The goal of this project is to explore the microscopic dynamics of hydrogen ions in proton conducting oxide materials. Proton conduction is a fundamental process that has attracted considerable attention based on important developments and applications in hydrogen energy research; therefore experimental characterization of transition rates and migration mechanisms is highly valuable. We employ an optical picosecond pump-probe technique that directly measures the vibrational lifetimes of hydrogen related defects. These measurements reveal that proton migration is highly coupled to infrared photon absorption and suggest a means to enhance proton conduction in important device applications.

Important Results:

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>>Proton Tunneling in KTaO3

We have found that the vibrational lifetimes in the perovskite oxide KTaO3 are extremely long lived and, unlike the semiconductors, the stretch mode decays by tunneling to the next near neighbor oxygen ion. The tunneling process in found to be phonon assisted-the vibration of the surrounding lattice alters the barrier shown in the figure to promote a higher tunnel probability.

The excited state proton-tunneling rate (kpt) has been extracted from the vibrational lifetime and shows a rate constant of about 10¹⁰ s^-1 at room temperature. This excied-state tunneling rate is found to be 7 orders of magnitude large than from the ground state in a similar proton conduction oxide [Phys. Rev. B 60, R3713 (1999)]

Reference:
E. J. Spahr, L. Wen, M. Stavola, L. A. Boatner, L. C. Feldman, N. H. Tolk, G. Lüpke, Proton Tunneling: A decay channel of the O-H stretch mode in KTaO3, Phys. Rev. Lett. 102, 2009, pp. 075506-9.

Funding: NSF, DoE, Jeffress Foundation