Oxygen non-stoichiometry, ordering and mobility in SrCoO3-δ perovskite

Abstract

Topotactic synthesis aims at new crystal structures through using compounds with related structures as precursors in which only chosen structural features are removed or introduced during each synthesis step. In the present work aqueous immersions of strong oxidizing agents, KMnO4/HCl, Na2S2O8, Br2/H2O and Br2/NaOH, were tried in adjusting the oxygen content of SrCoO3-δ (0.00 < δ < 0.71). A method, where the oxidation rate is controlled with temperature and H2O concentration of a Br2/H2O/CH3CN immersion, was developed. The key is the unfavourable (Keq ≪ 1) disproportionation reaction Br2 + H2O → Br- + BrO- + 2H+ introducing, at each time, only a small amount of the actual oxidizing agent, BrO-, into the immersion. Additionally, reduction of oxygen-rich SrCoO3-δ (0.00 < δ < 0.25) via low-temperature (200 °C < T < 275 °C) air annealing was found effective. Several series of multiphase SrCoO3-δ samples with varied δ were produced and investigated. A variety of oxygen-deficient SrCoO3-δ phases was observed using X-ray diffraction and iodometric titration. The number of the phases concluded between the single phases SrCoO2.50 (n = 2) and SrCoO2.875 (n = 8) is predicted by the AnBnO3n-1 homologous series. In addition to the n = 2, 4, 5, 8, 32 and ∞ members discovered earlier, signatures for the n = 6 and 7 members were detected and the n = 3 phase was clearly discovered. The n = 3 phase with a stoichiometry of SrCoO2.68(1) presents a tetragonal (t) unit cell with a superlattice at × bt × ct = ac × ac × 4ac and a strong c-axis elongation, as compared to an ideal cubic (c) perovskite unit cell. Oxygen intercalation into SrCoO3-δ lattice was studied through XANES spectroscopy. The weak chemical shift of the Co-L2,3 edge over the 0.50 > δ > 0.18 range indicates the oxidation process to significantly involve electron distribution within the oxide-anion sublattice. Analysis of the depth-resolved O-K XANES concludes the superoxide (O2-) to be most abundant at the high δ value. Should it be always present in the bulk, octahedrally coordinated cobalt is unobservable in the surface region of the highly oxygen-deficient samples. The surface region hosts also square-pyramidally coordinated cobalt even in the brownmillerite-type SrCoO2.50. An oxygen-intercalation mechanism was proposed in which (i) O2 is first absorbed on the surface as O2-, which is then (ii) reductively split into Ox- associated with square-pyramidally coordinated cobalt. Deeper in the bulk (iii) Ox- is eventually reoxidized to Oz- (0 < z < x) being attached to octahedrally coordinated cobalt.