Stable and Metastable Phase Diagram of the Two-Component System (CH<INF>3</INF>)<INF>3</INF>CCl−(CH<INF>3</INF>)CCl<INF>3</INF>:  An Example of Crossed Isodimorphism

The phase diagram of the two-component system 2-methyl-2-chloropropane ((CH<INF>3</INF>)<INF>3</INF>CCl) and 1,1,1-trichloroethane ((CH<INF>3</INF>)CCl<INF>3</INF>) has been determined by means of X-ray powder diffraction and thermal analysis techniques from 180 K to the liquid state. The isomorphism relationship between the stable orientationally disordered (OD) face-centered cubic (FCC) phase of (CH<INF>3</INF>)<INF>3</INF>CCl and the metastable OD FCC phase (monotropic behavior with respect to the OD rhombohedral stable phase) of (CH<INF>3</INF>)CCl<INF>3</INF> has been put into evidence throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] (L = liquid) for the whole range of composition. This equilibrium interferes with a rhombohedral (R) plus liquid ([R + L]) equilibrium, giving rise to a peritectic invariant. The validity of the well-known crossed isodimorphism formalism has been experimentally demonstrated, and a subsequent thermodynamic assessment has been performed to obtain the excess properties of the OD FCC and R phases. In addition excess properties of the R and FCC phases are compared, and the existence of short-range orientational order for the R phase is inferred.