The structural second virial coefficient

Abstract

The neutron scattering cross-section has been measured for low-density sulphur hexafluoride gas. The molecular term gives the S-F distance as 1·573 ± 0·008 Å. The intermolecular term is analysed in terms of the structural second virial coefficient [1]. Although a Lennard-Jones (LJ) 12–6 potential fits the second virial coefficient over a wide range of temperature it is clearly inconsistent with our structural data at 20°C. Two intermolecular potentials have been found which fit both the second virial and the structural second virial coefficient data. One is a LJ 28–7 potential, virtually that proposed by Hamann and Lambert [14]. The other is a simple anisotropic site-site potential. The sites are at the fluorine nuclei and have LJ 12–6 potentials between sites with parameters, ε/k _B K=61·39 and σ/Å=3·024. The site-site potential is preferred on physical grounds. Neither a simple average nor a Boltzmann average of our site-site potential, which are virtually LJ 28–7 and LJ 12–6 respectively, has any apparent connection with the previously proposed LJ potentials. Moreover our proposed site-site potential differs substantially from a previous one [19] which was found suitable for the plastic crystal phase of SF₆.

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M.B. Deraman

On leave from Tun Ismail Atomic Research Centre (Puspati), Kajang, Malaysia.