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 - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue European Journal of Inorganic Chemistry Année : 2010

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

Résumé

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: Fdequation imagem, 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.

Dates et versions

hal-01984406 , version 1 (17-01-2019)

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Citer

Cyrille Galven, Christophe Legein, Monique Body, Jean-Louis Fourquet, Jean-Yves Buzaré, et al.. 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. European Journal of Inorganic Chemistry, 2010, 2010 (33), pp.5272-5283. ⟨10.1002/ejic.201000599⟩. ⟨hal-01984406⟩
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