Ultrasonic study of the charge-fluctuation compound ${\mathrm{Sm}}_3{\mathrm{Te}}_4$

We have measured the ultrasonic velocity and attenuation coefficient of the rare-earth chalcogenide ${\mathrm{Sm}}_3{\mathrm{Te}}_4$ to examine the valence-fluctuation effect due to the coexistence of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions in the ratio of 1:2. The ultrasonic dispersion around 120 K indicates that the charge-fluctuation time obeys the activation-type temperature dependence $\tau ={\tau }_0\exp {(E/k}_{B}T)$ with a characteristic time $2\pi {\tau }_0=2.5\mathrm{}\times {10}^{-13}\sec$ and an activation energy $E=0.136\mathrm{}\mathrm{eV}.$ The absence of the phase transition due to the charge ordering in ${\mathrm{Sm}}_3{\mathrm{Te}}_4$ means a freezing of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions in random distribution at low temperatures. We find an elastic softening with $\ln T$ dependence below 15 K down to a spin glass transition around 1.3 K. This striking behavior is attributed to a two-level system due to the tunneling of $4f$ electrons among randomly distributed ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions. This result is similar to the ionic tunneling in amorphous glass compounds. Employing the group-theoretical analysis, we show some aspects of charge-fluctuation modes in ${\mathrm{Sm}}_3{\mathrm{Te}}_4$ and a possible mechanism for the charge glass state of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions.