Elastic constants of the orientationally disordered phase of C2F6 by Brillouin spectroscopy

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Abstract
The elastic constants and elasto‐optic coefficient ratios of the orientationally disordered β phase of C2F6 have been determined using the technique of high resolution Brillouin spectroscopy. Just below the triple point, at 169 K, the values of the elastic constants (in units of kbar) are C11=26.99±0.10, C12=19.32±0.10, and C44=4.44±0.04. The temperature dependence of all three elastic constants was also determined and appears to be linear for the temperature range investigated, with ΔC11/ΔT=−0.335±0.010, ΔC12/ΔT=−0.236±0.010, and 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.