Iron redox equilibrium, structure and properties of iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for 40Fe (2)O(3)-60P(2)O(5) (mol%) glasses melted at different temperatures. The Fe2 +/(Fe2+ + Fe3+) ratio increased from 17% to 50% as the melting temperature changed from 1150 degreesC to 1400 degreesC. The equilibrium constant, K, f or the reaction of Fe3+ being reduced to Fe2+ varied with temperature as In K = 9.40-1.58 x 10(4)/T, The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe(2)O(3)-60P(2)O(5) (mol%) gl asses did not change as the Fe2+/(Fe2+ + Fe3+) ratio changed. All of the pr operties did not change to any major degree with increasing the melting tem perature. The molar volume decreased while the density increased with incre asing Fe2+/(Fe2+ + Fe3+) ratio. It was found by DTA and XRD that two phases , Fe-3(P2O7)(2) and Fe-4(P2O7)(3), crystallized from the glass when the gla ss was heated in nitrogen, The crystallization behavior suggested that the amount of the crystal, Fe-3(P2O7)(2), may increase with increasing Fe2+/(Fe 2+ + Fe3+) ratio, which supported the opinion that there are some structura l similarities between the iron phosphate glass and the crystalline Fe-3(P2 O7)(2) in terms of the iron coordination number and bonding of the phosphat e groups. The decrease in de resistivity and increase in dielectric constan t and dielectric loss tangent, which occurred with increasing the Fe2+/(Fe2 + + Fe3+) ratio, were attributed to the increase of the electronic hopping from Fe2+ ions to Fe3+ ions. (C) 2001 Elsevier Science B.V. All rights rese rved.