Correlation between 31P chemical shift tensor and local structure in lithium cyclohexaphosphates Li6P6O18 x 3H2O and Li6P6O18.

To understand the surprising behavior between the variations of the P'-P-P" angles and the correlated variations of the O'-P-O" ones, two lithium cyclohexaphosphate compounds Li6P6O18 x 3H2O and Li6P6O18 are studied by solid state nuclear magnetic resonance (NMR) spectroscopy. The two compounds exhibit the same [P6O18]6- ring anions but with 3m or 1 internal symmetry, respectively. Such symmetries induce local distortions that are exhibited by NMR spectroscopy. One-dimensional (1D) NMR gives information on structural sites of 7Li and 31P ions and the crystallographic non-equivalencies are observed. Nevertheless, in the anhydrous compound, X-ray diffraction and NMR results do not completely agree and some discrepancy exists between the number of sites observed with the first technique and the number of lines exhibited in the NMR spectra either for 7Li or 31P nuclei. This problem is elucidated by using 2D double quantum NMR spectroscopy coupled with theoretical considerations. We find that the 31P chemical shift tensor is dependent on the deviations of the O-P-O angles from those in the regular tetrahedron. Within the same empirical model, we suggest that the surprising behavior between the variations of the P'-P-P" and the ones of the O'-P-O" is related to the overall charge on the PO4 group. We also find the positions of the isotropic lines for 7Li essentially depend on the site co-ordination of this nuclei.