STRUCTURAL STUDIES OF CYCLOHEXANE-IV

We report an extensive neutron powder diffraction study of the high-pr essure phase IV of cyclohexane-d12. Using constrained Rietveld refinem ent we deduce approximate atomic positions for this phase which are fo und to be in close accord with the predictions of energy-minimization calculations. The likely relationship between the unit cell of phase I V and that of the ambient-pressure phase II is also presented. We furt her demonstrate, by means of an X-ray powder diffraction study of cycl ohexane-h12, that phase IV can be produced in a metastable state at am bient pressure by rapid quenching to 77 K from the liquid. This findin g contradicts a previous report [Burns & Dacol (1984). Solid State Com mun. 51, 773-775] claiming that the ambient-pressure metastable phase is an orientational glass.