We present a first-principles study of the UO(3)·n(H(2)O) uranyl oxide hydrates, namely, schoepite (n = 2.25), metaschoepite (n = 2) and dehydrated metaschoepite (n = 1.75), which appear as the alteration U(VI) products of aqueous corrosion of nuclear fuel. For these compounds, the calculated enthalpy of formation is in good agreement with calorimetry and solubility measurements. We discuss the key electronic state factors behind the phase stability of uranyl oxide hydrates. An unexplored proton-transfer mechanism, which produces the H(3)O hydronium ions in UO(3)·nH(2)O, has been studied using ab initio molecular dynamics simulations at room temperature. For the hydronium ion, a very short lifetime of around 20 fs has been suggested.
A multiscale modeling approach is developed to compute the phase diagram of the RbF-CsF binary system. The mixing enthalpies of the (Rb,Cs)F solid and liquid solutions are evaluated using density functional theory and classical molecular dynamics calculations, respectively. For the solid solution, 18 different configurations are studied with density functional theory and the surrounded atom model is applied in order to compute the configurational partition function. We also measure the solidus and liquidus equilibria using differential scanning calorimetry. Finally the RbF-CsF phase diagram is constructed using the calculated excess free enthalpies of the solid and liquid solutions and a very good agreement with our experimental data is found.
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