Diffusion of Li atoms in LiMn2O4 - A structural point of view

The structure of LiMn2O4 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) over barm) is essentially of spinel-type containing various types of disorder; (1) the Li atoms are not located exactly at the tetrahedral 8a sites, but mainly distributed among four positions 0.14 angstrom apart from 8a, (2) some portion of Li atoms are further displaced toward the 16c octahedral interstices with a notable accumulation at the positions 0.35 angstrom apart from 16c, and (3) the 0 atoms also show a statistical distribution around their ideal 32e sites. The low-temperature modification adopts a 3 x 3 x I 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(4)O9(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(V). 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 3d 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 0 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. (C)2009 The Ceramic Society of Japan. All rights reserved.