Time-resolved in situ studies of oxygen intercalation into SrCoO2.5, performed by neutron diffraction and X-ray absorption spectroscopy

Electrochemical oxidation of the antiferromagnetically ordered SrCoO2.5, with brownmillerite-type structure, to the cubic ferromagnet SrCoO3, with perovskite structure, has been investigated in situ by neutron diffraction as well as by X-ray absorption fine structure (XAFS) spectroscopy in specially designed electrochemical cells. The neutron diffraction experiments were performed twice, using two different wavelengths (lambda = 1.2921(2) and 4.74 angstrom) in order to better discriminate structural and magnetic changes as functions of the charge transfer. From the neutron diffraction experiments, two intermediate phases, SrCoO2.75 and SrCoO2.82 +/- 0.07, were characterized. No superstructure reflections were observed for the corresponding SrCoO2.75 phase. Instead we observed here, for the first time, 3D oxygen ordering during an oxygen intercalation reaction, as established for SrCoO2.82 +/- 0.07, which can be described as a tetragonal unit cell, related to the perovskite cell by a approximate to 2(a root 2) and c approximate to 2a. The structure of this intermediate phase confirms the strongly topotactic character of the oxygen intercalation reaction. We were also able to prove, from in situ XAFS spectroscopy at the Co absorption edge, that the evolution of the Co valence state from formally +3 for SrCoO2.5 to +4 for the final reaction product (SrCoO3.0) does not proceed continuously but gives evidence for the formation of O- species for stoichiometries corresponding to SrCoO2.82 +/- 0.07. The use of neutrons (vs X-rays) in the diffraction experiments and the choice of the transmission (vs fluorescence) mode in the XAFS experiment guarantee that the obtained data well represent bulk and not just surface properties.