Quadrupolar susceptibility of NdOs4Sb12

Elastic constants of filled-skutterudite NdOs4Sb12 single crystals have been investigated by means of ultrasonic measurements. Transverse C-44 and longitudinal C-11 modes show Curie-type softening, which is proportional to the reciprocal temperature from 10K down to 0.9K. This softening can be explained in terms of the quadrupolar susceptibility modified by crystalline electric field with a Gamma(8) quartet ground state in O-h symmetry. The C-11 mode also shows a broad shoulder and a small dip around 45 and 13 K, respectively. A Debye-type frequency dependence of the anomalies in the longitudinal C-11 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. (c) 2006 Elsevier B.V. All rights reserved.