Grüneisen rule in cubic rare-earth cage systems: the examples of LaB6and LaPt4Ge12.

Abstract

Specific heat and thermal expansion properties are investigated in two non-magnetic rare-earth cage compounds, LaB₆and LaPt₄Ge₁₂, which represent extremes in guest-to-cage mass ratio. Using simplified phonons dispersions for the two lowest branches, a theoretical framework is proposed for the low temperature thermodynamic analysis of cage compounds. Within the quasi-harmonic approximation, the Grüneisen rule is found to break down even at low temperatures. However, under the influence of the flattened branches, it should be approximatively restored at intermediates temperatures. The model accurately describes LaB₆specific heat below 50 K. In the LaPt₄Ge₁₂case, the description is rapidly inadequate with increasing the temperature, which points to the interference of additional low frequency phonon branches. Subsequently, thermal expansion measurements are used to investigate the Grüneisen rule in these two compounds. As predicted, there appears to be distincts Grüneisen regimes at low temperature. This study will help distinguish between phonon and magnetic contributions to the thermal expansion in the RB₆and RPt₄Ge₁₂series.