Quadrupolar susceptibility of NdOs4Sb12

Elastic constants of filled-skutterudite NdOs4Sb12 single crystals have been investigated by means of ultrasonic measurements. Transverse C44 and longitudinal C11 modes show Curie-type softening, which is proportional to the reciprocal temperature from down to . This softening can be explained in terms of the quadrupolar susceptibility modified by crystalline electric field with a Gamma8 quartet ground state in Oh symmetry. The C11 mode also shows a broad shoulder and a small dip around 45 and , respectively. A Debye-type frequency dependence of the anomalies in the longitudinal C11 mode resembles an ultrasonic dispersion in PrOs4Sb12, which is expected to be due to the 'rattling' effect, a thermally activated off-center motion of the rare-earth atom in the Sb-cage.