QM investigations on perovskite-structured transition metal oxides:: bulk, surfaces and interfaces

We present the results of electronic-structure, ab initio calculations on a set of perovskite-structured transition metal oxides, in which the transition metal ion has electronic configuration ). We perform an analysis of the QM solution for the bulk materials, based on a phenomenological, tight binding-like examination of the band structures in reciprocal space. This treatment allows us to understand the trends in the properties of bulk perovskites as a function of their chemical composition; a parameter is defined, easily calculated from the band structure of the cubic phase, that controls 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. Investigations on the < 001 > surface termination of BaTiO3 and WO3 show that the electronic perturbation induced by the surface can couple with the ferro-/antiferro-electric (FE/AFE) distortional modes of the bulk materials in the surface and sub-surface regions. We have been able to attribute to different surface terminations an FE or AFE character. Finally, via the design of suitable perovskite/perovskite interfaces, we have combined materials with FE bulk behaviour with AFE surface terminations, and vice versa AFE bulk materials with FE surfaces. Our results show that the interface strain may significantly alter the behaviour of the support. The complexity of the systems investigated is already comparable to applications of FE materials in the field of computer memories.