Volume and temperature dependence of the Raman spectrum in phase II of solid CH4

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Abstract
Two samples of solid CH4 of different densities were grown. Grüneisen parameters of 2.6 and 3.1 were obtained for the librational mode and one translational mode in phase II. The former indicates an anisotropic intermolecular potential with a strong volume dependence. We conclude that the II–III phase transition involves a sudden, simultaneous tetragonal distortion of the lattice and ordering of the orientationally disordered molecules of phase II, with a slow reorientation of the ordered molecules. The librational mode of phase II does not go soft near the transition to the disordered phase I. The temperature dependence of the linewidth for this mode can be explained by a thermally activated reorientation of the ordered molecules in phase II with an activation energy of 59 cm−1.