Ultrasonic study of the charge-fluctuation compound Sm3Te4

We have measured the ultrasonic velocity and attenuation coefficient of the rare-earth chalcogenide Sm3Te4 to examine the valence-fluctuation effect due to the coexistence of Sm2+ and Sm3+ 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)=2.5X10(-13) sec and an activation energy E=0.136 eV. The absence of the phase transition due to the charge ordering in Sm3Te4 means a freezing of Sm2+ and Sm3+ ions in random distribution at low temperatures. We find an elastic softening with In 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 Sm2+ and Sm3+ 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 Sm3Te4 and a possible mechanism for the charge glass state of Sm2+ and Sm3+ ions.