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Keywords:

  • Anionic ordering;
  • Density-functional theory;
  • Oxynitrides, cobalt;
  • Phase stability;
  • Quantum-chemical studies

Abstract

We report electronic structure calculations using density-functional theory (local density approximation (LDA) and generalized gradient approximation (GGA); plane waves and muffin-tin orbitals; pseudopotentials and all-electron approaches) on non-stoichiometric CoNxO1–x oxynitride phases. The preference of the experimentally suggested zinc-blende structure type over the rock-salt type is confirmed and explained, on the basis of COHP (crystal orbital Hamilton population) chemical bonding analyses, by reduced Co–Co antibonding interactions in the ZnS structural alternative. A pressure-induced phase transition into the NaCl type, however, is predicted at approximately 30 GPa. Supercell calculations touching upon the exact composition and local structure of CoNxO1–x provide evidence for a broad range of energetically metastable compositions with respect to the zinc-blende-type boundary phases CoN and CoO, especially for the more oxygen-rich phases. All non-stoichiometric compounds are predicted to be metallic materials which do not exhibit significant magnetic moments. Likewise, there is no indication for anionic ordering such that random anion arrangements are preferred.