ioniccond_lect26

Electronic conductivity lets begin by comparing the

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Unformatted text preview: - Solid State Chemistry Ionic vs. Electronic Conductivity Let's begin by comparing the properties of ionic conductors with the conventional electronic conductivity of metals. Metals Conductivity Range = 10 S/cm < < 105 S/cm Electrons carry the current Conductivity Increases linearly as temperature decreases (phonon scattering decreases as T ) Conductivity Range = 10-3 S/cm < < 10 S/cm Ions carry the current Conductivity decreases exponentially as temperature decreases (activated transport) Chem 754 - Solid State Chemistry Solid Electrolytes 2 Defects In order for an ion to move through a crystal it must hop from an an occupied site to a vacant site. Thus ionic conductivity can only y onl occur if defects are present. The two simplest types of point defects are Schottky and Frenkel defects. Schottky Defect (i.e. NaCl) Na+ + Cl- Vna + VCl Frenkel Defect (i.e. AgCl) Ag+ VAg+ Ag+interstitial Chem 754 - Solid State Chemistry Ion Migration (Schottky Defects) Consider the movement of Na+ ions in NaCl via vacancies originating from Schottky defects. Note that the Na+ ion must squeeze through the lattice, inducing significant local distortion/relaxation. This is one factor that limits the mobility mobility of ions. A second factor that contributes is the relatively high high probability that the ion will jump back to it's original position, position,...
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