Hypersound Speeds in Sulfur Hexafluoride

Sound speeds were measured from 2 to 32 amagats in sulfur hexafluoride at 26°C by spontaneous Brillouin scattering. The observed dispersion can be accounted for entirely by the nonideality of the gas. It is concluded that vibrational contributions to the specific heat are completely relaxed. No evidence of rotational relaxation was observed. Comparison of these data by commonly used techniques with those determined from ultrasonic measurements is shown to be inadequate in the case of sulfur hexafluoride for the densities used. Comparison can be made using the exact thermodynamic equation and assuming relaxing specific heat. The difference between the smoothed experimental results and the exact calculated values is less than 0.7% over the density range studied.