The short range order of ions in clay minerals: Sm3+ coordination

Experimental techniques as neutron diffraction with isotopic substitution, extended X-ray absorption fine structure spectroscopy and quasielastic neutron scattering, in combination with molecular dynamics simulations, are usually applied to the study of non-crystalline 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 Sm3+ in the interlayer of hydrated synthetic montmorillonite and hectorite. The neutron diffraction results indicate that not all O atoms in the first coordination shell of the Sm3+ belong to water molecules, supporting the formation of the Sm3+ innersphere complex. On the other hand, the other techniques suggest that the adsorbed Sm3+ cations form outersphere 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 Sm3+ cationic form, forming outer-sphere adsorption complexes, another part is hydrolyzed and present in the interlayer space as Sm(OH)2+, Sm(OH)+ 2 or Sm(OH)03 species. The latter are more hydrophobic than Sm3+ cations and can be dehydrated and are able to stick to the clay surface.