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Title: Phase segregation in the Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} series

Abstract

Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} ferrites have been studied by means of X-ray powder diffraction in the whole composition range. Single-phase solid solution is found for x<0.09 and for x>0.63. At intermediate Sr content, phase segregation takes place. Compounds with x=<0.05 crystallize in the orthorhombic structure, space group Pbnm. Oxygen-deficient Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} with x>=2/3 are cubic or nearly cubic. The oxygen vacancies stabilize the cubic phase for x=2/3 whereas highly oxidized samples show an orthorhombic distortion, which has not been observed earlier. Magnetic and electrical properties have been measured for the single-phase solid solutions. Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} compounds with x>=2/3 order antiferromagnetically below {approx}100K. In the paramagnetic region, their susceptibility follows the Curie-Weiss law in all but SrFeO{sub 2.96} compound. These ferrites show semiconducting behavior in the electrical transport likely related to atomic disorder. We find that the conductivity activation energy becomes larger by increasing either the Gd content or the oxygen vacancies.

Authors:
 [1];  [2];  [2]
  1. Departamento de Fisica de la Materia Condensada, Instituto de Ciencia de Materiales de Aragon, CSIC-Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza (Spain). E-mail: jbc@unizar.es
  2. Departamento de Fisica de la Materia Condensada, Instituto de Ciencia de Materiales de Aragon, CSIC-Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
Publication Date:
OSTI Identifier:
20784935
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 179; Journal Issue: 3; Other Information: DOI: 10.1016/j.jssc.2005.12.023; PII: S0022-4596(05)00605-5; Copyright (c) 2005 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ACTIVATION ENERGY; ANTIFERROMAGNETISM; ELECTRICAL PROPERTIES; FERRITES; GADOLINIUM COMPOUNDS; ORTHORHOMBIC LATTICES; PARAMAGNETISM; PEROVSKITES; SEGREGATION; SOLID SOLUTIONS; SPACE GROUPS; STRONTIUM COMPOUNDS; VACANCIES; X-RAY DIFFRACTION

Citation Formats

Blasco, Javier, Stankiewicz, Jolanta, and Garcia, Joaquin. Phase segregation in the Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} series. United States: N. p., 2006. Web. doi:10.1016/j.jssc.2005.12.023.
Blasco, Javier, Stankiewicz, Jolanta, & Garcia, Joaquin. Phase segregation in the Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} series. United States. doi:10.1016/j.jssc.2005.12.023.
Blasco, Javier, Stankiewicz, Jolanta, and Garcia, Joaquin. Wed . "Phase segregation in the Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} series". United States. doi:10.1016/j.jssc.2005.12.023.
@article{osti_20784935,
title = {Phase segregation in the Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} series},
author = {Blasco, Javier and Stankiewicz, Jolanta and Garcia, Joaquin},
abstractNote = {Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} ferrites have been studied by means of X-ray powder diffraction in the whole composition range. Single-phase solid solution is found for x<0.09 and for x>0.63. At intermediate Sr content, phase segregation takes place. Compounds with x=<0.05 crystallize in the orthorhombic structure, space group Pbnm. Oxygen-deficient Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} with x>=2/3 are cubic or nearly cubic. The oxygen vacancies stabilize the cubic phase for x=2/3 whereas highly oxidized samples show an orthorhombic distortion, which has not been observed earlier. Magnetic and electrical properties have been measured for the single-phase solid solutions. Gd{sub 1-x}Sr{sub x}FeO{sub 3-{delta}} compounds with x>=2/3 order antiferromagnetically below {approx}100K. In the paramagnetic region, their susceptibility follows the Curie-Weiss law in all but SrFeO{sub 2.96} compound. These ferrites show semiconducting behavior in the electrical transport likely related to atomic disorder. We find that the conductivity activation energy becomes larger by increasing either the Gd content or the oxygen vacancies.},
doi = {10.1016/j.jssc.2005.12.023},
journal = {Journal of Solid State Chemistry},
number = 3,
volume = 179,
place = {United States},
year = {Wed Mar 15 00:00:00 EST 2006},
month = {Wed Mar 15 00:00:00 EST 2006}
}
  • 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
  • Bi{sub 1-x}Sr{sub x}FeO{sub 3-x/2} (I), Bi{sub 1-x}Sr{sub x}Fe{sub 1-x}MnxO{sub 3} (II), and Bi{sub 1-x}Ca{sub x}Fe{sub 1-x}Mn{sub x}O{sub 3} (III) solid solutions have been obtained. Their magnetization has been measured by X-ray and neutron diffraction and Moessbauer spectroscopy. According to the Moessbauer spectroscopy data, iron ions are in the trivalent state in system I. Near the concentration x {approx} 0.2, rhombohedral distortions (sp. gr. R3c) are transformed into tetragonal (P4/mmm). The symmetry of system II changes at x > 0.2 (R3c {sup {yields}} R3c), whereas orthorhombic distortions (R3c {sup {yields}} Pbnm) arise in system III at x > 0.2. The magneticmore » structure is antiferromagnetic (of G type). The samples of systems II and III exhibit weak ferromagnetism at x > 0.2 due to the Dzyaloshinski-Moriya interaction.« less
  • Two-ordered perovskites, Bi{sub 1/3}Sr{sub 2/3}FeO{sub 2.67} and Bi{sub 1/2}Ca{sub 1/2}FeO{sub 2.75}, have been stabilized and characterized by transmission electron microscopy, Moessbauer spectroscopy and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a{approx}b{approx}2a{sub p} and c{approx}3a{sub p} (S.G.: Pb2n or Pbmn) for the Sr-based compound and one with a{approx}b{approx}2a{sub p} and c{approx}8a{sub p} (S.G.: B222, Bmm2, B2mm or Bmmm) for the Ca-based one. The high-resolution transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeO{sub x}]{sub {infinity}} layer, suggesting that Bi{sub 1/3}Sr{sub 2/3}FeO{sub 2.67} and Bi{sub 1/2}Ca{sub 1/2}FeO{sub 2.75} behave differently compared to their Ln-based homolog.more » The HAADF-STEM images allow to propose a model of cation ordering on the A sites of the perovskite. The Moessbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compounds exhibit a high value of resistivity and the inverse molar susceptibility versus temperature curves evidence a magnetic transition at about 730 K for the Bi{sub 1/3}Sr{sub 2/3}FeO{sub 2.67} and a smooth reversible transition between 590 and 650 K for Bi{sub 1/2}Ca{sub 1/2}FeO{sub 2.75}. - Graphical abstract: Complex long-range ordering of Bi/Ae cations and oxygen/vacancies consisting of 12-fold and 32-fold enlargement of the perovskite unit cell is observed in Bi{sub 1/3}Sr{sub 2/3}FeO{sub 2.67} and Bi{sub 1/2}Ca{sub 1/2}FeO{sub 2.75}, respectively.« less
  • 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 atomicmore » 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.« less