Observation of magnetic transition around room temperature in the spinel Li[Mn1.96Li0.04]O4 by inelastic neutron scattering

The stoichiometric compound LiMn2O4, a material that is studied for its application as cathode in Li-ion batteries, is well known to show a structural (cubic to orthorhombic) phase transition around room temperature. This effect has always been attributed to the dynamic Jahn-Teller distortion due to the Mn3+-ions. This presumes a corresponding Verwey-like charge ordering: below the ordering temperature the electrons get localized, forming a regular Mn3+/Mn4+ ordered state. By disturbing the stoichiometry, e.g. through substitution of a tiny fraction of Li ions on the 16d Manganese sites, the charge ordering is 'frustrated', and the structural transition is suppressed. However, by inelastic neutron scattering on the lithium doped compound Li[Mn1.96Li0.04]O4 we measure a critical narrowing of the elastic peak around room temperature. Because the quasielastic broadening is purely magnetic, the only explanation seems to be the localization of electrons on the manganese ions