Ionic conduction in crystals

Fig, 7.15 Mechanisms of ionic conduction in crystals with defect structures (a) vacancy (Schoilky defect) mechanism, (b) interstitial (Frenkel defect) mechanism, (c) inlcrsthialcy (concerted Schottky-Frenkel) mechanism. 

Table 76. Expressions for the diffusion constant and the ionic conductivity in crystals... 

In an ideal ionic crystal, all ions are held rigidly in the lattice sites, where they perform only thermal vibratory motion. Transfer of an ion between sites under the effect of electrostatic fields (migration) or concentration gradients (diffusion) is not possible in such a crystal. Initially, therefore, the phenomenon of ionic conduction in solid ionic crystals was not understood. 

Figure 6.5 Arrhenius plots of ln(

The ionic charge carriers in ionic crystals are the point defects.1 2 23,24 They represent the ionic excitations in the same way as H30+ and OH-ions are the ionic excitations in water (see Fig. 1). They represent the chemical excitation upon the perfect crystallographic structure in the same way as conduction electrons and holes represent electronic excitations upon the perfect valence situation. The fact that the perfect structure, i.e., ground structure, of ionic solids is composed of charged ions, does not mean that it is ionically conductive. In AgCl regular silver and chloride ions sit in deep Coulomb wells and are hence immobile. The occurrence of ionic conductivity requires ions in interstitial sites, which are mobile, or vacant sites in which neighbors can hop. Hence a superionic dissociation is necessary, as, e.g. established by the Frenkel reaction ... 

S Atomistic simulation assisted synthesis and investigations The classical atomistic simulation techniques based on the pair potentials are suitable for the simulations of ceria nanoparticles even with a real sized model. Molecular d)mamics studies with several thousands of ions and up to hundreds of nanoseconds in a time scale have been carried out to interpret the diffusion, and crystal growth behaviors for pure and doped-ceria nanoparticles. Traditionally, the technique has been used to explore the oxygen ionic conductivity in ionic conductors such as ceria and zirconia (Maicaneanu et al., 2001 Sayle et al., 2006). 

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... 

Found values of a and other observed regularities of electrical conductivity variations for the OIL coincide mainly with the studied earlier for simple REO, including doped oxides. These results can be explained within the framework of theoretical model of equilibrium between electronic and ionic defects in crystals, which application for the analysis of defect structure of REO is reviewed in [1]. 

The ionic transport in solids is attributed to the hopping of ionic carriers between the equivalent positions in the crystal lattice. This mechanism is known as lattice diffusion and depends on the jumping distance and frequency of moved ions. The understanding of the influence of these factors on the ionic conductivity is very important for the development of material with enhanced ionic transport. The question of what is the limit of ionic conductivity in solids will be addressed by analyzing the ionic transport in cubic stabilized zirconia systems with different acceptor dopants. 

Figure 7.16 Airhenius-type plot of ln(<7T) versus 1 /T for ionic conductivity in a crystal containing only a small concentration of impurities. Note

In spite of these analogies the electrochemistry of solids is more complex than the electrochemistry in aqueous solutions. So it must be noted that apart from ionic conduction, solids often show an electronic conductivity, caused by electrons or electron defects, which may be predominant in many cases over the ionic conduction. In good solid electrolytes the conduction of the electrical current is caused exclusively by the ions—in most cases practically by only one kind of ion present in a crystal. 

The decreased kinetic energy of the electrons in the metallic bonds reduces the possibility of disordered electron movements, facilitating the transport of electrical charges and heat. It is these features of electrical and thermal conductivity that make the crystalline metallic bond differ from the ionic bond in crystals (e.g., structure of NaCl). 

Ruiz-Trejo E, Sirman JD, Baikov Ju M, Kilner JA (1998) Oxygen ion diffusivity, surface exchange and ionic conductivity in single crystal Gadolinia doped Ceria. Solid State Ion 113-115 565-569... 

There are very few studies of the ORR under hot and dry fuel cell operating conditions. Recently, methods have been devised to separate the mass transport effects from the kinetic effects [74, 75], but none of these have been applied to hot and dry fuel cell operation. These studies showed that, under fully humidified conditions up to 70°C, oxygen reduction had a tenfold higher specific performance for platinum black at 0.90 V compared to Pt on carbon as has been previously reported in the literature [76]. However, this significant benefit of platinum black is shown to rapidly decrease when the potential is shifted to lower, more fuel cell relevant potentials. This is manifested in the Tafel slope, which decreased from 360 to 47 mV/decade in the region where the overpotential was <0.35 V. The effect of hot and dry conditions has been studied in a 5 cm MEA where mass transport and kinetics are difficult to separate [73]. At 120°C, the Tafel slope is found to increase inversely with RH. It is speculated that this is due to the decrease in ionic conductivity in the electrode. RH can also influence water oxidation to form Pt-OH and Pt-O and thereby change the surface condition of the platinum crystals. 

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