Incorporation of cobalt and nickel into uranium oxide hydrates: growing needles in a sodium haystack

• Uranium oxide hydrates (UOH) incorporating Na + and Ni 2+ or Co 2+ have been synthesised • Achieved with rational design via the doping of Ni 2+ or Co 2+ into the Na-UOH system • Synchrotron crystallography revealed the detailed structures of these materials • Flexibility of the U 3 O 8 -type layer allows for the inclusion of both Na + and Co 2+ /Ni 2+ • Broadens the current knowledge of dual-cation UOH materials and SNF alteration The synthesis of two isostructural uranium oxide hydrate (UOH) materials, containing both sodium and either cobalt or nickel ions, has been achieved through a rational synthesis design. Through deliberate modification of a previously reported procedure to obtain a sodium-bearing UOH, the compounds Na 2 Co(H 2 O) 4 [(UO 2 ) 4 O 3 (OH) 4 ] 2 ( UOH-Co ) and Na 2 Ni(H 2 O) 4 [(UO 2 ) 4 O 3 (OH) 4 ] 2 ( UOH-Ni ) were synthesized, with their crystal structures obtained via synchrotron single-crystal X-ray diffraction and their vibrational properties revealed using Raman spectroscopy. Both materials were isostructural to a previously identified Mg-containing UOH material, with their U 3 O 8 -type layer taking on corrugated topology, allowing for the incorporation of both Na + and Co 2+ /Ni 2+ as the interlayer species. These new compounds, achieved through the addition of their corresponding acetate salts to the reaction solution, were isolated from a mixed phase system also containing crystals of Na 2 [(UO 2 ) 3 O 3 (OH 2 )] ( UOH-Na ) and α-UO 3 , highlighting the complex reactivity that is present under these hydrothermal conditions. Also revealed was the structural flexibility adopted by the Na + cations when incorporated into these systems, broadening the current knowledge of dual-cation UOH materials and their role in the alteration chemistry of spent nuclear fuel under hydrated and oxidative conditions in a geological setting.