X-ray absorption spectroscopy study of parent misfit-layered cobalt oxide [Sr2O2]qCoO2

• [Sr2O2]0.52CoO2 is a parent of misfit-layered cobalt oxides.
• It is obtained by ultra-high-pressure synthesis from Sr3Co2O6, Sr(OH)2∙6H2O and KClO3.
• Co-K and CoL XANES spectra reveal lower than expected Co-valence value.
• Sr-K XANES spectrum indicates that the [Sr2O2] block is not of simple rock-salt structure.
• This block most probably contains OH groups, i.e. [Sr2(O,OH)2]0.52CoO2.

Here we present a comprehensive X-ray absorption spectroscopy study carried out at Co-L2,3, Co-K, O-K and Sr-K edges for the parent misfit-layered cobalt oxide phase [Sr2O2]0.52CoO2; comparison is made to another misfit-layered oxide [CoCa2O3]0.62CoO2 and the perovskite oxide LaCoO3. A high-quality sample of [Sr2O2]0.52CoO2 was obtained through ultra-high-pressure synthesis using Sr3Co2O6 and Sr(OH)2∙8H2O as starting materials. Different dosages of KClO3 were mixed with the raw materials as an oxygen source and tested, but it was found that the window for the redox control of [Sr2O2]0.52CoO2 is rather narrow. From Co-K and Co-L2,3 spectra a mixed III/IV valence state is revealed for cobalt in [Sr2O2]0.52CoO2, but the average valence value is a little lower than in [CoCa2O3]0.62CoO2. Then, Sr-K spectrum indicates that the [Sr2O2] double-layer block in [Sr2O2]0.52CoO2 clearly deviates from the cubic SrO rock-salt structure, suggesting a more complicated coordination environment for strontium. This together with a somewhat low Co-valence value and the fact that the phase formation of [Sr2O2]0.52CoO2 required the presence of Sr(OH)2∙8H2O in the high-pressure synthesis suggest that the [Sr2O2] block contains OH groups, i.e. [Sr2(O,OH)2]0.52CoO2.

[Sr2O2]0.52CoO2 obtained through high-pressure synthesis is a parent of misfit-layered cobalt oxides, such as [CoCa2O3]0.62CoO2 or [MmA2O2+m]qCoO2 in general. Our comprehensive X-ray absorption spectroscopy study shows that both [Sr2O2]0.52CoO2 and [CoCa2O3]0.62CoO2 possess mixed III/IV valence cobalt, but the average Co-valence is a little lower in the former. This is tentatively believed to be due to OH groups replacing part of O2− ions in the [Sr2O2] layer block.Figure optionsDownload as PowerPoint slide