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