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

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 La_1−xMe_xCoO_3−δ and (La_1−y Me_y)₂CoO₄ were found to exist in the systems. The limiting composition of La_1−xMe_xCoO_3−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 La_1−xMe_xCoO_3−y decreases while x increases. La_1−xMe_xCoO_3−δ (Me = Sr, Ba) shows an ideal cubic structure at x=0.5. The stability range of (La_1−yMe_y)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 La_1−xSr_xCoO_3−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: La_1−x′ Sr_x′CoO_3−δ′=n La_1−x″Sr_x″CoO_3−δ″+m SrCoO_2.5+q/2 O₂ where x′>x″. The decomposition mechanism of La_1−xSr_xCoO_3−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: La_1−xSr_xCoO_3−δ′=r La_1−x′Sr_x′CoO_3−δ″+w (La_1−y′Sr_y′)₂CoO₄+v CoO+f/2 O₂. The results are shown in “logPo₂-composition” diagrams. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.