Re-examination of the neptunium-hydrogen system

New P-C-T studies have been made on the Np + H system, using ultrapure double-electrorefined metal. Measurements were carried out over six orders of magnitude in pressure, from 0.0005 to 70 bar. The solubility of hydrogen was found to be very low. Metal-dihydride plateaus were flat, the two-phase boundary extremely sharp; this occurred at the unusual value H/Np approx. = 2.13. The dihydride lattice expanded upon addition of hydrogen, also in contrast to other trivalent rare-earth and actinide hydrides. A rather narrow cubic/hexagonal two-phase region was found (only on dehydriding), together with an even narrower hexagonal phase region. The effect of the beta-gamma transition at 576/sup 0/C could be seen both in the curvature of the phase boundary and the partial molal enthalpy values. Entropies of formation were found to be nearly constant. The data below 576/sup 0/C can be described by the equation ln P(bar) = 13.297 - 13233/T(K), giving values, adjusted for the reaction 0.93 Np + H/sub 2/ = 0.93 NpH/sub 2.13/: ..delta..H/sub f/(NpH/sub 2.13/) = 118.3 kJ/mol (28.27 kcal/mol), and ..delta..S/sub f/(NpH/sub 2.13/) = 118.4 J/mol-K (28.3 cal/mol/-K). Integral heats and entropies are calculated for the entire system, and the unusual phase behavior is discussed in terms of the electronic structure