AN X-RAY-POWDER DIFFRACTION STUDY OF SYNTHETIC (FE,MN)(2)TIO4 SPINEL

The (Fe,Mn)(2)TiO4 spinel solid solution was synthesized in steps of 10 mole % at 950 degrees C, 1 bar total pressure and log f(O2) of -16.30. Electron microprobe analysis indicates that all synthesized Fe-Mn titanate spinels are homogeneous but contain an additional small magnetite component in solid solution. The magnetite content is 6.8 mole % for ulvospinel and decreases with increasing manganese. As stoichiometric (Fe,Mn)(2)TiO4 oxide mixtures were used as starting material, additional Fe-Mn ilmenite occurs in the run products as a result of the Fe3O4 solution in spinel. Powder X-ray diffraction data, collected in the range 15-160 degrees 2 theta with CuK alpha radiation, indicate that the solid solution is continuous at 950 degrees C with all members having the cubic crystal structure (space group Fd3m). Unit cell dimensions, oxygen positional factors, and isotropic displacement factors for Fe-Mn ulvospinel members were refined using the Rietveld method. The unit cell dimension increases linearly from 8.53374(8) Angstrom for ulvospinel to 8.68063(7) A for the Mn-endmember. The degree of inversion was estimated from X-ray powder data in three different ways, of which the method of optimizing the Rg factor in the Rietveld refinement procedure is considered as the most accurate one. The results indicate that the whole Fe-Mn ulvospinel solid solution is 82-92 % inverse at 950 degrees C. A change in degree of inversion with composition is not obvious from the data. The oxygen parameter u ranges from 0.2601(2) to 0.2615(2) within the solid solution. Calculated tetrahedral-oxygen bond lengths increase from 1.999(3) Angstrom for Fe2TiO4 to 2.046(3) Angstrom for Mn2TiO4, whereas the octahedral-oxygen bond lengths increase from 2.050(2) Angstrom to 2.079(2) Angstrom with X(Mn). Estimated effective ionic radii of Fe2+ and Mn2+ in spinel are slightly larger in IV-fold coordination and slightly smaller in VI-fold coordination than values reported in previous studies.