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Title: Characterization of La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} and La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} as a function of temperature by x-ray absorption spectroscopy

Abstract

A temperature-dependent x-ray absorption spectroscopy (XAS) study was performed to investigate the changes in electronic and atomic structure of La-deficient La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} (L7S2FO3) and stoichiometric La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} (L8S2FO3). La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} is a promising cathode material for intermediate operating temperature (700-800 deg. C) solid oxide fuel cells. Performance improvements have been shown by increasing the La- or A-site deficiency in this material but a clear understanding of the mechanisms responsible for this improvement are still needed. Here we report an x-ray absorption spectroscopy (XAS) study as a function of temperature to investigate electronic and atomic structure changes of La-deficient La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} (L7S2FO3) and stoichiometric La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} (L8S2FO3). In particular we have measured the temperature-dependent changes in oxidation state, bond distance, Fe coordination number, and oxygen vacancies for both compounds. L7S2FO3 contains 10% A-site vacancies compared to stoichiometric L8S2FO3, which has a fully occupied A site and thus some form of charge compensation is necessary in the former to maintain charge neutrality. X-ray absorption near edge spectroscopy shows the presence of Fe{sup 3+} and Fe{sup 4+} in both L7S2FO3 and L8S2FO3 (mixed valence) as established by comparison with model compounds. Studies frommore » room temperature to 850 deg. C show that Fe{sup 3+} dominates over Fe{sup 4+} in both materials with increasing dominance as the temperature is increased. Furthermore, the temperature-dependent study revealed the La-deficiency in L7S2FO3 leads to a higher concentration of both electron holes (i.e., more Fe{sup 4+} created) and oxygen vacancies, compared to the stoichiometric L8S2FO3. Analysis of the extended x-ray absorption fine structure shows that the Fe-O bond increases with the increase in temperature for both the systems.« less

Authors:
; ; ;  [1];  [2];  [3];  [2]
  1. Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720 (United States)
  2. (United States)
  3. (United States) and Department of Chemical Engineering, University of California, Berkeley, California 94720 (United States)
Publication Date:
OSTI Identifier:
20788001
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 73; Journal Issue: 11; Other Information: DOI: 10.1103/PhysRevB.73.115114; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ABSORPTION; ABSORPTION SPECTROSCOPY; BOND LENGTHS; CATHODES; COMPARATIVE EVALUATIONS; COORDINATION NUMBER; FERRITES; FINE STRUCTURE; HOLES; IRON IONS; LANTHANUM COMPOUNDS; OXYGEN; PERFORMANCE; SOLID OXIDE FUEL CELLS; STOICHIOMETRY; STRONTIUM COMPOUNDS; TEMPERATURE DEPENDENCE; VACANCIES; VALENCE; X RADIATION; X-RAY SPECTROSCOPY

Citation Formats

Deb, Aniruddha, Ralph, James M., Cairns, Elton J., Bergmann, Uwe, Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, and Stanford Synchrotron Radiation Laboratory, Sand Hill Road, Menlo Park, California 94025. Characterization of La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} and La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} as a function of temperature by x-ray absorption spectroscopy. United States: N. p., 2006. Web. doi:10.1103/PHYSREVB.73.1.
Deb, Aniruddha, Ralph, James M., Cairns, Elton J., Bergmann, Uwe, Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, & Stanford Synchrotron Radiation Laboratory, Sand Hill Road, Menlo Park, California 94025. Characterization of La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} and La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} as a function of temperature by x-ray absorption spectroscopy. United States. doi:10.1103/PHYSREVB.73.1.
Deb, Aniruddha, Ralph, James M., Cairns, Elton J., Bergmann, Uwe, Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, and Stanford Synchrotron Radiation Laboratory, Sand Hill Road, Menlo Park, California 94025. Wed . "Characterization of La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} and La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} as a function of temperature by x-ray absorption spectroscopy". United States. doi:10.1103/PHYSREVB.73.1.
@article{osti_20788001,
title = {Characterization of La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} and La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} as a function of temperature by x-ray absorption spectroscopy},
author = {Deb, Aniruddha and Ralph, James M. and Cairns, Elton J. and Bergmann, Uwe and Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439 and Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720 and Stanford Synchrotron Radiation Laboratory, Sand Hill Road, Menlo Park, California 94025},
abstractNote = {A temperature-dependent x-ray absorption spectroscopy (XAS) study was performed to investigate the changes in electronic and atomic structure of La-deficient La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} (L7S2FO3) and stoichiometric La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} (L8S2FO3). La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} is a promising cathode material for intermediate operating temperature (700-800 deg. C) solid oxide fuel cells. Performance improvements have been shown by increasing the La- or A-site deficiency in this material but a clear understanding of the mechanisms responsible for this improvement are still needed. Here we report an x-ray absorption spectroscopy (XAS) study as a function of temperature to investigate electronic and atomic structure changes of La-deficient La{sub 0.7}Sr{sub 0.2}FeO{sub 3-{delta}} (L7S2FO3) and stoichiometric La{sub 0.8}Sr{sub 0.2}FeO{sub 3-{delta}} (L8S2FO3). In particular we have measured the temperature-dependent changes in oxidation state, bond distance, Fe coordination number, and oxygen vacancies for both compounds. L7S2FO3 contains 10% A-site vacancies compared to stoichiometric L8S2FO3, which has a fully occupied A site and thus some form of charge compensation is necessary in the former to maintain charge neutrality. X-ray absorption near edge spectroscopy shows the presence of Fe{sup 3+} and Fe{sup 4+} in both L7S2FO3 and L8S2FO3 (mixed valence) as established by comparison with model compounds. Studies from room temperature to 850 deg. C show that Fe{sup 3+} dominates over Fe{sup 4+} in both materials with increasing dominance as the temperature is increased. Furthermore, the temperature-dependent study revealed the La-deficiency in L7S2FO3 leads to a higher concentration of both electron holes (i.e., more Fe{sup 4+} created) and oxygen vacancies, compared to the stoichiometric L8S2FO3. Analysis of the extended x-ray absorption fine structure shows that the Fe-O bond increases with the increase in temperature for both the systems.},
doi = {10.1103/PHYSREVB.73.1},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 11,
volume = 73,
place = {United States},
year = {Wed Mar 15 00:00:00 EST 2006},
month = {Wed Mar 15 00:00:00 EST 2006}
}
  • To study the local structure and oxygen ion conduction mechanism in (La{sub 0.6}Sr{sub 0.4})(Co{sub 0.2}Fe{sub 0.8})O{sub 3-{delta}} (LSCF) as a function of the oxygen partial pressure (P(O{sub 2})), in situ the Co and Fe K-edge X-ray absorption spectroscopy (XAS) was measured at elevated temperatures of 900 and 1000 K. The reduction of the Co and Fe valence, i.e., the oxygen content (3-{delta}) in LSCF, followed the change of P(O{sub 2}) from 1 to 10{sup -4} atm during{approx}4000 s. The quantitative analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) indicated that themore » Fe valence was higher than the Co valence at oxidative condition ({delta} Almost-Equal-To 0) in LSCF. Whereas the Co valence decreased more than the Fe valence after reduction of P(O{sub 2}) at both 900 and 1000 K. From the relaxation plots of the valence and the oxygen content (3-{delta}) for Co and Fe after changing P(O{sub 2}), we successfully determined D{sub chem} and E{sub a} of an oxygen ion migration around Co and Fe in LSCF. A structural model with and without oxygen vacancies and an oxygen ion conduction mechanism for LSCF are proposed based on these results. - Graphical abstract: A structural model with and without oxygen vacancies, and the oxygen ion conduction mechanism of LSCF were speculated. In other words, oxygen vacancies would form more preferentially around Co than Fe from the results of in situ XAS analysis during reduction, and oxygen ions needs to pass through at the vicinity of Fe from the results of D{sub chem} and E{sub a}. Highlights: Black-Right-Pointing-Pointer Study of the oxygen ion conduction mechanism in (La{sub 0.6}Sr{sub 0.4})(Co{sub 0.2}Fe{sub 0.8})O{sub 3-{delta}} (LSCF). Black-Right-Pointing-Pointer Using in situ X-ray absorption for study of valence and oxygen diffusion coefficient. Black-Right-Pointing-Pointer The oxygen vacancies should be preferentially localized around Co in LSCF. Black-Right-Pointing-Pointer The values of the dynamics parameters for Co and Fe are close to each other.« less
  • 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 ismore » 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.« less
  • The oxyfluorides La{sub 1-x}Sr{sub x}FeO{sub 3-x}F{sub x} have been prepared by fluorination of the precursor oxides La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} via a low temperature route using poly(vinylidene fluoride) (PVDF). The structures of the oxides and oxyfluorides were investigated in detail by the Rietveld analysis of powder diffraction data. The oxyfluorides crystallize in the space group Pnma for 0<x{<=}0.9 (SrFeO{sub 2}F itself is cubic, space group Pm-3m) and show a sort of two-step structural distortion for decreasing x. Furthermore, a structural comparison of the oxyfluorides with the oxides is given, revealing an increase of the volume per La{sub 1-x}Sr{sub x}FeX{sub 3}more » unit during fluorination, of which the magnitude highly depends on the value of x. - Graphical abstract: The crystal structures of the perovskites La{sub 1-x}Sr{sub x}FeO{sub 3-x}F{sub x} for x=0.8 (a), 0.5 (b) and 0.1 (c). Highlights: > Oxyfluorides La{sub 1-x}Sr{sub x}FeO{sub 3-x}F{sub x} were prepared via low temperature fluorination using PVDF. > A structural investigation of the compounds La{sub 1-x}Sr{sub x}FeO{sub 3-x}F{sub x} is presented in detail. > Differences in structure are discussed in comparison with the precursor oxides La{sub 1-x}Sr{sub x}FeO{sub 3-{delta}}.« less
  • We report here a detailed study of the system La{sub 1−x}Sr{sub x}FeO{sub 3−x}F{sub x}, by neutron powder diffraction- and magnetic-measurements. All the compounds are robust antiferromagnetics with ordering temperatures well above room temperature. Magnetic moments are shown to align parallel to the c-axis. FC-ZFC measurements indicate a small canting of the magnetic moments, resulting in a ferromagnetic component with a maximum for La{sub 0.5}Sr{sub 0.5}FeO{sub 2.5}F{sub 0.5}. We show that the system exhibits a composition-driven transition from a phase, for low fluorination levels (x≤0.5), with Pnma symmetry and the usual system of octahedral tiltings, to a phase with space groupmore » Imma for higher fluorine contents, where a correlated distortion of the oxygen octahedra plays a significant role. The consistency of the structural models, with respect to the expected continuity of the amplitudes of the different distortion modes and the invariance of their internal form, was monitored through the symmetry mode decomposition of the structures. - Graphical abstract: The crystal and magnetic structure of La{sub 0.5}Sr{sub 0.5}FeO{sub 2.5}F{sub 0.5}. Display Omitted - Highlights: • The compounds La{sub 1−x}Sr{sub x}FeO{sub 3−x}F{sub x} (x=1, 0.8, 0.5, 0.2) were studied by neutron diffraction. • A mode analysis shows lower symmetry for Sr rich compounds than previously reported. • SrFeO{sub 2}F crystallizes in the orthorhombic space group Imma. • Magnetic moments resulting from canting depend on metric distortion. • The magnetic structure was determined for La{sub 0.5}Sr{sub 0.5}FeO{sub 2.5}F{sub 0.5}.« less
  • Bi{sub 0.8}A{sub 0.2}FeO{sub 3} (A = La, Ca, Sr, Ba) multiferroics were studied using x-ray, neutron diffraction and magnetization techniques. All the samples crystallized in rhombohedral structure with space group R3c. The compounds exhibit antiferromagnetic (AFM) ordering at 300 K and no evidence of further structural or magnetic transition was observed on lowering of temperature below it. The magnetic structure of these substituted compounds are found to be collinear G-type AFM structure as against the non collinear incommensurate magnetic structure reported in the case of parent compound. The moments on Fe at 6 K are aligned along the a-axis inmore » the case of Ca-doped sample. With increase in the ionic radii of dopant, the moments are found to be aligned in the ac plane and the angle of tilt away from the a-axis increases. The observed change in the magnetic structure with substitution is attributed to the intrinsic structural distortion as evidenced by the change in the bond angle (Fe-O-Fe) and bond distances (Bi-O, Fe-O). It has been found that heterovalent substitution A{sup 2+} results in the formation of oxygen vacancies in the parent lattices as the possibility of Fe{sup 4+} ruled out by Mössbauer spectra recorded at room temperature. Higher value of remnant magnetization (0.4187 emu/g) and coercivity (4.7554kOe) is observed in Bi{sub 0.8}Ba{sub 0.2}FeO{sub 3} sample in comparison to other substituted samples revealing a strong correlation between ionic radii and magnetization.« less