M.I. de Barros Marques

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

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.

The structure of the first coordination shell of the yttrium ion in concentrated aqueous solutions of YBr3 and YCl3

Yttrium hydrates with eight-coordinated water molecules seem to be the predominant species in concentrated aqueous solutions of yttrium bromide and chloride. For molarities from approximately 2.1 down to 0.6M in H2O EXAFS experiments yield an average Y3+-O distance of 2.33+or-0.02 AA for the YBr3 and 2.34+or-0.02 AA for the YCl3. Investigations by means of Raman spectroscopy with substitution of the solvent by D2O for a 2.1M solution of YCl3 lead to an isotopic shift of 1.052+or-0.007 for the polarized band assigned to the stretching vibrational mode of the complex, which corroborates the idea of the presence of a (relatively) stable hydrate of the yttrium ion.

Local order in concentrated aqueous solutions of yttrium chloride and bromide

X-ray diffraction patterns of equimolecular concentrated aqueous solutions of yttrium chloride and bromide have been investigated from s=0.4 AA-1 up to s=15 AA-1. An assumed coordination of eight water molecules around each yttrium cation, at a distance of 2.36 AA, reproduces the intensity at high s values in agreement with the values obtained by some of the present authors from EXAFS experiments. Narrow interference maxima at s=(4 pi sin theta )/ lambda approximately=0.9 AA-1 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 the hydrated cations were built to simulate that local order.

EXAFS studies of local order in InBr3 aqueous solutions

EXAFS measurements at the In and Br k edges are reported for InBr3 aqueous solutions at various concentrations. The experimental results are in agreement with the structure of indium complexes proposed on the grounds of X-ray diffraction measurements for a concentrated aqueous solution of indium bromide. The analysis of the Fourier-filtered spectra shows that these complexes persist at least until the 1M concentration has been reached.