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

Highlights: • D0₉, B17 and C18 are the most stable phases for Cu₃N, CuN and CuN₂ respectively. • Other Cu₃N phases show similar stability to D0₉ and may be present during nitridation. • The stable Cu₃N phases are indirect-gap semiconductors with lower bulk modulus than Cu metal. • The GW₀ optical energy gap of Cu₃N(D0₉) significantly improves the DFT values. 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.