Oxygen and phosphorus coordination around iron in crystalline ferric ferrous pyrophosphate and iron-phosphate glasses wi
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Oxygen and phosphorus coordination around iron in crystalline ferric ferrous pyrophosphate and iron-phosphate glasses with UO2 or Na2 O C. H. Booth,a) P. G. Allen,b) J. J. Bucher, N. M. Edelstein, and D. K. Shuh Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
G. K. Marasinghe, M. Karabulut, C. S. Ray, and D. E. Day Department of Physics, Department of Ceramic Engineering, and the Graduate Center for Materials Research, University of Missouri, Rolla, Missouri 65409 (Received 19 August 1998; accepted 17 February 1999)
Fe K-edge x-ray absorption fine-structure (XAFS) measurements were performed on glass samples of (Fe3 O4 )0.3 (P2 O5 )0.7 with various amounts of Na2 O or UO2 . Near-edge and extended XAFS regions are studied and comparisons are made to several reference compounds. We find that iron in the base glass is ,25% divalent and that the Fe21 coordination is predominantly octahedral, while Fe31 sites are roughly split between tetrahedral and octahedral coordinations. Also, we measure roughly one Fe–O–P link per iron. Substitution of Na2 O or UO2 up to 15 mol% primarily affects the first Fe–O shell. The results are compared to data from the related material Fe3 (P2 O7 )2 . I. INTRODUCTION
Iron-phosphate glasses have many advantages over other phosphate glasses as a high-level nuclear waste storage medium.1–4 One important advantage is the excellent chemical durability, which is comparable to or better than that of borosilicate glasses. For lead-ironphosphate glasses, this durability is manifest by leach rates in aqueous solutions with pH between ,5 and 9 that are 100–1000 times lower than in borosilicate glasses,1,3 which encompasses almost all natural ground waters. Iron-phosphate glasses maintain this durability even when moderate amounts (,15 wt%) of simulated nuclear waste (including nonradioactive constituents) are vitrified into the glass.3,4 This durability has made ironphosphate glasses an alternative to methods currently under consideration by the United States Department of Energy for storage of high-level nuclear waste that contains significant amounts of iron and phosphorus.5 Other important advantages over the borosilicate glasses include a low melting point (,800 ±C), no tendency to devitrify at repository temperatures, and the abundance of Fe2 O3 and P2 O5 in nuclear waste streams.1 The latter point means that less additional material is needed to vitrify some waste, and therefore the overall volume of the waste form is smaller. Several models have been proposed to explain why a phosphate glass (normally easily hydrated) should become resistant to hydration when iron in added. Phosphate glasses typically include a backbone of (PO4 )32 a)
Address all correspondence to this author. Present address: Los Alamos National Laboratory, MS K764, Los Alamos, New Mexico 87545. e-mail: [email protected] b) Present address: Lawrence Livermore National Laboratory, P.O. Box 808, MS L-231, Livermore, California 94551. 262
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