Low-temperature structure and magnetic properties of the spinel LiMn2O4: Afrustrated antiferromagnet and cathode material

Powder neutron diffraction has been used to study the nature of the structu ral transition away from the Fd3m cubic structure upon cooling below simila r to 285 K in the spinel LiMn2O4. We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I4(1 )/amd for the material at 100 K. While complete segregation of the Mn3+ and Mn4+ ions is not possible in this space group, bond-valence analysis indic ates that the distribution of Mn3+ and Mn4+ ions is not random and that the re is a degree of charge segregation. Further, LiMn2O4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Dir ect current magnetic susceptibility measurements show field-cooled, zero-fi eld-cooled irreversibility at similar to 65 K and a maximum in zero-field-c ooled data at similar to 40 K. Neutron diffraction shows magnetic scatterin g in the form of a broad peak assigned to short-range order which develops above 100 K. Upon cooling to 60 K additional Bragg peaks are seen, signalin g long-range magnetic order. The Bragg peaks grow at the expense of the dif fuse feature on cooling to 10 K but the latter persists even at the lowest temperature studied which indicates that a significant fraction of the spin s still remain disordered. The magnetic Bragg peaks index on a tetragonal c ell which is 2a, 2b, and 4c with respect to the low-temperature tetragonal cell and contains 1152 spins. The large size and implied complexity of the magnetic structure is consistent; with both charge segregation and signific ant further neighbor exchange interactions.