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 solids are reviewed. Simulation of spectral intensities reveal that vibrational dynamics for protons are better represented with localised modes, rather than with conventional normal modes. Tunnelling transitions are observed for proton transfer along hydrogen bonds in various systems, either ionic in nature, like KHCO3, or molecular, like polyglycines. The phonon assisted tunnelling model is not confirmed. Free recoiling entities with proton mass have been observed in different solid-state protonic conductors. Electric charges of these entities and interaction with 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 tetrahedral structures instead of the trigonal structure anticipated for oxonium ions. (C) 1999 Elsevier Science B.V. All rights reserved.