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

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 Fm3̄m high temperature prototype and undergo a partial or total ordering at LT. An ordering process of NH4+ ions in CN=6 also occurs. The total entropy change corresponding to the complete sequence of PTs has been assigned to ∑ΔS=R(ln(8/1)+ln(2/1))=R ln 16 in good agreement with the experimental values. For elpasolites containing mixed monovalent cations (alkaline and ammonium cations) the behavior appears to be intermediate. The dependence of the transition temperature(s) under an external pressure has also been investigated. All these trends are emphasized for (AI)2BIGaF6 series.