The structure of LiMn
2O
4, its phase transition at around 290 K, and the diffusion mechanism of Li are discussed based on the recent synchrotron X-ray diffraction and molecular dynamics simulation studies. The high-temperature modification (cubic
Fd 3
m) is essentially of spinel-type containing various types of disorder; (1) the Li atoms are not located exactly at the tetrahedral 8
a sites, but mainly distributed among four positions 0.14 Å apart from 8
a, (2) some portion of Li atoms are further displaced toward the 16
c octahedral interstices with a notable accumulation at the positions 0.35 Å apart from 16
c, and (3) the O atoms also show a statistical distribution around their ideal 32
e sites. The low-temperature modification adopts a 3 × 3 × 1 superstructure (orthorhombic
Fddd) with respect to the high-temperature modification. The bond-length fluctuation has been observed along the pseudo-tetragonal Jahn-Teller distortion direction parallel to the
a axis in the heterocubane Mn2
4O9
4 cluster. The four Mn2 atoms in the heterocubane presumably shares three electrons in the
e-parentage low-energy-level orbitals by the double-exchange Zener mechanism. The time-averaged oxidation state for Mn2 is estimated to be +3.25. The heterocubane Zener polarons are isolated with each other and embedded in an ordered way in the charge-ordered matrix containing Mn1
III, Mn3
III, Mn4
IV and Mn5
IV. Two kinds of diffusion mechanisms for Li atoms are proposed for the high-temperature modification from the molecular dynamics simulation. One is a microscopic version of the classical picture for diffusion based on the concentration difference in diffusion species. This mechanism requires an activation energy of ca. 0.25 eV to jump over a saddle point at the bottleneck. The other mechanism is a diffusion accompanied by the local lattice distortion coupled with the 3
d electron transfer between a pair of nearby Mn atoms. This requires no activation energy for Li to pass through the bottleneck. The valence exchange between +3 and +4 in the neighboring Mn pair prompts the displacement of coordinating O atoms along the pseudo-tetragonal Jahn-Teller distortion direction, which presumably plays a principal role in opening the bottleneck of oxygen triangle along the diffusion pathway.
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