Phase equilibria in iron phosphate system

The main objectives of this research were to synthesize iron phosphate compounds, study phase equilibria in the iron phosphate system, focusing on the glass forming area, and investigate glass formability and properties based on the liquidus regions. Twelve iron phosphate compounds were successfully prepared and studied in terms of liquidus temperature or decomposition behavior. The liquidus surface of the Fe3PO7Fe(PO3)3 system was re-determined and found to be significantly different from that originally presented by Wentrup in 1935. Eutectic points exist at 58.0 mole% Fe2O3 (1070°C), 42.7 mole% Fe2O3 (925°C), and 37.0 mole% Fe2O3 (907°C). The latter two eutectic points bracket the conventional iron phosphate glass-forming range. The liquidus surface of the Fe2P2O7-Fe(PO3)2 system was also determined. A eutectic point exists at 52.8±0.5 mole% FeO and 935°C in the Fe2P2O7-Fe(PO3)3 system. Glass formation of iron-rich phosphate glasses (nominal Fe/P ratios between 1.0 and 1.6) requires a critical cooling rate in the range 10-10 oC/sec, compared to 1-10oC/sec for conventional iron phosphate melts (nominal Fe/P ratios near 0.50). The structures of the iron-rich phosphate glasses are based on isolated orthophosphate tetrahedra, similar to those found in α-FePO4. The stability of melts (with nominal Fe/P compositions between 0.50 and 0.67) against crystallization, described by the Angell and Weinberg parameters, generally decreases with increasing O/P and Fe/P ratios. The structures of crystalline and glassy iron phosphates were studied using Raman spectroscopy. The correlation of the structure of iron phosphate compounds and the Raman modes was summarized and established. The structural parameters (like P-O bond length) of glassy iron phosphates were predicted and discussed.