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Tracking of high-temperature thermal expansion and transport properties vs. oxidation state of cobalt between +2 and +3 in the La2Co1+z(Ti1-xMgx)(1-z)O-6-system
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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2012 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 32, 16269-16276 p.Article in journal (Refereed) Published
Abstract [en]

The high-temperature thermal expansion and electronic transport properties of the B-site substituted LaCoO3 with both variable oxidation state of cobalt between +2 and +3 (La2Co(Ti1-xMgx)O-6, 0 <= x <= 0.5) and variable Co3+-content relative to the other B-cations (La2Co1+z(Ti0.5Mg0.5)(1-z)O-6, 0.2 <= z <= 0.6) have been investigated. Based on the temperature dependence of the thermal expansion, electronic transport properties and Seebeck coefficient, three different groups of compositions according to their symmetries can be allocated. It was found that the thermal expansion coefficients (TECs) of the studied compounds are mainly dependent on the proportion of Co2+/Co3+. For La2Co(Ti1-xMgx)O-6, the TEC increases from similar to 9 (x = 0) to similar to 19 ppm K-1 (x = 0.5) with an increase of the oxidation state of cobalt from +2 to +3, respectively. The TECs of La2Co1+z(Ti0.5Mg0.5)(1-z)O-6, z = 0.2-0.6 with Co3+-only, remain constant at similar to 21 ppm K-1 independent of the cobalt content. Thermoelectric measurements of the system indicate that all samples in the system, except La2Co1.6(Ti0.5Mg0.5)(0.4)O-6, are p-type conductors over the whole temperature range, 300 < T < 1175 K. The conductivities were found to increase with an increase of both Co3+ and total cobalt content and are described with a small polaron hopping model. Due to an insignificant number of oxygen vacancies of La2Co1+z(Ti1-xMgx)(1-z)O-6 samples prepared in air at elevated temperatures, the investigated system is proposed as an excellent model system for the investigation of the influence of the Co oxidation state and stoichiometry on different properties in perovskite cobalt oxides.

Place, publisher, year, edition, pages
2012. Vol. 22, no 32, 16269-16276 p.
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-82397DOI: 10.1039/c2jm32550kISI: 000306708700016OAI: oai:DiVA.org:su-82397DiVA: diva2:567561
Note

AuthorCount:9;

Available from: 2012-12-19 Created: 2012-11-13 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Properties in New Complex Perovskite-Related Materials, a Matter of Composition and Structure
Open this publication in new window or tab >>Properties in New Complex Perovskite-Related Materials, a Matter of Composition and Structure
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Egenskaper hos nya komplexa perovskitrelaterade material, en fråga om sammansättning och struktur
Abstract [en]

This PhD thesis presents investigations of perovskite-related compounds in systems of interest for applications in components in solid oxide fuel cells. The compound compositions derive from substitutions in the parent compounds LaCoO3, LaCrO3 and SrFeO3.

Novel phases La2Co1+z(MgxTi1-x)1-zO6 were synthesized and investigated with regard to structure, thermal expansion, electronic and magnetic properties. The study focused on the composition lines La2Co(MgxTi1-x)O6 (z=0), where the oxidation state of Co nominally changes from +2 (x=0.0) to +3 (x=0.5), and La2Co1+z(Mg0.5Ti0.5)1-zO6, with a varying fraction of Co3+ ions. XANES data show that the Co ions in the system have discrete oxidation states of +2 and +3. The TEC increases with increasing x due to an increasing contribution from spin state transitions of the Co3+ ions. Novel compounds La2Cr(M2/3Nb1/3)O6 with M=Mg, Ni, Cu were synthesized and characterized with respect to structure and magnetic properties. XRPD and NPD data indicate Pbnm symmetry; however, SAED patterns and HREM images indicate a P21/n symmetry for M=Mg, and Cu. The magnetic measurements results were rationalized using the Goodenough-Kanamori rules.

Oxygen-deficient phases with x≥0.63 in SrxY1-xFeO3-δ and Sr0.75Y0.25Fe1-yMyO3-δ (M=Cr, Mn, Ni and y=0.2, 0.33, 0.5), were synthesized and characterized with respect to structure, oxygen content, thermogravimetry, TEC, conductivity and magnetic properties. Powder patterns of phases agree with cubic  perovskite structures. NPD data for x=0.75 reveal anisotropic displacement for the O atom, related to local effects from Fe3+/Fe4+ ions. SAED patterns for x=0.75 reveal the presence of an incommensurate modulation. The compounds start to lose oxygen in air at ~ 400°C. The TEC up to ~400°C for x=0.75 is ~10.5 ppm/K and increase to ~17.5 ppm/K at higher temperatures. The conductivity for x=0.91 is 164 S/cm at 400°C. Partial substitution of Fe by Cr, Mn or Ni does not increase the conductivity or decrease TEC.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University, 2013. 92 p.
Keyword
Perovskite, XANES, XRPD, NPD, TEC, TEM, SEM, TGA, SAED, HREM, thermal expansion, Seebeck coefficient, electronic conductivity, SOFC materials, magnetic susceptibility, Perovskit, XANES, XRPD, NPD, TEC, TEM, SEM, TGA, SAED, HREM, termisk expansion, Seebeck koefficient, elektrisk ledningsförmåga, SOFC material, magnetiska egenskaper
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-88793 (URN)978-91-7447-672-9 (ISBN)
Public defence
2013-04-29, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilThe Foundation for Baltic and East European StudiesKnut and Alice Wallenberg Foundation
Available from: 2013-04-08 Created: 2013-03-28 Last updated: 2013-04-02Bibliographically approved

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