Structure of methane and ethane at high pressure

The Journal of Chemical Physics

We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies and the USPEX ab initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.

Chemical Physics Letters, 2009

Solid methane (CH 4 ) was compressed up to 202 GPa at 300 K in a diamond-anvil cell. The crystal structure and equation of state over this entire range were determined from angle dispersive X-ray diffraction results. CH 4 undergoes phase transitions from rhombohedral to a simple cubic phase at 19 GPa and from simple cubic to a higher pressure cubic phase at approximately 94 GPa. This higher pressure cubic phase was stable to the maximum pressure investigated. Combined with previous optical measurements, it was found that at room temperature compressed CH 4 remains an insulator with cubic structure to 202 GPa.

Chemical Physics, 2010

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P2 1 2 1 2 1 , Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane ͑C 2 H 6 ͒ at 95 GPa, butane ͑C 4 H 10 ͒ at 158 GPa, and further, carbon ͑diamond͒ and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.

BIBECHANA, 2014

We study the structural and electronic properties of solid methane of space group P212121 at high pressure. The density-functional theory (DFT) based first-principles calculations within the Generalized Gradient Approximations (GGA) have been performed by using Quantum Espresso package. Our findings show that the solid methane in orthorhombic structure compresses fast at the first, and then slowly as a function of elevated hydrostatic pressure. The pressure-volume diagram agrees with the available previously reported data up to pressure of around 200 GPa. In orthorhombic structure, solid methane is a wide band gap insulator at low pressures (tens of GPa). The band gap decreases with increase in the pressure. At high pressure (around 900 GPa), the band gap decreases to semi-conductor range (1.78 eV). Our results reveal that methane to be metallic above the pressure coverage of the present study which is consistent to the interior of the giant planets. The band gap as a function of pr...

Journal of Physical Chemistry C, 2018

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate-the stable form of methane for pressures above 2 GPa at room temperature-by combining high-pressure Raman spectroscopy with ab-initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH 4 :D 2 O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20GPa. This happens along with a progesssive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

HAL (Le Centre pour la Communication Scientifique Directe), 2018

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate-the stable form of methane for pressures above 2 GPa at room temperature-by combining high-pressure Raman spectroscopy with ab-initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH 4 :D 2 O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20GPa. This happens along with a progesssive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

Geoscience Frontiers, 2011

The structural stability of methane hydrate under pressure at room temperature was examined by both in-situ single-crystal and powder X-ray diffraction techniques on samples with structure types I, II, and H in diamond-anvil cells. The diffraction data for types II (sII) and H (sH) were refined to the known structures with space groups Fd3m and P6 3 /mmc, respectively. Upon compression, sI methane hydrate transforms to the sII phase at 120 MPa, and then to the sH phase at 600 MPa. The sII methane hydrate was found to coexist locally with sI phase up to 500 MPa and with sH phase up to 600 MPa. The pure sH structure was found to be stable between 600 and 900 MPa. Methane hydrate decomposes at pressures above 3 GPa to form methane with the orientationally disordered Fm3m structure and ice VII (Pn3m). The results highlight the role of guest (CH 4)-host (H 2 O) interactions in the stabilization of the hydrate structures under pressure.

Journal of Raman Spectroscopy, 2007

New high-pressure experiments on the H 2 O-CH 4 system have been conducted for investigating the structure of methane hydrates (MH) under pressure. Interestingly, structure II (sII) MH was generated and locally coexisted with structure I (sI) up to 500 MPa. The Raman analysis of MH formation during the experiments allowed us to distinguish two possible evolutions: (1) a direct crystallization of sI MH from CH 4 gas and (2) an indirect evolution from gas to sII, followed by a sII-sI transition. Calculations show that H 2 O molar fraction is 2.9-25% lower in sII MH than in sI in the early stages of hydrate formation. We suggest that sII MH crystallize locally because the amounts of H 2 O available for sI MH generation are too low.

THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY, 1998

We discuss the application of ab initio molecular dynamics simulations results to a variety of simple molecular systems under pressure. In particular we consider the polymerization and subsequent amorphization of C,H2 crystals upon compression up to 50 GPa, the determination of the ground state structure of the broken symmetry phase of H2 in the pressure range 100-150 GPa, the fate of methane and ammonia along the isentrope of the middle ice layers of Neptune. We also discuss preliminary applications to 02 and CO.

The Journal of Physical Chemistry A, 2018

Samples of energetic material TEX (C 6 H 6 N 4 O 8) are studied using Raman spectroscopy and X-ray diffraction (XRD) up to 27 GPa pressure. There are clear changes in the Raman spectra and XRD patterns around 2 GPa related to a conformational change in the TEX molecule, and a phase transformation above 11 GPa. The molecular structures and vibrational frequencies of TEX are calculated by density functional theory based Gaussian 09W and CASTEP programs. The computed frequencies compare well with Raman spectroscopic results. Mode assignments are carried out using Vibrational Energy Distribution Analysis program, and also visualized in the Materials Studio package. Raman spectra of the high pressure phases indicate that the sensitivity of these phases is more than that of the ambient phase.