Crystal Structure of Mixed Np(V)-Ammonium Carbonate

Abstract

This work presents details of the synthesis, properties and structure of a novel neptunium carbonate (NH4)[NpO2CO3], a member of the M[AnO2CO3] (M = K, (NH4), Rb, Cs) class of compounds. Carbonates play an important role in the migration of actinides in the environment, and thus are relevant for handling and disposal of radioactive wastes, including spent nuclear fuel and vitrified raffinates. Knowledge of the crystallographic structure of these compounds is important for models of the environmental migration behavior based on thermodynamic descriptions of such chemical processes. (NH4)[NpO2CO3] crystals were obtained during long-term hydrothermal treatment of Np(VI) in aqueous ammonia at 250 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show that a single-phase sample containing only Np(V) was obtained. Structural features of (NH4)[NpO2CO3] were elucidated from single crystal X-ray diffraction and confirmed by vibrational spectroscopy. The results obtained are of interest both for fundamental radiochemistry and for applied problems of the nuclear fuel cycle.

Formation of Neptunium(V) Carbonates: Examining the Forceful Influence of Alkali and Alkaline Earth Cations

“…Structural data on Pu-containing solids are even scarcer than those for Np. However, it has already been shown that the structures of K- and NH 4 + -containing Pu­(V) and Np­(V) mixed carbonates coincide. ,, Consequently, the study of Np­(V) mixed carbonates would help to understand the potential formation and crystal structures of Pu­(V) mixed carbonates with both M­(I) and M­(II) cations.…”

“…Large cations (K, NH 4 + , etc.) form mixed Np(V) carbonates with a hexagonal neptunyl layer, 18,22,23,27,28 while small cations (Na + and Li + ) form mixed Np(V) carbonates with a orthorombic anionic layer. 21,25 The radius of Ca 2+ (1.00 Å) is close to that of Na + (1.02 Å), supporting this theory.…”

“…18,22 NH 4 NpO 2 CO 3 crystallizes in the same hexagonal crystal family. 23,24 Volkov et al studied Np(V) carbonates with various M(I) cations in the structure, 20−22,26−28 highlighting morphotropic transformations from hexagonal to orthorhombic crystal structure as one moves from K to Na in the series of pentavalent Np monocarbonates with Cs + , Rb + , NH 4 + , K + , Na + , and Li + . Although structural data for NaNpO 2 CO 3 are obtained from powder diffraction patterns, the refinement of its crystal group Pm2 1 n and atom positions was limited to anhydrous NaNpO 2 CO 3 .…”

“…Initial discussion on Np chemistry in carbonate media dates back to the 1950s and 1960s, with the earliest studies describing the preparation of hydrated KNpO 2 CO 3 through chemical precipitation in K 2 CO 3 media. , The structures and composition of M (I) NpO 2 CO 3 (M­(I) = Cs + , Rb + , NH 4 + , K + , Na + , and Li + ) compounds have been revealed through thermogravimetric analysis and laboratory X-ray diffraction (XRD). − Depending on the alkali cation radius, M (I) NpO 2 CO 3 phases with different crystalline structures could be formed. KNpO 2 CO 3 crystallizes in a hexagonal crystal family similar to pentavalent Pu and Am carbonates. , NH 4 NpO 2 CO 3 crystallizes in the same hexagonal crystal family. , Volkov et al studied Np­(V) carbonates with various M­(I) cations in the structure, − ,− highlighting morphotropic transformations from hexagonal to orthorhombic crystal structure as one moves from K to Na in the series of pentavalent Np monocarbonates with Cs + , Rb + , NH 4 + , K + , Na + , and Li + . Although structural data for NaNpO 2 CO 3 are obtained from powder diffraction patterns, the refinement of its crystal group Pm 2 1 n and atom positions was limited to anhydrous NaNpO 2 CO 3 .…”