MECHANISMS FOR HYDROGEN DIFFUSION IN TIO2

The predominant hydrogen-containing species in TiO2 (rutile) are OH- (hydroxyl) ions, in which the oxygen occupies a regular oxygen ion site, and the O-H bond is perpendicular to the c axis. It is proposed that diffusion of hydrogen parallel to the c axis proceeds by a proton jump from one O2- ion to another along the channel as represented by OH-O2-O2-H-O2-O2-HO-. It is also proposed that diffusion perpendicular to the c axis proceeds by a rotation of the OH- bond to move the proton from one channel to an adjacent channel, followed by a proton jump to another O2- ion in the same channel. From a potential-energy model, which includes a Morse function to represent the OH- bond, as well as electrostatic and repulsive terms, the activation energies for hydrogen and tritium diffusion parallel to the c axis were calculated to be (including a zero-point energy correction) 0.60 and 0.69 eV, respectively, in good agreement with the respective experimental values of 0.59 and 0.75 eV. The calculated activation energy for diffusion perpendicular to the c axis was 1.23 eV (no zero-point energy correction), as compared to the experimental values of 1.28 and 1.11 eV, respectively, for hydrogen and tritium. The calculated equilibrium orientation of the OH- ion in TiO2 and the calculated stretching frequency of this species were also in good agreement with the respective experimental results. © 1979 The American Physical Society.