Electron spin resonance determination of the redox state of iron and its relationship to radiation-induced defect centers in oxidized and reduced ZrF 4-based glasses

Abstract

Electron spin resonance (ESR) studies have been carried out on a series of ZrF 4-based glasses comprising a fixed base glass composition of the ZBLAN type doped with 0, 10, and 100 ppm FeF 3, each melted under three atmospheres: Ar, Ar + O 2, and O 2. The object of this exercise was to develop a free-standing method of quantitative analysis for Fe 3+ in these materials. The accomplishment of this objective required the fabrication of an absolute spin concentration standard consisting of a ZBLAN glass doped with 100 ppm Mn 2+ and exploration of the issue of the ESR “oscillator strength” of Fe 3+ in glasses. Competitive electron and hole trapping by Fe 2+, Fe 3+, and five intrinsic color centers was investigated following γ-irradiation at room temperature and 100-keV X irradiation at 77 K and sequences of isochronal anneals to higher temperatures. The oscillator strength of Fe 3+ in ZBLAN glasses was deduced to be 0.9±0.1 by quantitative comparison of the changes in ESR signal strength of Fe 3+ with the corresponding changes in signal strength of the color centers, which on the basis of elementary considerations were assumed to have oscillator strengths of 1.0. The redox ratio R = [Fe 3+]/[Fe] total was then shown to be 0.63±0.18 in glasses melted under continuous O 2 atmosphere. The uncertainty in R is due principally to difficulties in establishing the proper baseline for numerical integration of the experimental first-derivative spectra; prospects for improving the precision of this measurement are discussed.