Cation ordering in Li2Mn3MO8 spinels:: structural and vibration spectroscopy studies

Lithium-manganese oxide spinels with 1/4 manganese replaced by Mg, Ti, Co, Ni, Cu, Zn and Ga, yielding formula LiMn1.5M0.5O4 (or Li2Mn3MO8) have been prepared. Cationic ordering was known previously for M = Mg and Zn, resulting in a superstructure with primitive cubic symmetry. Given the poor chemical contrast of X-ray diffraction between Mn and Ti, Co, Ni, Cu or Ga, neutron diffraction studies were carried out. Evidence of cation ordering is found for M = Ni and Cu, but not for Ti, Co or Ga. These results are confirmed by FTIR and Raman spectroscopies. Doubly-substituted samples(Li0.5M0.5)[Mn1.5M0.5]O-4 (overall formula LiMn3M2O8) were also prepared for M = Mg and Zn. These do not form the primitive superstructure, a result ascribed to the lower manganese valence with respect to LiMn1.5M0.5O4. Zn-containing spinels give rise to an extensive Li/Zn cation inversion, which also shows up as additional high-frequency bands in IR and Raman spectroscopies. This investigation shows that the cell volume is determined by the average octahedral-site cation radius, and that the main driving force for octahedral cation ordering is the charge difference between Mn and M atoms. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.