2.06.3.3.2.1 Preparation

U2F9 (which is called uranium enneafluoride) is prepared by treating UF4 with UF6. The kinetics are faster (about 1 day) when the process is per­formed under higher pressure of UF6 and high temperature. Another way is to start from the penta- fluoride and decompose it into U2F9. It is less conve­nient because it includes the synthesis of pure pentafluoride which is less stable. U2F9 is the more stable intermediate fluoride under argon atmosphere without moisture. It forms black needles.

U4F17 was obtained by maintaining UF4 at 593 K and introducing UF6 at 2.3 x 103 Pa pressure

during 2 days.59 The diffraction pattern of this material resembling that of UF4 but containing uniquely differ­ent features is described as ‘distorted UF4.’ It is a black powder. This synthesis is to be avoided due to the corrosion problem at high temperature under UF6. It is easier to decompose higher intermediate fluorides such as UF5 or U2F9 than synthesize directly.

We have found no physical properties for these compounds in the literature. Mixtures of UF5-U2F9 have a tendency to melt at lower temperature than pure UF5.

2.06.3.3.2.2 Crystal structure

U2F9 as a body-centered cubic structure, space group I43m — Td with a = 8.4716(5) /A (Figure 12) and a theoretical density pth = 7.06gcm—3 62,63 This sym­metry was confirmed by high-resolution neutron powder diffraction data.54

U4F17 has a deformed UF4 lattice. The symmetry is monoclinic, space group C6h, C2/c, with a = 12.09 (0.08) A, b = 10.81(2) A, c = 8.29(4) A, and b = 128.0(8).12