Structural Studies of Bi2(FexAl1-x)4O9 Solid Solutions (0.1 ≤ x ≤ 1.0) Prepared by a Combined Mechanochemical/Thermal Synthesis

Bi-2(FexAl1-x)(4)O-9 solid solutions with 0.1 <= x <= 1.0 prepared by mechanochemical processing of alpha-Fe2O3/Bi2O3/Al2O3 mixtures and subsequent annealing were investigated by X-ray diffraction, Mossbauer spectroscopy, and EDX. A Rietveld refinement was made for the whole range of the solid solutions. The structure is orthorhombic with space group Pbam. Mossbauer spectroscopy reveals paramagnetic behavior for Bi-2(FexAl1-x)(4)O-9 with 0.1 <= x <= 1.0 at room temperature and indicates significant distortion of the octahedral iron coordination for aluminum-rich mixed crystals. The occupancy of octahedrally and tetrahedrally coordinated sites was determined by Mossbauer spectroscopy and Rietveld analysis. For x >= 0.5, the Mossbauer measurements as well as the Rietveld refinements show an almost random distribution of aluminum and iron cations over the two non-equivalent sites, whereas for x < 0.5, a slight preference of iron for the octahedral sites is observed.