Ultrasonic Investigation of Off-Center Rattling in Filled Skutterudite Compound NdOs4Sb12

The off-center rattling of Nd rare-earth ions in the filled skutterudite compound NdOs 4 Sb 12 has been investigated by ultrasonic measurement. The longitudinal C 11 mode for frequencies between 34 and 253 MHz shows a marked frequency dependence in elastic constant and ultrasonic attenuations at two different temperatures centered at around 45 and 15 K. The relaxational frequency dependence of ultrasonic dispersion reveals a thermally activated Γ 23 -type off-center motion of Nd-ions, involving local charge fluctuations with Γ 23 symmetry in the (OsSb 3 ) 4 cage. An attempt time τ 0,(1) =7.5×10 -12 s and an activation energy E 1 =337 K were obtained from fits to the dispersion at around 45 K. The presence of another dispersion at lower temperatures of around 15 K implies that the rattling in NdOs 4 Sb 12 has an additional low-energy excitation characterized by a lower activation energy E 2 ∼67 K with an attempt time τ 0,(2) =5.1×10 -11 s.