Low-Energy Optical Phonon Modes in the Caged Compound LaRu2Zn20

TL;DR

This study investigates atomic dynamics in LaRu2Zn20, revealing low-energy optical phonon modes associated with Zn atoms that may drive its structural transition at 150 K.

Contribution

The paper combines experimental measurements and first-principles calculations to identify low-energy optical phonon modes in LaRu2Zn20, linking them to structural instability.

Findings

Optical phonon modes at 3 meV and 7 meV identified by IXS.

Specific heat data consistent with two Einstein modes at 35 K and 82 K.

Low-energy Zn vibrations potentially cause structural transition.

Abstract

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.