Influence of synthesis conditions on microstructure and phase transformations of annealed Sr(2)FeMoO(6−x) nanopowders formed by the citrate–gel method

The sequence of phase transformations during Sr(2)FeMoO(6−x) crystallization by the citrate–gel method was studied for powders synthesized with initial reagent solutions with pH values of 4, 6 and 9. Scanning electron microscopy revealed that the as-produced and annealed powders had the largest Sr(2)FeMoO(6−x) agglomerates with diameters in the range of 0.7–1.2 µm. The average grain size of the powders in the dispersion grows from 250 to 550 nm with increasing pH value. The X-ray diffraction analysis of the powders annealed at different temperatures between 770 and 1270 K showed that the composition of the initially formed Sr(2)FeMoO(6−x) changes and the molybdenum content increases with further heating. This leads to a change in the Sr(2)FeMoO(6−x) crystal lattice parameters and a contraction of the cell volume. An optimized synthesis procedure based on an initial solution of pH 4 allowed a single-phase Sr(2)FeMoO(6−x) compound to be obtained with a grain size in the range of 50–120 nm and a superstructural ordering of iron and molybdenum cations of 88%.