NUKLEONIKA 2004, 49(4):131-136

The formation and stabilization of reduced silver species in the regularly interstratified clay minerals, trioctahedral smectite/chlorite (tri-Sm/Ch) and dioctahedral smectite/mica (di-Sm/M), have been studied by electron paramagnetic resonance (EPR) spectroscopy. Both minerals loaded with Ag⁺ cations after degassing and dehydration were g-irradiated at 77 K and monitored by EPR as the temperature increased. Some samples were exposed to water or methanol vapor after dehydration. In both hydrated and dehydrated samples only the doublets assigned to Ag⁰ atoms were observed with no evidence of the formation of Ag clusters. However, the EPR parameters of silver atoms in both matrices are different. In tri-Sm/Ch the narrow anisotropic EPR lines overlap with the broader isotropic lines, whereas in di-Sm/M only broad lines are recorded. The hyperfine splitting - A_iso(Ag⁰) is larger in tri-Sm/Ch than in di-Sm/M. Also the stability of Ag⁰ in both clay minerals is distinctly different. Ag⁰ doublet in di-Sm/M disappears completely above 230 K, whereas in tri-Sm/Ch it is still recorded at 310 K.
It is proposed, basing on the EPR results that Ag⁰ atoms appear at different sites in both matrices: - in tri-Sm/Ch in the middle of smectite interlayer and in hexagonal cavities in the silicate sheets of tetrahedron layer and in di-Sm/M in hexagonal cavities only. When samples had been exposed to methanol before irradiation, the silver clusters become stabilized in the interlayer sites. In tri- Sm/Ch matrix the silver dimer Ag₂⁺ formed by g-irradiation at 77 K is transformed to tetrameric cluster, Ag₄³⁺ at 150 K. In di-Sm/M the radiation-induced silver agglomeration proceeds in a similar way, but with a slower rate and Ag tetramer is formed only above 190 K. In both clay minerals, Ag₄³⁺ clusters decay above 250 K.