Персона: Ястребцев, Алексей Алексеевич
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Алексей Алексеевич
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- ПубликацияТолько метаданныеCrystalline structure formation and phase transitions in complex oxides Ln2Me2O7 (Ln = La - Lu; Me = Ti, Zr, Hf)(2019) Tsarenko, N. A.; Popov, V. V.; Yastrebtsev, A. A.; Menushenkov, A. P.; Попов, Виктор Владимирович; Ястребцев, Алексей Алексеевич; Менушенков, Алексей Павлович© 2019 Komarov Botanical Institute of the Russian Academy of Sciences. All rights reserved.Using X-ray diffraction and Raman spectroscopy, the authors of this paper investigated the formation and evolution of crystalline structure in complex oxides Ln2Me2O7 (Ln = La - Lu; Me = Ti, Zr, Hf), which occur as a result of calcination of amorphous precursors synthesized through coprecipitation from metal salt solutions. It is shown that the crystallization parameters and further phase transformations are largely governed by the cation radius ratio = rLn3+/rMe4+. In the case of zirconates and hafnates of rare earth elements (REE), nanocrystalline powders with fcc fluorite are initially formed during amorphous precursor crystallization. Further increase of the annealing temperature to _1,000 oC in the case of Ln = La - Dy leads to the phase transition fluorite → pyrochlore, the temperature of which rises and the intensity decreases as the REE radius gets smaller. The powders containing REEs with smaller cation radii retain their fluorite structure within the entire tempe rature range studied. For Yb2Zr2O7, it was found that _-phase was formed. In the case of rare earth titanates (Ln = Sm - Yb), crystallization of amorphous precursors directly resulted in a pyrochlore structure. For titanates of REEs with big cation radii (Ln = La - Nd), generation of layered perovskites was observed. Crystallite growth activation energies were calculated for the above mentioned types of phase transitions, which helped explain the sequence of structural transformations (amorphous phase → fluorite → pyrochlore) observed in rare earth zirconates and hafnates from the energy point of view. A good correlation was found between the X-ray diffraction and Raman spectroscopy data. It was concluded that Raman spectroscopy provides a more sensitive technique (versus X-ray diffraction) for studying the phase transitions occurring in complex rare earth oxides.
- ПубликацияТолько метаданныеComparative analysis of long- and short-range structures features in titanates Ln 2 Ti 2 O 7 and zirconates Ln 2 Zr 2 O 7 (Ln = Gd, Tb, Dy) upon the crystallization process(2019) d'Acapito, F.; Puri, A.; Castro, G. R.; Shchetinin, I. V.; Popov, V. V.; Menushenkov, A. P.; Ivanov, A. A.; Gaynanov, B. R.; Yastrebtsev, A. A.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Иванов, Андрей Анатольевич; Гайнанов, Булат Радикович; Ястребцев, Алексей Алексеевич© 2019 Elsevier Ltd We report a comparative study of Ln 2 M 2 O 7 (Ln = Gd, Tb, Dy; M = Ti, Zr) synthesized by combination of the sol-gel and coprecipitation methods followed by calcination at elevated temperatures. The long-range average crystal as well as short-range local structure in the samples were analyzed using X-ray diffraction, X-ray absorption spectroscopy (XAFS) and Raman spectroscopy. It has been found that Ln-Ti precursors crystallize directly to pyrochlore phase, whereas crystallization of Ln-Zr precursors to pyrochlore phase goes through intermediate fluorite phase. We have shown that X-ray absorption data make possible to detect the crystallization and phase transition features in Ln 2 M 2 O 7 such as splitting of Ln-O coordination shells and changes of the symmetry in local environment.
- ПубликацияТолько метаданныеПроцессы анионного и катионного упорядочения в соединениях Ln2M2O7 (Ln=La-Dy, Y; M=Zr, Hf)(2016) Ястребцев, А. А.; Ястребцев, Алексей Алексеевич; Менушенков Алексей Павлович