Determination of solid solution limits based on the thermal behaviour of aluminium substituted LiFe5O8

Despite extensive studies of substitutional solid solutions formed between the isostructural oxides of aluminium and iron, very few experiments on the system LiFe5O8/LiAl5O8 have been carried out [1-4]. One reason for this is that it is often difficult to synthesize the pure single-phase sample or because the crystallinity of the product decreases with increasing aluminium content. Synchrotron radiation opens the possibility for X-ray powder diffraction measurements with high angular resolution, that allow to fix the end-members of the LiFe5-yAlyO8 solid solution. In the present studies, the structural information on aluminium-substituted spinel lithium ferrites (LiFe4.5Al0.5O8, LiFe4.0Al1.0O8, LiFe3.5Al1.5O8), are obtained from powder diffraction patterns in an extended temperature range. Investigations on the high temperature phase transitions were carried out at the synchrotron beamline B2 at HASYLAB (DESY, Hamburg). The diffractometer was equipped with capillary furnace (STOE) and the on-site readable image-plate detector OBI [5]. Samples mounted into quartz capillaries of diameter 0.3mm were heated and cooled at the temperature range from 300K up to 1173K. The wavelength was 0.685356Å. Analysis of the data was performed with the GSAS software [6]. A series of aluminium-substituted spinel iron oxides was prepared by the solid state reaction of Li2CO3 (Merck) with the precursors, α-(Fe,Al)2O3, displayed a single crystalline phase with hematite structure, in different Li:M2O3 rations (M=Al+Fe), and underwent a thermal treatment in air, at 750°C and 950°C successively, for 5h. Figure 1: Observed, calculated and difference profiles resulting from the Rietveld analysis of synchrotron X-ray powder diffraction data of the one-phase LiFe4.5Al0.5O8 spinel solid solution at temperature 1003K. For LiFe4.5Al0.5O8 and LiFe4.0Al1.0O8 formation of single phase spinel solid solution was observed (Figure 1). Additionally for LiFe3.5Al1.5O8 Rietveld refinement revealed presence of the two phases system (doubling of the spinel X-ray lines, which could be fitted by the two cubic phases). Bot