Inhibition of Jahn–Teller cooperative distortion in LiMn2O4 spinel by transition metal ion doping

Abstract

The aim of this study is to determine the minimum amount of dopant that prevents the occurrence, near room temperature, of a Jahn–Teller (J–T) transition in the M-doped lithium manganese spinel of composition Li_1.02M_xMn_1.98−xO₄ with 0.00<x⩽0.06 and M = Ni²⁺, Co³⁺, Cr³⁺ or Ti⁴⁺. EPR spectra and magnetic susceptibility data are related to the valence state of M and Mn, and the homogeneous distribution of the dopant. We find that the spinel framework is remarkably sensitive to displaying low electronic and magnetic changes in its cationic sublattice due to cation substitution. The J–T distortion, which is associated with a sudden drop in conductivity with decreasing temperature, is suppressed by substituting 3% of Mn with Co³⁺ or Cr³⁺, or by adding an even smaller amount of Ni²⁺ (x = 0.02, or 1% substitution). However, this inhibition occurs only in samples with a ratio r = [Mn⁴⁺]/[Mn³⁺]1.18, i.e., a value larger than the ratio r = 1.106 we have with no doping (x = 0). As a consequence, doping with the tetravalent cation Ti⁴⁺, which always decreases the r value, does not suppress the J–T transition. We suggest that both the dopant ion and the Li⁺ in excess over the stoichiometric composition are located in 16d sites. The removal of the J–T transition in the Co³⁺ (x = 0.06) sample is also due to local disorder.