Methane plays an important role in fundamental and applicable sciences. Also, it is present in outer Solar planets such as Uranus and Neptune, as well as in the deep Earth, where its state is determined by pressures and temperatures. Therefore, the high-temperature -- high-pressure phase diagram of methane is of fundamental importance to many fields of physics, chemistry, planetary and geological sciences. As the simplest hydrocarbon, CH4 adopts a rich variety of crystal structures that have been extensively studied in a wide pressure and temperature range. (1-4) However, compared to other simple molecular solids its phase diagram and the relationship between solid phases and fluid states is surprisingly not that well established. (5- 10)

By combining the high-pressure high-temperature techniques with Raman spectroscopy we have studied the phase diagram of methane up to 40 GPa and 1200K, illustrating the complexity and (meta)stability of its phases. Our results indicate that there are two triple points between the solid and liquid states. Interestingly, we find that the P-T boundaries of the different phases are path dependent, so we will discuss their kinetics, formation and (meta)stability. More importantly, we demonstrate that the melting curve changes slope at one of the triple points and melting happens at temperatures considerably higher than previously thought.

[1] Bini, R., et al., JCMP, 103, (1995): 1353-1360.
[2] Maynard-Casely, H. E., et al., JCP, 133, (2010): 064504.
[3] Bykov, M. et al., PRB 104, (2021): 184105.
[4] Bini, R., et al., PRB, 55, (1997): 14800.
[5] Naumova, A., et al. JPCC 123, (2019): 20497-20501.
[6] Conway, Lewis J., and Hermann A., Geosciences 9, no. 5 (2019): 227.
[7] Hazen, R. M., et al., APL 37, (1980): 288-289.
[8] Abramson, Evan H. HPR, 31, (2011): 549-554.
[9] Yagi, T. , and Hiroto S., Proc. Jap. Aca, 66, (1990): 167-172.
[10] Lobanov, S. S., et al. Nat. Comm.4, (2013): 2446.

high pressure,Raman spectroscopy