PROTON DIFFUSION MECHANISMS

Mobility of H-atoms in hydrogen oxide bronzes and related systems has excited considerable interest in view of their possible applications in e.g. catalysis and electrochromic electrodes for display devices. In practical materials the structure of the host oxide may be ill-defined and intercrystallite regions may contribute significantly to observed (bulk) mobilities. This article aims to present a consistent picture (derived from the published work of many scientists internationally) of the atomic level phenomena responsible for intracrystallite H-atom mobility in well-characterized single phases. The detailed characterization of such phases is discussed by Dickens Chippindale (Chapter 7). 

Measurements have included relaxation times, and absorption spectra and (in cases where lines are already narrowed by translation of protonic species) application of combined magic-angle-rotation and multiple-pulse line-narrowing CRAMPS NMR). For the fastest self-diffusions (D 10 cm s ) direct measurement of D is possible using the pulsed field gradient (PFG) technique (see Chapters 26 28). 

Correct interpretation of variable temperature relaxation time measurements at more than one spectrometer operating frequency can lead to characterization of a dynamical process (e.g. for fast self diffusion, a continuous increase is seen in T2 at high temperatures and T2 7 above the minimum in 7, i.e. for D 10 cm s ). Results for phases with slower motions can be ambiguous due to sample decomposition at higher temperatures. 

For in-depth treatments of the backgrounds to these techniques the reader is referred to the books by Wolf (relaxation times), Abragam (broadline spectra) and Mehring (high resolution spectra of solids). 

For an in-depth treatment of the application of QNS techniques the reader is referred to the book by Bee  

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Choi, P. Jalani, N.H. Datta, R. Thermodynamics and proton transport in Nafion II. Proton diffusion mechanisms and conductivity. J. Electrochem. Soc. 2005, 152 (3), E123-E130. 

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