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

8.2 Crystalline and amorphous neptunium oxides and hydroxides 1298.2.6 Neptunium(III) oxideAccording to Ackermann et al. [66ACK/FAI], traces of a possible rare-earth type-A hexagonal phase formed on cooling hypostoichiometric NpO 2−x . Lattice parametersa = 4.234 × 10 −10 m, c = 6.10 × 10 −10 m were reported by Ackermann andChandrasekharaiah [75ACK/CHA]. However, Ritcher and Sari [87RIC/SAR] couldnot confirm this phase. The intermediate formation of Np 2 O 3 was reported to havebeen observed by photoelectron spectroscopy [88ITU], in situ, during the slow oxidationof clean metal surfaces. Morss [86MOR] estimated that, at 298 K under standardconditions, r G m for the disproportionation of Np 2 O 3 into metal and dioxide is approximately−85 kJ·(mol of Np 2 O 3 ) −1 . In the present review no chemical thermodynamicquantities are selected for Np 2 O 3 (s).8.2.7 Neptunium(III) hydroxideNo systematic study of ∗ K s,0 for the actinide(III) hydroxides as a function of temperatureor ionic strength has been made. No usable experimental values for Np(OH) 3 (s)appear to have been reported. In the NEA/TDB americium volume [95SIL/BID], thevalues of log10 ∗Ks,0 ◦ for Am(OH) 3 (s) = (17.0±0.6) (amorphous) and (15.2±0.6) (crystalline)have been chosen. It seems reasonable to assume that the solubility productdecreases slightly in the order U(III)>Np(III)>Pu(III)> Am(III)>Cm(III), probablyby less than a factor of two for each element. No further evaluation is made here.