Structural investigation of the aqueous Eu²⁺ ion: Comparison with Sr²⁺ using the XAFS technique

Structural parameters of the Sr²⁺ and, for the first time, of the Eu²⁺ ions in aqueous solution were determined by the XAFS method. For the Sr²⁺, 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 σ² = 0.0126 (5) Ų. These results were confirmed by an analysis performed with experimental phase and amplitude, extracted from the solid reference compound [Sr(H₂O)₈](OH)². The same theoretical approach was used for the analysis of the Eu²⁺ XAFS spectra in aqueous solution. This gives a first coordination shell of Eu²⁺ formed by 7.2 (3) water molecules, an Eu-O distance of 2.584 (5) Å, and a high Debye-Waller factor of σ² = 0.0138 (5) Ų. Whereas Eu³⁺ occurs as an equilibrium between the [Eu(H₂O)₈]³⁺ and the [Eu(H₂O)₉]³⁺ ions, Eu2+ occurs in aqueous solution as an equilibrium between a predominant [Eu(H₂O)₇]²⁺ ion and a minor [Eu(H₂O)₈]²⁺ species.