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1.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Mel'nikova S. V., Molokeev M. S., Laptash N. M., Misyul S. V.
Заглавие : A non-typical sequence of phase transitions in (NH4)3GeF7: optical and structural characterization
Коллективы : RFBR [15-02-02009 a]
Место публикации : Dalton Trans.: Royal Society of Chemistry, 2016. - Vol. 45, Is. 12. - P.5321-5327. - ISSN 1477-9226, DOI 10.1039/c5dt04907e. - ISSN 1477-9234(eISSN)
Примечания : Cited References:25. - The reported study was partially supported by RFBR, research project no. 15-02-02009 a
Предметные рубрики: Crystal-structure
Powder diffraction
NH4F
Fluorination
Аннотация: Single crystals of germanium double salt (NH4)3GeF7 = (NH4)2GeF6·NH4F = (NH4)3[GeF6]F were grown and studied by the methods of polarization optics and X-ray diffraction. The birefringence Δn = (no − ne), the rotation angle of the optical indicatrix ϕ(T) and unit cell parameters were measured in the temperature range 100–400 K. Three structural phase transitions were found at the temperatures: T1↓ = 279.2 K (T1↑ = 279.4 K), T2↑ = 270 K (T2↓ = 268.9 K), T3↓ = 218 K (T3↑ = 227 K). An unusual sequence of symmetry transformations with temperature change was established: P4/mbm (Z = 2) (G1) ↔ Pbam (Z = 4) (G2) ↔ P21/c (Z = 4) (G3) ↔ Pa (Z = 8) (G4). The crystal structures of different phases were determined. The experimental data were additionally interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account the critical and noncritical atomic displacements. Strengthening of the N–H⋯F hydrogen bonds can be a driving force of the observed phase transitions.
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2.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Aleksandrov K. S., Beznosikov B. V., Misyul S. V.
Заглавие : Successive phase-transitions in crystals of K2MGF4-type structure
Место публикации : Phys. Status Solidi A. - 1987. - Vol. 104, Is. 2. - P.529-543. - ISSN 0031-8965, DOI 10.1002/pssa.2211040204
Примечания : Cited References: 66
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3.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Aleksandrov K. S., Melnikova S. V., Misyul S. V.
Заглавие : Successive phase-transitions in ferroelastic cs2rbdyf6 with elpasolite-type structure
Место публикации : Phys. Status Solidi A. - 1987. - Vol. 104, Is. 2. - P.545-548. - ISSN 0031-8965, DOI 10.1002/pssa.2211040205
Примечания : Cited References: 6
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4.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Aleksandrov K. S., Beznosikov B. V., Misyul S. V.
Заглавие : Successive structural phase-transitions in layered crystals. 1. TlAlF4 type crystals
Место публикации : Ferroelectrics. - 1987. - Vol. 73, Is. 1. - P.201-220. - ISSN 0015-0193, DOI 10.1080/00150198708227918
Примечания : Cited References: 50
Аннотация: Rotational distortions due to octahedral tilts and possible ways of successive phase transitions are considered in crystals of TIAIF4-type structure. Crystallographical and group-theoretical analysis of possible transitions from the initial space group D14h was performed. It is shown that in some crystals of the studied type there exist the examples of distortional polytypism, i.e., different types of octahedral tilts in the neighbour layers which have been equivalent in the initial phase.
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5.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Aleksandrov K. S., Misyul S. V., Baturinets E. E.
Заглавие : Symmetrical analysis of structural phase transitions in crystals with the Oh 5 space group
Место публикации : Ferroelectrics. - 2007. - Vol. 354, Is. 1. - P.60-68. - ISSN 0015-0193, DOI 10.1080/00150190701454529
Примечания : Cited References: 13
Ключевые слова (''Своб.индексиров.''): complete condensate of order parameters--elpasolites--group-theoretical methods--phase transitions--elpasolites--experimental data--group-theoretical methods--order parameter--permutation representation--space groups--structural phase transition--ammonium compounds--crystals--halide minerals--phase transitions
Аннотация: Using the group-theoretical methods, the complete set of critical and noncritical order parameters arising at phase transitions in crystals with the Oh 5- Fm3m space group has been obtained and analyzed. The mechanical and permutation representations have been constructed, and their composition for all the right systems of points of the Oh 5 space group has been determined. Experimental data on structural phase transitions in ammonium cryolites and elpasolites have been considered.
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6.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Makarova I. P., Misyul S. V., Muradyan L. A., Bovina A. F., Simonov V. I., Aleksandrov K. S.
Заглавие : Anharmonic thermal atomic vibrations in the cubic phase of Cs//2NaNdCl//6 single crystals.
Место публикации : Phys Status Solidi B. - 1984. - Vol. 121, Is. 2. - P.481-486. - ISSN 0370-1972
Ключевые слова (''Своб.индексиров.''): crystals--x-rays - diffraction--anharmonic thermal atomic vibrations--cubic phase--phase transitions--cesium compounds
Аннотация: Thermal atomic vibrations in the cubic phase of Cs//2NaNdCl//6 single crystal undergoing the structural phase transition (PT) at 138 K are investigated from the data of X-ray diffraction obtained at 278, 218, and 148 K. A significant anisotropy of thermal vibrations and their deviations from harmonic behavior are found for the Cl-atoms of the structure. These atoms displace from their equilibrium positions below PT due to the tilts of the octahedral groups.
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7.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Drokina T. V., Petrakovskii G. A., Molokeev M. S., Misyul S. V., Bondarev V. S., Velikanov D. A., Frontzek M., Schefer J.
Заглавие : Crystal and magnetic structures, phase transitions in quasi-one-dimensional pyroxenes Na0.5Li0.5FeGe2O6
Коллективы : Ministry of Education and Science of the Russian Federation of Siberian Federal University
Место публикации : J. Magn. Magn. Mater.: Elsevier Science, 2015. - Vol. 385. - P.243-249. - ISSN 0304, DOI 10.1016/j.jmmm.2015.03.016. - ISSN 18734766(eISSN)
Примечания : Cited References:25. - Neutron beam time at the cold neutron powder diffractometer DMC at the Swiss spallation neutron source SINQ Paul Scherrer Institut, Switzerland is gratefully acknowledged. MF acknowledges funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 290605 (PSIFELLOW/COFUND). This work was also supported by Ministry of Education and Science of the Russian Federation of Siberian Federal University 2014 year (Assignment no. 3.2534.2014/K).
Предметные рубрики: CLINOPYROXENE-TYPE LIFEGE2O6
NAFESI2O6
LIFESI2O6
Ключевые слова (''Своб.индексиров.''): phase transition--magnetic structure--pyroxene--magnetic properties--calorimetric measurements
Аннотация: The possibility of cation substitution in pyroxenes allows the investigation the gradual change of structural and magnetic properties under doping in these compounds. Here, Na0.5Li0.5FeGe2O6 was prepared by the standard solid-phase reaction method and has been investigated using X-ray and neutron powder diffraction, and further characterized by magnetic and calorimetric measurements. The crystal structure of Na0.5Li0.5FeGe2O6 at 300 K is monoclinic C2/c (a=10.0333(1), b=8.8136(1), c=5.5295(9) Å, β=108.921(1)°). Calorimetric investigations indicate a displacive first order phase transition at T=271±1 K which is accompanied by the appearance of superstructure reflections in the X-ray patterns. At this transition Na0.5Li0.5FeGe2O6 undergoes a space group change from C2/c to P21/c (a=9.9692(3), b=8.8545(3), c=5.4752(2) Å, β=108.494(1)°). Magnetic order has been found below the Néel temperature TN≈18 K and has been refined from neutron diffraction. The quasi-low-dimensional magnetic spin system Na0.5Li0.5FeGe2O6 exhibits a collinear antiferromagnetic structure with the space group Pa21/c and the doubling of the unit cell along the crystallographic a-axis of the pyroxene crystal (propagation vector k=(1/2, 0, 0)). The critical modes which are responsible for the phase transition from C2/c to P21/c symmetry and for the magnetic transition from paramagnetic to antiferromagnetic state have been determined and their role in the transition is described. They are also the relevant mechanism driving the magnetic order in LiFeGe2O6.
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8.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Molokeev M. S., Bogdanov E. V., Misyul S. V., Tressaud A., Flerov I. N.
Заглавие : Crystal structure and phase transition mechanisms in CsFe2F6
Место публикации : J. Solid State Chem. - 2013. - Vol. 200. - P.157-164. - DOI 10.1016/j.jssc.2013.01.038
Аннотация: For the first time, structural phase transitions induced by the temperature were found in AxMxIIM(1−x)IIIF3 fluorides with the defect pyrochlore structure (Fdm, Z=8). The room temperature structure of CsFe2F6 was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100 K and 700 K has shown a succession of phase transitions. The Pnma (Z=4)→Imma (Z=4)→I41/amd (Z=4)→Fdm (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500–560 K. The mechanism of structural transition has been mainly associated with the rotation of (MF6) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations.
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9.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Flerov I. N., Tressaud A., Aleksandrov K. S., Couzi M., Gorev M. V., Grannec J., Melnikova S. V., Chaminade J. P., Misul S. V., Voronov V. N.
Заглавие : Effect of B3+ ion size on the phase transitions in Rb2KB3+F6 elpasolites series
Место публикации : Ferroelectrics: Gordon breach SCI publ LTD, 1991. - Vol. 124, Is. 1. - P.309-314. - ISSN 0015-0193, DOI 10.1080/00150199108209457
Аннотация: The physical properties of Rb2KScF6 crystal characterized by the medium size of B3+ atom in Rb2KB3+F6 series are studied. The two successive phase transitions from cubic into tetragonal and then into monoclinic phases are found. The results are discussed in view of the influence of B3+ atom size on structural transformations.
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10.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Chechin G. M., Sakhnenko V. P., Misyul S. V., Aleksandrov K. S.
Заглавие : Improper ferroelectric phase due to condensation of two order parameters in crystals of high-temperature superconductors
Место публикации : Ferroelectrics. - 1992. - Vol. 130, Is. 1. - P.155-162. - ISSN 0015-0193
Примечания : Cited References: 21
Предметные рубрики: TRANSITIONS
FERROELASTICS
RB3CD2CL7
Ключевые слова (''Своб.индексиров.''): ceramic materials--crystals--ferroelectricity--phase diagrams--order parameters--oxide superconductors--high temperature superconductors
Аннотация: Using group-theoretical methods it is shown that improper ferroelectric phase may arise in high-temperature superconductors (HTSC) where high-symmetry phases have space groups D4h1 or D4h17. Their appearance is caused by condensation of order parameters (OP's) transforming two irreducible representations (IR's). The most typical phase diagrams are considered, as well as temperature dependence of OP's and the basic structural mechanism of the ferroelectricity arising in the HTSC.
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