Weakly bound oxygen and its role in stability of solid solutions La1-x Sr x FeO3-δ

In the present study high-temperature X-ray diffraction, thermal analysis, and mass-spectrometry have been employed for investigation of samples in the La1-x Sr (x) FeO3-delta family (0 a parts per thousand currency sign x a parts per thousand currency sign 1), the materials being solid solutions having perovskite structure. It has been shown that the loss of oxygen by the samples on heating to 1200A degrees C in air (0 a parts per thousand currency sign x a parts per thousand currency sign 1) or in vacuum (x < 0.75) does not result in structural rearrangement of the solid solutions, but causes an increase in the lattice parameters. Heating of the compositions with x a parts per thousand yen 0.75 in vacuum affords phases with ordered vacancies. The observed structural evolutions (growth of the unit cell parameter and vacancy ordering) are reversible, and on re-oxidation (on cooling in air or additional heating of the "vacuum" samples in air atmosphere) the original parameters of the oxides are recovered. The amount of oxygen evolved on heating increases in vacuum or in helium atmosphere, as compared to air, and also grows with rising strontium content, but under experimental conditions does not reach the maximum possible value (delta = x/2).