Lithium location in NASICON-type Li+ conductors by neutron diffraction: II. Rhombohedral alpha-LiZr2(PO4)(3) at T=423 K

High-resolution powder neutron diffraction data have been collected T=423 K on the rhombohedral alpha phase of LiZr2(PO4)(3) (HRPD, ISIS Facility, UK). By Rietveld refinement analysis (space group R (3) over barc; Z=6; n=8.85493(3), c = 22.1440(1) Angstrom) and Fourier difference maps, lithium was found to be disordered over six positions related by (3) over bar symmetry in the large M' hole of the NASICON structure with centre at 0 0 0. The final agreement indexes are R-p=0.0352, wR(p) = 0.0436, R(F-2)= 0.1289 for 46 refined parameters. Li shows a very distorted tetrahedral coordination, with <Li-O> = 2.27 Angstrom; this result, together with the large number of empty sites available for hopping, due to disorder, accounts for the high Li+ mobility in the alpha phase. The order-disorder nature of the alpha-alpha' phase transition of LiZr2(PO4)(3) is discussed, and structural relations between NASICON and corundum alpha -Al2O3 are pointed out. (C) 2000 Elsevier Science B.V. All rights reserved