Investigation of the chemical bonding in nickel mixed oxides from electronic structure calculations

The chemical bonding in a series of nickel mixed oxides, namely Li₂NiO₂, Na₂NiO₂, tetragonal and orthorhombic La₂NiO₄ for Ni(II), LiNiO₂, NaNiO₂, LaNiO₃ for Ni(III), BaNiO₃ for Ni(IV) and LaNiO₂, 'LaSrNiO₃' for Ni(I), is investigated from the results of electronic structure calculations, including both band structure and local properties, i.e. atomic charges, covalent bond orders and atomic valences. The metallic contribution to the chemical bonding is modelled on the basis of the calculation of the delocalization degree of the covalent bond order, in terms of the existence of long range M-M interactions. As a main result, the increase of the formal oxidation state of nickel in these oxides involves an increase of both its atomic charge and its covalency. The highest covalency is found for the (III) and (IV) oxidation states, as the strongest ionicity is typical of Ni(I). A special emphasis is devoted to the chemical bonding in the perovskite like nickelates. In LaNiO₃, the metallicity is a 3D property, as in LaNiO₂ it is a 2D one. Comparing the chemical bonding in La₂NiO₄ and LaSrNiO₃ shows evidence for a decrease of the anisotropy of the perovskite-rocksalt intergrowth in the reduced nickelate.