Protonic Defects and Their Transport

we consider the protonic state of hydrogen. A proton is an elementary positive particle with radius of only m (10 times smaller than any other chemical species). Protons cannot persist in the free state in a chemical environment but are attracted into nearby electron clouds. In oxides and other oxidic materials, attraction will be to oxygen anions to form the hydroxide ion, OH. As such, the proton residing on a normal oxygen ion can be considered as an interstitial proton, Hj (but with the interstitial site strongly displaced towards a particular oxygen ion) or as a substitutional hydroxide ion, OHq. 

The predominating transport mechanism for such protons is by free proton jumps (Grotthus mechanism) between neighboring oxygen ions although, statistically, a concentration of oxygen vacancies will enable some protons to move as OH ions when the host oxygen ions jump to vacancies. 

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