Local lattice distortion caused by short range charge ordering in LiMn ₂O₄

We have performed powder neutron diffraction on 7Li-enriched sample of LiMn₂O₄ at 300 K. The crystal structure determined by Rietveld analysis is a cubic spinel with space group of Fd3m in which all Mn atoms are crystallograghically equivalent, consistent with many preceding studies. However, the atomic pair distribution function (PDF) of this compound can not be fitted by the cubic structure with space group of Fd3m satisfactorily, and it can be reproduced by the orthorhombic structure with Fddd. It corresponds with the structure of charge ordered phase below about 260 K, indicating a short range charge ordering. In the local structure determined by PDF analysis, two types of MnO₆ octahedra with long and short atomic distances between Mn and O atoms exist and their Mn-O distances are almost consistent with the distances in the charge ordered phase. From these results, valence electrons are localized at Mn sites like a glass even in the cubic phase, resulting in the non-metallic electrical conductivity.