Lattice parameters, thermal expansion coefficients, and vacancy density in solid CH4

We report an x-ray study of the lattice parameters and thermal expansion of polycrystalline samples of methane. The sizes of the volume changes at the phase transition and at melting are determined. Using the coefficients of thermal expansion obtained, we calculate the specific heat Cv and the Grüneisen constants. We analyze the contributions to the specific heat due to the translational and rotational motions of the molecules. From a comparison of the x-ray and dilatometric data for the thermal expansion coefficient near the melting temperature of CH4 we determine the density, volume, and enthalpy of formation of the equilibrium vacancies. The values obtained are close to those of the corresponding quantities for single vacancies in inert gas crystals .

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