Ca2+/vacancies and O2−/F− ordering in new oxyfluoride pyrochlores Li2xCa1.5−x□0.5−xM2O6F (M = Nb,Ta) for 0 ≤ x ≤ 0.5

Abstract

New oxyfluorides Li_2xCa_1.5−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-Ca_1.5□_0.5Nb₂O₆F forms and from powder samples for the others. This characterisation was completed by TEM and solid state ¹⁹F NMR experiments. For the Ca_1.5□_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-Ca_1.5□_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 Li_2xCa_1.5−x□_0.5−xTa₂O₆F (0 ≤ x ≤ 0.5) was prepared and the order–disorder phase transition observed for Ca_1.5□_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.