Ultrasonic Investigation of Off-Center Rattling in Filled Skutterudite Compound NdOs_4Sb_<12>(Condensed matter: structure and mechanical and thermal properties)

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The off-center rattling of Nd rare-earth ions in the filled skutterudite compound NdOs_4Sb_<12> has been investigated by ultrasonic measurement. The longitudinal C_<11> mode for frequencies between 34 and 253MHz shows a marked frequency dependence in elastic constant and ultrasonic attenuations at two different temperatures centered at around 45 and 15K. 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=337K were obtained from fits to the dispersion at around 45K. The presence of another dispersion at lower temperatures of around 15K implies that the rattling in NdOs_4Sb_<12> has an additional low-energy excitation characterized by a lower activation energy E_2〜67K with an attempt time τ_<0,(2)>=5.1×10^<-11>s.
社団法人日本物理学会の論文
2008-07-15