Hydrated halogenides of Eu(ii): structure, spectroscopy and charge density analysis

Abstract

Five hydrated halogenides of divalent europium, EuCl₂·2H₂O (tagged as Cl2), EuCl₂·6H₂O (Cl6), EuBr₂·H₂O (Br1), EuBr₂·6H₂O (Br6) and EuI₂·6H₂O (I6), were studied herein. The crystals are all isomorphous with analogous compounds of Sr. In Cl2, the metal cation is surrounded by 4 Cl⁻ anions and 4 water molecules; in Br1, it is surrounded by 7 Br⁻ anions and 2 water molecules; and in Cl6, Br6 and I6, it is surrounded by 9 water molecules, with infinite [Eu(H₂O)₆]^2∞+_∞ chains being formed. Spectroscopic properties of Cl2, Br1, Br6 and I6 were studied. Cl2 and Br1 are bright luminophores, whereas Br6 and I6 are weak emitters at room temperature. In the latter two compounds, long-wave emissions at about 550 nm were observed and theoretical calculations indicated that this was due to 6,7s → 4f transitions. Additionally, 4f → 4f transitions were observed in the excitation spectra of Br6 and I6 recorded at 77 K. Experimental charge density analysis was performed for Cl6, and the results show that Eu–O bonds in the [Eu(H₂O)₆]^2∞+_∞ polymeric aqua cation are more ionic than those in [Gd(H₂O)₉]³⁺.