Towards modelling speciation and crystal morphology: The hydration of plutonium hydride surfaces

Density Functional Theory (PBEsol + U + SOC) calculations were used to study the adsorption of molecular and dissociative water on PuH2 {100}, {110} and {111} surfaces up to the monolayer coverage. In general, we find that dissociated water has a more exothermic adsorption than molecular water, and that the {100} surface has a more exothermic adsorption followed by the {110} and {111} surfaces. Water was bidentate bridging on {100} and terminal on {110} and {111} surfaces. Hydroxyl ions were bidentate bridging on {100} and a mix of bridging and terminal on {110} and {111} surfaces, while the introduced hydrogen was exclusively in two or three coordinate bridging positions. This hydrogen from dissociated water displaced a surface hydride into the subsurface on the {111} surface. We then calculate the temperature of desorption of water from the surfaces, and the morphology of PuH2 particles from thermodynamic considerations. We finally constructed an adsorption isotherm for H2O on PuH2 surfaces, which shows that the surfaces load up sequentially in the order {100}, {110}, and {111}. The ratio between dissociated and molecular adsorbed water displayed a mix of species under all coverage conditions. For both the {110} and {111} surfaces, the fraction of dissociative adsorbed water increases as the coverage of water increases, i.e. as more water is loaded onto these surfaces, more of it is dissociated. This is not the case for the {100} surface where it does not show a clear pattern and remains approximately 1:1 dissociated to molecular water at the monolayer coverage.