  Proton motions in hydrated proton conducting perovskites
BaZr_{0.90}A_{0.10}O_{2.95} (A = Y and Sc) have been investigated using
quasielastic neutron scattering. The results reveal a localized motion on the
ps time scale and with an activation energy of ~10-30 meV, in both materials.
The temperature dependence of the total mean square displacement of the protons
suggests an onset of this motion at a temperature of about 300 K. Comparison of
the QENS results to density functional theory calculations suggests that for
both materials this motion can be ascribed to intra-octahedral proton transfers
occurring close to a dopant atom. The low activation energy, more than ten
times lower than the activation energy for the macroscopic proton conductivity,
suggests that this motion is not the rate-limiting process for the long-range
proton diffusion, i.e. it is not linked to the two materials significantly
different proton conductivities.
