A theoretical investigation of structural, electronic and optical properties of bulk copper nitrides

We present a first-principles density functional theory (DFT) study of the structural stability and electronic properties of bulk crystalline Cu4N, Cu3N, CuN and CuN2 in a set of twenty one different structural phases. By analysing the energetics of these systems, we show that D09, B17 and C18 are the most stable phases in the parameter space considered for the Cu3N, CuN and CuN2 stoichiometric series, respectively. This study predicts that other Cu3N phases (i.e. RhF3 and D02) have similar stability to D09, and may be present during the nitridation process. These stable Cu3N phases are found to be indirect band-gap semiconductors with lower bulk moduli, whereas CuN(B17) preserves the metallicity and has a larger bulk modulus than pure Cu. Furthermore, the optical spectra of the experimentally synthesized Cu3N phase (D09) is investigated by GW0 calculations within the random phase approximation to the dielectric tensor. The obtained optical energy band-gap significantly improves the DFT values and agrees with some experiments.