Diffusion of H and D in TiO2: Suppression of internal fields by isotope exchange

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Abstract
Measurements of H and D diffusion in TiO2, using the isotope exchange technique described in the preceding paper, are reported. Use of this technique resulted in diffusion which was accurately described by Fick’s law with a constant diffusion coefficient, as predicted theoretically, in sharp contrast to single ion diffusion, where dramatic departures from classical diffusion theory were observed. The measured diffusion coefficients for H were 1.8×10−3 exp(−0.59eV/kT) and 3.8×10−1 exp(−1.28eV/kT) and 3.8×10−1 exp(−1.28eV/kT) cm2/sec for diffusion ∥ and ⊥ to the c axis, respectively. Ionic conductivity measurements are reported, which agree well with the bulk diffusion measurements, and permitted us to extend the temperature range of the measurements for c−axis diffusion from 125 to 750 °C, corresponding to a range of more than four orders of magnitude in D. The measured diffusion parameters were found to be essentially independent of sample purity, although it was observed that significant concentrations of lattice defects sharply inhibited H diffusion.