Lattice effects in a cerium based clathrate compound Ce 3 Pd 20 Ge 6 with a cubic Cr 23 C 6 -type structure have been investigated by ultrasonic and thermal expansion measurements. Elastic softenings of (C 11 ϪC 12 )/2 and C 44 proportional to the reciprocal temperature 1/T above T Q1 ϭ1.25 K are well described in terms of the quadrupole susceptibility for the ground state ⌫ 8 quartet. A huge softening of 50% in (C 11 ϪC 12 )/2 and a spontaneous expansion ⌬L/Lϭ1.9ϫ10 Ϫ4 along the ͓001͔ direction in particular indicate the ferroquadrupole ordering of O 2 0 below T Q1 . The elastic anomalies associated with the antiferromagnetic ordering at T N2 ϭ0.75 K and the incommensurate antiferromagnetic ordering are also found. Notable frequency dependence of C 44 around 10 K is accounted for by the Debye-type dispersion indicating a ⌫ 5 rattling motion of an off-center Ce ion along the ͓111͔ direction with eight fractionally occupied positions around the 4a site in a cage. The thermally activated ⌫ 5 rattling motion obeying a relaxation time ϭ 0 exp (E/k B T) with an attempt time 0 ϭ3.1ϫ10 Ϫ11 sec and an activation energy Eϭ70 K dies out with decreasing temperature, and then the off-center tunneling state of Ce ion in the 4a-site cage will appear at low temperatures.

No abstract

No abstract

No abstract

No abstract