L.Ya. Gavrilova

Tuning oxygen content and distribution by substitution at Co site in 112 YBaCo2O5+δ: impact on transport and thermal expansion properties

Polycrystalline “112” ordered oxygen deficient double perovskites YBaCo2−xMexO5+δ (Me = Fe, Cu, Ni) were synthesized by a glycerol-nitrate route with 0.0 ≤ x ≤ 0.7 for Me = Fe, 0.0 ≤ x ≤ 0.6 for Me = Cu and x = 0.1 for Me = Ni. The combined X-ray diffraction, electron microscopy and thermo-gravimetric studies show that all these oxides exhibit the ap × ap × 2ap tetragonal structure (S.G. P4/mmm); moreover, the oxygen content increases continuously with x in the iron-substituted oxide YBaCo2−xFexO5+δ, whereas the opposite is observed for the copper phase YBaCo2−xCuxO5+δ. This difference, which is due to the more electropositive character of Fe3+ compared to Cu2+, is hindered in the Ni2+ case due to its inability to accommodate the pyramidal coordination. The changes of the conductivity of these compounds versus temperature are closely related to their oxygen loss, in agreement with a defect structure model suggested earlier. Thermal expansion measurements prove the absence of phase transition in all oxides within the temperature range studied. The chemical compatibility of YBaCo1.4Fe0.6O5+δ with the electrolyte Ce0.8Sm0.2O2−δ is also demonstrated.

Specific features of phase equilibriums in Ln–Ba–Fe–O systems

Formation of five-layered Ln2–εBa3+εFe5O15–δ phases [exhibiting nanoscale ordering with layer-by-layer location of the cations in the Ln–Ba–(Ln,Ba)–(Ln,Ba)–Ba–Ln perovskite-type structure] has occurred in the Ln–Ba–Fe–O (Ln = Y, Pr, Nd, Sm, Eu, and Gd) systems at 1100°С in air. Partial substitution of iron with cobalt (Ln2–εBa3+εFe5–yCoyO15–δ, Ln = Nd, Sm, Eu) has stabilized formation of the ordered structure. The oxygen content in the complex oxides has been determined in air over a wide temperature range by means of high-temperature thermogravimetry and iodometric titration. The change in oxygen content with temperature for the phases with five-layered ordering was significantly smaller than for the disordered phases.

Crystal Structure and Physicochemical Properties of Doped Lanthanum Manganites

Substituted lanthanum-strontium manganites La0.7Sr0.3Mn0.9Me0.1O3 ± δ (Me = Ti, Cr, Fe, and Cu) are obtained by standard ceramic and glycerin-nitrate techniques. High-temperature powder X-ray diffraction is employed to study the crystal structure of La0.7Sr0.3Mn0.9Me0.1O3 ± δ oxides. It is shown that in the range 298–1023 K in air, La0.7Sr0.3Mn0.9Me0.103 ± δ manganites crystallized in an orthorhombic cell (space group R-3c). The isobaric temperature dependences of unit cell parameters are determined. Thermal expansion coefficients are calculated for La0.7Sr0.3Mn0.9Me0.103 ± δ oxides. The conductivity of La0.7Sr0.3Mn0.9Me0.103 ± δ is studied as a function of temperature in the range 500 K ≤ T ≤ 1200 K in air. It is shown that substituting 3d metal for manganese considerably lowers the conductivity of basic La0.7Sr0.3Mn0.9O3 ± δ. The chemical stability of iron-substituted manganite La0.7Sr0.3Mn0.9Fe0.1O3 ± δ is studied with respect to the electrolyte material.