“…2 Among the various oxidation numbers available for actinide elements relevant to nuclear fuel recycling, the tetravalent state, An(IV), is highly important especially in terms of Pu 4+ separation by solvent extraction with tri-n-butyl phosphate (TBP). The limiting An 4+ complexes with NO 3 À are [An(NO 3 ) 6 ] 2À (An ¼ Th, [3][4][5][6][7] U, 8,9 Np, 10,11 Pu 12-14 ), which usually crystallize with various countercations having relatively low hydration enthalpy like heavy alkali metals and quaternary ammonium ions (NR 4 + , R ¼ H, alkyl). 15 To our best knowledge, all the reported compounds of An(IV) usually exhibit original colours dened by their respective 5f electronic congurations.…”