LOCAL ENVIRONMENT OF INTERCALATED LANTHANIDE IONS IN VERMICULITE

The local structure of interlayer lanthanide ions ion-exchanged into vermiculite clay has been determined as a function of the nature of the lanthanide ion and of the physical form of the sample (powders and oriented films) using X-ray absorption spectroscopy. Water molecules form the first coordination shell, with metal-oxygen bond lengths of 2.52 (La-O), 2.53 (Ce-O), 2.32 (Er-O) and 2.33 angstrom (Lu-O), very similar to the corresponding interatomic distances in aqueous lanthanide nitrate solutions. However, the coordination numbers of the heavier lanthanide ions are greater than those of the lighter ones, in contrast to the behaviour observed in solution. This variation is attributed to the participation of coordinated water in the transmission of local charges on the lanthanide ions to the vermiculite layer surface. Across the lanthanide series a significant difference in coordination number between La-vermiculite powder (6 neighbours) and film (9 neighbours), is reduced in the cerium sample (7.5 and 9, respectively) and reached a negligible variation in the heavier (Er, Lu) vermiculites. This is attributed to a diffusion phenomenon of water within the vermiculite interlayers. Preliminary a.c. conductivity measurements of Lu-vermiculite films are in the range of 10(-6) S cm-1.