Polymorphism of 2-methyl-2-chloropropane and 2,2-dimethylpropane (neopentane):: Thermodynamic evidence for a high-pressure orientationally disordered rhombohedral phase through topological p-T diagrams

The trimorphism of (CH3)(3)CCl, a member of the (CH3)(4-n)CCln series, is reexamined as a function of the temperature and pressure. Its topological p-T diagram is constructed resorting to the available thermodynamic and crystallographic data for the four observed crystalline solids and the data extrapolated from binary phase diagrams sharing compounds of this series. The p,T coordinates of the five stable triple points are calculated together with the slopes of the two-phase equilibrium curves. Through these results, the uncharacterized phase IV (a high-pressure orientationally disordered stable phase) is shown to coincide with the rhombohedral phase (R) which is the orientationally disordered stable phase for the members with n = 2, 3, and 4 at ordinary pressure. In addition, the existence of a isomorphous high-pressure R phase is postulated for the first member [neopentane, (CH3)(4)C] of the series.