An X-ray diffraction study of the crystal structures of hexamethylethane and its phase transitions

ISSN： 1362-3028

Source： Molecular Physics, Vol.37, Iss.5, 1979-05, pp. : 1333-1348

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Abstract

We have examined the Im 3 m -phase of hexamethylethane and low temperature phases β and δ, by Bragg and diffuse X-ray scattering. The Im 3 m phase fits well to a four thermal-parameter model at 188 and 295 K ( R W = 12 per cent). The data suggest that significant improvement may come only from inclusion of intermolecular correlation between centre-of-molecular-mass and the orientation of neighbouring molecules' ethane axes. This results in a ‘static' centre-of-mass positional disorder and a distribution in potential barriers to rotation about the molecular ethane axes. The β and metastable δ phases are tentatively identified as P 2 1 / c and P 4 2 / mnm , each derived from the Im 3 m structure by ordering of the molecular ethane axes, together with small distortions. The effect of crystal strain in stabilizing the δ-phase is discussed. Atom-atom intermolecular potentials are used to calculate the stabilities of various phases and the important correlations in the disordered Im 3 m phase.