Correlation effects in diffusion in solids
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8. — Summary
Correlation effects occur when the diffusing species (tracer, impurity) is moved by another particle (vacancy, interstitial).
Correlation effects in linear systems are so large that no diffusion occurs.
Correlation effects may also be important in impurity diffusion.
Correlation effects with tracer diffusion in ionic conductors can be seen in a comparison ofkTσ/Ne2 withDT. For some interstitial mechanisms the ratiokTσ/Ne2DT is greater than unity not only because of the correlation effect but also because the interstitialcy jump distance differs from the tracer jump distance.
«Indirect interstitial» or «interstitialcy» migration occurs in AgCl and AgBr, but it is necessary to suppose that non-collinear jumps occur as well as collinear jumps.
Hove’s dumb-bell model without rotations leads to the same ratio forkTσi/Ne2DTi as does the collinear interstitialcy mechanism.
Correlation effects in CsCl lattices may be very large if Frenkel disorder occurs.
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