  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.
