Structural characterisation and physical properties of Li MMnO 4 ( M=Cr, Ti) spinels

New spinel-type phases of general formula Li MMnO ₄ ( M=Cr, Ti), derived from LiMn ₂O ₄ by substitution of Mn ³⁺ by Cr ³⁺ or Mn ⁴⁺ by Ti ⁴⁺, have been obtained and characterised. Neutron diffraction refinements confirm that both phases crystallise in the Fd3m space group, giving the cation distributions [Li] _{8 a}[CrMn] _{16 d}O ₄ and [Li _0.66Ti _0.34] _{8 a}[Li _0.34MnTi _0.66] _{16 d}O ₄. 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 LiCrMnO ₄. 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.