On the structure and radiation chemistry of iron phosphate glasses: New insights from electron spin resonance, Mössbauer, and evolved-gas mass spectroscopy

Abstract

Several vitreous forms for immobilization of plutonium and/or high-level nuclear wastes have been surveyed by electron spin resonance (ESR) to gain insights into their atomic-scale structures and to look for signs of radiolytic decomposition resulting from exposures to γ-ray doses of 30 MGy. While preliminary results are reported for Defense Waste Processing Facility (DWPF) borosilicate compositions and an experimental lanthanum-silicate glass, this paper focusses primarily on a class of glasses containing 40–75 mol% P 2O 5 and up to 40 mol% Fe 2O 3. Each of the six diverse compositions investigated displayed characteristic ESR signals (not resembling those of the iron-containing phosphorus-free glasses) comprising combinations of an extremely broad “X resonance” and a narrow “Z resonance”, both centered near g=2.00 and both displaying nearly perfect Lorentzian line shapes (peak-to-peak derivative widths ∼300–600 mT and ∼30 mT, respectively, at 300 K). The X-resonance intensities in the air-melted glasses correlated linearly with Fe:P ratio up to [Fe 2O 3]/[P 2O 5] ≈ 0.6, where intensity values ∼1 spin/phosphorus were reached. Mössbauer studies showed that the [Fe 3+]/[Fe] tot ratio could be varied from 0.82 to 0.49 by raising the melting temperature in air from 1150°C to 1450°C and/or by employing mildly reducing atmospheres. The combined X + Z-resonance intensities were reduced to zero for [Fe 3+]/[Fe] tot less than ∼0.6, leaving only a much weaker spectrum attributable to Fe 3+ ions. The X and Z ESR signals of the iron phosphate glasses resemble nothing else in the literature except the correspondingly denoted signals in an iron-free amorphous peroxyborate (APB) preparation. The X and Z resonances in the latter are deemed to arise from superoxide ions (O 2 −) in the borate network and in a separated Na 2O 2 phase, respectively. An asymmetric Z resonance signal attributable to interstitial O 2 − species was a radiation-induced manifestation in phosphate glass of composition 50P 2O 5–20Fe 2O 3–23Li 2O–7CeO 2. Irradiated and unirradiated samples of this same glass were studied by ESR isochronal annealing and differential thermal analysis, revealing a one-for-one conversion of X to Z upon partial crystallization near 670°C and a Z → X reconversion upon partial remelting near 970°C. Heating to 1070°C in dry Ar resulted in a weight loss of ∼5 wt%, while quadrupole mass spectrometry (QMS) during ramped heating to 1550°C at a pressure of 10 −5 Pa revealed the evolution of O 2 molecules with (radiation sensitive) ion-current peaks near 1170°C and 1350–1400°C. To account for the totality of these and other results, it is suggested that air-melted iron phosphate glasses may contain macroscopic numbers of superoxide ions as an intrinsic chemical feature of their as-quenched structures. A specific four-connected phosphorus–oxygen glass network incorporating O 2 − ions is proposed.