| dc.description.abstract | Methane (CH4) is believed to be a major constituent of ice giants (Neptune and Uranus, for example), playing a significant role in the internal structure and dynamics of this type of planet. Thorough knowledge of CH4 is thus essential to fully understand ice giants. Neverthe-less, properties of solid CH4 at high pressure (P) and temperature (T) remain largely unknown. So far, several distinct high-pressure structures for solid CH4 have been proposed, including P212121, Pnma, Cmcm, P21/c, C2/c, and R3, ranging from 20 to 145 atoms per unit cell. Ther-mal properties of these proposed structures, however, are unexplored.
In this thesis, we present our computational study for the thermal properties of solid me-thane at high P–T conditions up to 200 GPa and 1800 K. Our calculations are based on density functional theory (DFT), and all the above-mentioned proposed structures are considered. For nonzero temperature (T /= 0), the equation of state, density, bulk modulus, thermal expansivity, molar heat capacity, Grüneisen parameter, and bulk sound velocity are computed within qua-si-harmonic approximation (QHA). Remarkably, our calculations indicate that thermal proper-ties of all the proposed structures are nearly the same, despite their distinct structural properties. The underlying reason is that phonon spectra of all these structures are highly similar. Our re-sults thus suggest that effects of atomic structures on the thermal properties of solid CH4 are minimal. Therefore, our calculations provide reliable information for the thermal parameters of solid CH4 at high P–T conditions of planetary interiors. | en_US |