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.