Raman spectrum of phase III of solid CH4 in the lattice and intramolecular regions

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Abstract
Two samples of solid CH4 were grown under high pressures. The pressures of these samples at 10 K place them in phases III and IV, respectively. The Raman spectra of these samples in the lattice and intramolecular regions have been observed. These spectra provide no evidence of a III–IV phase transition. The lattice lines are assigned to translational or librational modes on the basis of their observed mode Grüneisen parameters. The translational modes are then assigned on the basis of a three‐site structure recently proposed for phase III. The intramolecular bands of CH4 in phase III contain broad and sharp lines. The broad lines are assigned to lattice sidebands based on the volume dependence of their frequencies. The sharp lines are then assigned to the pure vibrational modes of three types of molecules in phase III.