The effect of sol-gel-synthesis temperature on the structure and magnetic properties of Sr-substituted La$_{0.5}$Sr$_{0.5}$FeO$_{3-\gamma}$

The effect of temperature used at the final stage of sol-gel synthesis method on the structure, Fe valence states, and Neel temperature of Sr-substituted La$_{0.5}$Sr$_{0.5}$FeO$_{3-\gamma}$ has been studied using transmission electron microscopy, X-ray diffraction, Mössbauer and Raman spectroscopy, as well magnetic measurements. The samples were synthesized by sol-gel method in air at the temperatures of 1100$^\circ$C and 1300$^\circ$C and then annealed in the vacuum in the temperature range of 200–650$^\circ$C to deoxygenize and deoxidate Fe. Oxygen yield and transition from Fe$^{4+}$ to Fe$^{3+}$ under vacuum annealing have been found out to occur slower in the samples synthesized at higher temperature. Therefore the transformation of the initial rhombohedral phase to the cubic one finishes at the higher annealing temperature. This is explained by the difference in the sizes of the initial crystals formed at different synthesis temperatures. After long final annealing at 650$^\circ$C, the samples become single-phase with a cubic structure in both cases. It is confirmed by the results of Mössbauer and Raman spectroscopy as well magnetic measurements. The Neel temperatures have been determined for all samples under study. The results obtained by all used methods correlate well between themselves.