Abstract
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 Sr2CrMoO6-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 Sr2CrMoO6-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.
Highlights
Complex oxides with the double perovskite structure are the unique class of materials, having a number of exotic properties, such as ferrimagnetism with high values of Curie temperature (TC) and the half-metal nature [1−3]
It has been determined as a result of investigations of dα/dt = f(T) that amplitude values of the derivative of the degree of conversion of the phases SrCrO3, SrMoO4 and Sr2CrO4 correspond to the temperatures T1 = 920 K, T2 = 990 K and T3 = 1045 K, correspondingly, at which their crystallization rates are maximal (Fig. 5, b)
The saturation magnetization value is 14.4 А×m2×kg-1 at Т = 7 K, in the magnetic field with the induction В ≥ 6 T (Fig. 4, inset). It was determined during the investigation of the sequence of phase transformations during a crystallization of the strontium chrome-molybdate, that SrCrO3, SrMoO4 and Sr2CrO4 are the main concomitant compounds
Summary
Complex oxides with the double perovskite structure are the unique class of materials, having a number of exotic properties, such as ferrimagnetism with high values of Curie temperature (TC) and the half-metal nature [1−3]. This fact demonstrates the nonreproducibility of the ferrimagnetic properties, which is caused by the uncontrollability of the defect formation processes in the double perovskite structure [22−24] It has been determined on the base of analysis of the research results [12, 19, 20, 22, 24] that it is quite problematic to obtain the single-phase Sr2CrMoO6 compound, due to the presence of the secondary phases, such as SrMoO4, SrCrO3, and it is practically impossible to remove the SrMoO4 compound. Strict correlations, connecting the functional characteristics of materials with the conditions of their obtaining, are practically absent in the realized investigations In this case, a formation of the single-phase Sr2CrMoO6 compound under conditions of control over the defect formation processes and, the compound with reproducible physico-chemical properties needs analysis of the phase transformations occurring in the batch as well as studies of the kinetics of the degree of conversion of double perovskite during its crystallization. The present article is dedicated to establish a correlation between the rate of phase transformations and the degree of phase transformation of strontium chrome-molybdate, which will allow for a controlled change in the phase composition of the synthesized ceramic with reproducible physico-chemical properties
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