Frenkel defects, solid electrolytes

Fig. 2 Schematic of Frenkel-conducting solid electrolyte on negative polari2sed electrode and resulting defect distributions. Defect concentrations exaggerated for illustration.

Diffusion and migration in solid crystalline electrolytes depend on the presence of defects in the crystal lattice (Fig. 2.16). Frenkel defects originate from some ions leaving the regular lattice positions and coming to interstitial positions. In this way empty sites (holes or vacancies) are formed, somewhat analogous to the holes appearing in the band theory of electronic conductors (see Section 2.4.1). 

In some ionic crystals (primarily in halides of the alkali metals), there are vacancies in both the cationic and anionic positions (called Schottky defects—see Fig. 2.16). During transport, the ions (mostly of one sort) are shifted from a stable position to a neighbouring hole. The Schottky mechanism characterizes transport in important solid electrolytes such as Nernst mass (Zr02 doped with Y203 or with CaO). Thus, in the presence of 10 mol.% CaO, 5 per cent of the oxygen atoms in the lattice are replaced by vacancies. The presence of impurities also leads to the formation of Schottky defects. Most substances contain Frenkel and Schottky defects simultaneously, both influencing ion transport. 

FIGURE 8.9. Distribution of the majority, Frenkel pair, defects near the grain boundaries between an SE (or an MIEC in which the majority charged defects are ionic), and a second phase. Specific example assumes M to be a mobile ion. (a) A(insulator)/MX(MIEC or SE). (b) Two different MIECs or SEs with a common mobile ion M. (c) Two grains of the same MIEC or SE material. (Adapted fi om Maier, J., in Recent Trends in Superionic Solids and Solid Electrolytes, Chandra, S. and Laskar, A., Eds., Academic Press, New York, 1989, 137.)... 

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