Mechanical milling of the (a-Fe2O3)x(a-Al2O3)1 x system : an x-ray diffraction and Mössbauer spectral study

The system (a-Fe2O3)x(a-Al2O3)1 x was subjected to 24 h of high-energy ball-milling in different milling media and velocities, with x varying in the 0.10 – 0.50 range. Stainless-steel vial with stainless-steel balls and alumina vial with zirconia rods were alternately used in a planetary mill apparatus, at velocities ranging from 100 to 600 rpm. The phases present in the milled samples were characterized by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). For the stainless-steel milling medium, the results revealed that the final products are extremely dependent on the starting concentration of hematite and on the milling velocity. Iron contamination, originated from abrasion in the metallic medium (MM), was detected for low x and, also, for intermediate x at higher milling velocities. Solid solutions of the (Fe,Al)2O3 type, with different iron concentration ranges, were identified in both milling media. In the ceramic vial, a medium in which milling velocity is also found to be a decisive factor, the FeAlO3 phase, non-isostructural to the precursors, was also recognized in addition to solid solutions. Low-temperature Mössbauer measurements verified the occurrence of four magnetic iron sites in this compound