CHEMICAL AND MAGNETIC CHARACTERIZATION OF SPINEL MATERIALS IN THE LIMN2O4-LI2MN4O9-LI4MN5O12 SYSTEM

Lithium manganospinels belonging to the LiMn2O4-Li2Mn4O9-Li4Mn5O12 sys tem 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 preparatio n 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 tr ansformation, 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 o r 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 f erromagnetic behavior (Li4Mn5O12, theta = + 40 K, n(Mn) + = 4.0), wher e the lattice parameter is not too small. (C) 1996 Academic Press, Inc .