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

Abstract Two new solid solutions Li 2– x La (1+ x )/3 □ (2 x –1)/3 B 2 O 6 F (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 Li 1.00 La 0.67 □ 0.33 Ta 2 O 6 F and Li 1.25 La 0.58 □ 0.17 Nb 2 O 6 F. It shows that both structures adopt the pyrochlore type A 2 B 2 X 6 X′ 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 BO 6 octahedra built the classical pyrochlore [B 2 X 6 ] 2– array, while the F – ions are strictly ordered on the 8b positions, which correspond to the centre of the pyrochlore cages. The Li + , La 3+ 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 19 F solid‐state NMR techniques. The HREM study reveals significant population fluctuations in the 16d site. 19 F MAS NMR spectroscopy evidences six different environments for the fluoride ion, [FLi 3 □] (or [FLi 4 ]), [FLi 3 La], [FLi 2 La□], [FLi 2 La 2 ], [FLiLa 2 □] and [FLa 2 □ 2 ], whereas fifteen different types of environment are expected if a random distribution of Li + , La 3+ 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 La 3+ ions and vacancies content (high x value) because of the low stability of environments rich in Li + or La 3+ ions. It has been shown that La(Li)–F distances decrease with the CN of F – ions, from [FLi 3 La] to [FLi 2 La□] and from [FLi 2 La 2 ] to [FLa 2 □ 2 ] environments. Fluoride ions occupying tetrahedra involving vacancies are brought closer to the corners occupied by Li + and/or La 3+ . Finally, the stability of the oxyfluoride pyrochlore phases is discussed.