Synthesis and Phase Characterization of a Solid Solution Series between β-Fe2(PO4)O and Fe4(PO4)3(OH)3

Abstract

An intermediate solid solution series was synthesized in the quaternary system FeO–Fe2O3–P2O5–H2O between the compositions of Fe2(PO4)O and Fe4(PO4)3(OH)3 hydrothermally at 586°C and 0.3GPa. Quantitative XRPD studies in combination with chemical bulk analysis reveal that under these conditions an intermediate solid solution series is formed in the above-mentioned quarternary system. It was found to range between 0.18≤x≤0.60 according to a stoichiometry of Fe3+4−xFe2+3x (PO4)3(OH)3−3xO3x. In the range of 0.02≤x≤0.18 a two-phase field exists under these conditions between the limiting member of the intermediate solid solution and Fe4(PO4)3(OH)3 with a small miscibility region in direction of Fe2(PO4)O. On the other hand, the hypothetical end-member Fe2(PO4)O could not be synthesized under these conditions. Using standard X-ray powder diffraction in combination with the Rietveld method, the members of the solid solution were found to crystallize analogously to β-Fe2(PO4)O in the tetragonal space group I41/amd. The solid solutions are characterized by a disordered distribution of Fe2+, Fe3+, and vacancies over the single octahedrally coordinated Me (8f) site.