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THE CRYSTAL STRUCTURES OF X(NpO₂)(PO₄)(H₂O)₃ (X = K⁺, Na⁺, Rb⁺, NH₄⁺) AND THEIR RELATIONSHIP TO THE AUTUNITE GROUP

Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, U.S.A.

E-mail address: pburns{at}nd.edu

	ABSTRACT

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Four Np⁶⁺ phosphates with compositions X(NpO₂)(PO₄)(H₂O)₃, X = K⁺, Na⁺, Rb⁺, NH₄⁺, have been synthesized by mild hydrothermal methods, and their structures have been determined. Single-crystal X-ray-diffraction data were collected using MoK radiation and an APEX CCD detector. The Np⁶⁺ structures containing K⁺, Na⁺, Rb⁺, and NH₄⁺ (KP1, NaP1, RbP1, NH₄P1) were refined to agreement indices R₁ 0.042, 0.051, 0.066, and 0.023 calculated for 831, 856, 798, and 857 unique observed reflections, respectively. KP1 crystallizes in space group P4/ncc, with a 6.9564(8), c 17.826(4) Å, V 862.6(2) ų, Z = 4. NaP1 also is tetragonal, P4/ncc, with a 7.005(1), c 16.986(4) Å, V 833.6(3) ų, Z = 4. RbP1 is tetragonal, P4/ncc, with a 7.031(4), c 17.87(2) Å, V 884(1) ų, Z = 4. NH₄P1 crystallizes in space group P4/ncc with a 6.9843(6), c 18.023(3) Å, V 879.2(2) ų, Z = 4. Each of these structures contains sheets of composition [(NpO₂)(PO₄)]^– consisting of Np⁶⁺ in square bipyramidal coordination and phosphate tetrahedra. These sheets are topologically identical to the well-known uranyl phosphate sheets of the autunite group. The monovalent cations and H₂O group are located in the interlayer, where they balance the charge on the sheets and provide linkages between the sheets.

Keywords: neptunium phosphate, synthetic compounds, crystal structure, neptunyl phosphate sheets, autunite group.

	INTRODUCTION

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More than 40 species of uranyl phosphates and arsenates have been described that are members of the autunite and meta-autunite groups (Finch & Murakami 1999). The relatively low solubility of the uranyl phosphates of these groups in groundwater has important implications for the mobility of uranium in phosphate-bearing systems (Murakami et al. 1997). Precipitation of these minerals can be achieved at emplaced reactive barriers in the subsurface that are designed to limit the mobility of uranium in groundwater systems (Sowder et al. 1996). Soils contaminated by actinides, such as those at the Fernald site in Ohio where uranium ore was processed (Buck et al. 1996), and the K1300 locality of the DOE–K 25 site at Oak Ridge, Tennessee (Roh et al. 2000), have been found to contain minerals of the autunite group.

The fate and transport of ²³⁷Np are important for the safe, permanent disposal of nuclear waste in a geological repository contained in the unsaturated zone (Lieser & Muhlenweg 1988, Silva & Nitsche 1995). With a half-life of 2.14 x 10⁶ years, ²³⁷Np is a significant contributor to the total potential dose of radiation associated with such a repository system (Silva & Nitsche 1995). Expected conditions in a geological repository within the unsaturated zone suggest that Np will be most mobile in the pentavalent state. Pentavalent Np exhibits a higher solubility than Np⁴⁺ and could be transported as a dissolved constituent of groundwater. Engineered systems that limit the mobility of ²³⁷Np may be important in the design components of a repository (Kaszuba & Runde 1999). The mobility of Np in a repository could also be influenced by the precipitation of Np-rich phases, adsorption of Np onto mineral surfaces or colloids, or through the incorporation of Np into secondary uranyl phases (Silva & Nitsche 1995, Burns et al. 1997b, 2004). The formation of uranyl and neptunyl phosphates in or near a repository could decrease the release of ²³⁷Np from the repository.

The crystal-chemical characteristics of U⁶⁺, Np⁵⁺, and Np⁶⁺ are generally similar, as all usually exist as nearly linear dioxo cations [(UO₂)²⁺, (NpO₂)⁺, and (NpO₂)²⁺] with typical bond-lengths of 1.79(4), 1.83(2) and 1.75(2) Å, respectively (Burns et al. 1997a). The actinyl ion in crystal structures is further coordinated by four, five, or six equatorial ligands, giving square, pentagonal, and hexagonal bipyramids that are capped by the O atoms of the actinyl ions. Given the similar size of cations Np⁶⁺ and U⁶⁺ and geometries of polyhedra, it is reasonable to expect that analogous Np⁶⁺ and U⁶⁺ compounds will be isostuctural. Furthermore, (NpO₂)²⁺ should substitute readily for (UO₂)²⁺ within the structures of uranyl minerals (Burns et al. 1997b). We are conducting a detailed study of the crystal chemistry of Np⁵⁺ and Np⁶⁺, and are delineating relationships with the crystal chemistry of U⁶⁺ (Forbes & Burns 2005, 2006, Forbes et al. 2006). Here, we present the details of the synthesis and structures of the Np⁶⁺ phosphates X(NpO₂)(PO₄)(H₂O)₃ (X = K⁺, Na⁺, Rb⁺, NH₄⁺) and compare them to their analogous uranyl mineral structures.

	PREVIOUS WORK

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Hexavalent Np compounds with the formula X(NpO₂)(PO₄)(H₂O)_3–4, X = H₃O⁺, NH₄⁺, Li⁺, Na⁺, K⁺, and X[(NpO₂)(PO₄)]₂(H₂O)_6–9, X = Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, have been synthesized (Weigel & Hoffmann 1976). These syntheses yielded violet powders for the monovalent cations and light brownish powders for the divalent cations. The identity of these materials was established by X-ray powder diffraction, but their structures have not been reported. The compounds presented here appear to be identical to some of those synthesized by Weigel & Hoffmann (1976), on the basis of comparison of their calculated and observed X-ray powder-diffraction patterns.

	EXPERIMENTAL METHODS

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Hydrothermal synthesis

The reagents KCl (Fisher Scientific Lot #78390), NaCl (Aldrich Chemical Lot #12926EN), RbCl (Acros Chemical Lot #A011718701), and NH₄Cl (J.T. Baker Lot #30960) were used as received. Apatite, Ca₅(PO₄)₃F, from the Liscombe deposit, Ontario, or a 1.6 M H₃PO₄ solution (Baker Scientific Lot #350820), were used as the source of phosphate in each experiment. A stock solution was prepared by dissolving Np⁵⁺ in a 1 M HCl solution. CAUTION: ²³⁷Np represents a serious health risk due to the emission of alpha and gamma radiation. All studies were conducted in the hot laboratories of Argonne National Laboratory, which has appropriate equipment and personnel for handling such materials. All reactions took place in 7 mL Teflon cups with screw-top lids. After loading the reactants, the tightly closed Teflon cups were placed in 125 mL Teflon-lined Parr reaction vessels. Fifty mL of ultrapure water was added to each vessel to provide counter-pressure during heating. The reactions took place in a gravity convection oven for one to two weeks. The vessels were allowed to cool slowly to room temperature before removal from the oven. In each case, red platy crystals with maximum diameters of approximately 200 to 800 µm and a dark green precipitate resulted. Additional details of the syntheses are given in Table 1.

Single crystals of Np⁶⁺ phosphates containing K⁺, Na⁺, Rb⁺, and NH₄⁺ (KP1, NaP1, RbP1, NH₄P1) were mounted on glass fibers and placed on a Bruker three-circle X-ray diffractometer equipped with either an APEX or APEX II CCD detector. A sphere of three-dimensional data was collected at room temperature for each crystal using graphite-monochromatized MoK radiation and frame widths of 0.3° in for KP1, RbP1, and NH₄P1 and 0.6° in for NaP1. The X-ray data were collected with count-times per frame of 60 seconds for NaP1 and 10 seconds for KP1, RbP1, and NH₄P1. Unit- cell parameters were refined by least-squares techniques using the Bruker SMART or APEX II software. The Bruker SAINT software was used for data integration as well as correction for Lorentz and polarization. Semi-empirical corrections for absorption were applied by modeling the crystals as plates using the Bruker program XPREP. Reflections with plate-glancing angles less than 3° were discarded. Selected crystallographic data and refinement information are provided in Tables 2 and 3.

The final structure-model for KP1 includes anisotropic displacement parameters for the cations, and isotropic parameters for the lighter anions. In the case of NaP1, anisotropic displacement parameters were refined for Np and P only, owing to positional disorder present at the Na site. The final structure-models of RbP1 and NH₄P1 include anisotropic displacement parameters for all atoms.

The occupancies for the Na site in NaP1 and the NH₄ site in NH₄P1 were constrained to provide an electroneutral structure. The H₂O groups occupy 75% of these sites, and the cation accounts for the remaining 25%. The H atom positions of the interlayer H₂O group were refined with the constraint that O–H bonds be ~0.96 Å for KP1. In the case of NaP1, the electron density around the O(4) atom, which corresponds to the H₂O group, is distributed over two closely spaced sites. The O atom was modeled as two sites with O(4A) 25% occupied and O(4B) 50% occupied, with a Na⁺ cation accounting for the remaining 25%. The O(4A) and O(4B) sites are separated by 1.04(3) Å, indicating that both cannot be occupied locally. Atomic positional parameters are given for all atoms in Tables 4 for KP1, NaP1, RbP1, and NH₄P1. Table 5 contains selected interatomic distances for KP1, NaP1, RbP1, and NH₄P1, respectively. Tables of structure factors have been sent to the Depository of Unpublished Data on the MAC web site [document Np phosphates CM45_471].

	DESCRIPTIONS OF THE STRUCTURES

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View larger version (45K): [in this window] [in a new window]   FIG. 1. Polyhedral representation of the neptunyl phosphate sheets in X(NpO2)(PO4)(H2O)3, X = K+, Na+, Rb+, NH4+. The neptunyl polyhedra (shown in gray) share corners with the phosphate tetrahedra to form the autunite-type sheet.

View larger version (51K): [in this window] [in a new window]   FIG. 2. Projection of the structure of X(NpO2)(PO4)(H2O)3, X = K+, Na+, Rb+, NH4+ along [100] showing the interlayer sites (illustrated as spheres). The interlayer cations (K+, Na+, Rb+, or NH4+) occupy 25% of the sites, and the remainder is occupied by H2O groups.

	DISCUSSION

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Markovi et al. 1988

A solid-solution series has been hypothesized among uramphite, meta-ankoleite, and chernikovite, (H₃O)(UO₂)(PO₄)(H₂O)₃ (Ross & Evans 1965). The H₃O⁺, NH₄⁺, and K⁺ play similar structural roles in these compounds. All occur randomly distributed over the four general interlayer sites. Similarities of the ionic radii of these species allow the K⁺ cation to substitute into the H₂O sites with little strain or disturbance to the structure (Ross & Evans 1965). Structural data obtained for the synthetic analogue of meta-ankoleite indicate a disordered structure at room temperature, with K⁺ randomly occupying four of the 16 general sites that are otherwise filled with H₂O groups (Fitch & Cole 1991). Similar disorder of the interlayer cations is found in all four compounds presented here.

The X-ray powder-diffraction patterns for the Np⁶⁺ phosphates synthesized by Weigel & Hoffman (1976) were indexed and gave unit-cell dimensions that are smaller than the compounds reported here. Powder X-ray-diffraction patterns of the monovalent compounds yielded a tetragonal symmetry with lattice constants a 6.93(3) to 6.96(2) Å, c 8.76(6) to 9.16(7) Å, V 421.2 to 439.9 ų and Z = 2. The single-crystal X-ray-diffraction data for the K⁺, Na⁺, NH₄⁺, and Rb⁺ compounds require doubling of the unit cell in the c direction.

The compounds synthesized by Weigel & Hoffman (1976) were formed by the reaction of Np⁶⁺O₂(NO₃)₂ with a H₃PO₄ solution. Our synthesis began with Np⁵⁺O₂ ⁺ in 1 M HCl solution combined with apatite or H₃PO₄ as the phosphate source. The oxidation of Np⁵⁺ to Np⁶⁺ most likely occurred by the disproportionation reaction:

The equilibrium of the disproportionation reaction is strongly dependent on the acid concentration, with K = 4 x 10^–7 in 1 M perchloric acid (Fahey 1986). Our compounds were synthesized in solutions with a pH of 0 to 0.5, indicating that the disproportionation reaction is likely. In addition, the presence of complexing agents of Np in solution may further shift the equilibrium of this reaction. Substitution of 1 M HClO₄ for 1 M H₂SO₄ shifts the equilibrium constant from 4 x 10^–7 to 2.4 x 10^–2 (Fahey 1986). In studies concerning the electrode reaction-kinetics of NpO₂⁺/NpO₂²⁺ , Yamamura et al.(2005) found that the reaction is irreversible in sulfuric and acetic acid and reversible in perchloric, nitric and hydrochloric acid. The redox behavior of Np in phosphoric acid has not been studied, but may also shift the equilibrium of the reaction and affect the disproportionation of Np⁵⁺ in solution.

	AKNOWLEDGEMENTS

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This research was supported by the National Science Foundation Environmental Molecular Science Institute at the University of Notre Dame (EAR02–21966). We thank Drs. Lynne Soderholm and S. Skanthakumar for their help while conducting research at Argonne National Laboratory, and Dr. Andrew Locock for his advice concerning synthesis techniques for autunite-type structures. We thank associate editor Prof. S. Krivovichev and Prof. S. Obbade and an anonymous referee for their comments and helpful suggestions on the manuscript. We also thank Prof. R.F. Martin for providing beneficial annotations.

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Received May 31, 2006 ,revised manuscript accepted October 19, 2006.

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