Raman spectral studies of uranyl sulfate and its urea complex structural isomers

Using Raman spectroscopy, the authors have studied two forms of the dioxouranium(VI) sulphate complexes of the urea molecule, the bis(urea)dioxouranium(VI) sulphates α- and β-UO₂SO₄·2CO(NH₂)₂. Raman spectra were recorded using λ = 488 and 647.1 nm laser excitation lines. For λ = 448 nm, the crystals exhibit strong, structured luminescence, with peaks separated by 857 cm⁻¹ (α-form) and 853 cm⁻¹ (β-form). For λ = 647.1 nm, very intense Raman spectra are obtained which are similar but slightly different and can be used to distinguish each form. A comparison with the urea and uranyl sulphate hydrate (USH) spectra has clarified the Raman band assignment of the two parent α- and β forms. Contributions of the urea donor ligand and the sulphate have been identified. Charge transfer to uranium and the uranium—oxygen force constant in the uranyl ion have been evaluated. The results agree with previous X-ray structural investigations.