Enthalpies of formation and phase stability relations of USi, U 3 Si 5 and U 3 Si 2
U–Si intermetallic compounds have drawn great attention due to their potential application as nuclear fuels. However, the thermodynamic properties and phase equilibria of this binary system from ambient to high temperature conditions are not fully understood. Via high temperature oxidative drop calorimetry and detailed characterization of the initial and final phases, we have experimentally determined the standard enthalpies of formation of USi and U 3 Si 5.07 at 298 K to be –43.2 ± 6.2 and –43.8 ± 9.0 kJ/mol·atom, respectively. The energetics of the tetragonal USi (t-USi, space group I 4/ mmm ) phase has also been calculated with Density Functional Theory (DFT) for the first time. Combining the obtained formation enthalpies with the heat capacities measured previously, we assessed the thermodynamic stability of t-USi relative to a phase assemblage of two other U–Si phases, U 3 Si 5.07 and U 3 Si 2 , from ambient temperature to 1200 K. The tetragonal USi is thermodynamically more stable than U 3 Si 5.07 + U 3 Si 2 , which supports previously published phase diagram (H. Okamoto and T. Massalski, 1990 [1]): specifically, at least one stable USi phase exists when the U content is 50 at.%. In conclusion, further thermodynamic and phase equilibrium studies are needed for a more comprehensive understanding of the U–Si system across broader compositional and temperature ranges.