Year: 2003
Type: article
Abstract: Lattice effects in a cerium based clathrate compound ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{20}{\mathrm{Ge}}_{6}$ with a cubic ${\mathrm{Cr}}_{23}{\mathrm{C}}_{6}$-type structure have been investigated by ultrasonic and thermal expansion measurements. Elastic softenings of ${(C}_{11}\ensuremath{-}{C}_{12})/2$ and ${C}_{44}$ proportional to the reciprocal temperature $1/T$ above ${T}_{\mathrm{Q}1}=1.25$ K are well described in terms of the quadrupole susceptibility for the ground state ${\ensuremath{\Gamma}}_{8}$ quartet. A huge softening of 50% in ${(C}_{11}\ensuremath{-}{C}_{12})/2$ and a spontaneous expansion $\ensuremath{\Delta}L/L=1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ along the [001] direction in particular indicate the ferroquadrupole ordering of ${O}_{2}^{0}$ below ${T}_{\mathrm{Q}1}.$ The elastic anomalies associated with the antiferromagnetic ordering at ${T}_{\mathrm{N}2}=0.75$ 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 ${\ensuremath{\Gamma}}_{5}$ rattling motion of an off-center Ce ion along the [111] direction with eight fractionally occupied positions around the 4a site in a cage. The thermally activated ${\ensuremath{\Gamma}}_{5}$ rattling motion obeying a relaxation time $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\mathrm{exp}{(E/k}_{B}T)$ with an attempt time ${\ensuremath{\tau}}_{0}=3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ sec and an activation energy $E=70$ 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.
Language: Latvian
Subfield: Condensed Matter Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W98409142