New Oxyfluoride Pyrochlores Li2–xLa(1+x)/3□(2x–1)/3B2O6F (B = Nb, Ta): Average and Local Structure Characterization by XRD, TEM and 19F Solid‐State NMR Spectroscopy

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Abstract
Two new solid solutions Li2–xLa(1+x)/3□(2x–1)/3B2O6F (B = Ta, Nb) have been synthesized in powder form by solid‐state synthesis. Their composition domain corresponds to 0.6 ≤ x ≤ 1.1 for the Ta compounds and to 0.7 ≤ x ≤ 1.0 for the Nb compounds. The structural study has been performed by XRD on two single crystals with the compositions Li1.00La0.67□0.33Ta2O6F and Li1.25La0.58□0.17Nb2O6F. It shows that both structures adopt the pyrochlore type A2B2X6X′ structure [SG: Fd$\bar {3}$ m, Z = 8, a = 10.448(1) Å for the Ta compound and a = 10.448(7) Å for the Nb compound]. The BO6 octahedra built the classical pyrochlore [B2X6]2– array, while the F– ions are strictly ordered on the 8b positions, which correspond to the centre of the pyrochlore cages. The Li+, La3+ ions and vacancies located in the hexagonal tunnels are assumed to occupy statistically the A sites in the 16d position. The local structure has been more precisely studied by HREM and 19F solid‐state NMR techniques. The HREM study reveals significant population fluctuations in the 16d site. 19F MAS NMR spectroscopy evidences six different environments for the fluoride ion, [FLi3□] (or [FLi4]), [FLi3La], [FLi2La□], [FLi2La2], [FLiLa2□] and [FLa2□2], whereas fifteen different types of environment are expected if a random distribution of Li+, La3+ and vacancies in the 16d site is assumed. The solid solution composition domains are limited for the highest Li+ ions content (low x value) and for the highest La3+ ions and vacancies content (high x value) because of the low stability of environments rich in Li+ or La3+ ions. It has been shown that La(Li)–F distances decrease with the CN of F– ions, from [FLi3La] to [FLi2La□] and from [FLi2La2] to [FLa2□2] environments. Fluoride ions occupying tetrahedra involving vacancies are brought closer to the corners occupied by Li+ and/or La3+. Finally, the stability of the oxyfluoride pyrochlore phases is discussed.

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