Pressure-temperature stability of the van der Waals compound (H2)4CH4

The melting curve of (H2)4CH4, an extremely hydrogen-rich compound in the hydrogen-methane system, was determined using optical microscopy and Raman spectroscopy at high pressure (6 GPa) and low temperature (10 K) in a diamond anvil cell. The experimental data was fit with a Simon-Glatzel equation: P (GPa) = 2.67 × 10−3T (K)1.36 − 0.771. The high hydrogen content (50.2 wt% including the hydrogen in methane, 33.4 wt% only considering the molecular hydrogen) makes this material of interest for hydrogen fuel storage. We found that (H2)4CH4 can be quenched to ambient helium pressure of 1 atm at 10 K and decomposes at approximately 23 K.