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Ястребцев, Алексей Алексеевич

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Ястребцев

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Алексей Алексеевич

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Только метаданные
Comparative neutron and X-ray diffraction analysis of anionic and cationic ordering in rare-earth zirconates (Ln = La, Nd, Tb, Yb, Y)
(2020) Beskrovnyi, A. I.; Neov, D. S.; Shchetinin, I. V.; Ponkratov, K. V.; Yastrebtsev, A. A.; Popov, V. V.; Menushenkov, A. P.; Ястребцев, Алексей Алексеевич; Попов, Виктор Владимирович; Менушенков, Алексей Павлович
© 2020 Elsevier B.V.The influence of the cation radii ratio (γ = 1.37–1.61) and temperature of the isothermal annealing on the specific features of fluorite – pyrochlore phase transition in Ln2O3: ZrO2 = 1:2 (Ln = La, Nd, Tb, Yb, and Y) systems was investigated by means of Neutron (NPD) and X-ray (XRD) powder diffractions as well as Raman spectroscopy. It was shown that with a decrease in the radius of the Ln cation in the La → Nd → Tb → Y → Yb series, the following sequence of phase transitions is observed: the pyrochlore structure → the structure of defect fluorite → the rhombohedral δ-phase. Significant advantages of NPD over XRD are demonstrated for determining the structural features of the anion sublattice. A detailed description of the parameters of the vibrational modes in Raman spectra for samples with a pyrochlore type of ordering is given, depending on the refined structural parameters.
Только метаданные
Effect of the Chemical and Phase Composition of Nanocrystalline Gadolinium Complex Oxides on the Propane Cracking Process
(2020) Markova, E. B.; Kovtun, S. O.; Tereshina, T. A.; Shchetinin, I. V.; Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Molokova, A. Y.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Ястребцев, Алексей Алексеевич
© 2020, Pleiades Publishing, Ltd.Abstract: Effect of the chemical composition and crystal structure type of gadolinium complex oxides on their catalytic activity is studied. It is shown that nanocrystalline powders Gd2Ti2O7, Gd2Zr2O7, and Gd2Hf2O7 form highly symmetrical face-centered сubic crystal structures having localized (in Gd2Ti2O7) and delocalized (in Gd2Zr2O7 and Gd2Hf2O7) oxygen vacancies. At the same time, low-symmetrical crystal structures are formed in Gd2(WO4)3 (the monoclinic structure) and Gd2(МоO4)3 (a mixture of monoclinic and rhombic structures). Catalytic runs show that formation of the cubic structure contributes to an increase in the degree of conversion of propane and causes a shift in cracking temperatures to lower values. Formation of this type of nanocrystalline oxides facilitates the dehydrogenation reaction with propylene selectivity up to 80% at temperatures up to 700 К. Formation of the mixture of monoclinic and rhombic structures in Gd2(МоO4)3 leads to a shift in the degree of conversion to the catalytic temperature range of 700–900 К, in which the dehydrogenation reaction predominates (80%). In the case of the monoclinic structure of Gd2(WO4)3 in the same cracking temperature ranges the process of degradation occurs in parallel; this decreases propylene selectivity to 50% and promotes an increase in the yield of ethylene to 30%.
Только метаданные
Rearrangement in the local, electronic and crystal structure of europium titanates under reduction and oxidation
(2020) Khramov, E. V.; Svetogorov, R. D.; Kurilkin, V. V.; Murzin, V. Y.; Popov, V. V.; Menushenkov, A. P.; Molokova, A. Y.; Ivanov, A. A.; Rudakov, S. G.; Boyko, N. V.; Yastrebtsev, A. A.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Иванов, Андрей Анатольевич; Рудаков, Сергей Геннадьевич; Бойко, Надежда Владимировна; Ястребцев, Алексей Алексеевич
© 2020 Elsevier B.V.We studied powder samples of europium titanates Eu1−x 2+Eux 3+TiO3+x/2 prepared by sol-gel and coprecipitation methods with subsequent thermal treatment of precursors at different conditions (reducing or oxidizing atmosphere, temperature, exposure time). We have revealed a radical rearrangement in the local, electronic and crystal structure of europium titanates depending on synthesis conditions. The combination of synchrotron X-ray diffraction (s-XRD), X-ray absorption (XANES and EXAFS), infrared and Raman spectroscopies supplemented by photoluminescence and simultaneous thermal analysis was used in our study. It is shown that the combined XANES and thermogravimetric analysis gives clear evidence of a change in the oxidation state of europium cations from Eu2+ to Eu3+. The comparison of s-XRD, XANES, and EXAFS data allowed us to explain the local structure rearrangement during the phase transition from cubic perovskite Eu2+TiO3 (space group Pm3¯m) to pyrochlore phase Eu2 3+Ti2O7 (space group Fd3¯m) through intermediate layered perovskite Eu2 3+Ti2O7 with monoclinic structure (space group P21) under annealing in oxidizing atmosphere. The Raman spectroscopy study pointed to significant changes in the local structure of the anionic sublattice upon the observed phase transitions. The analysis of luminescence spectra has shown that Eu3+ cations occupy positions with low local site symmetry in all synthesized compounds regardless of the type of the crystal structure.
Только метаданные
A XAFS investigation of amorphous-to-crystalline and fluorite-to-pyrochlore phase transitions in Ln2M2O7 (Ln = Gd, Tb, Dy; M = Ti, Zr)
(2020) d'Acapito, F.; Puri, A.; Popov, V. V.; Menushenkov, A. P.; Ivanov, A. A.; Yastrebtsev, A. A.; Gaynanov, B. R.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Иванов, Андрей Анатольевич; Ястребцев, Алексей Алексеевич; Гайнанов, Булат Радикович
© 2019 Elsevier LtdAmorphous-to-crystalline and fluorite-to-pyrochlore phase transitions in complex oxides Ln2M2O7 (Ln = Gd, Tb, Dy; M = Ti, Zr) prepared by sol-gel and coprecipitation methods were studied using x-ray absorption spectroscopy in combination with x-ray diffraction and Raman spectroscopy. The Ln titanates crystallize directly from the amorphous state to the pyrochlore structure, while the Ln zirconates form the intermediate fluorite phase first. We have found that both XANES and EXAFS data reflect the considerable difference between the crystallization processes of Ln titanates and zirconates. In particular the evolution of the K–Ti pre-edge features is a good indicator of the phase transition from amorphous to crystalline state. The splitting of the first Ln-O shell in the FT modulus of L3-Ln edge EXAFS spectra in Ln titanates can be regarded as a fingerprint of the ordered pyrochlore structure formation which is accompanied also by specific changes in the main features of the XANES spectra. For the Ln zirconates these features are less pronounced.
Только метаданные
Magnetization of Crystalline and Amorphous Phases of R(2)Ti(2)O(7)and R2Zr2O7(R = Gd, Dy, Tb)
(2020) Kuznetsov, A. V.; Churkin, O. A.; Popov, V. V.; Shchetinin, I. V.; Ivanov, A. A.; Yaroslavtsev, A. A.; Gaynanov, B. R.; Yastrebtsev, A. A.; Chernysheva, O. V.; Alekseev, P. A.; Menushenkov, A. P.; Кузнецов, Алексей Владимирович; Чуркин, Олег Алексеевич; Попов, Виктор Владимирович; Иванов, Андрей Анатольевич; Гайнанов, Булат Радикович; Ястребцев, Алексей Алексеевич; Чернышева, Ольга Викторовна; Алексеев, Павел Александрович; Менушенков, Алексей Павлович
We present the study of frustrated magnets R2M2O7, where R = Gd, Dy, Tb and M = Ti, Zr. Magnetic susceptibility chi and low-temperature magnetization of perfectly ordered pyrochlores R2Ti2O7, partially disordered fluorites R2Zr2O7, and their completely disordered amorphous phases were measured and analyzed. We have found that structural disorder weakens the exchange interaction of the rare-earth ions. Magnetic moments of dysprosium and terbium, obtained from nearly saturated magnetization, are slightly affected by the disorder while the gadolinium moment is completely independent of it. The local structure of the compounds, including oxygen environment of the rare-earth ions, is discussed in terms of the influence of the crystal electric field on the magnetic properties of R2M2O7.
Только метаданные
Crystalline 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.
Только метаданные
The effect of the synthesis conditions on the structure and phase transitions in Ln2(MoO4)3
(2021) Svetogorov, R. D.; Tsarenko, N. A.; Ponkratov, K. V.; Ognevskaya, N. V.; Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Molokova, A. Y.; Rudakov, S. G.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Ястребцев, Алексей Алексеевич; Рудаков, Сергей Геннадьевич
© 2020 Elsevier Masson SASThe effect of the lanthanide cation type and calcination temperature on the structure and phase transitions in Ln2(MoO4)3 molybdates was studied using synchrotron X-ray diffraction, infrared and Raman spectroscopy, photoluminescence, simultaneous thermal analysis and inductively coupled plasma atomic emission spectroscopy. It was found that the crystallization of the precursors at 500∘ C/3 h results in the formation of nanocrystalline powders with the monoclinic structure (sp. gr. I112/b (15)) for Ln2(MoO4)3 (Ln = La–Ho), and a mixture of orthorhombic (sp. gr. Pnca(60)) and tetragonal (sp. gr. I41/a(88)) phases for Yb2(MoO4)3. A further increase in temperature to 1000∘ C leads to a significant rearrangement of the crystal structure, which is largely determined by the lanthanide type. The diagram of phase relations in the Ln2(MoO4)3 molybdates as a function of the Ln3+ cation radius and the calcination temperature has been refined. According to the results of Raman and IR spectroscopy, MoO4 tetrahedra are the main structural units in Ln molybdates. The increased hygroscopicity of heavy lanthanide molybdates (Ln = Ho, Yb) is associated with the presence of empty cavities in the orthorhombic structure. The photoluminescence spectra have shown that EuOn polyhedra in Eu2(MoO4)3 are distorted, and Eu3+ ions occupy several nonequivalent crystallographically different sites with a low symmetry point group.
Только метаданные
Influence of the lanthanide on the composition and structure of hydrated Ln chromates
(2021) Svetogorov, R. D.; Ognevskaya, N. V.; Tsarenko, N. А.; Seregina, O. N.; Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Dubyago, F. E.; Rudakov, S. G.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Ястребцев, Алексей Алексеевич; Рудаков, Сергей Геннадьевич
© 2021 Institute of Physics Publishing. All rights reserved.The effect of the Ln3+ cation type on the composition and structure of a wide range of hydrated lanthanide chromates (Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Yb, Y) has been studied. It was found that hydrated Ln chromates can be divided into several groups in terms of their composition and structure. The synthesized chromates of light lanthanides crystallize into heptahydrates [Ln2(CrO4)3(H2O)5]·2H2O (Ln = La, Pr, Nd, Sm) and tetrahydrate Eu2(CrO4)3·4H2O with a monoclinic structure (sp. gr. Р21/с). The chromates of heavy lanthanides (Ln = Gd, Dy, Ho, Yb, Y) remain X-ray amorphous.
Только метаданные
The synthesis and studies of crystal/local structures and morphology of hydrated molybdenum oxides
(2021) Khramov, E. V.; Zubavichus, Y. V.; Shchetinin, I. V.; Ponkratov, K. V.; Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Molokova, A. Y.; Pisarev, A. A.; Попов, Виктор Владимирович; Менушенков, Алексей Павлович; Ястребцев, Алексей Алексеевич; Писарев, Александр Александрович
© 2021 Elsevier Inc.The crystal/local structure and morphology of hydrated molybdenum oxides particles obtained by acidic precipitation of molybdates solutions were studied using X-ray diffraction, X-ray absorption spectroscopy, scanning electron microscopy, Raman and IR spectroscopy. The precipitation of ammonium heptamolybdate solutions (pH = 1.00) at room temperature result in the formation of poorly crystallized hydrated molybdenum oxides. At the same time, acidification of the sodium molybdate solutions does not lead to the appearance of precipitates for several days. Heating to 90 °C and aging the reaction suspension prepared from both ammonium heptamolybdate and sodium molybdate results in the formation of prismatic h-MoO3 particles with the hexagonal crystal structure. It was shown that distorted octahedra MoO6 are the basic structural units in the formed hydrated molybdenum oxides. Along with the presence of Mo[dbnd]O and Mo–O bonds, the presence of water molecules, hydroxyl ions, and NH4+ (or Na+) ions was detected in the structure of hydrated MoO3. The type of Mo salt and acid significantly affects the morphology of h-MoO3 particles. Large h-MoO3 particles are formed by aggregation of small ones and Ostwald ripening due to the “dissolution - precipitation” process.
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Reversible Valence Transition Eu-3+ ? Eu-2+ ? Eu-3+ in EuxEux Ox (2)+ (3)+ (M = Ti, Zr, Hf) 1-MO3+x /2 : An Analysis of XAFS and XRD Data
(2022) Menushenkov, A. P.; Popov, V. V.; Kuznetsov, A. V.; Molokova, A. Y.; Yastrebtsev, A. A.; Gaynanov, B. R.; Rudakov, S. G.; Менушенков, Алексей Павлович; Попов, Виктор Владимирович; Кузнецов, Алексей Владимирович; Ястребцев, Алексей Алексеевич; Гайнанов, Булат Радикович; Рудаков, Сергей Геннадьевич
The regularities of completely reversible valence transition Eu3+. Eu2+. Eu3+ in complex europium oxides have been investigated using a combination of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XANES + EXAFS) synchrotron methods. The influence of the type of M4+ cations on the reduction and oxidation, as well as the formation and evolution of the crystalline and local structure of the (M = Ti, Zr, Hf) compounds is established. It is shown that oxidation of initially reduced Eu2+MO3 samples leads to the formation of mixed-valence (MV) state Eu2+/Eu3+ with inhomogeneous charge distribution, causing a phase transition with formation of partially amophized intermediate metastable phases Eu3+MO3.5 having a monoclinic structure (sp. gr..1211). An increase in the oxidative annealing temperature to 1000 degrees. facilitates further phase transition with the formation of stable cubic phases Eu M2O7.