Sequence of phase transformations at the formation of the strontium chrome-molybdate compound

The sequence of phase transformations in the process of crystallization of the Sr₂CrMoO₆ by the solid-phase technique from a stoichiometric mixture of simple oxides SrCO₃ + 0.5Cr₂O₃ + MoO, has been investigated. It was determined that the synthesis of the strontium chrome-molybdate proceeds through a series of sequential-parallel stages. By means of the differential thermal analysis and thermogravimetric analysis data, it has been established that five clearly expressed endothermal effects were observed in the temperature range 300−1300 K. It was found that during the studies of the phase transformations sequence in the process of the double perovskite synthesis, SrCrO₃, SrMoO₄ and Sr₂CrO₄ are the main concomitant compounds. Herewith, it has been observed that with the annealing temperature increase from 300 to 1270 K, the complex compounds SrCrO₄, SrCrO₃ (350−550 К) and SrMoO₄, Sr₂CrO₄ (600−750 К) are emerging initially and practically simultaneously. It has been revealed with a subsequent temperature increase that in the temperature range 940−1100 К, the SrMoO₄, Sr₂CrO₄ and SrCrO₃ phase concentration dramatically drops with the emerging and growth of the Sr₂CrMoO_6-δ double perovskite. With that in the range up to 1120–1190 К, the main XRD reflexes intensity for the SrCrO₃ and SrMoO₄ lowers substantially, and their content in the samples at 1170 К is no more than 7.9%. During a consideration of the derivative of the SrCrO₃, SrMoO₄ and Sr₂CrO₄ phase transformation degree (|(dα/dt)|_mах), at which their crystallization rates are maximal, it has been determined that |(dα/dt)|_mах for the Sr₂CrO₄ corresponds to the maximal temperature 1045 К, which indicates the presence of considerable kinetic difficulties at the formation of the Sr₂CrO₄ phase. Thereafter this phase does not disappear and at its appearance the slowing down of the double perovskite growth takes place. On the base of investigations of the phase transformations dynamics for the obtaining of the single-phase Sr₂CrMoO_6-δ compound with the superstructural ordering of the Cr/Mo cations and improved magnetic characteristics, the SrCrO₃ and SrMoO₄ precursors were used with combined heating modes.