Structural phase transition of Cu₃N under high pressure

We performed the high-pressure angle-resolved X-ray diffraction experiments on Cu₃N up to 36.0 GPa. It was found that a crystal structure phase transition begins to take place at ∼5.5 GPa on Cu₃N. Under high pressure, Cu₃N transforms to a new simple tetragonal structure denoted by [Cu(1)₃Cu(2)_x]N_(1+x3), because parts of Cu and N ions occupy the vacant sites in the crystal lattice. In this structure, Cu(1) and Cu(2) denote the primal Cu position and the vacant position in ReO₃ type Cu₃N, respectively. The content x is increasing with pressure, indicating that the phase transition is a gradual change. When pressure is greater than 12 GPa, x keeps 1 up to the maximum pressure, so Cu₃N completely transforms to a body-center tetragonal structure, where the N sites are filled with 13 N ions.