  The caged compound LaRu2Zn20 exhibits a structural transition at TS =150 K,
whose driving mechanism remains elusive. We have investigated atomic dynamics
by the measurements of specific heat C and inelastic X-ray scattering (IXS).
The lattice part of the specific heat Clat divided by T3, Clat/T3, shows a
broad peak at around 15 K, which is reproduced by two Einstein modes with
characteristic temperatures of 35 K and 82 K, respectively. IXS measurements
along the [111] and [110] directions reveal optical phonon modes at 3 meV (35
K) and 7 meV (80 K), respectively, whose values agree with the values of
Einstein temperatures. The first principles calculation has assigned the phonon
modes at 3 meV as the optical modes of Zn atoms located at the middle of two La
atoms. The low-energy vibration of the Zn atom perpendicular to the there-fold
axis is thought to lead the structural instability of LaRu2Zn20.
