Magnetic signature of charge ordering in Li[Mn 1.96 Li 0.04 ]O 4 and Li 0.2 [Mn 1.96 Li 0.04 ]O 4

The stoichiometric compound LiMn2O4 is known to show charge ordering with well-defined Mn3+ and Mn4+ sites just below room temperature (RT). Above RT the electrons are hopping rapidly between sites. For lithium-ion batteries the material Li[Mn1.96Li0.04]O4 is technologically relevant. Due to the small amount of Li on the Mn site, the low-T regular ordering of the Mn charge appears to be destroyed completely, as is evidenced by neutron diffraction in the magnetically ordered state. However, the charges are still fixed in an irregular fashion, as can also be deduced from 7Li nuclear magnetic resonance measurements. In the lithium-extracted compound Li0.2[Mn1.96Li0.04]O4, predominantly Mn4+ is present. Neutron diffraction in the magnetically ordered state shows a well-defined antiferromagnetic ordering, with doubling of the unit cell in three directions. Clearly (a trivial type of) charge ordering occurs now, since almost only Mn4+ is present.