STRUCTURAL FEATURES OF IRON PHOSPHATE-GLASSES

The structures and valence states of iron ions in several iron phospha te glasses with batch compositions similar to 40Fe(2)O(3)-60P(2)O(5) ( mol%) have been investigated using Mossbauer spectroscopy, X-ray absor ption fine-structure spectroscopy (XAFS), X-ray photoelectron spectros copy (XPS), differential thermal (DTA) and thermo-gravimetric (TGA) an alysis and X-ray and neutron diffraction. Mossbauer spectra show that a redox equilibria corresponding to an Fe(II)/[Fe(II) + Fe(III)] ratio of 0.2-0.4 is reached under processing conditions described in this p aper. Even though the valence state of iron ions in the glass appears to be insensitive to the oxygen content in the melting atmosphere, the Fe(II) content can be increased within the observed range of redox eq uilibria by increasing the partial pressure of a reducing gas in the m elting atmosphere. Large amounts of Fe(II), Fe(II)/[Fe(II) + Fe(III)] greater than or equal to 0.4, appear to be detrimental to the glass-fo rming ability of the iron phosphate melts. The local structure of the iron phosphate glasses appears to be related to the short range struct ure of crystalline Fe-3(P2O7)(2) which consists of a network of (Fe3O1 2)(-16) clusters. These clusters consist of one iron(II) ion and two i ron(III) ions in sixfold coordination with near-neighbor oxygen ions. The (Fe3O12)(-16) clusters are interconnected via (P2O7)(-4) groups. C ompared to other phosphate glasses, the proposed structure for iron ph osphate glasses contain a smaller number of P-O-P bonds, a feature whi ch is believed to be responsible for the unusually good chemical durab ility of iron phosphate glasses. (C) 1997 Elsevier Science B.V.