Local order in aqueous solutions of lanthanum chloride and bromide by x-ray diffraction, EXAFS and Raman spectroscopy

Abstract

Concentrated aqueous solutions of lanthanum chloride (3.4 molal) and bromide (3.2 molal) were investigated by x-ray diffraction, EXAFS and Raman spectroscopy. The observation of narrow maxima of intensity in the diffraction patterns is well interpreted as arising from positional correlations between the hydrated cations present in these solutions. Raman spectra, exhibiting a single polarized band (~350 cm-1), suggest that only one kind of aggregate is observed in these solutions. EXAFS results allowed us to establish the distances of (plausibly) nine water molecules around each lanthanum cation. The persistence of lanthanum hydrates has been followed from very concentrated solutions (~3 molal) up to diluted ones (~0.5 molal) by x-ray diffraction as well as by EXAFS. Previous investigations are discussed, too.