ioniccond_lect26

High activation energy decreases carrier mobility

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Unformatted text preview: arrier for jumping between between neighboring sites. High activation energy decreases carrier mobility, very stable sites (deep potential energy wells) lead to carrier localization. localization. 3. The structure should have solid framework, preferable 3D, permeated by open channels. The migrating ion lattice should be "molten", so that a solid framework of the other ions is needed in order to prevent the entire material from melting. 4. The framework ions (usually anions) should be highly polarizable. polarizable. Such ions can deform to stabilize transition state geometries Chem 754 - Solid State of the migrating ion through covalent interactions. Chemistry Molten Sublattice (1/2 Melting) In the best ionic conductors one ion becomes so mobile that for all intensive purposes those ions are in a "molten" state. This behavior can be seen in part from the entropies of the observed phase transitions, where the Ag (and F respectively) sublattice melts prematurely. (poor ionic conductor) -AgI -AgI (excellent ionic conductor) T = 146 C, S = 14.5 J/mol-K J/mol-AgI molten AgI S = 11.3 J/mol-K J/molCompare with the an entropy of melting of 24 J/mol-K for NaCl. J/molNaCl. solid PbF2 molten PbF2 S = 16.4 J/mol-K J/molCompare with the an entropy of melting of 35 J/mol-K for MgF2 J/molChem 754 - Solid State Chemistry 7 Ag+ Ion Conductors Stable be...
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This note was uploaded on 06/11/2011 for the course CHEM 101 taught by Professor Stegemiller during the Spring '07 term at Ohio State.

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