chemical thermodynamics of neptunium and plutonium - U.S. ...

9.1 Neptunium halide compounds 137and these are the selected values. The input data used and a complete table of thermalfunctions up to 2000 K are given by Rand and Fuger [2000RAN/FUG]. The input dataare very similar to those suggested by Konings and Hildenbrand [98KON/HIL], whichappeared after the current assessment was completed.9.1.2.6 Intermediate fluorides and NpF 5 (cr)No solid fluorides between NpF 4 (cr) and NpF 5 (cr), analogous to the wellestablisheduranium compounds U 4 F 17 (cr), U 2 F 9 (cr) have been found, (cf.plutonium fluorides). However NpF 5 (cr) has been prepared by both reductionof NpF 6 [70COH/FRI, 80BAL/YEH, 82BRO/WHI, 93MAL/WIL] and oxidationof NpF 4 [75DRO/SER, 78DRO/SER]. Cohen et al. [70COH/FRI] andBrown et al. [82BRO/WHI] used iodine dissolved in IF 5 to reduce NpF 6 , whileBaluka et al. [80BAL/YEH] used PF 3 as a reductant; Drobyshevskii et al.[75DRO/SER, 78DRO/SER] reacted NpF 4 and KrF 2 .Malmet al.[93MAL/WIL]havestudied the reduction of NpF 6 in some detail; their preferred method of preparationwas to react an anhydrous HF(aq) solution containing NpF − 6 ions with BF 3 and LiF. Arange of colours is reported for NpF 5 (cr), from creamy-white through bluish-white toyellow-brown. The latter material was shown to contain at least 98% Np(V) from itsMössbauer spectrum. NpF 5 (cr) is isomorphous with the high temperature tetragonalform of UF 5 (cr) (stable above ca. 398 K), space group I4/m with cell parametersa = 6.5358 × 10 −10 mandc = 4.4562 × 10 −10 m[93MAL/WIL]. This structurewas retained at 133 K [82BRO/WHI], suggesting that NpF 5 (cr) may not undergo thetransformation to the different low-temperature tetragonal structure, space group I42d,that occurs in UF 5 (cr).No thermodynamic data have been reported for NpF 5 (cr), but approximate valueshave been derived from the observation by Malm et al. [93MAL/WIL] that disproportionationto NpF 6 (g) and NpF 4 is not appreciable below 591 K, a much highertemperature than for UF 5 (cr) which begins to lose UF 6 (g) at 423 K.The standard entropy and heat capacity have been assumed to be close to those forthe isostructural UF 5 (cr, α) [92GRE/FUG]:Sm ◦ (NpF −15, cr, 298.15 K) = (200 ± 15) J·K−1·molCp,m ◦ (NpF 5, cr, T) = (126 + 3 × 10 −2 T − 1.9 × 10 5 × T −2 −1) J·K−1·mol(298.15 to 600 K)If the pressure of NpF 6 (g) in the reaction2NpF 5 (cr) Å NpF 4 (cr) + NpF 6 (g)is assumed to be 0.1 bar at 591 K, the derived value of f Hm ◦ (NpF 5, cr, 298.15 K)is −(1941 ± 5) kJ·mol −1 , where the given uncertainty excludes those of NpF 4 (cr)and NpF 6 (g). The latter are independent, and the selected value for the enthalpy offormation of NpF 5 (cr) is: f Hm ◦ (NpF 5, cr, 298.15 K) = −(1941 ± 25) kJ·mol −1