Ion–ion and ion–water interactions in an aqueous solution of erbium bromide (ErBr3). A differential anomalous x-ray scattering study

A differential anomalous x-ray scattering (DAS) study of a three molal aqueous solution of erbium bromide $({\mathrm{ErBr}}_{\mathrm{3}})$ is reported. X-ray scattering at various energies below the $L_3$ edge of erbium and the $K$ edge of bromine was used to elucidate the ion–ion and the ion–water interaction in this solution and to get more precise information on the local environment of both ions. After an intricate data evaluation procedure the differential atom pair correlation function (DAPCF) and its “negative image” the complement atom pair correlation function (CAPCF), were obtained. The analysis of the DACPF and the CAPCF provides no considerable inner-shell complexing of the cation and the anion, but a well defined and stable hydratation sphere around the triply charged ${\mathrm{Er}}^{\mathrm{3+}}$ –ion was found, which is in good agreement with results of conventional x-ray studies, extended x-ray absorption fine structure (EXAFS) measurements and Raman spectroscopy. In addition the analysis provides ${\mathrm{Er}}^{\mathrm{3+}}{\mathrm{–Br}}^{\mathrm{1-}}$ interaction in the second coordination sphere and an overlap of the hydratation spheres of cat- and anions as well as of two anions. On the basis of these results a two-dimensional model of the ion–ion and the ion–water interaction is presented.