Low-Temperature Structure and Magnetic Properties of the Spinel LiMn₂O₄:  A Frustrated Antiferromagnet and Cathode Material

Powder neutron diffraction has been used to study the nature of the structural transition away from the Fd 3 m cubic structure upon cooling below ∼285 K in the spinel LiMn 2 O 4 . We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I 4 1 / amd for the material at 100 K. While complete segregation of the Mn 3+ and Mn 4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn 3+ and Mn 4+ ions is not random and that there is a degree of charge segregation. Further, LiMn 2 O 4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Direct current magnetic susceptibility measurements show field-cooled, zero-field-cooled irreversibility at ∼65 K and a maximum in zero-field-cooled data at ∼40 K. Neutron diffraction shows magnetic scattering 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, signaling long-range magnetic order. The Bragg peaks grow at the expense of the diffuse feature on cooling to 10 K but the latter persists even at the lowest temperature studied which indicates that a significant fraction of the spins still remain disordered. The magnetic Bragg peaks index on a tetragonal cell 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 significant further neighbor exchange interactions.