Raman study of phase transitions in compressed methane using moissanite anvil cells

Thorough investigations using Raman spectra of solid methane (CH${}_4$) were conducted using moissanite anvil cells in the range of 1$-$43 GPa at room temperature to identify high-pressure phases. Two interesting phases were uncovered, one with a narrow pressure range of 1 GPa centered at around 12.5 GPa and another with a range of 3 GPa centered at around 40.5 GPa. Our results do not support the existence of a phase transformation around 25 GPa. The high-quality Raman spectra, obtained by using single-crystal samples, narrow the boundaries of seven methane phases at pressures up to 43.2 GPa.

Figure 1

Raman spectra of methane at key pressures from 1.0 to 43.2 GPa in different phases.

Figure 2

Pressure dependence of Raman peaks (cm${}^{-1}$) of methane at room temperature. The vertical dashed lines indicate the phase transformations.

Figure 3

Raman spectra of solid CH${}_4$ in the pressure range of 12.0$-$13.1 GPa. The arrows indicate vibrational mode.

Figure 4

Raman spectra of solid CH${}_4$ in the pressure range of 22.1$-$42.2 GPa. The arrows indicate mode evolution.