Relationships between the crystal chemistry and magnetic properties of Np 5+ sulfates
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Relationships between the crystal chemistry and magnetic properties of Np5+ sulfates. Tori Z. Forbes1, Peter, C. Burns1,2, L. Soderholm1,2 and S. Skanthakumar2 1 Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556 2 Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 ABSTRACT Np5+ sulfate compounds were synthesized using hydrothermal methods and their structures were determined using single crystal X-ray diffraction. Several compounds were found to contain cation-cation interactions between the Np polyhedra. Magnetic susceptibility measurements of two Np5+ sulfate compounds containing cation-cation interactions exhibit ferromagnetic ordering. The presence of the cation-cation interactions may provide a superexchange pathway through which magnetic interactions can be enhanced. INTRODUCTION Understanding the basics of the structural chemistry of neptunium is vital for addressing its geochemical behavior and risk assessment of nuclear waste disposal[1-3]. Unfortunately there are very few published crystal structures that contain neptunium in its most environmentally important oxidation state, Np5+. Neptunium has four common oxidation states – III, IV, V, and VI, with pentavalent neptunium, present as a linear dioxo cation, [O=Np=O]+, the most stable moiety in solution [1, 4]. Actinide (VI) compounds such as uranium almost invariably contain approximately linear (AnO2)2+ dioxide cations that are further coordinated by four, five, or six ligands, giving square, pentagonal, or hexagonal bipyramids [5-7]. Little is known about the crystal structures of actinide (V) compounds, but of the structures reported, most contain a linear (AnO2)+ dioxide cation similar in geometry to those found for the hexavalent state. To compensate for the residual of negative charge on the (NpO2)+ ion relative to that of 2+ (UO2) , the neptunyl cation may be involved in significant cation-cation interactions. Cationcation interactions, initially postulated to occur in solution [8] involve the coordination of the oxo anion of one actinyl moiety as an equatorial ligand of a different actinyl polyhedron. [9, 10]. Such interactions have been found to be relatively common in the reported structures that contain Np5+, but are much more unusual in the case of U6+ [11]. Bonding requirements of the dioxo ions are nearly satisfied by the bonding within the uranyl cation alone, and therefore do not normally bond to other high-valent cations. Linkage of uranyl bipyramids with other polyhedra is almost invariable through the equatorial vertices of the bipyramids [7]. The potential for cation-cation interactions in Np5+ structures suggests significant divergence from the crystal chemistry and structural connectivities of U6+ and suggests the possibility that these materials may exhibit interesting physical properties. We are undertaking a detailed study of Np5+ crystal chemistry, focusing on compounds with environmentally relevant chemical compositions, to develop better insigh
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