Location and lattice dynamics of a proton in the perovskite structure (Phys. Status Solidi B 9/2016)

In the history of physics, many of the physical laws and concepts are found deeply rooted in geometry. The work by Deng and Zhang (pp. 1688–1696 ) shows another example: based on a simple but solid experimental fact, a geometrical analysis can be used to characterize the domain of a proton location in a perovskite structure ABO 3 by using a hard‐sphere point charge model. Two intra‐ and one inter‐octahedron processes with different energy barriers of ⟨ E 1 ⟩, ⟨ E 2 ⟩ and ⟨ E 3 ⟩ have been studied, respectively. The order of the energy barriers is predicted as ⟨ E 3 ⟩ > ⟨ E 1 ⟩ ≥ ⟨ E 2 ⟩ by studying the bonding changes in these processes and the charge imbalance between the acceptor dopant ion and the B type ion. The inter‐octahedron hopping, ⟨E 3 ⟩, being predicted as temperature and doping dependent, is shown to be a necessary and rate limiting step by using percolation and charge imbalance argument. The predicted energy order agrees quite well with the result of a quasi‐elastic‐neutron scattering experiment which however is in contradiction with some earlier theoretical results. The small polaronic behaviour of a proton in a perovskite structure as well as the red‐shift and broadening of the O–H infrared absorption peak are studied by using several newly developed formulae. These phenomena are revealed as indications of the quasi‐free state of a proton instead of the formation of hydrogen bond, an observation recognized also by many experimentalists.