Prediction of Unknown Uranyl Oxide Hydrate Structure Types: Comparison of Calculated and Measured XRD Powder Patterns
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PREDICTION OF UNKNOWN URANYL OXIDE HYDRATE STRUCTURE TYPES: COMPARISON OF CALCULATED AND MEASURED XRD POWDER PATTERNS R.J. FINCH,* F.C. HAWTHORNE* AND R.C. EWING** *Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada "**Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 U.S.A. ABSTRACT The accurate identification of uranium-bearing corrosion products from spent fuel leaching studies is often difficult because the XRD powder patterns for many U(VI) phases are closely similar. Standard pattern-matching techniques commonly used for phase identification (e.g., JCPDS) make confident phase identification difficult and can oversimplify complex mixtures of UOHs. Refinement methods that fit the full XRD powder pattern, such as Rietveld, may be the only methods for accurate UOH identification, especially for data from sample mixtures. Rietveld refinement of XRD powder data for uranium oxides can be accomplished for phases whose structures are well defined. Unfortunately, many of the uranyl oxide hydrates are still ill-defined, both structurally and compositionally. The uranyl oxide hydrates have large unit cells and relatively low symmetries, as well as close structural similarities, all of which complicates the simultaneous refinement of mixed UOH phases. Modeling peak shape profiles can also hamper refinements of mixed-UOH samples due to extreme peak overlap. Because of these difficulties, Rietveld structure determination for UOHs from sample mixtures remains intractable, although structure determination may be possible for fine-grained, single-phase samples. Nonetheless, Rietveld refinement is the most promising method for accurately identifying the component phases in a mixture, but the unknown UOH structures must be determined before this technique can be applied successfully. INTRODUCTION The uranyl oxide hydrates (UOHs) are a group of structurally related minerals formed during the oxidative corrosion of uraninite, UO 2+x. These phases are also reported from corrosion and dissolution studies on U0 2 and spent U0 2 nuclear fuel (SNF). Dehydration and alteration of the UOHs in nature are extensive. Their alteration behavior can be understood in terms of their crystal structures, but, in nature and the laboratory, the UOHs commonly form as fine-grained, polycrystalline mixtures, often precluding structural determination by single-crystal XRD methods and making confident identification difficult or impossible. Several of the UOH structures have been solved, and most of these phases display significant structural similarities. The method of Rietveld [I] fits the full XRD powder pattern, and it may refine multiple phases simultaneously. The Rietveld method can also be used to solve unknown structures provided a suitable starting model is employed. A comparison of the known UOH structure types with the unit cell parameters and compositions of several unknown UOH phases can provide model structures for the refinement or solution of unknown structures. This study
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