Perovskite structure metal-oxygen sublattice

We now examine results concerning ferroelectric-like (FE) distortions in a pseudocubic perovskite phase, in which the oxygen sublattice is kept fixed in its undistorted cubic coordinates, and only the transition metal ion M displaced within its octahedron. The pseudocubic is another idealised phase, for which no direct comparison with experiment can be made, but it enables us to compare different functionals in describing structural distortions. For each combination of composition and Hamiltonian reported in Table 3, we examined an FE distortion, in which the M ion is displaced along the [001] direction. Each phase is based on the equilibrium lattice parameter for the cubic phase (Table 1). This idealised distortion makes reference to the tetragonal phase of FE materials such as BaTi03 and KNb03 [99]. 

Oxides with close-packed oxygen lattices and only partially filled tetrahedral and octahedral sites may also facilitate diffusion of metal ions in the unoccupied, interstitial positions. Finally, even large anions may diffuse interstitially if the anion sublattice contains structurally empty sites in lines or planes which may serve as pathways for interstitial defects. Examples are rare earth sesquioxides (e.g. Y2O3) and pyrochlore-type oxides (e.g. La2Zr207) with fluorite-derived structures and brownmillerite-type oxides (e.g. Ca2Fe205) with perovskite-derived structure. 

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