FLEROV, Igor N.; GOREV, Mikhail V.; GRANNEC, Jean; TRESSAUD, Alain

doi:10.1016/S0022-1139(02)00068-4

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FLEROV, Igor N.
L.V. Kirensky Institute of Physics

GOREV, Mikhail V.
L.V. Kirensky Institute of Physics

GRANNEC, Jean
Institut de Chimie de la Matière Condensée de Bordeaux [ICMCB]

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Journal of Fluorine Chemistry. 2002, vol. 116, n° 1, p. 9-14

Elsevier

Résumé en anglais

Sequences of phase transitions (PTs) occurring in (AI)2BIMIIIF6 elpasolites have been studied via X-ray diffraction, differential scanning calorimetry (DSC) and adiabatic calorimetry at low temperature (LT). The substitution of NH4+ ions for alkaline ions in sites with either 6 or 12 coordination number considerably affects both transition temperature and entropy change associated with the transition(s). Two main mechanisms are responsible for the PTs: displacive transitions, as for the Rb2KMIIIF6 series, in which the transitions are associated with small octahedral tilts and are followed by a rather small entropy change; order-disorder transitions of rigid octahedra, as for (NH4)2NH4MIIIF6. In this latter case, the fluoride atoms located at the apices of the octahedra are disordered onto several possible positions in the cubic...< Réduire

Mots clés en anglais

Fluoro-elpasolites

Ordered perovskites

Structural phase transitions

Order-disorder

Displacive transition

Entropy

Ferroelastics

Pressure effect

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Role of metal fluoride octahedra in the mechanism of phase transitions in A2BMF6 elpasolites

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