Neutron powder diffraction study of methane deuterohydrate by the maximum entropy method

To examine the nature of thermal motions of the CH₄ molecules in the methane deuterohydrate (8CH₄·46D₂O), the scattering length density distribution was observed by the maximum entropy method (MEM) using neutron powder diffraction data measured in the temperature range of 7-185 K. We drew the scattering-length density distribution as three dimensional graphic images and used the same isosurface level for all temperatures. The negative scattering length density, corresponding to the H atoms of CH₄, was observed only in the large cage. The positive scattering length density attributed to the C atom of CH₄ was observed at the center of each cage. With an increase in temperature, the negative and positive scattering length densities in the large cage disappear. The positive scattering length density remains at the center of the small cage regardless of temperature. These results strongly indicate that the motions of CH₄ depend on the cage size and geometry.