X-ray absorption investigation of the valence state and electronic structure of La{sub 1-x}Ca{sub x}CoO{sub 3-{delta}} in comparison with La{sub 1-x}Sr{sub x}CoO{sub 3-{delta}} and La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}

Journal Article · Thu Dec 15 00:00:00 EST 2011 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2011.09.027· OSTI ID:21612810

Haas, O ^[1]; Ludwig, Chr ^[2]; Bergmann, U ^[3]; Singh, R N ^[4]; Braun, A; Graule, T ^[5]

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1. Energy and Material Research Consulting, CH-6648 Minusio (Switzerland)
2. General Energy Research, Paul Scherrer Institut, CH-5232 Villigen PSI (Switzerland)
3. Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025 (United States)
4. Banaras Hindu University, Chemistry Department, Varanasi 221005 (India)
5. Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for High Performance Ceramics, CH-8600 Duebendorf (Switzerland)

3d metal K-shell X-ray absorption spectra of perovskites with the composition La{sub 1-x}Ca{sub x}CoO{sub 3-{delta}} (x=0, 0.2, 0.4, 0.5, 0.6, 0.8), La{sub 1-x}Sr{sub x}CoO{sub 3-{delta}} (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) and La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} (x=0, 0.2, 0.4, 0.5, 0.6, 0.8) are compared on the basis of pre-edges, white line features and extended fine structures. The measurements were performed at 300 K and for La{sub 1-x}Ca{sub x}CoO{sub 3-{delta}} also at temperatures as low as 10-20 K. Going to low-temperature the measurements indicate an increase in t{sub 2g}{sup Low-Asterisk} and a decrease in e{sub g}{sup Low-Asterisk} orbital occupancy, which is most accentuated in the LaCoO{sub 3} sample. Virtually no Co K-edge shift was observed for the La{sub 1-x}Ca{sub x}CoO{sub 3-{delta}} and La{sub 1-x}Sr{sub x}CoO{sub 3-{delta}} compounds and the Co-O distances are also not significantly reduced when La{sup 3+} is partially substituted by Ca{sup 2+} or Sr{sup 2+}. From the pre-edge features of these perovskites we are tended to conclude that the t{sub 2g}{sup Low-Asterisk} orbitals are less, and the e{sub g}{sup Low-Asterisk} orbitals are more occupied with increasing x in the Ca and Sr substituted compounds, whereas the total d-electron density is not changing. These results indicate that cobalt prefers a valence state of 3{sup +} in these Co perovskites. This could also be confirmed with iodometric titrations. The Fe perovskites behave differently. In contrast to the Co perovskites, for La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} perovskites the Fe K-edge is shifted, the pre-edge features intensity is increasing and the Fe-O bond length is decreasing with increasing x. The valence states of the iron in the La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} perovskites in fact increase as much as x increases. - Graphical abstract: Co K and Fe K pre-edge of La{sub 1-x}Ca{sub x}CoO{sub 3-{delta}} and La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} perovskites one of the evidences in favor of {delta}=x/2 for the Co-perovskites and {delta}=0 for the Fe-perovskites. Highlights: Black-Right-Pointing-Pointer XAS a valuable tool to evaluate the valence states of Co and Fe perovskites. Black-Right-Pointing-Pointer For La{sub 1-x}Ca{sub x}CoO{sub 3-{delta}} and La{sub 1-x}Sr{sub x}CoO{sub 3-{delta}} perovskites {delta} is close to x/2. Black-Right-Pointing-Pointer For La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} series {delta} is close to 0. Black-Right-Pointing-Pointer Discussion of the x dependency of the pre-edge bands.

• OSTI ID: 21612810
• Journal Information: Journal of Solid State Chemistry, Vol. 184, Issue 12; Other Information: DOI: 10.1016/j.jssc.2011.09.027; PII: S0022-4596(11)00520-2; Copyright (c) 2011 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ABSORPTION
ABSORPTION SPECTROSCOPY
COBALT
COMPARATIVE EVALUATIONS
FINE STRUCTURE
IODOMETRY
IRON
K SHELL
PEROVSKITES
VALENCE
X RADIATION
X-RAY SPECTRA
X-RAY SPECTROSCOPY
CHEMICAL ANALYSIS
ELECTROMAGNETIC RADIATION
ELECTRONIC STRUCTURE
ELEMENTS
EVALUATION
IONIZING RADIATIONS
METALS
MINERALS
QUANTITATIVE CHEMICAL ANALYSIS
RADIATIONS
SORPTION
SPECTRA
SPECTROSCOPY
TITRATION
TRANSITION ELEMENTS
VOLUMETRIC ANALYSIS