Structure and vibrational properties of methane up to 71 GPa

TL;DR

This study combines experimental and computational methods to determine the structure and vibrational properties of methane at pressures up to 71 GPa, revealing a trigonal R3 phase with detailed hydrogen atom positions.

Contribution

It identifies the high-pressure phase of methane as trigonal R3 and provides detailed hydrogen atom positions using first principles calculations, expanding understanding of methane's structural behavior under extreme conditions.

Findings

High-pressure phase of methane is trigonal R3.

Hydrogen atom positions were determined from first principles.

Raman spectra match calculated vibrational modes.

Abstract

Single-crystal synchrotron X-ray diffraction, Raman spectroscopy, and first principles calculations have been used to identify the structure of the high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa. The structure of HP phase is trigonal R3, which can be represented as a distortion of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The positions of hydrogen atoms in HP phase have been 19 obtained from first principles calculations. The molecules occupy four different crystallographic sites in phases B and eleven sites in the HP phase, which result in splitting of molecular stretching modes detected in Raman spectroscopy and assigned here based on 22 a good agreement with the Raman spectra calculated from the first principles.