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Properties in New Complex Perovskite-Related Materials, a Matter of Composition and Structure
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
2013 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Egenskaper hos nya komplexa perovskitrelaterade material, en fråga om sammansättning och struktur (Swedish)
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 [en]
Perovskite, XANES, XRPD, NPD, TEC, TEM, SEM, TGA, SAED, HREM, thermal expansion, Seebeck coefficient, electronic conductivity, SOFC materials, magnetic susceptibility
Keyword [sv]
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: urn:nbn:se:su:diva-88793ISBN: 978-91-7447-672-9 (print)OAI: oai:DiVA.org:su-88793DiVA: diva2:613672
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
List of papers
1. Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1-x)1-zO6
Open this publication in new window or tab >>Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1-x)1-zO6
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2011 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 184, no 1, 177-190 p.Article in journal (Refereed) Published
Abstract [en]

Perovskite-type cobaltates in the system La(2)Co(1+z) (Mg(x)Ti(1-x))(1-z)O(6) were studied for z=0 <= x <= 0.6 and 0 <= x <= 0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 degrees C. The space group symmetry of the structure changes from P2(1)/n via Pbnm to R (3) over barc with both increasing Mg content and increasing Co content. The La(2)Co(Mg(x)Ti(1-x))O(6) (z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0 <= x <= 0.5 a linear decrease in the L(3)/(L(3)+ L(2))Co-L(2.3) edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co(2+) ion content.

Keyword
Cobaltates, perovskite, X-ray diffraction, neutron diffraction, magnetic susceptibility, electron diffraction, XANES
National Category
Inorganic Chemistry Physical Chemistry Materials Chemistry
Research subject
Inorganic Chemistry; Materials Chemistry
Identifiers
urn:nbn:se:su:diva-46139 (URN)10.1016/j.jssc.2010.11.002 (DOI)000286774600026 ()
Projects
Studier av koboltbaserade perovskiter för användning i bränsleceller
Funder
Swedish Research Council, 2007-4348
Available from: 2010-11-26 Created: 2010-11-21 Last updated: 2017-12-12Bibliographically approved
2. 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
Open this publication in new window or tab >>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
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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.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-82397 (URN)10.1039/c2jm32550k (DOI)000306708700016 ()
Note

AuthorCount:9;

Available from: 2012-12-19 Created: 2012-11-13 Last updated: 2017-12-07Bibliographically approved
3. Synthesis and characterisation of the novel double perovskites La2CrB2/3Nb1/3O6, B = Mg, Ni, Cu
Open this publication in new window or tab >>Synthesis and characterisation of the novel double perovskites La2CrB2/3Nb1/3O6, B = Mg, Ni, Cu
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2012 (English)In: Materials research bulletin, ISSN 0025-5408, E-ISSN 1873-4227, Vol. 47, no 9, 2449-2454 p.Article in journal (Refereed) Published
Abstract [en]

The novel perovskites La2CrB2/3Nb1/3O6, B = Mg, Ni, and Cu have been synthesised at 1350 degrees C in air via the citrate route. Rietveld refinements using neutron powder diffraction (NPD) data showed that the compounds adopt the GdFeO3 type structure with space group Pbnm, and unit cell parameters a approximate to b approximate to root 2 x a(p) and c approximate to 2 x a(p), where a(p) approximate to 3.8 angstrom. Selected area electron diffraction (SAED) of B = Ni and Cu samples confirmed space group Pbnm. However, distinct reflections forbidden in Pbnm symmetry, but allowed in the monoclinic sub-group P2(1)/n and unit cell parameters a approximate to b approximate to root 2 x a(p) and c approximate to 2 x a(p), beta approximate to 90 degrees were present in SAED patterns of B = Mg sample. This indicates an ordering of the B-cations within the crystal structure of La2CrMg2/3Nb1/3O6. High-resolution electron microscopy (HREM) study indicating uniform, without formation of clusters, ordering of B-cations in the crystallites of La2CrMg2/3Nb1/3O6. Magnetic susceptibility measurements show that the compounds are antiferromagnetic (with some glass or spin clustering effects due to additional ferromagnetic interactions between the B-cations) with T-N for La2CrB2/3Nb1/3O6, B = Mg, Ni, Cu being 90, 125 and 140K, respectively.

Keyword
Inorganic compounds, Oxides, X-ray diffraction, Neutron scattering, Crystal structure, Magnetic properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-81827 (URN)10.1016/j.materresbull.2012.05.019 (DOI)000309081400052 ()
Note

AuthorCount:11;

Available from: 2012-11-07 Created: 2012-11-01 Last updated: 2017-12-07Bibliographically approved
4. Synthesis and characterisation of perovskite SrxY1-xFeO3-d (=.63≤x<1) and Sr0.75Y0.25Fe1-yMyO3-d (M=Cr, Mn, Ni) (y= 0.2, 0.33, 0.5)
Open this publication in new window or tab >>Synthesis and characterisation of perovskite SrxY1-xFeO3-d (=.63≤x<1) and Sr0.75Y0.25Fe1-yMyO3-d (M=Cr, Mn, Ni) (y= 0.2, 0.33, 0.5)
Show others...
2013 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 200, 30-38 p.Article in journal (Refereed) Published
Abstract [en]

Oxygen-deficient ferrates with the cubic perovskite structure SrxY1-xFeO3-d were prepared in air (0.71 ≤ x ≤ 0.91) as well as in N2 (x=0.75 and 0.79) at 1573K. The oxygen content of the compounds prepared in air increases with increasing strontium content from 3-δ=2.79(2) for x=0.75 to 3-δ=2.83(2) for x=0.91. Refinement of the crystal structure of Sr0.75Y0.25FeO2.79 using TOF neutron powder diffraction (NPD) data shows high anisotropic atomic displacement parameter (ADP) for the oxygen atom resulting from a substantial cation and anion disorder. Electron diffraction (ED) and high-resolution electron microscopy (HREM) studies of Sr0.75Y0.25FeO2.79 reveal a modulation along <100>p with G± ~0.4<100>p indicating a local ordering of oxygen vacancies. Magnetic susceptibility measurements at 5-390K show spin-glass behaviour with dominating antiferromagnetic coupling between the magnetic moments of Fe cations. Among the studied compositions, Sr0.75Y0.25FeO2.79 shows the lowest thermal expansion coefficient (TEC) of 10.5 ppm K-1 in air at 298-673K. At 773-1173K TEC increases up to 17.2 ppm K-1 due to substantial reduction of oxygen content. The latter also results in a dramatic decrease of the electrical conductivity in air above 673K. Partial substitution of Fe by Cr, Mn and Ni according to the formula Sr0.75Y0.25Fe1-yMyO3-δ (y=0.2, 0.33, 0.5) leads to cubic perovskites for all substituents with y=0.2. Their TECs are higher in comparison with un-doped Sr0.75Y0.25FeO2.79. Only M=Ni has increased electrical conductivity compared to un-doped Sr0.75Y0.25FeO2.79.

Keyword
Perovskites, neutron diffraction, electron diffraction, high-temperature conductivity, thermal expansion, magnetic susceptibility
National Category
Inorganic Chemistry Materials Chemistry
Research subject
Inorganic Chemistry; Materials Chemistry
Identifiers
urn:nbn:se:su:diva-88792 (URN)10.1016/j.jssc.2013.01.008 (DOI)000317158000005 ()
Projects
Studies of cobalt based perovskites for cathode materials in solid oxide fuel cells.
Funder
Swedish Research Council, 2007-4348
Available from: 2013-03-28 Created: 2013-03-28 Last updated: 2017-12-06Bibliographically approved

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