Local Lattice Distortion Caused by Short Range Charge Ordering in LiMn$_2$O$_4$

TL;DR

This study reveals that LiMn₂O₄ exhibits local lattice distortions due to short-range charge ordering at room temperature, with localized electrons causing non-metallic behavior despite a cubic structure.

Contribution

It demonstrates the presence of short-range charge ordering and local lattice distortions in LiMn₂O₄ at 300 K using neutron diffraction and PDF analysis, highlighting electron localization.

Findings

Short-range charge ordering exists at room temperature.

Local lattice distortions involve MnO₆ octahedra with varying Mn-O distances.

Electrons are localized at Mn sites, leading to non-metallic conductivity.

Abstract

We have performed powder neutron diffraction on $^7$Li-enriched sample of LiMn$_2$O$_4$ at 300 K. The crystal structure determined by Rietveld analysis is a cubic spinel with space group of $Fd\bar{3}m$ in which all Mn atoms are crystallograghically equivalent, consistent with many preceding studies. However, the atomic pair distrubution function (PDF) of this compound can not be fitted by the cubic structure with space group of $Fd\bar{3}m$ 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 MnO$_6$ 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.