22.08.2015   Neutron Scattering Applications and Techniques

The Perovskite Structure

The basic chemical formula of perovskite-type oxides is ABO3, where A represents a relatively large cation with oxidation state +1, +2, or +3, and B is a cation with oxidation state between +1 and +7. For proton-conducting perovskites, A is usually +2 (e. g. Sr2+ or Ba2+) and B is usually +4 (e. g. Ce4+ or Zr4+). The “ideal” perovskite structure is defined by a cubic lattice of corner-sharing BO6 octahedra and 12-fold coordinated A site ions, see Fig. 9.3a. A cubic structure is, however, typically observed only if the sizes of the cations are compatible with the sizes of their respective interstices, and if not, the perovskite adopts a structure of lower symmetry. The deviation from cubic symmetry may be quantified with the

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Fig. 9.3 a Schematic of the cubic ABO3-type perovskite structure showing the octahedrally coordinated B4+ ions within BO6 octahedra and the 12-fold coordinated A2+ ions. b Schematic of doping with In3+ at the Zr4+ site in cubic-structured BaZrO3, followed by the incorporation of protons through hydration in a humid atmosphere. c Schematic of the two principal steps of the proton-conduction mechanism in hydrated perovskites

Goldschmidt factor, tG, which has a value of one or close to one for a cubic perovskite [25].[12]