The interplay between thermodynamics and mechanical properties in the transformation of studtite, (UO2)(O-2)(H2O)(2)center dot 2H(2)O, into metastudtite, (UO2)(O-2)(H2O)(2), two important corrosion phases observed on the surface of uranium dioxide exposed to water, is revealed using density functional perturbation theory. Phonon calculations within the quasi-harmonic approximation predict that the standard entropy change for the (UO2)(O-2)(H2O)(2)center dot 2H(2)O -> (UO2)(O-2)(H2O)(2) + 2H(2)O reaction is Delta S-0 = +80 J center dot mol(-1)center dot K-1 for the production of water in the liquid state and +389 J center dot mol(-1)center dot K-1 for water vapor. Similar to bulk H2O(l), the bulk modulus of (UO2)(O-2)(H2O)(2)center dot 2H(2)O increases with temperature, contrasting with (UO2)(O-2)(H2O)(2) which features the typical Anderson-Gruneisen temperature dependence of oxide solids. Upon removal of interstitial H2O in studtite, the most important changes in the shear modulus, the parameter limiting the mechanical stability, arise in the planes normal to chain propagation directions. The present findings have important implications for the dehydration of other hygroscopic materials.
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