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The superionic conductor Ag3SI was studied by129I-Mossbauer spectroscopy in the rhombohedral γ-phase and in the cubic β-phase at temperatures between 4.2 K and 180 K. In the low-temperature γ-phase, one observes well below the γ-β phase transition at 157 K a motional-narrowing-like decrease of the electric-quadrupole interaction, roughly linear with temperature, which points to a diffusional motion of the Ag+ ions already in the γ-phase. Diffusional effects are also reflected by anomalous decrease of thef-factor in the same temperature region, which originates from a restricted short-range diffusional displacement of the iodide ions induced by the hopping Ag+ ions.

The127I- and129I-Mossbauer resonances are applied to study the chemical nature of the interhalogen molecules ICI and IBr in stage-1 and stage-2 graphite intercalation compounds (GICs) and, for sake of comparison, in their solid molecular modifications. For both GIC systems one observes, in support of a model for the intercalate structure derived from recent X-ray absorption studies, two sites with an intensity ratio 1∶1 and with similar isomer shifts and electric-quadrupole splittings. The charge transfer mechanisms are discussed in conjunction with complementary X-ray absorption and photoemission studies.