Electronic Transport in Semimetallic Cerium Sulfide

The electrical resistivity $\rho$ and Seebeck coefficient $S$ have been measured between 10 and 1000°K in ${\mathrm{Ce}}_{3-x}{\mathrm{S}}_4$ with values of $x$ ranging from 0 to 0.30. The electrical behavior is semimetallic and can be fitted, in large measure, to the equations of conventional transport theory for $S\left(T\right)\mathrm{}$ and $\rho \mathrm{}\left(T\right)\mathrm{}$. However, examination of the electrostatic effects of the vacancies shows that they introduce large effective charges which are poorly screened. Consequently, there are wide fluctuations in potential in the crystal which cast doubt on a literal interpretation of the theoretical equations; at present, they must be regarded as providing a largely empirical description of the experimental results. There is a relatively large residual resistivity which indicates a very large cross section per vacancy in samples with small values of $x$. This can be accounted for by the abnormally large screening distance. In the temperature range below 100°K, anomalies are observed in $S\left(T\right)\mathrm{}$ and $\rho \mathrm{}\left(T\right)\mathrm{}$ in samples with small vacancy concentrations. Anomalies in $S\left(T\right)\mathrm{}$ seem to be caused by phonon drag. The cause of the resistivity anomalies is not yet clear; we consider the possibilities that they are caused by local lattice vibrations or by spin scattering by electrons in the $4f$ shell of the cerium ions.