The metal oxide compounds Sr2FeMoO6–d systems with an ordered double perovskite structure due to their unique and extremely important magnetotransport and magnetic properties are among the most promising materials for spintronic devices. In the present work, we investigated the correlation between the citrate-gel synthesis conditions (pH of initial solutions and annealing temperature) and the microstructure, phase transformations and magnetic properties of the Sr2FeMoO6–d nanopowders. According to the results the average grain size of the powders in the dispersion grows from 250 to 550 nm with increasing of pH values. Single-phase nanosized Sr2FeMoO6–d powders had various degrees of superstructural ordering of Fe3+ and Mo5+ (P = 65% for pH = 4, P = 51% for pH = 6 and P = 20 % for pH = 9). With increasing of pH, the Fe2+ concentration increases from 63% to 72%, and the Fe+3 concentration drops from 37% to 28%. According to the results of investigations of magnetization temperature dependence in Sr2FeMoO6–d powders a metastable superparamagnetic state was established at TS<19 K in low-dimensional grains. An optimized synthesis procedure, based on an initial solution of pH = 4, has allowed obtaining a single-phase Sr2FeMoO6–d compound having grain size in the range of 50—120 nm and a superstructural ordering of iron and molybdenum cations of 88%. The optimum conditions of synthesis of nanopowders strontium ferromolybdate allow for the directional change of the phase composition of the synthesized nanosized ceramic with reproducible physical and chemical properties.

Поступила в редакцию 1 апреля 2019 © Белорусский государственный университет информатики и радиоэлектроники, 2020 Аннотация. Настоящая статья посвящена изучению механизмов деградации структуры сегнетоэлектрика состава Pb1−хBaхZr0,53Ti0,47O3−δ. На основании результатов исследований по влиянию температурно-временных режимов синтеза на степень структурного совершенства твердого раствора Pb0,85Ba0,25Zr0,53Ti0,47O3− установлено, что при его отжиге (Т = 420 К и t = 15-100 ч) наблюдается смещение положения и уширение рентгеновского рефлекса (112), что обусловлено аморфизацией и увеличением упругих напряжений в кристаллической решетке материала в ходе отжига. Показано, что при увеличении температуры отжига до 520 К, во временном интервале 15-100 ч, кристаллическая решетка материала деформируется за счет образования в ней дефектов типа VPb, VО и [V О •• 2Pb / i ], что проявляется в уменьшении соотношения интенсивностей I(hkl) дифракционных рефлексов (112) и (211). Предполагается, что в результате этого отжига на межзеренных границах накапливается избыток свинца, который способствует локальному появлению жидкой фазы. Данная фаза, участвуя в переносе компонентов материала, не только ускоряет взаимодействие реагирующих компонентов, но и снижает устойчивость их кристаллических решеток из-за образования в них точечных дефектов. В этом случае состояние решетки определяется как появлением вакансий по кислороду (VО), свинца (VPb), ассоциатов типа [V О •• 2Pb / i ], так и возникновением различных упругих напряжений, что способствует ослаблению химических связей между атомами, обусловленных внесенной дефектностью в анионную подрешетку твердого раствора Pb0,85Ba0,25Zr0,53Ti0,47O3− при десорбции кислорода. Ключевые слова: цирконат-титанат свинца-бария, твердый раствор Pb0,85Ba0,25Zr0,53Ti0,47O3−, сегнетоэлектрические и диэлектрические свойства, дефекты, вакансии, межзеренные границы, жидкая фаза, рентгено-фазовый анализ. Конфликт интересов. Авторы заявляют об отсутствии конфликта интересов.

Single-phase Sr2FeMoO6-δ samples with different-degreesuperstructiral ordering Fe/Mo cations superstructural ordering (P, 76, 86 and 93 %) were obtained by the solid-phase technique. Based on the results of measuring the magnetic characteristics in the samples, we found that an increase in magnetization (26.41, 32.36 and 42.66 A·m2·kg–1), magnetic moment (1.33, 3.07 and 3.58 μB/f.u.) and Curie temperatures (422, 428 and 437 K) withparameter P (76, 86 and 93 %) can be explained by the presence of antistructural defects, as well as antiferromagnetic inclusions. This determines the redistribution of electron density, which is accompanied by the change in electronic configuration of a part of Fe/Mo cations. Based on the temperature dependences of the magnetic moment of the samples measured in ZFC and FC modes, and on small-angle polarized neutron scattering (SANS), we found that the samples are in a magnetically inhomogeneous state. An important result to mention is that we detected the difference between the slope of the SANS curves of samples with different oxygen content, which demonstrates a different microstructure of inhomogeneities. The main inhomogeneities are magnetic inclusions with the dimensions depending on the superstructural ordering of Fe/Mo cations. According to the Porod law, it was shown that the Sr2FeMoO6-δ samples with wave vector values 0.1 > q > 0.002 Å–1 contain polydisperse grains with a smooth surface. For q > 0.1 Å–1 a deviation from the Porod law is observed, confirming the presence of magnetic inhomogeneities with a diameter < 6 nm in the grains.

The kinetics of phase contents modification in the process of SrBaFeMoO6–δ crystallization from a stoichiometric mixture of SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3 simple oxides using the solid phase method has been investigated. In the temperature region of 300–1200°С, a number of endotermic effects have been detected. Herewith, the first one (with maximum around 552°С) and the third one (with maximum around 743°С) are accompanying by the significant decrease of the mass of specimen. In the temperature range of 946–1200°С, the mass change of specimen is practically not observable, while the thermal effect is still present, and the specimen remains not single-phase one. This indicates the difficulty of the flow of solid phase reactions with the formation of solid solution of barium-strontium ferromolybdate. During analysis of the change of the phase composition consisting of a mixture of initial reagents of stoichiometric relation SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3, it has been observed that with increasing temperature, complex compounds BaMoO4, SrFeO3 appear almost simultaneously, then SrBaFeMoO6–δ appears consequently. Thus, the compounds BaMoO4 и SrFeO3, are structure forming for the solid solution of barium-strontium ferromolybdate. With further temperature increase up to 770°С the formation of new compound ВаFeO3 with disappearing SrFeO3 was detected. In this case, the amount of double perovskite increases faster than that of barium molybdate. The main accompanying compounds at the crystallization of the SrBaFeMoO6–δ double perovskite solid solution are BaMoO4 and SrFeO3. It was established that at the initial stage of the interaction, the resulting solid solution of barium-strontium ferromolybdate is enriched with iron and its composition changes during the reaction in the direction of an increase of the molybdenum content, as in the case of other precursor combinations.

Sr2FeMoO6–δ single-phase samples without Fe/Mo cations superstructural ordering (P) and with Curie temperature 407 K were obtained by the solid-phase technique. According to the XRD data, the growth dynamics of the parameter P is nonlinear. In this case, the process of reaching maximum values of P (Pmax) is long and its rate is several times lower than the change of the oxygen index 6–δ. It was found that with increasing temperature of isothermal annealing, P increases and reaches maximal values 88 % at T = 1320 K for 120 h, Pmax = 92 % at T = 1420 K for 100 h, while Pmax = 90 % at T = 1470 K for 45 h. One can assume that the lower values of Pmax at T = 1470 K than at T = 1420 K are due to the influence of thermal energy on the destruction of chain ordering of Fe and Mo cations placed in staggered order. Based on the analysis of P time dependences, two relaxation processes can be found and the dP/dt = ¦(t) can be divided into two regions – I and II. In the region I the relaxation time is shorter than that in the region II. The point is that the ordering of cations in the –O–Fe–O–Mo–O chains in the region I requires atomic displacements by approximately one interatomic distance, whereas in the region II cation displacements occur over long distances with the formation of long[1]chain long-range ordering.