Structural Investigation of the Aqueous Eu2+ Ion: Comparison with Sr2+ Using the XAFS Technique

Structural parameters of the Sr 2+ and, for the first time, of the Eu 2+ ions in aqueous solution were determined by the XAFS method. For the Sr 2+, the use of an improved theoretical approach led to a first shell coordination number of 8.0 (3), a Sr−O distance of 2.600 (3) Å and a Debye−Waller factor of σ 2 = 0.0126 (5) Å 2 . These results were confirmed by an analysis performed with experimental phase and amplitude, extracted from the solid reference compound [Sr(H 2 O) 8 ](OH) 2 . The same theoretical approach was used for the analysis of the Eu 2+ XAFS spectra in aqueous solution. This gives a first coordination shell of Eu 2+ formed by 7.2 (3) water molecules, an Eu−O distance of 2.584 (5) Å, and a high Debye−Waller factor of σ 2 = 0.0138 (5) Å 2 . Whereas Eu 3+ occurs as an equilibrium between the [Eu(H 2 O) 8 ] 3+ and the [Eu(H 2 O) 9 ] 3+ ions, Eu 2+ occurs in aqueous solution as an equilibrium between a predominant [Eu(H 2 O) 7 ] 2+ ion and a minor [Eu(H 2 O) 8 ] 2+ species.