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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

焦绿石 八面体 化学 结晶学 离子 人口 固溶体 固态 晶体结构 立体化学 物理化学 相(物质) 社会学 人口学 有机化学
作者
Cyrille Galven,Christophe Legein,Monique Body,J.L. Fourquet,Jean‐Yves Buzaré,F. Le Berre,Marie‐Pierre Crosnier‐Lopez
出处
期刊:European Journal of Inorganic Chemistry [Wiley]
卷期号:2010 (33): 5272-5283 被引量:16
标识
DOI:10.1002/ejic.201000599
摘要

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.

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