Raman and brillouin spectroscopic studies of single crystals of CH₄ and CD₄

Gregoryanz, Eugene (1998) Raman and brillouin spectroscopic studies of single crystals of CH₄ and CD₄. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A technique has been developed for obtaining single crystals of molecular crystals in solid phases which can be reached only from other solid phases. The technique was used to produce large (≈50 mm³) single crystals of CH₄ -I and CD₄-I in a liquid helium cryostat cell, which then were successfully cooled down below the phase transition temperatures. The crystals were studied by the means of Raman and Brillouin spectroscopies. X-ray Laüe diffraction techniques were applied to determine the orientation of the crystals in the laboratory frame. The elastic constants of CH₄-I, II and CD₄-I, II and III were determined as a function of temperature from 90 K to 15 K. The possible space groups of CD₄-III were determined on the basis of Brillouin data. -- Bulk adiabatic moduli and shear moduli were calculated from the elastic constants as functions of temperature. The elastic constants were used to calculate the acoustic velocities in high symmetry planes, and also the Debye temperature which is in agreement with previous results. The rotational specific heat of each species was calculated as a function of temperature. -- Raman spectra of the fundamental modes ν₁ and ν₃ were recorded and their shifts and full width at half height were determined as functions of temperature through the phase transition.

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