Relaxation of polaronic charge carriers in lithium manganese spinels

Lithium manganese spinels Li1+xMn2-xO4, 0 <= x <= 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 degrees C or 800 degrees C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn2O4 exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 degrees C and broadened over a range of temperature in sample sintered at 750 degrees C. In the low temperature phase of LiMn2O4, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, X >= 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons. (c) 2007 Elsevier B.V. All rights reserved.