الملخص الإنجليزي
We report on our attempts to synthesize bulk and nanocrystalline singlephased pure and Ti**-/Cr**-doped lithium ferrite (LiFeO2) using the conventional solid state and mechanosynthesis techniques. The structural influence of cationdoping and the reduction of particle size to the nanometer scale on the formation of different LiFeO2 polymorphs are discussed. The formation of LiFeO2 polymorphs is followed with XRD, TGA, SEM and TEM techniques, whereas their structural characterization is done using XRD Rietveld structure refinement and Mössbauer spectroscopy. To our knowledge, this is the first study where cation-doped LiFeO2 has been synthesized via solid-state routes starting form a mixture of Li2CO3 and cation - doped a-Fe2O3 rather than a mixture of Li2CO3, pure a-Fe2O3 and the oxide/carbonate of the cation to be doped.
Single-phased bulk a-LiFeO2 was successfully obtained using the ceramic technique at 800°C (20 h). The lattice parameter "a" was found to be 4.15478 and the average crystallites size ~ 300 nm. The prolonged heating (20 h) was found to cause a slight change in anionic distribution of a-LiFeO2 with ~ 10% O" ions occupying unusual crystallographic sites. No milling-induced structural changes were observed in nanocrystalline particles produced by milling the a-LiFeO2 prepared at 800°C (20 h). With increasing milling time, the lattice parameter "a" and the microstrain of these nanocrystalline a-LiFeO2 increase, whereas their average crystallites size decreases reaching a minimum of ~ 13 nm at 75 h and increases afterwards. Unexpectedly, rather than showing a paramagnetic/superparamagnetic behavior with decreasing particle size, a-LiFeO2 shows unusual magnetism which we attribute to partial nanoscale short range antiferromagnetic clustering of the magnetic moments where the super-exchange magnetic interaction is lost.
Cation-doping a-Fe2O3 either with Ti44 or Cr3+ was found to speed up the reaction leading to the formation of cation-doped LiFeO2 polymorphs at significantly lower temperatures relative to those at which the pure material is formed.
Tito doping was found to inhibit the formation of single-phased LiFeO2 in bulk and nanocrystalline forms. It also inhibits the formation of the y-LiFeO2 ploymorph as an intermediate phase before the formation of a-LiFeO2. In Ti-doped aLiFeO2, the Ti** ions occupy crystallographic sites with different symmetry relative to those of Fe**. Both the lattice parameter and crystallite size of bulk Ti-doped aLiFeO2 are larger than their counterparts for the nanocrystalline form.
It was possible to form single phased bulk Cr-doped a-LiFeO2 at 600°C (3 h) in which the Crtions, unlike the Ti** ions, replace the missing Fe* ions at their usual crystallographic positions. The lattice parameter and average crystallites size were found to be 4.15658 and 75 nm respectively. Mechanosynthesis starting from Crdoped a-Fe2O3 and Li2CO3 was found to lead to the formation of a mixture of minority (6%) Cr-doped a-LiFeO2 and (6%) LiFesOs phases and a majority (88%) yLiFeO2 phase. For the y-LiFeO2 phase the lattice parameters are a = 4.0469 Å and c = 8.7301 Å and the average crystallite size is ~ 70 nm.
