Characteristic features of oxygen exchange in lanthanum-strontium manganites doped with iron

Based on the data of thermogravimetric analysis the values of the oxygen index (3–δ) in the manganite of the La_0.7Sr_0.3Mn_0.9Fe_0.1O_3-δ composition, obtained by solid-phase reaction technique, have been calculated. The analysis of oxygen sorption-desorption curves showed that the processes of oxygen release and absorption at pO₂ = 10 Pa and pO₂ = 400 Pa are not reversible. The minima of the derivative dδ/dt = f(T) corresponding to the maxima of the oxygen extraction rate indicate the complex character of changes in the oxygen desorption rate from manganite. The decrease in the heating and cooling rate from 6.6 to 2.6 K/min resulted in a significant change in the value ∆δ, indicating the dependence of anion mobility on the oxygen concentration in the magnet structure. It has been revealed that in the La_0.7Sr_0.3Mn_0.9Fe_0.1O_3-δ manganite the oxygen desorption kinetics is well described by the exponential dependence on the Cramers model, which implies no return of desorbed oxygen to the sample. This model indicates the non-stationarity of the diffusion flux through the barrier during desorption of oxygen from samples. The calculation of the activation energy of oxygen desorption by the Merzhanov method at various partial pressures of oxygen has shown that at the initial stage of oxygen extraction from La_0.7Sr_0.3Mn_0.9Fe_0.1O_3-δ, the activation energy of oxygen desorption has a minimum value (Е_а = 103.7 kJ/mol at δ = 0.005) and as the concentration of oxygen vacancies increases, it rises reaching saturation (Е_а = 134.3 kJ/mol at δ = 0.06). It is assumed that with an increase in the concentration of oxygen vacancies, an interaction occurs between them, followed by the processes of their ordering with the formation of associates.

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