Ultrasonic measurement on the La-substituted PrOsSb system

We present the relevant elastic constants for a single crystal of the La-substituted Pr 1 - x La x Os 4 Sb 12 system for x = 0.4 by means of ultrasonic measurements. A characteristic softening toward low temperature was observed below around 30 K in the temperature dependence of the elastic constants. In addition, a clear increase was observed around 40 K. These results can be reasonably explained by the 4f electric state of the Pr ion formed by the crystalline electric field effect combined with a rattling motion of an off-center Pr ion in a cage of Sb-icosahedron.