Structural study and properties of the alkali metal, nitrosyl, and ammonium hepta- and octafluorouranates(VI)

Abstract

The thermal decomposition of the heptafluorouranates(V1) of the alkali metals is shown to take place in two steps. The first step gives the octafluorouranates(V1) and UF6, and the decomposition rate is noticeable at temperatures above 100, 130, 150, and 210 " C for the Na, K, Rb, and Cs salts, respectively. The second step for NazUFs yields pure N a F and UF6 above 300 OC, whereas the decomposition temperatures for the K, Rb, and Cs salts are above 300, 350, and 400 OC, respectively. Depending on the decomposition conditions, F2 and M2UF7 (M = K, Rb, Cs) or Fz, UF6, and M3UFs are formed. The heptafluorouranates(V1) of all the cations studied, except for ammonium, were shown to exhibit dimorphism.The parameters of their cubic form were obtained and are as follows: KUF7; a = 5.22 8,; RbUF7; a = 5.385 A; CsUF7; a = 5.517 A; NOUF7; a = 5.334 A; NH4UF7; a = 5.393 A; NaUF-i(fccub), a = 8.511 A, Z = 4. The x-ray pattern of the low-symmetry form of CsUF7 just below the solid transition temperature (15 f 1 "C) was indexed with a tetragonal cell where a = 5.50 8, and c = 5.37 A. The x-ray diagrams of the low symmetry form of the other MUF7 salts were not indexed, whereas those of the octafluorouranates were indexed with orthorhombic cells. The vibrational spectra of the hepta-and octafluorouranates were found to be very dependent on the temperature, and for the same temperature on the cation size. In the solids at high temperature the disordered F positions are likely to be averaged to give pseudo-Dsh and o h symmetry structures for the UF7-and UFs2-ions, respectively. At lower temperature, as the motions are frozen out, the observed spectra for the hepta-and octafluorouranates arise from structures of symmetry no higher than CzU and D2d, respectively. The ions UF7-and UFs2-were characterized in nitrosyl or cesium fluoride H F solutions, which were found to exchange F-ions with these anions. Based on observation of the chemical exchange between UF6 and UF7-and on a comparative study of the WF7-ion, a fluoride ion transfer mechanism is also found for UF7-dissolved in acetonitrile. Some trends observed in this study, like the thermal decomposition temperatures or the relative symmetries, are thought to arise from the differences in the cation-anion interaction. This interaction is stronger with smaller cations, which results in more distorted anions, less ionic U-F bonds, and paradoxically less stable complexes.

Bromine trifluoride oxide. Vibrational spectrum, force constants, and thermodynamic properties

“…The starting materials KBrF4Ó and G2ÁsF6 were obtained by literature methods.2,3 The vacuum lines, spectrometers, instruments, and sampling techniques used at C.E.N have previously been described. 4,5 At Rocketdyne, the BrF30 was prepared in a passivated sapphire reactor which was connected through a small-volume Teflon FEP U-trap to a stainless steel Teflon FEP vacuum system. The crude BrF30 was collected in this U-trap by fractional condensation at -20 °C and was further purified by briefly pumping off the more volatile components at 0°.…”