Structural characterisation and physical properties of LiMMnO4 (M=Cr, Ti) spinels

New spinel-type phases of general formula LiMMnO4 (M=Cr, Ti), derived from LiMn2O4 by substitution of Mn3+ by Cr3+ or Mn4+ by Ti4+, have been obtained and characterised. Neutron diffraction refinements confirm that both phases crystallise in the Fd3m space group, giving the cation distributions [Li]8a[CrMn]16dO4 and [Li0.66Ti0.34]8a[Li0.34MnTi0.66]16dO4. Electrical conductivity has been examined by various techniques showing that these materials behave as semiconductors. The electrochemical behaviour indicates different oxidation-reduction steps in both cases concomitant with the insertion/deinsertion of lithium in non-reversible processes. X-ray diffraction patterns show that the above process is topotactic in LiCrMnO4. Magnetic data and neutron diffraction measurements show that no long-range magnetic ordering is present, suggesting a spin-glass transition for M=Cr at low temperature, while for M=Ti the presence of non-magnetic ions in the octahedral sublattice provokes an inherent magnetic frustration.