Vibrational Lifetimes and Decay Mechanisms of O-H Stretch Modes in Metal Oxide Proton Conductors

In proton conducting metal oxides, measurements of the O-H and O-D vibrational lifetimes show that proton transfer processes successfully compete with multi-phonon decay, in contrast to conventional semiconductors, such as Si or Ge, where the latter dominates. In perovskite KTaO$_{3}$, the O-H stretch mode decays via a phonon-assisted proton-tunneling process involving the O-Ta-O bending motion. In rutile TiO$_{2}$, the local oscillatory motion of the proton quickly couples to a wag-mode-assisted classical transfer process along the c-channel with a jump rate of $>$1 THz. The transfer rates in these oxides are 7 to 9 orders of magnitude larger than that due to thermally activated diffusion at room temperature which is significant for renewable energy applications of proton conducting oxides.