Ionic conductivity superionic conductors

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

Crystalline ionic conductors. Superionic conductors have already been briefly introduced in Section 1.2.2.2. They have been known for quite a long time, and a major NATO Advanced Study Institute on such conductors was held as early as 1972 (van Gool 1973). Of course, all ionic crystals are to a greater or lesser extent ionically conducting - usually they are cationic conductors, because cations are smaller than anions. Superionic conductors typically have ionic conductivities lO" times higher than do ordinary ionic crystals such as KCl or AgCl. 

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... 

Because of the high values of conductivity which in individual cases are found at room temperature, such compounds are often called superionic conductors or ionic superconductors but these designations are unfounded, and a more correct designation is solid ionic conductors. Strictly unipolar conduction is typical for all solid ionic conductors in the silver double salts, conduction is due to silver ion migration, whereas in the sodium polyaluminates, conduction is due to sodium ion migration. 

One important group of conducting solids is the so-called superionic or fast ion conductors. These are characterized with a relatively high ionic conductivity (>10-2-10-1 Sm 1) in substantially disordered crystal lattices and/or near grain boundaries in polycrystalline materials. Typical examples include - j3 -alumina,... 

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. 

There is no strict boundary between superionic and other ion conductors. In the literature the level of ionic conductivity accepted as such boundary varies from 10-1 to 10 Sm-1. 

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. 

With the conductivity of an aqueous electrolyte (e.g., IN KCl) serving as a reference, comparable conductivities can be achieved in solid electrolytes under certain conditions. Some of the best solid ionic conductors, commonly referred to as superionic conductors , have resistivities comparable to those of aqueous electrolytes at room temperature (e.g., RbAg4l5 and single crystal MgO-stabilized 6"-alumina). However, they are either in the form of single crystals, which is impractical for most applications, or composed of very expensive and relatively unstable materials. Resistivities comparable to those of aqueous electrolytes can be achieved in solid electrolytes at higher temperatures in both superionic conductors like 6"-alumina (i.e., 300°C) and normal ionic conductors such as stabilized zirconia (800-1000°C), stabilized cerium oxide (>800 C), and stabilized bismuth oxide (>600°C). Sodium ion conducting glasses are much less conductive than polycrystalline 8 -alumina. 

The data in Table 1 also illustrate several additional facets of solid state ionic conduction. First the fast (or super ) ion conductors are charaeterized by very low activation energies for conduction (12-36 kj/mol) compared to the normal ionic conductors (> 60 kJ/mol). The low activation energy for superionic conduction is a major contributing factor to the high ionic conductivity at relatively low temperatures. 

The family of Ag + and Cu + superionic conductors have been extensively studied for many decades, using a wide range of experimental and computational techniques see also Chapter 7. They are principally of interest for fundamental reasons, as model systems in which to characterize the nature ofthe dynamic disorder and to probe the factors which promote high values of ionic conductivity within the solid state. Their commercial applications are generally limited by factors such as chemical stability, the high cost of silver, and their relatively high mass when compared, for example, to lithium-based compounds. 

Of all the fluorite-structured halides, P-PbF2 has attracted the most attention, since it has the lowest superionic transition temperature (Tc = 711 K). The ionic conductivity and structural properties of a number of compounds based on PbF2 have been studied, with the ternary compound PbSnF4 possessing one of the highest values of ionic conductivity of any F ion conductor at ambient temperature (<7, = 10 S cm- [76]). 

Ionic conductivity in ionic crystals, 336f in semiconductors, 129 superionic conductors, 316 Ionic radius, rj, 314ff comparison with core radius, r., 363 nonuniqueness, 314 of Pauling and Zachariasen, 3l5f values and trends, 316, and Solid Stale Table Ion softening, 33Iff omission in electrostatic energy, 303 lonicity, 43. See also Polarity lonicity theory, 173 validity, 190... 

Materials that show exceptionally high ionic conductivity are referred to as fast ion conductors or superionic conductors. The conductivities are comparable to those of electrolyte solutions, but are still low compared with metal-like electron conductivity. There are two important types of fast ion conductor ... 

There are numerous publications concerning the theory and applications of superionic conduction(see also the Preface). Table 3.1 gives the main electrical parameters of some typical superionic conductors. Superionic conductivity has been observed for a number of ions, Ag", Li+, Na+, K+, Cu+, Pb +, F", O ", NH4+, H+(H20) the lowest , has been found in Agl-based materials. Some compounds exhibit both high ionic and electronic conductivity. Glassy materials are usually poorer conductors and have higher activation energy than crystalline materials of similar composition. 

There is also P(to) = toa(to) where a(co) is the frequency dependent conductivity (see Chapter 25). In the case of a low conductivity ionic solid, P(co) is typically of an oscillating behaviour (a peak at = coq, more or less damped, see Fig. 11.2) and there are only very rare diffusive events which contribute to P(co = 0). In a liquid, the spectrum is centred at (u = 0, since all the particles diffuse. When a liquid becomes (more) viscous a pseudo-oscillating behaviour may be observed (c), while the oscillator damping in a superionic conductor may decrease the difference between the time of flight between two sites and the time of oscillation on a site (Fig. 30.2), leading to a quasi-liquid state ". In order to simplify the model, either the diffusive or the oscillatory behaviour is assumed to be predominant. The choice may depend on the supposition of relaxation time... 

If the gap in the forbidden band of the semiconductor film is wide enough and exceeds the decomposition voltage of the supporting electrolyte, the film substance would behave as a pure ionic conductor—superionic conductor, if the ion conductivity were high. 

Superionic conductor Solid electrolyte with very high ionic conductivity, comparable to liquid electrolytes. 

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