The crystal structure of methane phase III

Abstract

Methane is the simplest organic molecule, and like many supposedly simple molecular materials it has a rich phase diagram. While crystal structures could be determined for two of the solid phases, that of the low temperature phase III remained unsolved. Using high-resolution neutron powder diffraction and a direct-space Monte Carlo simulated annealing approach, this fundamental structure has now finally been solved. It is orthorhombic with space group Cmca, and 16 molecules in the unit cell. The structure is closely related to that of phase II, yet is no subgroup of it.

Orientational isotopic effects in the thermal conductivity of CH4∕CD4 solid solutions

“…Its structure (orthorombic symmetry group Cmca) is not cubic and is very close to the structure of phase II [4]. The orientational phase transformation at T II III -= 22.1 K is a first-order phase transition with a volume jump of 0.63% [5,4], which is much larger than that during the I-II phase transition. The considerable difference between the rotational constants, different total nuclear spins of the CD 4 and CH 4 molecules and the weak molecular field are the factors that generate strong orientational fluctuations in the vicinity of the phase transition temperature [6,7].…”

“…The considerable difference between the rotational constants, different total nuclear spins of the CD 4 and CH 4 molecules and the weak molecular field are the factors that generate strong orientational fluctuations in the vicinity of the phase transition temperature [6,7]. In solid CH 4 -CD 4 solutions phase III can exist under the equilibrium vapor pressure in a wide range of temperatures at CH 4 concentrations c above 0.15 [2].…”

“…Under equilibrium vapor pressure solid CH 4 experiences one phase transition from orientationally disordered phase I (plastic phase with cubic symmetry) to phase II with partial orientational order, at T I II -= 20.4 K. Apart from phases I and II with T I II -= 27.4 K, CD 4 can have a state with a complete orientational order (phase III). Its structure (orthorombic symmetry group Cmca) is not cubic and is very close to the structure of phase II [4]. The orientational phase transformation at T II III -= 22.1 K is a first-order phase transition with a volume jump of 0.63% [5,4], which is much larger than that during the I-II phase transition.…”

“…The strong isotopic effects observed in the properties of these substances [3] are first of all connected with the rotational degrees of freedom, the quantum statistics of nuclear spins and the weak anisotropic molecular interaction. The phase transformations from the orientational disorder (phase I) to the partial orientational order (phase II) and then from phase II to the completely orientationally ordered phase (phase III) occurs under equilibrium vapor pressure in pure CD 4 and its concentrated solutions with CH 4 , Kr, Xe and at P > 200 bar in pure CH 4 . Deuteration considerably affects the dynamics of the molecules in the orientationally ordered phases of solid methane, mainly because the rotational constant B / Ik B = h 2 (I is the moment of inertia) of CD 4 (B = 3.75 K) is half as high as that of CH 4 ( .…”

First-principles study of solid methane at high pressure

“…It is tetrahedral molecule with C-H bond 1.09 Å and H-C-H bond angle 109.5 o in phase without orientational ordering. In spite of the simplicity of its molecular structure, it has a complex and poorly understood phase diagram [13][14][15][16][17][18][19][20][21][22][23][24]. Furthermore, depending on the pressure and temperature, different phases are produced due to the degree of freedom of orientations.…”