LI-6 AND LI-7 NMR IN HIGH-TEMPERATURE PHASES OF LIXV2O5 BRONZES 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 lit hium site occupancies in the different phases. The spectra are influen ced by interactions from both the quadrupole coupling and the paramagn etic shift due to the dipole interactions of the Li nuclear moment wit h the paramagnetic vanadium ion moments. Both the magnitudes and relat ive orientation of the quadrupole (Q) and anisotropic shift (S) tensor s have been determined by iterative fitting of the Li-6 and Li-7 MAS N MR line shapes at the two magnetic field strengths. The large differen ce between the Li-6 and Li-7 quadrupole moments is found to be particu larly useful for an accurate determination of the NMR interaction para meters. Calculations of the quadrupole and paramagnetic shift coupling parameters have been performed by using a point monopole and point di pole 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 t hat the unpaired electrons become increasingly delocalized over the va nadium atoms as the lithium content decreases in the beta and beta' ph ases. On the other hand, the Q tensor is used to test several atomic e lectric charge distributions. NMR is found to be a valuable experiment al validation test for quantum chemistry computations.