anhydrous uranyl fluoride

The ideal structure of UO₂F₂ is rhombohedral with one molecule per unit cell. The unit rhombohedron has dimensions a = 5.755 ± 0.003 kX., [alpha] = 42° 47' ± 3'. The space group is Rm and the atomic positions are: 1 U in (0, 0, 0), 2 O in ±(u, u, u) with u = 0.122, 2 F in ±(v, v, v) with v = 0.294. Each uranium atom is bonded to two oxygen atoms with U-O = 1.91 A. and to six fluorine atoms with U-F = 2.50 A. The structure is of layer-structure type. Planes of uranium atoms are 5.22 A. apart and the axis of the uranyl group is normal to these planes. The fluorine atoms are 0.61 A. above and below the plane containing the uranium atoms. Weak O-O and O-F bonds hold the layers together. All actual samples of UO₂F₂ are found to show stacking disorder, the order increasing with the heat treatment. The stacking disorder gives rise to anomalous features in the diffraction pattern. It is shown that the theory of X-ray diffraction in crystals with stacking disorder is capable of explaining all observed anomalies.