Gr\"uneisen rule in cubic rare-earth cage systems : the examples of LaB$_6$ and LaPt$_{4}$Ge$_{12}$

TL;DR

This paper investigates the thermal expansion behavior of rare-earth cage systems like LaB6 and LaPt4Ge12, linking cage dynamics and orbital degeneracy effects through experimental data and a simple vibrational model.

Contribution

It introduces a model for phononic thermal expansion in cage systems and highlights the role of cage-specific crystal field effects on orbital degeneracy.

Findings

Thermal expansion reflects cage-specific crystal field contributions.

Vibrational model explains low-temperature thermal expansion.

Orbital degeneracy influences system properties at low temperatures.

Abstract

In some intermetallic compounds, the crystallographic structure allows for an unusual latitude of movement of lanthanide ions inside so-called cages. Examples of such magnetic cage systems include the rare-earth hexaborides RB6 and filled skutterudites RPt4Ge12 series. In both instances, the rare-earth site, at the center of the cage, is of high symmetry, which may preserve some degree of orbital degeneracy. A deviation from the cage center lifts this degeneracy, yielding an interplay between the rare-earth movement and the system properties in the paramagnetic range. In particular, the low temperature thermal expansion should reflect this cage-specific crystal field contribution. This study presents an experimental investigation of the thermal expansion in the paramagnetic range of some elements in the aforementioned series. From a simple model for the vibrations in cage systems, a description of the phononic thermal expansion is proposed, within the quasi-harmonic approximation. Accounting for this non-magnetic contributions helps discern the influence of the orbital degeneracy at low temperature.