Loading protons into the perovskite structure depends on acceptor doping at the B site [26]. Acceptor doping, such as when In is substituted for Zr in BaZrO3, creates an oxygen-deficient structure, which under elevated temperature and humidity absorbs water dissociatively so that oxygen vacancies fill with hydroxyl groups and the remaining protons bind to other oxygens in the structure [26]. In the Kroger — Vink notation [27] this reaction is written

H2O + Vo + O*, 2(OHO), (9.1)

where VO denotes an oxygen vacancy, O* denotes a lattice oxygen, and OHO denotes a proton bound to a lattice oxygen (the superscripts • and x denote positive and neutral charges, respectively). The protonation process is shown schematically in Fig. 9.3b. At elevated temperature, the protons are not stuck to any particular oxygen but move rather freely from one oxygen to another, as described further below, resulting in a high conductivity of the material.