LI-6 AND LI-7 NMR IN HIGH-TEMPERATURE PHASES OF LIXV2O5 BRONZES (0.2-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1)

A series of high-temperature phases of LixV2O5 bronzes (0.2 less-than-or-equal-to x less-than-or-equal-to 1) has been studied by Li-6 and Li-7 NMR. Static spin-echo and MAS NMR experiments have been carried out at two magnetic field strengths (4.7 and 7.1 T). It is shown that the MAS technique permits a very accurate direct determination of the lithium site occupancies in the different phases. The spectra are influenced by interactions from both the quadrupole coupling and the paramagnetic shift due to the dipole interactions of the Li nuclear moment with the paramagnetic vanadium ion moments. Both the magnitudes and relative orientation of the quadrupole (Q) and anisotropic shift (S) tensors have been determined by iterative fitting of the Li-6 and Li-7 MAS NMR line shapes at the two magnetic field strengths. The large difference between the Li-6 and Li-7 quadrupole moments is found to be particularly useful for an accurate determination of the NMR interaction parameters. Calculations of the quadrupole and paramagnetic shift coupling parameters have been performed by using a point monopole and point dipole model, respectively. The S tensor is related to the positions of the unpaired electrons in the different crystalline phases: while the electronic localization in the gamma phase is confirmed, it is shown that the unpaired electrons become increasingly delocalized over the vanadium atoms as the lithium content decreases in the beta and beta' phases. On the other hand, the Q tensor is used to test several atomic electric charge distributions. NMR is found to be a valuable experimental validation test for quantum chemistry computations.