On the structure of Li3Ti2(PO4)3
Résumé
The structure of the Nasicon-type phase Li3Ti2(PO4)3, obtained by chemical lithiation of LiTi2(PO4)3, has been characterised using neutron diffraction for the long range structure and 7Li NMR for more local information. The lithium atoms were precisely located from the neutron diffraction data, using nuclear difference Fourier maps. The lithium ions, which were known to be in the large M2 cavity, occupy two M3 and M′3 subsites (distorted tetrahedra) with occupation factors of 2/3 and 1/3, respectively. From these two intermediate sites, it was shown that the diffusion pathway between two M1 sites in these Nasicon-type structures consists of a set of seven face-sharing tetrahedra. A variable temperature 7Li MAS NMR study showed a set of signals due to a distribution of environments for a given Li+ ion, in terms of occupied or vacant M3/M′3 sites in its vicinity. Elevation of the temperature to 353 K leads to a reversible exchange of these signals, due to fast hopping of Li between the two sites within a given M2 cavity.