The Effect of Pressure on the Negative Thermal Expansion of Solid Methane

TL;DR

This paper investigates how pressure influences the negative thermal expansion of solid methane at low temperatures, focusing on rotational tunnelling modes and their thermodynamic properties.

Contribution

It provides a theoretical calculation of pressure effects on negative thermal expansion and the Gruneisen parameter for rotational tunnelling modes in solid methane.

Findings

Pressure increases the magnitude of negative thermal expansion peaks.

Peak positions shift to lower temperatures with increasing pressure.

The Gruneisen parameter for tunnelling modes is large, negative, and pressure-dependent.

Abstract

The effect of pressure on the thermal expansion of solid CH$_4$ is calculated for the low temperature region where the contributions from phonons and librons can be neglected and only the rotational tunnelling modes are essential. The effect of pressure is shown to increase the magnitude of the peaks of the negative thermal expansion and shifts the positions of the peaks to the low-temperature region, which goes asymptotically to zero temperature with increasing pressure. The Gruneisen thermodynamical parameter for the rotational tunnelling modes is calculated. It is large, negative, and increases in magnitude with rising pressure.