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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : ZEIN N. E., ZINENKO V. I., FEDOROV A. S.
Заглавие : ABINITIO CALCULATIONS OF PHONON FREQUENCIES AND DIELECTRIC-CONSTANTS IN A(4)B(6) COMPOUNDS
Место публикации : Phys. Lett. A: ELSEVIER SCIENCE BV, 1992. - Vol. 164, Is. 1. - P115-119. - ISSN 0375-9601, DOI 10.1016/0375-9601(92)90916-A
Примечания : Cited References: 20
Предметные рубрики: IV-VI COMPOUNDS
CRYSTAL-STRUCTURE
Аннотация: The transverse and longitudinal optical frequencies at q = 0, the dielectric constant and the effective charges are calculated for the A4B6 type family of the semiconducting compounds GeTe, SnTe and PdTe. The calculations are performed in the framework of the density functional method with the use of the norm-conserving "first-principles" pseudopotentials. The estimated ferroelectric phase transition temperatures and a number of other calculated quantities turned out to be in good agreement with the experimental data. The variations of the electronic structure in the row PbTe-GeTe, which promote the ferroelectric phase transition for GeTe, are discussed.
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Flerov I. N., Aleksandrov K. S., Tressaud A., Grannec J., Couzi M.
Заглавие : Phase transitions in elpasolites (ordered perovskites)
Место публикации : Materials Science and Engineering R: Reports. - 1998. - Vol. 24, Is. 3. - P.81-151. - ISSN 0927-796X
Примечания : Cited References: 174
Предметные рубрики: ELASTIC NEUTRON-DIFFRACTION
MII = CO
RAMAN-SCATTERING
THERMODYNAMIC PROPERTIES
CRYSTAL-STRUCTURE
SINGLE-CRYSTALS
RB2KMIIIF6 ELPASOLITES
CUBIC PEROVSKITES
TEMPERATURE FORM
HIGH-RESOLUTION
Ключевые слова (''Своб.индексиров.''): crystal atomic structure--crystal growth--crystal lattices--crystallization--crystallography--ferroelectricity--hydrostatic pressure--order disorder transitions--single crystals--thermodynamics--elpasolites--landau theory--perovskite
Аннотация: Many compounds with general chemical formula A2BB?X6 and with tolerance factor value t < 1 crystallize in the elpasolite (ordered perovskite) structure. Different preparative methods have been considered. For example, the Bridgman technique is the most suitable method of growing single crystals of halogenated elpasolites with monoatomic cations. Group-theory and symmetry analyses have allowed us to build a scheme of possible structural distortions connected with octahedral rotations and displacements of ions. Experimental observations have proven that ferroelastic phase transitions occurring most frequently in elpasolites are due to small octahedra tilts. Quantitative analysis allows the possibility of describing these transitions in the framework of the thermodynamic Landau theory. Hydrostatic pressure and the substitution of atoms in different crystallographic sites allow to change significantly the succession and the temperature of phase transitions. The hypothesis of bond stresses is able to predict the possible occurrence of a phase transition for a particular elpasolite and/or the role of the octahedra rotations in the transformation. The mechanism of phase transitions in elpasolites and related compounds has also been considered. One can assume that a displacive mechanism associated with a strong anharmonicity plays a large role in phase transitions occurring in compounds with monoatomic cations. On the other hand, for instance, in cryolites with ammonium cations, the transitions originate from an order-disorder phenomenon. В© 1998 Elsevier Science S.A. All rights reserved.
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Flerov I. N., Gorev M. V., Sciau P.
Заглавие : Heat capacity and p-T phase diagrams of the ordered perovskites Pb2MgWO6 and Pb2CoWO6
Место публикации : J. Phys.: Condens. Matter: IOP PUBLISHING LTD, 2000. - Vol. 12, Is. 5. - P.559-567. - ISSN 0953-8984, DOI 10.1088/0953-8984/12/5/304
Примечания : Cited References: 25
Предметные рубрики: CRYSTAL-STRUCTURE
X-RAY
TRANSITIONS
PRESSURE
Аннотация: The heat capacities of Pb2MgWO6 and Pb2CoWO6 and the effect of hydrostatic pressure on the phase transitions have been studied on powdered samples by means of an adiabatic calorimeter and DTA performed under pressure, respectively. Thermodynamic parameters, such as change of total excess entropy, latent hear and hysteresis of the phase transition temperature were determined. Entropies and p-T phase diagrams are discussed in connection with structural data.
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Lundin U., Sandalov I., Eriksson O., Johansson B.
Заглавие : Modification of the standard model for the lanthanides
Место публикации : Solid State Commun.: PERGAMON-ELSEVIER SCIENCE LTD, 2000. - Vol. 115, Is. 1. - P7-12. - ISSN 0038-1098, DOI 10.1016/S0038-1098(00)00126-5
Примечания : Cited References: 33
Предметные рубрики: ELECTRONIC-STRUCTURE CALCULATIONS
DENSITY-FUNCTIONAL THEORY
PARTICLE BAND-STRUCTURE
AB-INITIO CALCULATIONS
FERROMAGNETIC NICKEL
COHESIVE PROPERTIES
CRYSTAL-STRUCTURE
MOTT INSULATORS
FERMION SYSTEMS
PR METAL
Ключевые слова (''Своб.индексиров.''): metals--electronic band structure--band structure--elastic moduli--kohn-sham scheme--rare earth elements
Аннотация: We show that incorporation of strong electron correlations into the Kohn-Sham scheme of band structure calculations leads to a modification of the standard model of the lanthanides and that this procedure removes the existing discrepancy between theory and experiment concerning the ground state properties. Within the picture suggested, part of the upper Hubbard f-band is occupied due to conduction band-f-mixing interaction (that is renormalized due to correlations) and this contributes to the cohesive energy of the crystal. The lower Hubbard band has zero width and describes fermionic excitations in the shell of localized f-s. Fully self-consistent calculations (with respect to both charge density and many-electron population numbers of the f-shell) of the equilibrium volume V-0 and the bulk modulus of selected lanthanides have been performed and a good agreement with experiment is obtained. (C) 2000 Elsevier Science Ltd. All rights reserved.
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Flerov I. N., Gorev M. V.
Заглавие : Entropy and the mechanism of phase transitions in elpasolites
Место публикации : Phys. Solid State: AMER INST PHYSICS, 2001. - Vol. 43, Is. 1. - P127-136. - ISSN 1063-7834, DOI 10.1134/1.1340198
Примечания : Cited References: 33
Предметные рубрики: ORDERED PEROVSKITE PB2MGTEO6
ELASTIC NEUTRON-DIFFRACTION
THERMODYNAMIC PROPERTIES
CUBIC PEROVSKITES
CRYSTAL-STRUCTURE
HIGH-RESOLUTION
PB2MGWO6
CSPBCL3
Аннотация: The phase transitions in series of crystals with the general formulas A(2)BB'X-6 (X = F, Cl, Br, or CN) and Pb2BB'O-6 that belong to the elpasolite family (space group Fm(3) over bar m) are analyzed. The influence of the size and the shape of cations and anions on the entropy and the mechanism of structural distortions is discussed. (C) 2001 MAIK "Nauka/Interperiodica".
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Gorev M. V., Flerov I. N., Sciau P.
Заглавие : Heat capacity and the p-T phase diagram of Pb2MgTeO6 elpasolite
Место публикации : Phys. Solid State: AMER INST PHYSICS, 2001. - Vol. 43, Is. 2. - P345-349. - ISSN 1063-7834, DOI 10.1134/1.1349486
Примечания : Cited References: 17
Предметные рубрики: ORDERED PEROVSKITE PB2MGTEO6
CRYSTAL-STRUCTURE
PB2COWO6
PB2MGWO6
Аннотация: The heat capacity of Pb2MgTeO6 is measured in the temperature range 80-300 K. It is found that the heat capacity exhibits an anomaly associated with the phase transition at T-0 = 186.9 K. The thermodynamic parameters of the structural transformation are determined. The effect of hydrostatic pressure up to 0.5 GPa on the phase transition temperature is examined. (C) 2001 MAIK "Nauka/Interperiodica".
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Gorev M. V., Flerov I. N., Bondarev V. S., Sciau P.
Заглавие : Studies of the thermodynamic properties of the ordered perovskites Pb2CdWO6 and Pb2YbTaO6 within a broad temperature range
Место публикации : Phys. Solid State: AMER INST PHYSICS, 2002. - Vol. 44, Is. 2. - P353-357. - ISSN 1063-7834, DOI 10.1134/1.1451027
Примечания : Cited References: 18
Предметные рубрики: CRYSTAL-STRUCTURE
PB2COWO6
PB2MGWO6
PHASE
HEAT
Аннотация: This paper reports on measurements of the specific heat of Pb2CdWO6 made at temperatures ranging from 80 to 750 K and of Pb2YbTaO6 within the 350- to 700-K temperature range. First-order phase transitions from the cubic phase at 677.3 and 581 K, respectively, were observed, and their thermodynamic characteristics were determined. The entropy change on the phase transitions is close to Rln4 for both compounds. The results obtained are discussed in terms of the model of position disordering of the lead ions. It was established that below 350 K, Pb2CdWO6 can exist in two states, stable and metastable, depending on the sample thermal prehistory. (C) 2002 MAIK "Nauka/Interperiodica".
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Balaev A. D., Bayukov O. A., Vasil'ev A. D., Velikanov D. A., Ivanova N. B., Kazak N. V., Ovchinnikov S. G., Abd-Elmeguid M. M., Rudenko V. V.
Заглавие : Magnetic and electrical properties of Fe1.91V0.09BO4 warwickite
Разночтения заглавия :авие SCOPUS: Magnetic and electrical properties of Fe1.91V 0.09BO4 warwickite
Место публикации : J. Exp. Theor. Phys.: AMER INST PHYSICS, 2003. - Vol. 97, Is. 5. - P989-995. - ISSN 1063-7761, DOI 10.1134/1.1633954
Примечания : Cited References: 22. - This study was supported in part by the Russian Foundation for Basic Research (project no. 03-02-16286) and by the Federal Program “Integration” (project no. B0017)
Предметные рубрики: ELECTRONIC-STRUCTURE
CRYSTAL-STRUCTURE
FE2OBO3
CHAINS
FE2BO4
FEBO3
Ключевые слова (''Своб.индексиров.''): electric conductivity--magnetic variables measurement--magnetization--single crystals--hopping conductivity--warwickite--iron compounds
Аннотация: We have performed a complex investigation of the structure and the magnetic and electrical properties of a warwickite single crystal with the composition Fe1.91V0.09BO4 . The results of Mossbauer measurements at T = 300 K indicate that there exist "localized" (Fe2+, Fe3+) and "delocalized" (Fe2.5+) states distributed over two crystallographically nonequivalent positions. The results of magnetic measurements show that warwickite is a P-type ferrimagnet below T = 130 K. The material exhibits hopping conductivity involving strongly interacting electrons. The experimental data are analyzed in comparison to the properties of the initial (unsubstituted) Fe2BO4 warwickite. The entire body of data on the electric conductivity and magnetization are interpreted on a qualitative basis. (C) 2003 MAIK "Nauka / Interperiodica".
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Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Volkov N. V., Sablina K. A., Bayukov O. A., Eremin E. V., Petrakovskii G. A., Velikanov D. A., Balaev A. D., Bovina A. F., Boni P., Clementyev E.
Заглавие : Magnetic properties of the mixed-valence manganese oxide Pb(3)Mn(7)O(15)
Разночтения заглавия :авие SCOPUS: Magnetic properties of the mixed-valence manganese oxide Pb 3Mn7O15
Место публикации : J. Phys.: Condens. Matter: IOP PUBLISHING LTD, 2008. - Vol. 20, Is. 5. - Ст.55217. - ISSN 0953-8984, DOI 10.1088/0953-8984/20/5/055217
Примечания : Cited References: 16
Предметные рубрики: CRYSTAL-STRUCTURE
PHASE-SEPARATION
Ключевые слова (''Своб.индексиров.''): crystal structure--magnetization--manganese compounds--phase interfaces--x ray diffraction--manganese oxide--space groups--temperature range--single crystals
Аннотация: A Pb(3)Mn(7)O(15) single crystal has been grown by the flux method and studied using x-ray diffraction and magnetization measurements. The crystal is hexagonal (P6(3)/mcm space group, Z = 4) and exhibits a pronounced layered nature. Along the [001] direction (c axis), the structure consists of layers of edge-sharing MnO(6) octahedra. Pairs of Mn atoms occupy the octahedral sites located between layers forming 'bridges' along the c axis, which link neighboring Mn layers. The magnetic properties of the crystal have been investigated using ac and dc magnetization measurements in the temperature range 2-900 K at magnetic fields up to 90 kOe. The experimental data obtained suggest that in the temperature region under study several different magnetic phases can be distinguished. Down to similar to 250 K, the crystal is in the paramagnetic state. Below this temperature, short-range antiferromagnetic ordering apparently starts forming within Mn layers, although a transition to long-range magnetic order occurs at 70 K. The magnetization data obtained leads us to conclude that this state is canted antiferromagnetic with moments lying in the basal plane of the crystal. In addition, below 20 K the crystal undergoes one more magnetic transition that corresponds to spin reorientation.
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10.

Вид документа : Статья из журнала
Шифр издания :
Автор(ы) : Zvyagina G. A., Zhekov K. R., Bezmaternykh L. N., Gudim I. A., Bilych I. V., Zvyagin A. A.
Заглавие : Magnetoelastic effects in terbium ferroborate
Место публикации : Low Temp. Phys. - 2008. - Vol. 34, Is. 11. - P.901-908. - ISSN 1063-777X, DOI 10.1063/1.3009584; \b Физика низких температур
Примечания : Cited References: 17
Предметные рубрики: CRYSTAL-STRUCTURE
ND
Ключевые слова (''Своб.индексиров.''): acoustic wave velocity--antiferromagnetic materials--elastic moduli--exchange interactions (electron)--iron compounds--magnetoelastic effects--solid-state phase transformations--terbium compounds
Аннотация: The behavior of the elastic moduli and sound absorption in a terbium orthoborate single crystal at low temperatures is studied. The components of the tensor of the elastic moduli of this system are determined. A structural phase transition and a transition of the magnetic subsystem into an antiferromagnetically ordered state appear in the temperature dependences of the sound velocities and absorption. The magnetic field dependences of the velocities of transverse sound exhibit singularities in the form of jumps at a magnetic field equal to the field of the spin-flop transition of the antiferromagnetic subsystem. Theoretical analysis shows that the observed behavior of the acoustic characteristics are associated not with the rare-earth subsystem of ferroborate but rather with the renormalization of the exchange interaction between iron ions as a result of the magnetoelastic coupling.
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