Supertransferred Hyperfine Fields at⁷Li:  Variable Temperature⁷Li NMR Studies of LiMn₂O₄-Based Spinels

The temperature dependence of the 7 Li NMR shift was measured for LiMn 2 O 4, LiMn 2 - y Ni y O 4 ( y = 0.1, 0.25, 0.33), LiMn 2 - y Co y O 4 ( y = 0.25, 0.5, 1.0), Li[Mn 2 - y Li y ]O 4 ( y = 0.1, 0.33), and λ-MnO 2 spinel oxides. The 7 Li 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 7 Li 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 7 Li nucleus in the lithium-substituted samples. Replacement of manganese by either cobalt or nickel also results in a larger STH field at the 7 Li 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 LiMn 2 O 4 spinel oxide.