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
✍ Scribed by Cyrille Galven; Christophe Legein; Monique Body; Jean-Louis Fourquet; Jean-Yves Buzaré; Françoise Le Berre; Marie-Pierre Crosnier-Lopez
- Publisher
- John Wiley and Sons
- Year
- 2010
- Tongue
- English
- Weight
- 773 KB
- Volume
- 2010
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Two new solid solutions Li 2-x La (1+x)/3 ᮀ (2x-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 Յ x Յ 1.1 for the Ta compounds and to 0.7 Յ x Յ 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 XЈ structure [SG: Fd3 ¯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 sig-[a]