Orientationally degenerate (OH)$^-$ centers in perovskite-type proton-conducting oxides: Low-temperature thermodynamics and H/D isotope effects

It is shown that proton-associated dipole centers $\mathrm{(OH)^-}$ can provide significant, glass-like, contributions to the low temperature properties of perovskite-type proton conducting oxides, such as $\mathrm{ABO_{3-y}}$ doped with cations of lower valence. These contributions result from the splitting of the orientationally degenerated states of dipole centers by random crystal fields and protons tunneling. It is demonstrated that H/D (or H/T) substitution leads to large and abnormal isotope effects for the properties caused by $\mathrm{(OH)^-}$ degenerated states.