Cation Ordering and Oxygen Stoichiometry in Double Perovskite Systems, (Sr,La)2FeTaO6−δ and LaBaCo2O5+δ
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Aalto-yliopiston teknillinen korkeakoulu |
Doctoral thesis (article-based)
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en
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Verkkokirja (1915 KB, 44 s.)
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Inorganic chemistry publication series,
9
Abstract
The present thesis consists of five publications, reviewed with relevant literature data. Different types of double perovskites were investigated and consequently, two new compounds are introduced accompanied by their extensive characterization. The B-site ordered double perovskite (Sr1-xLax)2FeTaO6 (x = 0..0.3) achieved by RIII-for-SrII substitution is described by its synthesis, crystallographic features and physical properties. The degree of order of Fe and Ta cations is determined from the X-ray diffraction data after determining the crystal structure. The degree of order is achieved to a practically full extent in the highest-doped samples with respect to the disordered parent phase. The decrease in the Fe oxidation state and its local coordination is analyzed by 57Fe Mössbauer spectroscopy and Fe and Ta L-edge XANES spectroscopy, combined with wet-chemical analysis. The physical properties are found to be similar to those of the disordered parent phase, indicating paramagnetism at room temperature with insulating behavior. The synthesis of the A-site and oxygen-vacancy ordered LaBaCo2O5+δ (δ = 0.5) compound is presented for the first time. The crystal and magnetic structure is determined by neutron diffraction and the microstructure probed by electron diffraction and high-resolution transmission microscopy. The properties resemble the previously known layered cobalt oxides but a partial A-cation and oxygen-vacancy disorder is evidenced. The compound undergoes a spontaneous magnetization at ∼320 K which is the highest measured compared to other similar compounds. In addition, a new polymorph of the δ = 1 phase is described. This structure consists of ordered, nanometer-sized domains affecting the crystal lattice and the physical properties. The chemical pressure effect was realized in both A- and B-site ordered perovskites. For the B-site ordered compounds, the band structures of halfmetallic A2FeMoO6 (A = Ca, Sr, Ba) were calculated and compared to the literature results. As a main result, a decrease of the partial density of states -comparable to the Fe and Mo valence states- is observed, supporting the reported experimental data. For the A-site ordered compounds, RIII cation substitution with precise oxygen engineering in RBaCo2O5+δ (δ = 0.5) system is demonstrated. A partial A-cation and oxygen-vacancy order is studied by crystal-chemical features and physical properties. While the disorder has an effect on the unit cell in addition to magnetic and transport properties, oxygen engineering plays a significant role in fine-tuning of the same properties.Description
Supervising professor
Karppinen, Maarit, Prof.Other note
Parts
- [Publication 1]: E.-L. Rautama, T.S. Chan, R.S. Liu, J.M. Chen, H. Yamauchi, and M. Karppinen. 2006. Electron-doping through LaIII-for-SrII substitution in (Sr1−xLax)2FeTaO6: Effects on the valences and ordering of the B-site cations, Fe and Ta. Journal of Solid State Chemistry, volume 179, number 1, pages 111-116. © 2005 Elsevier Science. By permission.
- [Publication 2]: E.-L. Rautama, J. Lindén, H. Yamauchi, and M. Karppinen. 2007. Metal valences in electron-doped (Sr,La)2FeTaO6 double perovskite: A 57Fe Mössbauer spectroscopy study. Journal of Solid State Chemistry, volume 180, number 2, pages 440-445. © 2006 Elsevier Science. By permission.
- [Publication 3]: Eeva-Leena Rautama, Philippe Boullay, Asish K. Kundu, Vincent Caignaert, Valérie Pralong, Maarit Karppinen, and Bernard Raveau. 2008. Cationic ordering and microstructural effects in the ferromagnetic perovskite La0.5Ba0.5CoO3: Impact upon magnetotransport properties. Chemistry of Materials, volume 20, number 8, pages 2742-2750.
- [Publication 4]: E.-L. Rautama, V. Caignaert, Ph. Boullay, Asish K. Kundu, V. Pralong, M. Karppinen, C. Ritter, and B. Raveau. 2009. New member of the "112" family, LaBaCo2O5.5: Synthesis, structure, and magnetism. Chemistry of Materials, volume 21, number 1, pages 102-109.
- [Publication 5]: Eeva-Leena Rautama and Maarit Karppinen. 2010. R-site varied series of RBaCo2O5.5 (R2Ba2Co4O11) compounds with precisely controlled oxygen content. Journal of Solid State Chemistry, volume 183, number 5, pages 1102-1107. © 2010 Elsevier Science. By permission.