We present the reuslts of ab-initio Hartree-Fock calculations on a set of perovskite-structured transition metal oxides, in which the transition metal ion has electronic configuration d~[0]. The analysis of thesolution, based on a Tight Binding-like examination of the calculated baud structures in reciprocal space, allows us to understand the trends in the properties of perovskites as a functions of their chemical composition; a parameter is defined, which is easily calculated from the band structure of the cubic phase, that quantifies the extent of covalence in the M-O interaction. Ferroelectric-like distortions from the cubic phase are seeded by a symmetry breaking around either a M or an O ion of the structure; the electronic perturbation is then transferred to the neighbouring sites via a delocalisation of the #pi# M-O bonding levels in the valence band.
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