The dynamical picture of the proton-conduction mechanism in hydrated perovskites started to emerge in the mid-nineties and is mainly based on results obtained from molecular-dynamics simulations [28-31] and quasielastic neutron scattering [32, 33]. On a local scale, the protons jump between neighbouring oxygens, with an intermediate reorientational motion of the — OH group in between jumps (a sche­matic is shown in Fig. 9.3c), whereas at the longer length-scale the protons diffuse as a series of such jumps and reorientations. However, the effect of dopant atoms on the local chemistry and structure, as well as the resultant symmetry reduction and proton-defect interactions, complicate the description of the proton conductivity and such effects are not completely understood for even the simplest perovskite systems.

A key parameter for long-range proton diffusion is the hydrogen bonding experienced between a proton and a neighbouring oxygen, since the transfer between neighbouring oxygens is a hydrogen-bond mediated process, whereas the reorientational motion of the — OH group requires the breaking of such bonds. The proton diffusivity is further affected by the vibrational dynamics of the proton. More specifically, the proton performs localized O-H stretch and O-H wag vibrations, which may be seen as precursors to the transfer and reorientational step, respec­tively. There is also a coupling of the proton motions to the phonons of the perovskite host lattice [30, 34]. Hence, the hydrogen bonding, vibrational dynamics, and long-range proton diffusion are, most likely, strongly correlated.