Iron redox equilibrium, structure and properties of iron phosphate glasses

Abstract

Iron redox equilibrium, structure and properties were investigated for 40Fe 2O 3–60P 2O 5 (mol%) glasses melted at different temperatures. The Fe 2+/(Fe 2++Fe 3+) ratio increased from 17% to 50% as the melting temperature changed from 1150°C to 1400°C. The equilibrium constant, K, for the reaction of Fe 3+ being reduced to Fe 2+ varied with temperature as ln K=9.40–1.58×10 4/ T. The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe 2O 3–60P 2O 5 (mol%) glasses did not change as the Fe 2+/(Fe 2++Fe 3+) ratio changed. All of the properties did not change to any major degree with increasing the melting temperature. The molar volume decreased while the density increased with increasing Fe 2+/(Fe 2++Fe 3+) ratio. It was found by DTA and XRD that two phases, Fe 3(P 2O 7) 2 and Fe 4(P 2O 7) 3, crystallized from the glass when the glass was heated in nitrogen. The crystallization behavior suggested that the amount of the crystal, Fe 3(P 2O 7) 2, may increase with increasing Fe 2+/(Fe 2++Fe 3+) ratio, which supported the opinion that there are some structural similarities between the iron phosphate glass and the crystalline Fe 3(P 2O 7) 2 in terms of the iron coordination number and bonding of the phosphate groups. The decrease in dc resistivity and increase in dielectric constant and dielectric loss tangent, which occurred with increasing the Fe 2+/(Fe 2++Fe 3+) ratio, were attributed to the increase of the electronic hopping from Fe 2+ ions to Fe 3+ ions.