Low-Energy Optical Phonon Modes in the Caged Compound LaRu2Zn20
  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. 
K. Wakiya, T. Onimaru, S. Tsutsui, K. T. Matsumoto, N. Nagasawa, A. Q.   R. Baron, T. Hasegawa, N. Ogita, M. Udagawa, and T....
cond-mat.str-el cond-mat.mtrl-sci