Phase Equilibria in the Fe2O3-P2O5 System

Four ferric phosphate compounds were identified in the Fe2O3-P2O5 system and the liquidus surfaces in the subsystems Fe3PO7-FePO4, FePO4-Fe-4(P2O7)(3), and Fe-4(P2O7)(3)-Fe(PO3)(3) were determined. The results are significantly different from those presented by Wentrup in 1935. Fe3PO7 is the stable ferric oxo-phosphate compound, not Fe4P2O11, and Fe3PO7 decomposes in air at 1090 degrees C. The congruent melting point of FePO4 (1208 degrees C) is similar to what was reported, but Fe-4(P2O7)(3) melts congruently at 945 degrees C, about 300 degrees C lower than claimed by Wentrup. Fe(PO3)(3), for which the melting temperature has not been previously reported, melts congruently at 1205 degrees C. Eutectic points exist at 58.0 mol% Fe2O3 (1070 degrees C), 42.7% Fe2O3 (925 degrees C), and 37.0% Fe2O3 (907 degrees C). The latter two eutectic points bracket the conventional glass-forming range for iron phosphate melts under consideration as alternative hosts for nuclear wastes.