Theory of thermoelectric effects in dilute alloys

The low-temperature thermoelectric coefficient is calculated for a model of fixed impurities, Debye phonons, and free electrons using an expansion in the inverse powers of valence $Z$. It is shown that the cancellation of the electron-phonon corrections to the thermopower found in the previous papers of this series for the case of isotropic scattering on impurities does not occur in the general case. The convergence of the large-valence expansion is shown to be fast even for small values of $Z\gtrsim 1$. The first term of the expansion gives a simple approximate formula for the thermopower. For a class of model impurity potentials, the accuracy of this formula is found to be of order 5%.