Superionic protonic conductivity

Modelling of superionic conduction has received relatively little attention. Experimental studies, concerned mainly with the measurements of 

We would like to thank Dr J. F. Gouyet for fruitful discussions and critical reading of the manuscript. 

Ngai and C. B. Wright (eds), Relaxation in Complex Systems (U.S. Government Printing Office, Washington DC (1985)) 205-20. 

Dieterich, in High Conductivity Solid Ionic Conductors, Recent Trends and Applications, T. Takahashi (ed) (World Scientific, Singapore (1989)) 17-44. 

Cheradame, in lUPAC Macromolecules, H. Benoit and B. Rempp (eds) (Pergamon Press, Oxford (1982)). 

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The rearrangement of the hydrogen-bonded network at the superionic phase transitions studied in the framework of the lattice-gas-type model, which has been described in the previous subsection, makes it possible to evaluate the main aspects of proton conductivity in the mentioned compounds. 

The numerical estimate [150] of the proton conductivity (195) derived in the framework of the model described above is in good agreement with the experimental results. This is the direct theoretical justification of the experimentalists remark [152,153,157] that the reorientation of ion groups should assist the proton transport in some superionic conductors. 

Therefore, in cases when the value of J is too small, the proton conductivity can still be very substantial due to the fluemating absorption and radiation of phonons by charged carriers. This allows also the application of the approach developed above to compounds in which hopping of heavy carriers occurs by comparatively distant structural groups (for example, 0.4 to 0.8 nm, which takes place in some superionic crystals). 

J.J. Kweon, R. Fu, E. Steven, C.E. Lee, N.S. Dalai, High field MAS NMR and conductivity study of the superionic conductor LiH2P04 critical role of physisorbed water in its protonic conductivity, J. Phys. Chem. C 118 (2014) 13387—13393. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

In good conductors, the diffusion coefficient can attain a value similar to that in aqueous solution or in the molten state ( 10 cm s" ). The corresponding value of the ionic mobility is about 10 cm /Vs. This has been observed for many non-protonic conductors (the main examples are given in Table 4.1) and for some protonic materials (Table 4.2). If the number of mobile ions is suflSciently high, superionic conductivity occurs. ... 

Ph. Colomban and A. Novak, Proton transfer and superionic conductivity in solids and gels, J. Mol. Struct. Ill (1988) 277-308. 

Chemists often reason in terms of dielectric permittivity (e) or infrared spectra (a) while electrical engineers and physicists consider tangent loss (tg = e"/s ) and alternative current conductivity (free charges (where the complex impedance formalism is well-suited) and bound charges (where the description of the permittivity s = e (co) — ie"(co) is more suitable) can be distinguished. Such a distinction is not straightforward in superionic conductors, however, in which some conducting species are in a quasi-liquid state and in particular, for proton conductors. In the latter, the proton (attached... 

These models appear adequate for metal hydrides or non- or poorly-conductive halides (oxides). However, the hopping model is of limited value in superionic conductors since hopping and residence times are not so very different. Nevertheless, in the case of protonic conductors, it can... 

Progress in the understanding of superionic conduction is due to the use of various advanced techniques (X-ray (neutron) diffuse scattering, Raman spectroscopy and a.c.-impedance spectroscopy) and-in the particular case of protons - neutron scattering, nuclear magnetic resonance, infrared spectroscopy and microwave dielectric relaxation appear to be the most powerful methods. A number of books about solid electrolytes published since 1976 hardly mention proton conductors and relatively few review papers, limited in scope, have appeared on this subject. Proton transfer across biological membranes has received considerable attention but is not considered here (see references for more details). 

A nonstoichiometric mixture was produced from a poly-oxometalate and a heteropolyadd to make a proton conductor with the general formula of H3 xM POM. On the Walden plot, this mixture is above the line predicted by the classical Walden rule and in the superionic liquid region, indicating conduction is occnning by a more efficient conduction mechanism than the Walden mechanism, possibly by the Grotthus mechanism. ... 

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