Quantum‐Chemical Studies on the Geometric and Electronic Structures of Bertholloide Cobalt Oxynitrides

Abstract

AbstractWe 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.