Order-disorder competition in the Sc2VO5+δ system. Review and experimental pursuit of A3+2VO5 and Sc2VO5+δ-inspired solid phases

Abstract

This work was motivated by the proclivity of oxygen-deficient metal oxides to enable oxide transport at high temperatures. This property depends on the composition and metal-oxygen connectivities within the structure and requires vacant crystallographic sites. The project initially focused on non-conducting derivatives of oxygen defect fluorites AO2-δ, namely split-cation A2BO5-type superstructures, but eventually turned into a broader experimental and theoretical investigation of formation regimes and metal-oxygen connectivities of these phases and, to a degree, their structural competitors – A2B2O7 and A4B3O12. At the core of this work is the defect fluorite superstructure Sc2VO5+δ with tunable δ. It belongs to the structurally diverse yet scarce A2VO5 family of compounds. The high-temperature tetragonal polymorph t-Sc2VO5+δ is predominantly ordered – cation-ordered and oxygen-vacancy-ordered, and its vanadium sublattice is charge-ordered. Doping of the vanadium sublattice with commensurate cations Ge, Ti, Sn, and Cr replaced either V3+…4+ or V5+ in it in proportion to the octahedral ionic size of the dopant, also affecting δ and the average cation ordering. Increasing the content of the dopant led to formation of competing phases – Sc2B2O7 and Sc4B3O12, which are also discussed. The low-temperature cubic polymorph c-Sc2VO5+δ is fully disordered on average but partially ordered on the local scale, insofar as its vanadium sublattice accumulates smaller V5+ species. This cubic phase can be oxidatively obtained from either its tetragonal polymorph or a cation-disordered bixbyite Sc2VO4.5, in both cases inheriting the average ordering scheme of its precursor. Key phases were investigated with a tandem of ex-situ and in-situ average and local structure probes, such as X-ray and neutron powder diffraction, X-ray absorption near-edge structure spectroscopy, and X-ray and neutron total scattering. Results led to a discussion on the evolution of coordination and connectivities in the cation sublattices, oxygen driven phase transitions, limit of oxygen concentration, oxide vacancy structure, the crucial role of the cation radius in phase formation, and the order-disorder interplay. Discussed vanadates with 2:1 and 1:1 stoichiometric ratio of A3+ and aliovalent vanadium were compared across their joint phase diagram and discussed with the help of stability maps.