Structural studies of solid methane at high pressures

We present computer simulations of liquid and solid phases of condensed methane at pressures below 25 GPa, between 150 and 300 K, where no appreciable molecular dissociation occurs. We used molecular dynamics (MD) and metadynamics techniques and empirical potentials in the rigid molecule approximation, whose validity was confirmed a posteriori by carrying out ab initio MD simulations for selected pressure and temperature conditions. Our results for the melting line are in satisfactory agreement with existing measurements. We find that the fcc crystal transforms into a hcp structure with four molecules per unit cell (B phase) at about 10 GPa and 150 K, and that the B phase transforms into a monoclinic high pressure phase above 20 GPa. Our results for solid/solid phase transitions are consistent with those of Raman studies but the phase boundaries estimated in our calculations are at higher pressure than those inferred from spectroscopic data.

Section: Discussionmentioning

confidence: 99%

Theoretical investigation of methane under pressure

Donadio

Hohl

Galli

2009

“…At 5.4 GPa and room temperature, methane transforms to methane A, 3 to which a rhombohedral structure has been assigned. 13 The structure of methane A has been fully determined with x-ray single crystal and neutron powder diffraction. 14 It has 21 molecules in a rhombohedral unit cell which are described by 39 different crystallographic sites for all the carbon and hydrogen atoms.…”

Section: Introductionmentioning

confidence: 99%

The crystal structure of methane B at 8 GPa—An α-Mn arrangement of molecules

Maynard-Casely

Lundegaard

Loa

et al. 2014

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ∼8 GPa. We find this substructure to be cubic with space group $I\bar 43m$I4¯3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.911(1) Å at 8.3(2) GPa, which is a factor of √2 larger than had previously been proposed by Umemoto et al. [J. Phys.: Condens. Matter 14, 10675 (2002)]. The substructure as now solved is not related to any close-packed arrangement, contrary to previous proposals. Surprisingly, the arrangement of the carbon atoms is isostructural with that of α-manganese at ambient conditions.

“…6 All previous structural studies of methane A have been based on spectroscopic and powder diffraction experiments. Both methane IV and A were first interpreted as having tetragonal structures, but later powder diffraction data 14 showed methane A to have a rhombohedral unit cell ͓a = 8.643͑1͒ Å and ␣ = 89.40͑2͒°at 7.0 GPa and room tem-perature͔. On the basis of density in relation to methane I, and symmetry considerations, Nakahata et al 14 assigned 21 molecules to the unit cell.…”

Section: Introductionmentioning

confidence: 99%

“…Both methane IV and A were first interpreted as having tetragonal structures, but later powder diffraction data 14 showed methane A to have a rhombohedral unit cell ͓a = 8.643͑1͒ Å and ␣ = 89.40͑2͒°at 7.0 GPa and room tem-perature͔. On the basis of density in relation to methane I, and symmetry considerations, Nakahata et al 14 assigned 21 molecules to the unit cell. They also pointed out the similarities to high-pressure phases of CCl 4 ͑phase Ib͒ and CF 4 ͑phase I͒, which have rhombohedral unit cells with ␣ Ϸ 89.5°, believed to contain 21 molecules ͑see Sec.…”

Section: Introductionmentioning

confidence: 99%

The distorted close-packed crystal structure of methane A

Maynard-Casely

Bull

Guthrie

et al. 2010

We have determined the full crystal structure of the high-pressure phase methane A. X-ray single-crystal diffraction data were used to determine the carbon-atom arrangement, and neutron powder diffraction data from a deuterated sample allowed the deuterium atoms to be located. It was then possible to refine all the hydrogen positions from the single-crystal x-ray data. The structure has 21 molecules in a rhombohedral unit cell, and is quite strongly distorted from the cubic close-packed structure of methane I, although some structural similarities remain. Full knowledge of this structure is important for modeling of methane at higher pressures, including in relation to the mineralogy of the outer solar system. We discuss interesting structural parallels with the carbon tetrahalides.