Phase stability of U₅Si₄, USi, and U₂Si₃ in the uranium–silicon system

The intermetallic compound U₃Si₂ has received interest in recent years for use as accident tolerant fuels in light water reactors due to its combination of high thermal conductivity, reasonably high melting point, and high uranium density compared to UO₂. Although U₃Si₂ is well characterized there are ucertainties about the remaining phases in the U–Si system which could be potential fission products of the silicide fuel. In an effort to better detail the U–Si binary phase diagram, the 44–60 at.% Si region of the U–Si phase diagram was investigated experimentally by analyzing samples with nominal compositions U/Si = 5/4, U/Si = 1/1 and U/Si = 2/3 using SEM-EDS, XRD, and high temperature time-of-flight neutron diffraction, supported by DFT calculations. Both the U/Si = 5/4 and the U/Si = 2/3 samples were composed of two phases, U₃Si₂ + USi and USi + U₃Si₅ for U/Si = 5/4 and U/Si = 2/3, respectively. Both U₅Si₄ and U₂Si₃ were found from DFT calculations to have imaginary phonon frequencies and an enthalpy of formation above the U–Si convex hull, and thus likely to be thermodynamically unstable compared to the other phases, consistent with the experimental results. In the literature there are three different structures that are proposed for the USi phase, and for the first time, high temperature neutron diffraction measurements were collected on the compound synthesized with the U/Si = 1/1 composition to verify the correct structure. The structure of the USi phase was confirmed to be tetragonal with the I4/mmm space group from room temperature to 1100 °C. The USin crystal structure has 6 U atom sites and 8 Si atoms sites of which two are partially occupied.