Negative Thermal Expansion in Solid Deuteromethane

TL;DR

This paper calculates the negative thermal expansion of solid deuteromethane at low temperatures, revealing a giant peak and unusual pressure effects that enhance quantum behavior.

Contribution

It provides the first detailed calculation of negative thermal expansion in solid CD4 considering rotational tunneling modes at low temperatures.

Findings

Giant negative thermal expansion peak at mK temperatures.

Pressure increases quantum effects in solid CD4.

Negative Grüneisen parameter indicates strong quantum behavior.

Abstract

The thermal expansion at constant pressure of solid CD$_4$ III is calculated for the low temperature region where only the rotational tunneling modes are essential and the effect of phonons and librons can be neglected. It is found that in mK region there is a giant peak of the negative thermal expansion. The height of this peak is comparable or even exceeds the thermal expansion of solid N$_2$, CO, O$_2$ or CH$_4$ in their triple points. It is shown that like in the case of light methane, the effect of pressure is quite unusual: as evidenced from the pressure dependence of the thermodynamic Gr\"{u}neisen parameter (which is negative and large in the absolute value), solid CD$_4$ becomes increasingly quantum with rising pressure.