Effects of interconfiguration fluctuations upon the far-infrared and dc-transport properties of the thulium-chalcogenides

The far-infrared reflectivity and dc resistivity of $\mathrm{Tm}X(X=\mathrm{T}\mathrm{e},\mathrm{S}\mathrm{e},\mathrm{S})$ were measured as a function of temperature from 1 K to room temperature; the Hall constant of TmSe was also measured. Most of the properties of TmTe were semiconductorlike with a reststrahlen band at 120 ${\mathrm{cm}}^{-1\mathrm{}}$. However, the low-temperature resistivity could not be fit by a thermal activation model and weak, sample-dependent structure appeared in the reflectivity above the reststrahlen band above 2.5 K. Interconfiguration fluctuations are suggested as the cause. The reflectivities of TmSe and TmS are much more metallic; TmS shows a possible weak reststrahlen band at ∼ 190 ${\mathrm{cm}}^{-1\mathrm{}}$. The dc transport properties are not like those of a simple metal in their temperature dependences, but may be explained by Kondo scattering from ${\mathrm{Tm}}^{3+}$ ions in the temperature range studied.