Mössbauer and optical spectroscopy of phosphoniobate and phosphotantalate glasses doped with iron

Abstract

Glasses of the nominal compositions.xNb₂TO₅-(1 - x) (95mol% NaPO₃-5% Na₂B₄O₇) (x = 0, 10.3, 20.6, 30.0, 36.5 or 43.0) and x-Ta₂O₅-(1 - x) (95mol% NaP0₃-5mol%Na₂B₄O₇) (x = 0, 5.0, 9.3 or 14.0) were prepared by conventional quenching of the melts. Raman spectroscopy shows that introduction of T₂O₅ (T = Ta or Nb) in smali amounts gives rise to the formation of ‘isolated’ TO₆ octahedra. As the concentration of T₂O₅ increases, the spectra suggest the formation of corner-sharing TO₆, octahedra and finally, for the glasses richest in T₂O₅, of a three-dimensional association of cornersharing, less distorted octahedra. All these considerations lead to the conclusions that both niobium and tantalum in octahedral coordination act as network formers strengthening the glass network. The near-infrared spectra have provided information on the coordination geometries of iron(II) that has been used for the interpretation of the Móssbauer spectra. In this way the simultaneous presence of iron(III) and iron(II) in both tetrahedral and octahedral coordination has been proved. The effects of increasing amounts of T₂O₅ and of the alkali elements and of increasing temperature on the iron hyperfine parameters has been studied to check the modifcations of the glass network.