Prediction of Stable Ground-State Uranium Nitrides at Ambient and High Pressures

Uranium nitrides have been the subject of intense research owing to their potential applications as advanced nuclear fuels. However, the phase diagram of the U-N system at low temperature and high pressure still remains unclear. In this paper, we explore extensively the phase diagram of the U-N system up to 150 GPa based on first-principles swarm structure searches. The phase diagrams of the experimentally known stoichiometries like U2N3 and UN2 are refined. At zero temperature and pressure, the experimentally observed CaF2-type UN2 is found to transform into another new I41/amd-type UN2, which is related to the dynamical instability originated from Peierls mechanism. Two new stable high-pressure phases of U2N3 and UN2 are identified for the first time. Besides, several new chemical stoichiometries (UN4, UN3, U3N5 and U2N) are found to have stability fields on the U-N phase diagram. The pressure-induced phase transitions for the U-N system are further investigated. The peculiar structural features such as N2-dimers, planar SO3-like N(N)3 units, non-coplarnar zigzag N4 units, and zigzag U chains are found in U-N compounds under pressure. Our results on the structure exploring provide a better understanding of the structural characteristics and physical properties of uranium nitrides under pressure.