Ethane and methane at high pressures: structure and stability

TL;DR

This study combines experimental and theoretical methods to investigate the structure and stability of ethane and methane at high pressures up to 120 GPa, revealing phase transitions and structural details.

Contribution

It provides the first experimental determination of ethane's high-pressure phases and validates theoretical predictions with combined x-ray and Raman data.

Findings

Ethane crystallizes at 2.7 GPa in a disordered phase.

A pressure-induced phase transition in ethane occurs at 13.6 GPa to a monoclinic phase.

Equations of state for ethane and methane are established, supporting stability analysis.

Abstract

We have performed a combined experimental and theoretical study of ethane and methane at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the USPEX ab-initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (Phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on a good agreement of the experimental results and theoretical predictions. For methane, our XRD measurements are in agreement with the previously reported high-pressure structures and EOS. We have determined the equations of state of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.