Static and Dynamic Correlation Factors

The above considerations relied on a site exchange describable by simple chemical kinetics and a random walk behavior over large distances in a homogeneous medium subject to a small driving force. In the following we will briefly consider some of the most important complications to this picture. 

In general in each experiment individual deviations from random walk can occur. Noteworthy is the tracer correlation factor (f) (cf. Section VI.3. ///.) which is caused by the non-ubiquity of the jump partner (defect). If an ion has just changed its site from x to x via hopping into a vacancy and so releasing a vacancy at x, a further vacancy is not immediately available and the dance of a particle between x and x  

205) this is not the case for a vacancy migrating in an electrical experiment, nor for an isotope in a tracer experiment moving according to an interstitial mechanism (Fig. 42 bottom). The relation between D and Dq then reads 

Whereas such correlation factors are typically between 0.5 and 1, severe discrepancies between D and DQ can occur if the mechanisms differ. This happens in the case that defect contribute in different valence states, in particular if neutral transport is involved (see below). More generally then Eq. (126) has to be replaced by3,4 

In alkali hydroxides extended ring mechanism for proton motion can occur leading to a tracer propagation but not to any charge transport. The Haven ratio (H) amounts to several thousand in such cases.216 

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