Brillouin scattering studies of rotation-translation coupling effects in some simple molecular crystals

Askarpour, Vahid (1991) Brillouin scattering studies of rotation-translation coupling effects in some simple molecular crystals. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The role of rotation –translation (RT) coupling was studied in large single crystals of α-carbon monoxide, γ-oxygen, cyclooctane I and methane I by observing the behavior of acoustic phonons as a function of temperature particularly as the order-disorder phase transition is approached. To this end the adiabatic elastic constants were determined by the technique of high resolution Brillouin spectroscopy. – Large single crystals of α-carbon monoxide, γ-oxygen, methane I and cyclooctane I were grown and cooled in a low temperature cryostat. The quality of the crystals and the orientation of their crystal axes were dete3rminded by analyzing x-ray Laue diffraction photographs. Laser light of 5145 °A was incident along the axis of the cell and the scattered light was analyzed at 90° by a Brillouin spectrometer. – The adiabatic elastic constants of α-CO at 60.9 K were determined RT coupling appeared no to be significant in the q=0 wavevector regime and the α-CO crystals effectively behaved like a rare gas solid in this regime. The orientations of the fcc and hcp phases were correlated. – The adiabatic elastic constants of γ-O₂ were determined at 49.0 K, 46.5 K, 45.6 K and 44.0 K. The spectrum, close to the triple point temperature, exhibited unusually strong elastic isotropy and transverse component intensity. Cyclooctane I was studied for comparison to γ-oxygen since it had the same structure and RT coupling was known to be strong in this solid. The Brillouin spectra were recorded at 284.2 K, 274.2 K, 254.2 K, 234.2 K and 224.2 K. Cyclooctane I was also very isotropic and only one transverse component was ever observed for all the orientations and temperatures studied in this experiment. It was therefore concluded that the unusual features in the Brillouin spectra of γ-oxygen and cyclooctane I are caused by strong RT coupling in their peculiar structure. – The elastic constants of methane I single crystals were determined at 72. K, 50.9 K, 38.5 K and 25.0 K. It was noted that RT coupling effects become less significant as the rotational ordering is approached and methane I behaves like a rare gas solid at temperatures below 70 K. It was noted that RT coupling effects become less significant as the rotational ordering is approached and methane I behaves like a rare gas solid at temperatures below 70 K. Reduction in the value of C₁₂ was observed at 25.0 K which appeared to indicate the onset of the phase transition.

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