High-pressure phase transition and properties of Cu₃N: An experimental and theoretical study

In-situ X-ray and neutron diffraction investigations on Cu₃N indicate the onset of a high-pressure phase transition at about 5 GPa. The tetragonal cell parameters of the high-pressure phase reveal a discontinuous volume decrease of about 20 %. The phase transition, is reversible, with a hysteresis of about 2 GPa. Subsequent ex-situ investigations in a multi-anvil press evidence a reversible re-formation of ambient pressure Cu₃N from XRD patterns. The structure refinement with nitrogen atoms disordered in distorted octahedral voids of a tetragonal body-centered copper substructure leads to an occupation of approximately 1/3 and thus to a composition of Cu ₃N_1.o(1). Optical absorption measurements (IR-VIS) up to 10 GPa indicate a semiconductor-metal transition. Density-functional based total energy calculations concerning the proposed, high-pressure phase of Cu ₃N strongly support the experimental findings of a pressure-induced phase transition above 6 GPa to a structure with a copper tetragonal body-centered sublattice and nitrogen atoms in distorted octahedral voids. However, the calculations identify a need for an ordered alternative to provide the tetragonal distortion within the range of the observed c/a ratio. The resulting lattice parameters and the transition pressure fit with the measured data. For the case of an ordered occupation of the copper bet octahedral voids, all observed properties are in good agreement with the calculations.