The short-range order of ions in clay minerals: Sm3+ coordination
Résumé
Experimental techniques as neutron diffraction (ND) with isotopic substitution (NDIS), extended X-ray absorption fine structure spectroscopy (EXAFS), and quasielastic neutron scattering (QENS), in combination with molecular dynamics (MD) simulations, are usually applied to the study of noncrystalline solids, but they are also very useful for the study of complex systems, where the short range order provides an insight of its structure and dynamics. Here, they are used in the study of the coordination of the Sm3þ in the interlayer of hydrated synthetic montmorillonite and hectorite. The ND results indicate that not all oxygen atoms in the first coordination shell of the Sm3þ belong to water molecules, supporting the formation of theSm3þ inner-sphere complex.On the other hand, the other techniques suggest that the adsorbed Sm3þ cations form outer-sphere complexes with the clay surface. The hypothesis making compatible all results is that there are different Sm species adsorbed in the clay interlayer: a part of Sm is in the Sm3þ cationic form, forming outer-sphere adsorption complexes, another part is hydrolyzed and present in the interlayer space as Sm(OH)2þ, SmðOHÞþ 2 , or SmðOHÞ0 3 species. The latter are more hydrophobic than Sm3þ cations and can be dehydrated and are able to stick to the clay surface.