New oxyfluorides Li2xCa1.5−x□0.5−xM2O6F (M = Nb, Ta), belonging to the cubic pyrochlore structural type (Z = 8, a
≈ 10.5 Å), were synthesized by solid state reaction for 0 ≤
x
≤ 0.5. XRD data allowed us to determine their structures from single crystals for the two α and β-Ca1.5□0.5Nb2O6F forms and from powder samples for the others. This characterisation was completed by TEM and solid state 19F NMR experiments. For the Ca1.5□0.5M2O6F (x = 0) pyrochlore phases, the presence of a double ordering phenomenon is demonstrated, involving on one hand the Ca2+ ions and the vacancies and on the other hand the oxide and the fluoride anions which are strictly located in the 8b sites of the Fdm aristotype space group. The Ca2+ ions/vacancies ordering leads to a reversible phase transition, α (P4332) ↔
β (Fdm). The 19F NMR study strongly suggests that, in the β-phases, the fluoride ions are only on average at the centre of the Ca3□ tetrahedron. It shows that slightly different Ca–F distances occuring in α-Ca1.5□0.5Nb2O6F may be related to a more difficult thermal ionic and vacancies diffusion process than in the tantalate compound. This may explain the hysteresis phenomenon presented by the phase transition. A solid solution Li2xCa1.5−x□0.5−xTa2O6F (0 ≤
x
≤ 0.5) was prepared and the order–disorder phase transition observed for Ca1.5□0.5M2O6F compounds disappears for all the other compositions where less or no more vacancies exist in the 16d sites. In the LiCaM2O6F compounds, the 19F NMR study allows us to determine the Ca2+ and Li+ ions distributions around the fluoride ions and shows that the [FLi2Ca2] environment is clearly favoured.
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