Рубрики:
ELASTIC NEUTRON-DIFFRACTION
MII = CO
RAMAN-SCATTERING
THERMODYNAMIC PROPERTIES
CRYSTAL-STRUCTURE
SINGLE-CRYSTALS
RB2KMIIIF6 ELPASOLITES
CUBIC PEROVSKITES
TEMPERATURE FORM
HIGH-RESOLUTION
Кл.слова (ненормированные):
Crystal atomic structure -- Crystal growth -- Crystal lattices -- Crystallization -- Crystallography -- Ferroelectricity -- Hydrostatic pressure -- Order disorder transitions -- Single crystals -- Thermodynamics -- Elpasolites -- Landau theory -- Perovskite
ELASTIC NEUTRON-DIFFRACTION
MII = CO
RAMAN-SCATTERING
THERMODYNAMIC PROPERTIES
CRYSTAL-STRUCTURE
SINGLE-CRYSTALS
RB2KMIIIF6 ELPASOLITES
CUBIC PEROVSKITES
TEMPERATURE FORM
HIGH-RESOLUTION
Кл.слова (ненормированные):
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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Держатели документа:
L.V. Kirensky Institute of Physics, Krasnoyarsk 660036, Russian Federation
Inst. Chim. Matiere Cond. B., Universite Bordeaux I, 33608, Pessac Cedex, France
Lab. de Phys.-Chim. Molec., Universite Bordeaux I, 33405, Talence Cedex, France
Доп.точки доступа:
Flerov, I. N.; Флёров, Игорь Николаевич; Aleksandrov, K. S.; Александров, Кирилл Сергеевич; Tressaud, A.; Grannec, J.; Couzi, M.