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

High-resolution powder neutron diffraction data have been collected T=423 K on the rhombohedral α phase of LiZr2(PO4)3 (HRPD, ISIS Facility, UK). By Rietveld refinement analysis (space group R3c; Z=6; a=8.85493(3), c=22.1440(1) Å) and Fourier difference maps, lithium was found to be disordered over six positions related by 3 symmetry in the large M′ hole of the NASICON structure with centre at 0 0 0. The final agreement indexes are Rp=0.0352, wRp=0.0436, R(F2)=0.1289 for 46 refined parameters. Li shows a very distorted tetrahedral coordination, with <Li–O>=2.27 Å; this result, together with the large number of empty sites available for hopping, due to disorder, accounts for the high Li+ mobility in the α phase. The order–disorder nature of the α–α′ phase transition of LiZr2(PO4)3 is discussed, and structural relations between NASICON and corundum α-Al2O3 are pointed out.