No 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.

Ahstnet X-ray diffraction panems of equimolecular concentrated aqueous solutions of yttrium chloride and bromide have been investigated from s = 0.4 A-' up to s = 15 .&-I, An assumed coordination of eight water molecules around each ymium cation, at a distauce of 2.36 A. reproduces the intensity at high s values in agreement with the values obtained by some of the present authors from WAFS experiments. Narrow interference maxima at s = (4n sin0)jX E 0.9 A-' may be interpreted as the existence of a local order of the hydrated yttrium cations surrounded by anions and free water molecules. Structural models with and without close packing of lhe hydrated cations were built to simulate fhat local order.

Concentrated aqueous solutions of salts constituted by divalent ions combined with monovalent counter-ions were investigated by x-ray diffraction and Raman spectroscopy at room temperature. The salts studied were strontium, barium, and magnesium chlorides and bromides, and lithium and caesium sulphates. For many of these solutions, intensity maxima were detected in their x-ray diffraction patterns close to 0.6–1 Å−1. Interpretation of these maxima, pre-peaks, is discussed, taking into account the results of previous investigations of the authors on concentrated aqueous solutions of trivalent cations. In these solutions the pre-peaks appear to be narrower and much more intense. In the ionic solutions studied here, a slight contrast between values of the scattering power of two spatial domains is suggested as the origin of the observed pre-peak. One of these comes from an accumulation of scattering power around the ion with the higher electric charge, locally organized into a subtle close packing; the other comes from the holes of this structure. Molecular models of the structure of these electrolytes are used to demonstrate the plausibility of this interpretation.