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 Cu3N1.0(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 Cu3N 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 bct octahedral voids, all observed properties are in good agreement with the calculations.