ON THE STRUCTURE AND RADIATION-CHEMISTRY OF IRON PHOSPHATE-GLASSES - NEW INSIGHTS FROM ELECTRON-SPIN-RESONANCE, MOSSBAUER, AND EVOLVED GAS MASS-SPECTROSCOPY

Several vitreous forms for immobilization of plutonium and/or high-lev el nuclear wastes have been surveyed by electron spin resonance (ESR) to gain insights into their atomic-scale structures and to look for si gns of radiolytic decomposition resulting from exposures to gamma-ray doses of 30 MGy. While preliminary results are reported for Defense Wa ste Processing Facility (DWPF) borosilicate compositions and an experi mental lanthanum-silicate glass, this paper focusses primarily on a cl ass of glasses containing 40-75 mol% P2O5 and up to 40 mol% Fe2O3. Eac h of the six diverse compositions investigated displayed characteristi c ESR signals (not resembling those of the iron-containing phosphorus- free glasses) comprising combinations of an extremely broad ''X resona nce'' and a narrow ''Z resonance'', both centered near g = 2.00 and bo th displaying nearly perfect Lorentzian line shapes (peak-to-peak deri vative widths similar to 300-600 mT and similar to 30 mT, respectively , at 300 K). The X-resonance intensities in the air-melted glasses cor related linearly with Fe:P ratio up to [Fe2O3]/[P2O5] approximate to 0 .6, where intensity values similar to 1 spin/phosphorus were reached. Mossbauer studies showed that the [Fe3+]/[Fe](tot) ratio could be vari ed from 0.82 to 0.49 by raising the melting temperature in air from 11 50 degrees C to 1450 degrees C and/or by employing mildly reducing atm ospheres. The combined X + Z-resonance intensities were reduced to zer o for [Fe3+]/[Fe](tot) less than similar to 0.6, leaving only a much w eaker spectrum attributable to Fe3+ ions. The X and Z ESR signals of t he iron phosphate glasses resemble nothing else in the literature exce pt the correspondingly denoted signals in an iron;free amorphous perox yborate (APB) preparation. The X and Z resonances in the latter are de emed to arise from superoxide ions (O-2(-)) in the berate network and in a separated Na2O2 phase, respectively. An asymmetric Z resonance si gnal attributable to interstitial 0; species was a radiation-induced m anifestation in phosphate glass of composition 50P(2)O(5)-20Fe(2)O(3)- 23Li(2)O-7CeO(2). Irradiated and unirradiated samples of this same gla ss were studied by ESR isochronal annealing and differential thermal a nalysis, revealing a one-for-one conversion of X to Z upon partial cry stallization near 670 degrees C and a Z --> X reconversion upon partia l remelting near 970 degrees C. Heating to 1070 degrees C in dry Ar re sulted in a weight loss of similar to 5 wt%, while quadrupole mass spe ctrometry (QMS) during ramped heating to 1550 degrees C at a pressure of 10(-5) Pa revealed the evolution of Oz molecules with (radiation se nsitive) ion-current peaks near 1170 degrees C and 1350-1400 degrees C . To account for the totality of these and other results, it is sugges ted that air-melted lion phosphate glasses may contain macroscopic num bers of superoxide ions as an intrinsic chemical feature of their as-q uenched structures. A specific four-connected phosphorus-oxygen glass network incorporating O-2(-) ions is proposed, (C) 1998 Elsevier Scien ce B.V. All rights reserved.