Fast ionic conductors

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... 

Considerable spread is also observed in reported enthalpies of transition in single-component systems. As an example, the reported enthalpy of the first-order transition giving the fast ionic conductor phase of Agl at 420 K are compared in Table 10.5. In general, the agreement between the results obtained by adiabatic or... 

A large body of literature has been accumulated over the last three decades concerning so-called fast ionic conductors. Fast ionic conductors have an ionic conductivity (Fig. 15-8) comparable to that of moderately concentrated aqueous ionic solutions (ca. 0.1-1 moll1). Fast ionic conduction is found in solid electrolytes and semiconducting crystals. Although known for quite some time, these materials became really interesting when solids were discovered which showed the unexpected high... 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

We mentioned before that not all fast ionic conductors possessing a large fraction of disordered ions arrive at the disordered state via a phase transformation. SrF2, BaF2, and, in particular, FbF2 are increasingly disordered in the anionic sublattice. The defect fraction amounts to 10%, 20%, and 40% respectively at 700 K [M.H. Dickens, et al. (1980)]. In a strict and systematic sense, this type of disorder is not... 

One largely investigated system in the field of silver ion conductivity is the Rbl-AgI system, including the fast ionic conductor, RbAgJj [19,28,29], Structural investigations of these compounds started 40 years ago and the field has been active during the last four decades. The mechanism of conduction and the favored conduction pathway in ionic conductors is important for the development of solid-state batteries [28],... 

Both types of ions can be carriers of electric current in ionic solids but the majority of fast ionic conductors discovered are cationic conductors, for the reasons previously explained. 

Superionics — called also superionic conductors or fast - ionic conductors, are a group of solid materials with a high ionic - conductivity (>10 2-10 1 Sm 1). This state is characterized by the rapid - diffusion of a significant fraction of one of the constituent species (mobile - ions) within an essentially rigid framework formed by other components, typically at elevated temperatures. In many cases, the superionic state is considered as intermediate between normal solids and liquid -> electrolytes. 

The idea that ions can diffuse as rapidly in a solid as in an aqueous solution or in a molten salt may seem astonishing. However, since the 1960s, a variety of solids that include crystalline compounds, glasses, polymers, and composite materials with exceptionally high ionic conductivities have been discovered. Materials that conduct anions (e.g. and 0 ) and cations including monovalent (e.g. H+, Fi+, Na+, Cu+, Ag+), divalent, and even trivalent and tetravalent ions have been synthesized. A variety of names that have been used for these materials include solid electrolytes, superionic conductors, and fast-ionic conductors. Solid electrolytes arguably provides the least misleading and broadest description for this class of materials. 

The specific character of a fast ionic conductor (FIC) arises from the value n of the concentration of the charge carriers with respect to o the total number of ions. 

Zucker, U. H., and Schulz, H. Statistical approaches for the treatment of anharmonic motion in crystals, II. Anharmonic thermal vibrations and effective atomic potentials in the fast ionic conductor lithium nitride (LiaN)., 4cla Cryst. A38, 568-576 (1982). 

Synchrotron radiation is widely used for EXAFS. Rao and Gopalakrishnan [2] have reviewed the results obtained in studies of amorphous materials, catalysts, fast ionic conductors and intercalation compounds. 

G. E. Murch, The Haven ratio in fast ionic conductors. Solid State Ionics, 7 (1982) 177-198. J. Maier and G. Schwitzgebel, Theoretical treatment of the diffusion coupled with... 

Since the X-ray diffraction studies of Zintl et al. , these members of the family of intermetallic compounds have been of special interest because some of their chemical properties are unusual for intermetallic phases. Many experimental investigations have been reported for binary and ternary B32 type compounds. Besides the crystal structure " , the thermodynamic behavior , electrical conductivity ", magnetic susceptibility , NMR data elastic constants - and optical properties have been studied. Additionally for LiAl electrochemical investigations have been performed in view of the recent interest in fast ionic conductors " . ... 

Kudo, T. and Kawamura, J., Fast ionic conductors, in Materials for Energy Conversion Devices, Eds. S.S. Sorrell, J. Nowotny, and S. Sugihara, Cambridge, Woodhead Publishing, 2005, Chapter 7. 

In general, most fast ionic conductors are characterized by the following two common structural features. First, a highly ordered, immobile sublattice provides the framework and defines continuous open channels for ion... 

Since the ionic conductivity and diffusivities are related by the Nemst-Einstein relationship, what governs one governs the other. Fast ionic conductors are a class of solids in which the ionic conductivity is much larger than the electronic conductivity. For a solid to exhibit fast ion conduction, the concentration and mobility of ionic defects must be quite large. The band gap of the material must also be quite high to minimize the electronic contribution to the overall conductivity. The electronic conductivity depends critically on the concentration of free electrons and holes. There are essentially three mechanisms by which mobile electronic carriers can be generated in a solid ... 

Hotzel G. and Weppner W., Application of fast ionic conductors in solid state galvanic cells for gas sensors. Solid State Ionics, 18/19, 1223-1227, 1986. 

There are a number of review articles summarizing and critically examining the NMR studies of FlCs, and many of them contain some discussion of fast protonic conductors. Many features of the NMR studies are similar for protonic and other fast ionic conductors. However, there are also aspects peculiar to FPCs, an important one being the influence of local dynamics. In this article, we shall be specifically concerned with NMR studies of local motions in FPCs in contrast to the long range diffusion which is being discussed elsewhere in this book (see p. 412). 

This is a general review on fast ionic conductors but contains a fairly detailed section on NMR.)... 

Seifert applying a combination of DTA, X-ray and emf measurements (Seifert and Thiel 1982). Further preparative and X-ray work was done in the rare-earth halide systems with A" =NH in order to imderstand the synthetic uniqueness of the anmionium ion (Meyer 1994), and in the systems with Li, Na, Cu, and Ag aiming at new fast ionic conductors. 

Solid electrolytes have also been variously described as Fast Ionic Conductors or Superionic conductors and may cover ionic conductivities within the range of 10 to 1 S/cm with activation energies of 0.1 to 2eV/atom. The levels of ionic conductivity achieved in many of these solid electrolytes are well below their melting points and the values are more typical of liquids than solids. In contrast to liquid electrolytes such as the aqueous electrolytic solutions or molten salts, the mobile ions in a solid are limited to one sublattice such that one ionic component can move through a rigid framework provided by the other components. 

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