Hydration of Undoped Perovskites

Although such equiUbria for undoped oxides are, as already said, necessarily a basis for dissolution of protons in the doped cases, their thermodynamics—a combination of hydration of oxygen vacancies and Schottky equilibrium—is little studied. 

5 Proton Conductiyity in Selected Classes Of Non-Perovskite Oxides and Phosphates 

Proton solubility and conduction are known also for many non-perovskite classes of oxides, comprising mainly fluorite-related structures and structures with oxide ion tetrahedra. 

Among the fluorite-related oxides, the MO2 oxides (M = Zr, Hf, Ce) exhibit very little bulk solubility of protons, and the acceptor-doped oxides remain pure 

There has recently been much interest around proton conduction in condensed phosphates. Lanthanum metaphosphate (La(P03)3) exhibits a modest proton conductivity [59] whereas diphosphates of tetravalent metals, e.g., SnP207 and TiP207, appear to exhibit a high proton conduction peak at intermediate temperatures (around 200°Q. The effect is reportedly enhanced by substituting In for the tetravalent cation, and the conductivity can exceed 0.1 S/cm [60]. It is uncertain what is the defect or doping mechanism behind these behaviors. The same materials exhibit a lower, temperature-dependent conductivity above 400°C, tentatively attributed to protons from hydrolysis of the diphosphate groups [61]. 

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