A Brillouin spectroscopic study of the I-II phase transition in CH₄

Kelly, Robin G. (1992) A Brillouin spectroscopic study of the I-II phase transition in CH₄. Masters thesis, Memorial University of Newfoundland.

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Abstract

The effects of rotation-translation (RT) coupling in solid methane (CH₄), especially near the 1-11 phase transition, have been investigated using die technique in high resolution Brillouin spectroscopy. Large single crystals of methane were grown in a liquid helium cryostat and were successfully cooled down to 15.5 K using a novel cooling technique. Lane X-ray diffraction photographs were taken to determine crystal quality and the orientation of the crystal axes. Radiation from an argon laser was incident along the axis of the cell, while the scattered radiation was analyzed at 90° by a Brillouin spectrometer. -- The temperature dependence of the adiabatic elastic constants, the elasto optic coefficients, the bulk moduli and shear moduli have been determined in the temperature range i5.5 K < T < 90.4 K. The elastic constants all show acoustic anomalies at the transition temperature of 20.4 K. A theoretical analysis has shown that these anomalies are consistent with a decrease in RT coupling from the high temperature phase I to the low temperature phase II. Through an analysis of acoustic velocities in high symmetry directions, CH₄ -II has shown a strong similarity to the rare gas solids, especially argon at low temperatures.

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