1 - The hydrogen bond and chemical parameters favouring proton mobility in solids

Summary

Proton conduction in solids via hydrogen bonding was suggested in 1950 by Ubbelohde & Rogers. Later, the Grotthuss mechanism or translocation was proposed to explain the conductivity of para-electric potassium dihydrogen phosphate, KDP, and claimed soon after also for H₃O + ClO₄^–, oxonium perchlorate, OP. Reviews that classified protonic superionic conductors, PSC, appeared from 1980 and were based successively on the ion-exchange properties of materials, their structures and their conduction mechanisms.

In this chapter, the first part (Sections 1.1–1.3) deals with the conduction mechanisms, while the second part (Sections 1.4 and 1.5) points out the significant structural and chemical factors leading to the different conduction mechanisms. The hydrogen bond is a common feature and serves as Ariadne's thread.

From ionic to protonic conduction

Ionic conduction

Ionic conductors may be divided into three classes depending on their defect concentrations (i) dilute point defects, dpd (∼ 10¹⁸ defects cm^–3), (ii) concentrated point defects, cpd, (∼ 1O^18–20 cm^–3) and (iii) liquid-like or molten salt sublattice materials, mss, (∼ 10²² cm^–3).

For dilute and concentrated point defect materials, examples are, respectively, NH₄C;O₄ (σ_250°C = 10^–9 Ω^–1 cm^–1, E = 1.4 eV) and CeF₃ (σ_24o°c = 6 × 10^–4 Ω^–1 cm^–1, E = 0.26 eV). The conductivity occurs by an ion hopping mechanism.

In molten salt sublattice materials, practically all the ions in the sublattice are available for motion with an excess of available sites per cation as in e.g. Na β-alumina, (σ_25°c = 1.4 × 10^–2 Ω^–1 cm^–1, E = 0.16 eV).