Ab initio calculations of non-stoichiometric copper nitride, pure and with palladium

We present first principles calculations of copper nitride by using periodic density functional theory within a plane-wave ultrasoft pseudopotential scheme. The insertions of extra Cu and/or Pd atoms in the empty sites, vacancy reorganization, and substitution of Cu by Pd atoms were studied. We have used an equivalent reduced-symmetry 2 × 2 × 2 Cu3N-like cubic super-cell. Small Cu and/or Pd concentrations and vacancy rearrangements in the copper sub-lattice were conveniently calculated in these low-symmetry cells. We cover probable situations like: the occupation of the initially empty copper sites by (1) copper atoms, and by (2) palladium; (3) the relocation of vacancies in the copper sub-lattice; and (4) the substitution of small quantities of copper by palladium atoms in the copper sub-lattice. The equilibrium volumes and energies after relaxing the atomic positions are compared to those of intrinsic copper nitride. We found that the most stable arrangement corresponds to the ideal stoichiometric Cu3N. We also found that any deviation from this ideal configuration shift the semiconductor state to a metallic or semi-metallic one.

Research highlights▶ The most stable arrangement corresponds to the Cu3N-anti ReO3 structure. ▶ Formation energy of Cu32Vac0N8 and Cu24Pd8Vac0N8 are very similar. ▶ The biggest volume in the compound is Cu31Pd1Vac0N8/. ▶ Small amount introduction of extra metal atoms in copper nitride is possible.