Redox behaviour of polyvalent ions in the ppm range

High purity fluoride phosphate glasses FP 10 (35 mol% AlF3, 30 CaF2, 15 SrF2, 10 MgF2, 10 Sr(PO3)(2) with a large transmission range from the vacuum ultraviolet to the infrared were doped with polyvalent metal ions (Fe, Cu, Ce, Cr, Pb, Ni) in the ppm range. Their redox behaviour was investigated under normal and strong reducing melting conditions depending on temperature. Only one redox state was observed for Ni2+ and Pb2+. From the other metals, the following redox equilibria could be detected: Fe3+/Fe2+, Ce4+/Ce3+, Cu2+/Cu+/Cu-0, Cr3+/Cr2+. A linear relation between the log C-ox/C-red and 1/T was found in the temperature range 900-1200 degrees C. In samples melted under normal conditions in air atmosphere, the higher redox state dominates but is strongly affected by the raw materials. By reducing melting conditions, the redox equilibrium could be shifted more than 99.5% to the lower oxidation state, Fe2+, Ce3+ and Cu+. Absorption, luminescence excitation and emission spectra were used detect Ce3+, Cu+, Pb2+ and Cr3+ to low quantities (<0.1 ppm).