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

Abstract

Measurements of Hand D diffusion in Ti0 2 • 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 1.8X 10-3 exp( -O.5geV/kT) and 3.8X 10-1 exp( -1.28eV/kT) cm 2 /sec for diffusion II and 1 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.

Hydrogen Diffusion and Stabilization in Single-Crystal VO2 Micro/Nanobeams by Direct Atomic Hydrogenation

“…Direct imaging of the nanobeams allows us, through the difference in optical properties between metallic and insulating states, to measure the rate of rapid hydrogen diffusion along the beam growth direction (rutile c-axis; monoclinic a-axis), with no detectable evidence for hydrogen diffusion in the transverse directions. In analogy with rutile TiO 2 , this anisotropy of diffusion is expected 28,29 , as we discuss below. We find that the diffusion rate along the favored direction depends very strongly on whether the material is in the monoclinic or rutile phases.…”

Dense Ceramic Membranes for Hydrogen Separation

“…Oxides containing transition metals are therefore appropriate alternatives. There are indications based on conductivity measurements that TiO 2 could be a possible candidate [83], but no direct measurements of hydrogen permeability have been reported. Titanates, in general, however, are interesting because there are a number of materials classes that accommodate oxygen vacancies and may dissolve protons.…”

Small polaron characteristics of an OD center in TiO2studied by infrared spectroscopy

“…1,2 (2) H i diffuses readily along the large interstitial channel in the c-direction, which makes its existence at room temperature puzzling. [3][4][5] (3) OH and its isotopic siblings OD and OT exhibit strong fundamental stretch modes with unusually large anharmonicity. 6 Furthermore, recent EPR experiments on neutral H i as well as on F substituting for O in mildly reduced TiO 2 show a characteristic that is unusual in most materials, especially semiconductors: the spin associated with the unpaired electron in each case is localized on a single Ti, rather than being spread out over many sites.…”