Ionics

, Volume 4, Issue 3–4, pp 309–315 | Cite as

Phase equilibria in the La-Me-Co-O (Me=Ca, Sr, Ba) systems

  • V. A. Cherepanov
  • L. Ya. Gavrilova
  • L. Yu. Barkhatova
  • V. I. Voronin
  • M. V. Trifonova
  • O. A. Bukhner
Article

Abstract

The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 °C. Two types of solid solution of general composition La1−xMexCoO3−δ and (La1−y Mey)2CoO4 were found to exist in the systems. The limiting composition of La1−xMexCoO3−δ lies at x=0.8 for Me = Sr, Ba and between 0.3–0.5 for Me = Ca. It is shown that the rhombohedral distortion of the perovskite type La1−xMexCoO3−y decreases while x increases. La1−xMexCoO3−δ (Me = Sr, Ba) shows an ideal cubic structure at x=0.5. The stability range of (La1−yMey)2CoO4 was found to be 0.25≤y≤0.35 for Me = Ca, 0.3≤y≤0.55 for Me = Sr and 0.3≤y≤0.375 for Me = Ba. All phases have tetragonal K2NiF4-type crystal structure. Based on the XRD and neutron diffraction patterns of quenched samples, the phase diagrams (Gibbs triangles) are constructed for all systems. The phase equilibrium at low oxygen pressure is shown for the example of the La-Sr-Co-O system. The decomposition mechanism of La1−xSrxCoO3−δ at 1100 °C for the samples with 0.5<x<0.8 within the oxygen pressure range −0.678>log(Po2)>−2.25 can be written as follows: La1−x′ Srx′CoO3−δ′=n La1−x″Srx″CoO3−δ″+m SrCoO2.5+q/2 O2 where x′>x″. The decomposition mechanism of La1−xSrxCoO3−δ for the samples with x < 0.5 within the oxygen pressure range −2.25>log(Po2)>−3.55 changes and can be written as follows: La1−xSrxCoO3−δ′=r La1−x′Srx′CoO3−δ″+w (La1−y′Sry′)2CoO4+v CoO+f/2 O2. The results are shown in “logPo2-composition” diagrams.

Keywords

Oxygen Crystal Structure Phase Diagram Solid Solution Diffraction Pattern 

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Copyright information

© IfI - Institute for Ionics 1998

Authors and Affiliations

  • V. A. Cherepanov
    • 1
  • L. Ya. Gavrilova
    • 1
  • L. Yu. Barkhatova
    • 1
  • V. I. Voronin
    • 2
  • M. V. Trifonova
    • 1
  • O. A. Bukhner
    • 1
  1. 1.Department of ChemistryUral State UniversityYekaterinburgRussia
  2. 2.Institute for Metal PhysicsUral Branch of RASYekaterinburgRussia

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