Supertransferred hyperfine fields at 7Li:: Variable temperature 7Li NMR studies of LiMn2O4-based spinels

The temperature dependence of the Li-7 NMR shift was measured for LiMn2O4, LiMn2-yNiyO4 (y = 0.1, 0.25, 0.33), LiMn2-yCoyO4 (y = 0.25, 0.5, 1.0), Li[Mn2-yLiy]O-4 (y = 0.1, 0.33), and lambda-MnO2 spinel oxides. The Li-7 NMR shift can be separated into temperature-independent and -dependent components. The temperature-dependent shift follows the Curie-Weiss behavior of the bulk magnetic susceptibility. The temperature-independent shift is attributed to contributions from van Vleck and diamagnetic susceptibilities. Pauli susceptibility may also contribute to the temperature-independent shift in the nickel- and cobalt-substituted spinels. Supertransferred hyperfine (STH) coupling constants were derived from the 7Li NMR shifts and bulk magnetic susceptibility data. The progressive increase in average nominal manganese oxidation state from +3.5 to +4 results in an increase in the supertransferred hyperfine field at the Li-7 nucleus in the lithium-substituted samples. Replacement of manganese by either cobalt or nickel also results in a larger STH field at the Li-7 nuclei. The increase in STH field for the lithium-, nickel-, and cobalt-substituted spinel oxides may arise from a greater covalence in these materials relative to the parent LiMn2O4 spinel oxide.