2.06.3.3.1.1 Properties and preparation of UF5

UF5 was initially prepared by Ruff through the reac­tion of UCl5 with anhydrous HF. More recent studies have been made on the reaction of gaseous UF6 on UF4 as a powder:

UF4 + UF6 ^ 2UF5

However, a low temperature (T< 453 K for pUF6 < 1bar (105Pa)) is required to avoid the decomposition to U2F9. This often means a very slow transformation (several days depending on the specific surface). UF5 has a white color but with this technique a gray or black color powder is often obtained because of presence of U2F9.

Asada has more recently prepared UF5 at 643 K but under 3 bars (3 x 105Pa) UF6.53 However, corrosion problems have been experienced.

Probably the best way to obtain pure UF5 from UF4 and UF6 is to allow evaporation of UF5 at 573-673 K and condensation at ^346-408 K. Blue UF5 needles are then obtained.

UF5 is often considered a key product because it may be involved in clogging reactors (flame reactor or fluidized bed) when no excess F2 is maintained. UF5 has a tendency to polymerize. This very impor­tant property tends to explain why the reactivity of

U(V) fluorides is lower than that of U(IV). Also UF5 melts at a low temperature compared to UF4.

UF5 has two crystalline forms that are both tetrag­onal (Figure 10):

• High-temperature a-UF5 that has an I41m sym­metry with a = 6.5259(3) JA and c = 4.4717(2) JA, which was confirmed by high-resolution neutron powder diffraction data of Howard et a/.54

• Low-temperature p-UF5 that has an I42d symmetry with a = 11.469(5) JA and c = 5.215(2) 3A (a = 11.456 (2) A, c = 5.195(1) A, Z = 8).55

The transition temperature is 398 K, independent of the UF6 pressure. The a-phase is obtained from the p form when slowly raising the temperature (18 h at 458 K). The reverse transformation form a to p has not yet been achieved.

UF5 can be separated from U(VI) compounds using anhydrous acetonitrile dissolution.56 Again this property is linked to the polymerization trend of UF5.