11 - Anhydrous materials: oxonium perchlorate, acid phosphates, arsenates, sulphates and selenates

Summary

Proton-containing anhydrous materials are interesting proton conductors as they are stable to relatively high temperatures and contain fewer protons that do hydrates. Their conduction mechanism is thus expected to be easier to understand, and the proton diffusion may be investigated over a rather wide temperature range. However, the conduction mechanism can become complicated when the ‘melting’ temperatures of the protonic species and the non-protonic sublattice are close.

Oxonium perchlorate

Oxonium perchlorate H₃OClO₄ is the first solid proton conductor whose conductivity, σ_300K = 3.4 × 10^–4 Ω^–1 cm^–1, has been found to be comparable to that of a liquid electrolyte. It is also one of the first materials in which the oxonium ion has been identified. The compound is very hygroscopic and photosensitive. A glassy state can be obtained by quenching the molten salt (T_m = 55 °C)³. The crystalline phase undergoes a first order transition on cooling, at 249 K, with a slight volume increase. The low temperature phase is monoclinic with P2₁/n space group and four formula units in the unit cell. The structure consists of layers of oxonium and perchlorate ions hydrogen bonded by two short (O … O = 0.263 and 0.261 nm) and one long (O … O = 0.271 nm) hydrogen bond (Fig. 11.1).