Combustion synthesis of chromium-substituted lithium ferrites Li0.5Fe2.5−xCrxO4 (x≤2.0): Rietveld analysis and magnetic measurements

Pure and chromium-substituted lithium ferrites Li0.5Fe2.5−xCrxO4 (x≤2.0) have been made in air by self-propagating high-temperature synthesis (SHS), a combustion process involving the reaction of lithium peroxide, iron oxide, chromium oxide and iron or chromium metal powders. Reactions were performed in zero field and in an applied magnetic field of 1.1 T. Both the zero field and the applied field SHS reactions proceeded with a synthesis wave which spread out from the point of initiation. Notably the applied field synthesis waves were faster and hotter than their zero field counterparts. The products from the SHS reaction were ground and sintered at 1150°C for 2 h. Rietveld analysis of the powder X-ray data on the sintered samples showed that in all cases a cubic spinel ferrite was produced which showed a decrease in the lattice parameter with increasing Cr content. The x=0 applied field sample showed 87% superstructured and 13% disordered LiFe2.5O4 whilst the zero field sample showed 30% superstructured LiFe2.5O4 and 70% disordered LiFe2.5O4. X-ray line broadening indicated crystallites of order 70–90 nm in all samples. Electron microprobe analysis and EDAX showed that the samples were homogeneous and had the expected Fe to Cr ratios. Mössbauer and magnetic hysteresis data showed a significant change in sublattice occupancy and net magnetisation with Cr content. Coercive forces in the Cr-doped ferrites were larger than in pure compositions. Samples prepared in an applied magnetic field exhibited smaller coercivity and saturation magnetisation compared to those prepared in zero magnetic field.