Correlated ionic motion in solid electrolytes: Tests of Smoluchowski dynamics and conductivity relations

The conductivity of a one-dimensional model for solid electrolytes of framework type is calculated using both the Langevin and Smoluchowski equations. The Smoluchowski conductivity is generally larger than that from the (more general) Langevin approach; they become identical only in the strong damping limit. The inversion of computed or observed carrier densities to obtain an effective potential is generally straightforward for one dimension, but the derivation of the conductivity from this potential is easy only in the strong-damping extreme. The vibrational spectra of most ionic conductors indicate that the quantititve validity of the Smoluchowski equation is dubious for them.