Stability of U5Si4 phase in U-Si system: Crystal structure prediction and phonon properties using first-principles calculations

Abstract

U-Si systems have recently received considerable attention due to the potential application of U3Si2 as a high-density fuel under an accident tolerant fuel initiative. However, the thermodynamic stability of the more recently reported adjacent U5Si4 phase is uncertain and could play a significant role in fuel performance. In this work, the enthalpy of formation of the phase predicted by density functional theory (DFT) using the DFT + U formalism is used with an evolutionary algorithm (USPEX) to evaluate stability and possible atomic structures for U5Si4. The structure of U-Si convex hull phases and the confirmed U3Si2 structure were predicted providing confidence in the reliability of the evolutionary algorithm, as well as to obtain a convex hull with comparable enthalpies of formation. Subsequently, the code was applied to the U5Si4 composition for cells with 18 and 36 atoms, predicting a 36-atom hexagonal symmetrized unit cell with space group P6/mmm as the lowest-energy configuration, agreeing with that experimentally reported for U5Si4. Yet, phonon calculations using the density functional perturbation theory formalism, demonstrated that the predicted structure is dynamically unstable, exhibiting negative vibrational modes for the uranium. These indicated that generated shears are directed toward the formation of potential uranium octahedral sites, analogous to those occupied by carbon atoms in U20Si16C3. It was thus concluded that omitting the U5Si4 phase from assessed U-Si phase equilibria is currently justified.