Brillouin scattering experiments on single crystals of cyclohexane

Ahmad, Syed Faiz (1977) Brillouin scattering experiments on single crystals of cyclohexane. Masters thesis, Memorial University of Newfoundland.

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Abstract

Brillouin scattering experiments were carried out on single crystals of cyclohexane (C₆H₁₂) in the plastic phase, near the triple point, and adiabatic elastic constants were evaluated. Four single crystals were grown in a quartz cell. The crystals were rotated and their orientations with respect to the laboratory frame of reference were determined using X-ray Laue transmission methods. Light (514.5 nm wavelength) from an Ar-ion laser was incident along the vertical axis of the cylindrical cell, and scattering at an angle of 90° was analyzed and recorded by photon counting techniques using a piezoelectrically scanned Fabry-Perot interferometer, photomultiplier tube and a data acquisition and stabilization system. -- The spectra usually showed a quasilongitudinal component and a single, very much weaker, quasitransverse component; only three spectra showed both transverse components. By least-squares fitting 59 different frequency shift measurements, the adiabatic elastic constants were determined to be: at 279.2 ± 0.2 K (in units of 10⁹ N m⁻²), -- C₁₁ = 2.8(57) , C₁₂ = 2.4(72) , C₄₄ = 0.35(8) -- with an estimated absolute uncertainty of 2%. This resulted in an adiabatic bulk modulus Bs = 2.60 x 10⁹ N m⁻², and an elastic anisotropy factor A = 1.86. -- The results have been compared with those obtained by ultrasonic measurements on polycrystalline samples of C₆H₁₂ , with those of other plastic molecular crystals, and also with rare-gas and non-plastic molecular crystals. This is the only experiment where the elastic constants of a plastic crystal of complex molecules have been studied very close to the triple point. It was further established that, even at the triple point, the crystal remained anisotropic.

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