Abstract. The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 ~ Two types of solid solution of general composition LavxMexCoO~_8 and (Lal.y Mey)2CoO 4 were found to exist in the systems. The limiting composition of Lal_xMexCoO3 ~ 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 Lal_xMe• decreases while x increases. Lal_xMexCoO3_~ (Me = Sr, Ba) shows an ideal cubic structure at x -0.5. The stability range of (La1_yMey)2CoO 4 was found to be 0.25 < y < 0.35 for Me = Ca, 0.3 _ log(Po2) > -2.25 can be written as follows: Lal_ x, Sr• ~, = n Lal_x,,Srx,,CoO3_5,, + m SrCoO2.5 + q/2 O: where x' > x". The decomposition mechanism of Lal_xSrx CoO3_~ 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: Lal_xSr• ~, = r Lax_x,Srx,COO3_5,, + w (Lal_y,Sry,)2CoO 4 + v CoO + f/2 02. The results are shown in "logPo2-composition" diagrams.

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The investigation of the system Sm-Ba-Fe-O in air has allowed an oxygen deficient perovskite Sm 2-ε Ba 3+ε Fe 5 O 15-δ (δ=0.75, ε=0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (a p = 3.934Å), the combined HREM/ EELS study shows that this phase is nanoscale ordered with a quintuple tetragonal cell, "a p × a p × 5a p ". The nanodomains exhibit a unique stacking sequence of the A-site cationic layers along c, namely "Sm-BaBa/Sm-Ba/Sm-Ba-Sm", and are chemically twinned in the three crystallographic directions. The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications. They may be related to the particular distribution of oxygen vacancies in the lattice and to the 3d 5 L configuration of iron.

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