General introduction

For solid protonic conductors the broad term ‘protonic species’ includes all those hydrogen-containing entities formed, perhaps only transiently during the conduction process, by ionic or molecular groups of sufficient basicity. This description includes the wide range of anionic protonated species in anhydrous protonic conductors: OH–, protonated tetrahedral ions such as HSeO4–, HPO42– and dimeric XO4 groups such as H(SeO4)23–, in addition to the cationic proton hydrates [H(H2O)n+, including H3O+, H5O2+, H7O3+ etc., and combinations of hydrogen with nitrogen: ammonium NH4+, hydrazinium N2H5+ and hydrazinium (2 +) (dihydrazinium or hydrazonium), N2H62+. Structural and spectroscopic data on the anionic protonic species encountered in anhydrous materials are included in later chapters and will not be referred to further here, except in cases where the counterion is ammonium or a proton hydrate.

Detailed information on the proton hydrate series in the solid state is available, and the reader is referred to articles reviewing structural studies and vibrational spectroscopic studies of proton hydrates, but also general overviews, the most recent being the comprehensive review of 1986.

The extensiveness of the proton hydrate series is not mirrored in the analogous nitrogen containing family. Of the isoelectronic pairs OH3+/NH4+ and O2H5+/N2H7+, in the solid state only the former is known.