Ultrasonic Investigation of Off-Center Rattling in Filled Skutterudite Compound NdOs<SUB>4</SUB>Sb<SUB>12</SUB>
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The off-center rattling of Nd rare-earth ions in the filled skutterudite compound NdOs<SUB>4</SUB>Sb<SUB>12</SUB> has been investigated by ultrasonic measurement. The longitudinal <I>C</I><SUB>11</SUB> 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<SUB>23</SUB>-type off-center motion of Nd-ions, involving local charge fluctuations with Γ<SUB>23</SUB> symmetry in the (OsSb<SUB>3</SUB>)<SUB>4</SUB> cage. An attempt time τ<SUB>0,(1)</SUB>=7.5×10<SUP>−12</SUP> s and an activation energy <I>E</I><SUB>1</SUB>=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<SUB>4</SUB>Sb<SUB>12</SUB> has an additional low-energy excitation characterized by a lower activation energy <I>E</I><SUB>2</SUB>∼67 K with an attempt time τ<SUB>0,(2)</SUB>=5.1×10<SUP>−11</SUP> s.
- Journal of the Physical Society of Japan
Journal of the Physical Society of Japan 77(7), 074607-074607, 2008
THE PHYSICAL SOCIETY OF JAPAN