Brillouin spectroscopic studies of the orientationally disordered phase of C₂F₆

Andrews, Gordon Todd (1992) Brillouin spectroscopic studies of the orientationally disordered phase of C₂F₆. Masters thesis, Memorial University of Newfoundland.

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Abstract

The elastic constants and elasto-optic coefficient ratios of the orientationaIly disordered β-phase of C₂F₆ have been determined using the technique of high resolution Brillouin spectroscopy. Just below the triple point, at 169K, the values of the elastic constants (in units of kbar) are: -- C₁₁ = 26.9 ± 0.10, -- C₁₂ = 19.32 ± 0.10, -- C₄₄ = 4.44 ± 0.04. -- The elasto-optic coefficient ratios at 169K are: -- ρ12/ρ11 = 1.09 ± 0.09, -- ρ44/ρ11 = 0.12 ± 0.01. -- The temperature dependence of all three elastic constants was also determined over the range 169K to 144K. C₁₁ and C₁₂ appear to have linear temperature dependences, with ΔC₁₁/ΔT = -.335 ± 0.010 kbar/K, and ΔC₁₂/ΔT = -0.236 ± 0.010 kbar/K. C₄₄, however, appears to be independent of temperature in this range, with ΔC₄₄/ΔT = 0.005 ± 0.011 kbar/K. -- Values of the ratios of acoustic velocities in high symmetry directions in the crystal indicate near perfect elastic isotropy and that moderately strong rotation-translation coupling is present in β-C₂F₆. The extent of the coupling is of the order of that observed in plastic (CH₂CN)₂. This is in contrast to the structurally similar orientationally disordered phase of SF₆, which behaves elastically like a rare gas solid.

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