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

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dc.contributor Aalto-yliopisto fi
dc.contributor Aalto University en
dc.contributor.author Karvonen, Lassi
dc.date.accessioned 2012-08-31T07:45:41Z
dc.date.available 2012-08-31T07:45:41Z
dc.date.issued 2011
dc.identifier.isbn 978-952-60-4307-4 (PDF)
dc.identifier.isbn 978-952-60-4306-7 (printed)
dc.identifier.issn 1799-4942
dc.identifier.uri https://aaltodoc.aalto.fi/handle/123456789/5030
dc.description.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. en
dc.format.extent Verkkokirja (1385 KB, 43 s.)
dc.format.mimetype application/pdf
dc.language.iso en en
dc.publisher Aalto University en
dc.relation.ispartofseries Aalto University publication series DOCTORAL DISSERTATIONS , 92/2011 en
dc.relation.haspart [Publication 1]: Lassi Karvonen, Samuli Räsänen, Hisao Yamauchi, and Maarit Karppinen. 2007. Chemical oxidation of SrCoO3−δ. Chemistry Letters, volume 36, number 9, pages 1176-1177. en
dc.relation.haspart [Publication 2]: Lassi Karvonen, Hisao Yamauchi, and Maarit Karppinen. 2008. Homologous series of SrCoO(3n−1) ⁄ n perovskites obtained through Br2 oxygenation of SrCoO2.5. Chemistry of Materials, volume 20, number 22, pages 7143-7147. en
dc.relation.haspart [Publication 3]: Lassi Karvonen, Markus Valkeapää, Ru-Shi Liu, Jin-Ming Chen, Hisao Yamauchi, and Maarit Karppinen. 2010. O-K and Co-L XANES study on oxygen intercalation in perovskite SrCoO3−δ. Chemistry of Materials, volume 22, number 1, pages 70-76. en
dc.relation.haspart [Publication 4]: Lassi Karvonen, Songhak Yoon, Paul Hug, Hisao Yamauchi, Anke Weidenkaff, and Maarit Karppinen. 2011. The n = 3 member of the SrCoO(3n−1) ⁄ n series of layered oxygen-defect perovskites. Materials Research Bulletin, volume 46, number 9, pages 1340-1345. © 2011 Elsevier. By permission. en
dc.relation.haspart [Publication 5]: S. Shafeie, J. Grins, S. Ya. Istomin, L. Karvonen, S. A. Chen, T. H. Chen, J. M. Chen, A. Weidenkaff, M. Karppinen, T. Sirtl, and G. Svensson. 2011. Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1−x)1−zO6. Journal of Solid State Chemistry, volume 184, number 1, pages 177-190. © 2010 Elsevier. By permission. en
dc.relation.haspart [Publication 6]: Hisao Yamauchi, Lassi Karvonen, Takayuki Egashira, Yoshiaki Tanaka, and Maarit Karppinen. 2011. Ca-for-Sr substitution in the thermoelectric [(Sr,Ca)2(O,OH)2]q[CoO2] misfit-layered cobalt-oxide system. Journal of Solid State Chemistry, volume 184, number 1, pages 64-69. © 2010 Elsevier. By permission. en
dc.subject.other Chemistry
dc.title Oxygen non-stoichiometry, ordering and mobility in SrCoO3-δ perovskite en
dc.type G5 Artikkeliväitöskirja fi
dc.contributor.school Kemian tekniikan korkeakoulu fi
dc.contributor.department Kemian laitos fi
dc.contributor.department Department of Chemistry en
dc.subject.keyword soft chemistry en
dc.subject.keyword Chimie Douce en
dc.subject.keyword low-temperature oxygen (de)intercalation en
dc.subject.keyword oxygen non-stoichiometry en
dc.subject.keyword perovskites en
dc.subject.keyword cobalt oxides en
dc.subject.keyword XANES en
dc.identifier.urn URN:ISBN:978-952-60-4307-4
dc.type.dcmitype text en
dc.type.ontasot Väitöskirja (artikkeli) fi
dc.type.ontasot Doctoral dissertation (article-based) en
dc.contributor.supervisor Karppinen, Maarit, Academy Prof.

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