title: Two-component system CCl4 + (CH3)3CBr: extrema in equilibria involving orientationally disordered phases.
abstract: Phase equilibria involving orientationally disordered (OD) and liquid phases of the two-component system between carbon tetrachloride (CCl4) and 2-methyl-2-bromomethane ((CH3)3CBr) have been determined by means of X-ray powder diffraction and thermal analysis techniques from 210 K up to the liquid state. The isomorphism relation between the OD stable face-centered cubic (FCC) phase of (CH3)3CBr and the metastable FCC phase of CCl4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition, despite the monotropy of the FCC phase for the CCl4 component with respect to its OD rhombohedral (R) stable phase. A continuous series of OD R mixed crystals is found, which confirms the R lattice symmetry of the OD phase II of (CH3)3CBr, for which the crystallographic results have been long-time misinterpreted. X-ray patterns of such a phase were indexed according to the recent single-crystal results obtained by Rudman (Rudman, R. J. Mol. Struct. 2001, 569, 157). In addition, some experimental evidences are given to confirm the number of molecules per unit cell (Z = 21). The thermodynamic assessment reproduces coherently the phase diagram for the stable [R + L] and [R + FCC] two-phase equilibria as well as for the partially metastable [FCC + L] two-phase equilibrium and provides a set of data for the thermodynamic properties of nonexperimentally available phase transitions of pure components. Surprisingly, the phase equilibrium involving R and FCC OD phases appears as one of the very few showing a solid-solid equilibrium with two extremes.
authors.affiliation: Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Catalonia, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Barrio
authors.forename: L C
authors.initials: LC
authors.lastname: Pardo
authors.forename: J Ll
authors.initials: JL
authors.lastname: Tamarit
authors.forename: Ph
authors.initials: P
authors.lastname: Negrier
authors.forename: J
authors.initials: J
authors.lastname: Salud
authors.forename: D O
authors.initials: DO
authors.lastname: López
authors.forename: D
authors.initials: D
authors.lastname: Mondieig