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–Fe4(P2O7)3, and Fe4(P2O7)3–Fe(PO3)3 were determined. The results are significantly different from those presented by Wentrup in 1935. Fe3PO7 is the stable ferric oxophosphate compound, not Fe4P2O11, and Fe3PO7 decomposes in air at 1090°C. The congruent melting point of FePO4 (1208°C) is similar to what was reported, but Fe4(P2O7)3 melts congruently at 945°C, about 300°C lower than claimed by Wentrup. Fe(PO3)3, for which the melting temperature has not been previously reported, melts congruently at 1205°C. Eutectic points exist at 58.0 mol% Fe2O3 (1070°C), 42.7% Fe2O3 (925°C), and 37.0% Fe2O3 (907°C). The latter two eutectic points bracket the conventional glass-forming range for iron phosphate melts under consideration as alternative hosts for nuclear wastes.