Chemical and magnetic characterization of spinel materials in the LiMn2O4-Li2Mn4O9-Li4Mn5O12 system

Lithium manganospinels belonging to the LiMn2O4-Li2Mn4O9-Li4Mn5O12 system were prepared by solid-state reaction of various lithium salts and manganese sources at temperatures ranging from 400 to 900 degrees C. Their exact stoichiometry, which is highly dependent on the preparation temperature, was determined by X-ray diffraction, atomic absorption spectroscopy, and redox titration, Nearly stoichiometric LiMn2O4 shows a reversible phase transition at 30 degrees C upon heating and at 10 degrees C upon cooling, which is associated with a tetragonal-cubic transformation, An increase in the average oxidation state of manganese from 3.5+ to 4+ in the two systems Li1-deltaMn2-2 delta (0 less than or equal to delta less than or equal to 0.11) and Li1+deltaMn2-deltaO4 (0 less than or equal to delta less than or equal to 0.33) results in (i) the suppression of the Jahn-Teller distortion observed around room temperature for stoichiometric LiMn2O4 and (ii) a progressive passage from antiferromagnetic (LiMn2O4, theta = - 266 K, n(Mn) + = 3.5) to ferromagnetic behavior (Li4Mn5O12, theta = + 40 K, n(Mn) + = 4.0), where the lattice parameter is not too small. (C) 1996 Academic Press, Inc.