Triuranium heptaoxides: Heat capacities and thermodynamic properties of [alpha]- and [beta]-U3O7 from 5 to 350[deg]K

Abstract

Low temperature heat capacities have been measured by adiabatic calorimetry on two phases with composition UO2.33 designated [alpha]- and [beta]-U3O7. They were obtained by oxidation of UO2 at 135 and 165[deg]C, respectively. [beta]-U3O7 was subsequently heat treated at 225[deg]C. Both substances possess UO2-like structures, apparently tetragonally deformed, with c/a = 0.986 for the face-centered uranium lattice of [alpha]-U3O7 and c/a = 1.031 for that of [beta]-U3O7. Both have normal and almost equal heat capacities over the measured range, except for a small lambda-type anomaly at 30.5[deg]K in [alpha]-U3O7. At 298.15[deg]K the values of the practical entropy, S0, and the free energy function, -(F0-H00)/T, are 19.73 and 9.66 cal gfw-1[deg]K-1 for [alpha]-UO2.333, and 19.96 and 9.77 cal gfw1[deg]K-1 for [beta]-UO2.333, respectively. These new data are correlated with structural and magnetic properties and thermodynamic data for other uranium oxides.