Vibrational characterization of the various forms of (solvated or unsolvated) mobile proton in the solid state. Advantages, limitations and open questions

A didactic review of (Raman, infrared and neutron) vibrational spectroscopy procedures to study mobile protonic species (H2O, H3O+, H5O2+, “H+”, NH4+, etc.) in solid hydrates, crystals and ceramics is proposed on the basis of paper published since 1970s. Three materials are taken as representative examples: hydrated uranyl phosphate (HUP), oxonium, hydroxonium and ammonium beta- and beta“-aluminas and nominally anhydrous strontium/barium zirconate perovskite. Particular attention is given to the advantages of isotopic substitution and dilution measurements as a function of temperature and partial ion exchange. The vibrational signatures that we consider as being able to serve as references for the different kinds of proton species observed in the proton conductors are presented. We also discuss the signature of protonic species giving no vibrational signature - or whose Raman and infrared signatures are too weak to be clearly detected – that need to be better characterized and understood. The presence of a strong incoherent inelastic neutron scattering background appears characteristic of (mobile) proton conductors.