New proton dynamics in solids revealed by vibrational spectroscopy with neutrons

Recent studies with the incoherent inelastic neutron scattering (INS) technique of proton dynamics in various solid-state protonic conductors are reviewed. Free recoiling entities with proton mass have been observed in different materials: manganese dioxides, coals, graphite nitric-acid intercalation compounds, polyanilines and polypyrolles, beta-alumina. The electric charges of these entities and their interaction with the electrons of the host lattice are tentatively related to protonic and electronic conductivity. New protonic species have been characterised. In gamma-MnO2, charge compensating protons in Mn4+ vacancies form (H+)(4) entities, with tetrahedral structure, rotating freely inside oxygen octahedra. In beta-alumina, protonic entities are amenable to a tetrahedral structure instead of the trigonal structure anticipated for oxonium ions. Hydroxonium ions are not observed in hydrated analogues. (C) 1999 Elsevier Science B.V. All rights reserved.