Eu²⁺ and Gd³⁺ Distributions in Fluorohafnate Glasses Studied by Electron Paramagnetic Resonance

We have studied electron paramagnetic resonance spectra of europium- and gadolinium-activated ZBLAN-type fluorohafnate glasses, using the composition 58HfF₄ · 20BaF₂ · 2LaF₃ · 3AlF₃ · 17NaF as an example. The ratio of the concentration of free Eu²⁺ and Gd³⁺ ions to that of ions in clusters has been quantitatively evaluated for the first time. The percentage of free ions has been shown to increase significantly with decreasing activator concentration. At activator concentrations of 1.25 mol % EuF₂ and 1 mol % GdF₃, the activator ions predominantly form clusters and only a small fraction of Eu²⁺ and Gd³⁺ are present as individual ions, whereas at 0.1 mol % EuF₂ the concentration of free ions is comparable to that of ions in the clusters.