Smith, D, Hakeem, M, Parisiades, P, Maynard-Casely, HE, Foster, D, Eden, D, Bull, DJ 
ORCID: https://orcid.org/0000-0001-9406-2031, Marshall, ARL, Adawi, AM, Howie, R, Sapelkin, A, Brazhkin, VV and Proctor, JE ORCID: https://orcid.org/0000-0003-3639-8295
2017,
'Crossover between liquid-like and gas-like behaviour in CH4 at 400 K'
, Physical Review E (PRE), 96 (5)
, 052113.
Abstract
We report experimental evidence for a crossover between a liquid-like state and a gas-like state in fluid methane (CH4). This crossover is observed in all of our experiments, up to 397 K temperature; 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and X-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line - a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquid-like and gas-like properties. On the liquid-like side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gas-like side this competes with the attractive Van der Waal’s potential leading the vibration frequency to decrease as pressure is increased.
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