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

118 8. Neptunium oxygen and hydrogen compounds and complexeshave used f H ◦ m (NpO 2(OH) 2 (cr)) to estimate the enthalpy of formation of the anhydrousoxide to be −1058 kJ·mol −1 , and they further estimated an oxygen pressureof 10 14 Pa would be required to stabilise the solid. Similarly, using values fromGrenthe et al. [92GRE/FUG] and f H ◦ m (NpO 2(OH) 2 (cr)) from the present review, f H ◦ m (NpO 3(cr)) = −(1068 ± 7) kJ·mol −1 , and if the difference in the entropiesof NpO 3 (cr) and NpO 2 (OH) 2 (cr) is approximately the same as for the correspondinguranium compounds, f G ◦ m (NpO 3(cr)) =−(990 ± 10) kJ·mol −1 . By comparisonwith the Gibbs energies of formation for Np 2 O 5 (cr) and NpO 2 (cr), it appears that synthesisof this anhydrous oxide from the lower oxides and oxygen gas is not feasibleat practical pressures (see also [77MAL]) and no thermodynamic data are selected forNpO 3 (cr) in the present review.8.2.3.2 Dioxoneptunium(VI) hydrated oxides and hydroxidesThe uranium(VI)-water system has been found to be quite complex [92GRE/FUG],and even with the sparse experimental data available it is evident that several differentsolids can be found in the corresponding neptunium(VI)-water system.A hydrated neptunium(VI) oxide or hydroxide can be synthesised bypassing ozone/oxygen mixtures through suspensions of NpO 2 OH(am) at 90 ◦ C[64BAG/LAI, 75BEL/IL’]. The solid produced by this method may also contain someNp(VII) [74CHA/MAT]. The resulting precipitate (which has an X-ray diffractionpattern similar to that of uranium trioxide dihydrate [64BAG/LAI]), when dried inair at 100 to 105 ◦ C, is converted to a solid that was reported [64BAG/LAI] tobeisostructural with β-UO 2 (OH) 2 (cr) (on the basis of the X-ray diffraction pattern).Infrared and proton magnetic resonance spectroscopic results [75BEL/IL’] addedevidence in favour of considering this compound to be a true hydroxide with no waterof hydration. However, Kato et al. [96KAT/KIM] have recently reported preparationof a neptunium trioxide monohydrate by neutralisation of an acidic solution ofNp(VI). Their solid has an X-ray diffraction pattern and an infrared spectrum thatdiffer markedly from those of the solid of Bagnall and Laidler [64BAG/LAI] ofthe same stoichiometry and from those of other hydrated solids reported previously[64BAG/LAI, 75BEL/IL’, 75CHA/MAT]. For the purposes of the present review,these two different solids with the same apparent stoichiometry are distinguishedby referring to the dried solid of Bagnall and Laidler as “NpO 2 (OH) 2 (cr)”, and tothe solid of Kato et al. as “NpO 3·H 2 O(cr)”. Chaikhorskii, Matuzenko and Leikina[75CHA/MAT] also reported formation of an amorphous solid, NpO 3· 4H 2 O(am),on storage of the dihydrate over water in a sealed vessel for 12-15 days at “roomtemperature”. It appears the Np(VI) hydrated oxides and hydroxides may be a fairlycomplicated system, amd more work remains to be done to clarify which compoundsare stable.Moskvin [71MOS3] reported the solubility of “NpO 2 (OH) 2 (s)” (at 20 ◦ C) as a functionof pH (I ≪ 0.1M, self medium with small amounts of an unidentified acid). Thissolid was apparently not well characterised, nor does it appear that the material was reexaminedafter equilibration with the aqueous solutions. However, it appears likely thatunder the stated experimental conditions the solid was primarily the hydrated hydrox-