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

Chapter 13Neptunium group 2(alkaline-earth) compoundsSeveral ternary oxides of alkaline earth elements with neptunium have been reported,and general synthesis and X-ray diffraction data for these compounds have been summarisedbyFahey[86FAH]and references therein. However, chemical thermodynamicdata for these compounds are relatively limited.The thermodynamic quantities calculable from the available experimental data areexclusively enthalpies of formation. Gibbs energies would, of course, be more usefulfor thermodynamic modelling. However, in this review no entropy (or Gibbs energy)values are estimated for the mixed alkaline earth/neptunium oxides nor for the mixedalkali metal/neptunium oxides (Chapter 14). Entropies determined experimentally foruranium solids (e.g. γ − UO 3 (cr)) give a clear indication that entropies for M(VI)oxides are likely to be greater than those predicted based on “Latimer type” contributionsderived from entropies of M(III) or M(IV) solids (see [92GRE/FUG], especiallyAppendix D). However, NpO 3 (cr) does not exist, and no information is available fromdirect measurements on the hydrated Np(VI) oxides.13.1 Neptunium strontium compounds13.1.1 Sr 3 NpO 6 (cr)Morss et al.[83MOR/WIL] measured the enthalpy of solution of Sr 3 NpO 6 (cr) in 1.0 MHCl, obtaining f H ◦ m (Sr 3NpO 6 , cr, 298.15 K) =−(3125.3 ± 5.8) kJ·mol −1 .UsingCODATA compatible auxiliary data the value f H ◦ m (Sr 3NpO 6 , cr, 298.15 K) = −(3125.8 ± 5.9) kJ·mol −1is obtained from a recalculation in this review. The value is accepted here.13.2 Neptunium barium compounds13.2.1 Ba 3 NpO 6 (cr)Morss et al.[83MOR/WIL] measured the enthalpy of solution of Ba 3 NpO 6 (cr) in 1.0 MHCl, obtaining f H ◦ m (Ba 3NpO 6 , cr, 298.15 K) =−(3086.0 ± 7.7) kJ·mol −1 .Using289