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=8.85 493(3), c = 22.1440(1) Angstrom) and Fourier difference maps, lithium was f ound to be disordered over six positions related by (3) over bar symmetry i n the large M' hole of the NASICON structure with centre at 0 0 0. The fina l agreement indexes are R-p=0.0352, wR(p) = 0.0436, R(F-2)= 0.1289 for 46 r efined parameters. Li shows a very distorted tetrahedral coordination, with <Li-O> = 2.27 Angstrom; this result, together with the large number of emp ty sites available for hopping, due to disorder, accounts for the high Limobility in the alpha phase. The order-disorder nature of the alpha-alpha' phase transition of LiZr2(PO4)(3) is discussed, and structural relations be tween NASICON and corundum alpha -Al2O3 are pointed out. (C) 2000 Elsevier Science B.V. All rights reserved.