Abstract
Single crystals of four new layered uranyl phosphates, including three oxyfluoride-phosphates, were synthesized by molten flux methods using alkali chloride melts, and their structures were determined by single-crystal X-ray diffraction. Cs11[(UO2)12(PO4)3O13] (1) and Rb11[UO2)12(PO4)3O12F2] (2) contain uranyl phosphate layers exhibiting a new sheet topology that can be related to that of β-U3O8, while Cs4.4K0.6[(UO2)6O4F(PO4)4(UO2)] (3) and Rb4.4K0.6[(UO2)6O4F(PO4)4(UO2)] (4) contain layers of a known isomer of the prominent phosphuranylite topology. The location of the fluorine in structures 2-4 is discussed using bond valence sums. First principles calculations were used to explore why a pure oxide structure is obtained for the Cs containing phase (1) and in contrast an oxyfluoride phase for the Rb containing phase (2). Ion exchange experiments were performed on 1 and 2 and demonstrate the ability of these structures to exchange approximately half of the parent alkali cation with a target alkali cation in an aqueous concentrated salt solution. Optical measurements were performed on 1 and 2 and the UV-vis and fluorescence spectra show features characteristic of the uranyl group.
Highlights
Nuclear power has been well-established for several decades and, studies continue to develop a deeper understanding of the nuclear fuel cycle, including exploring improved methods of both long-term and short-term waste storage, and continuing to investigate the processes of radionuclide leaching into surrounding ecosystems.Layered Uranyl Oxyfluoride-PhosphateFor these reasons, it is advantageous to further expand our understanding of uranium coordination chemistry, in extended structures, as this can give us insights in understanding intermediate phases in the nuclear waste cycle, identifying potential structures useful in nuclear waste processing or storage, and possible pathways within the environment for the migration of U6+ and other actinides.Nature often gives a good indication of what chemical compounds can be made synthetically
Phosphuranylite and autunite minerals are the primary classes of phosphorus containing minerals, as phosphorus bearing uranium minerals make up nearly a quarter of all identified uranium minerals
Three new uranyl phosphate oxyfluorides, and one uranyl phosphate, have been synthesized by molten flux methods using alumina crucible reaction vessels and alkali chloride fluxes. Their structures were determined by single crystal X-ray diffraction (SXRD) and the presence of fluorine was confirmed by qualitative EDS
Summary
Nuclear power has been well-established for several decades and, studies continue to develop a deeper understanding of the nuclear fuel cycle, including exploring improved methods of both long-term and short-term waste storage (zur Loye et al, 2018), and continuing to investigate the processes of radionuclide leaching into surrounding ecosystems. Cs4.4K0.6[(UO2)6O4F(PO4)4(UO2)] (3) and Rb4.4K0.6[(UO2)6O4F(PO4)4(UO2)] (4) were obtained from reactions of 0.5 mmol UF4, 0.2 mmol AlPO4, and 5 mmol CsCl or RbCl and 5 mmol KCl loaded into an alumina crucible covered with an alumina plate held in place by rubber cement These were heated and slow cooled to 450◦C and produced a yellow crystalline product identified as a mixture of the newly reported F containing phases and either CsxK4−x[(UO2)3(PO4)2O2] orRb1.4K2.6[(UO2)3(PO4)2O2] that are visibly indistinguishable. Labeling all sites within the coordination sphere of the U sites as O in structures 2-4, does not result in charge balance, as there is an excess of negative charge This could not be resolved by reasonable models of the alkali cation disorder and this observation, along with the identification of F in all three structures by EDS in both powdered and singly crystalline forms, confirms the presence of F. In the case of the gases O2, F2, and Cl2, we use tabulated values for the standard entropies (Chase, 1998)
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