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.

From the Two-Component System CBrCl3 + CBr4 to the High-Pressure Properties of CBr4

“…43 For the compounds belonging to the series CBr 4-nCl n , n ) 0, 2, 3 (compound with n ) 1 seems to be chemically unstable), it has been shown the existence of an OD R phase. 16,19,41,42 Similar findings have been found within the (CH 3 ) 4-n CCl n , n ) 0,...,4 series. 36,[44][45][46] In addition, OD R mixed crystals between such compounds are known to exist for a noticeable range of composition.…”

Connecting the normal pressure equilibria of the two-component system CCl(CH3)3+CBrCl3 to the pressure–temperature phase diagrams of pure components

“…Similar behaviour has been recently found for the system CCl 4 + CBr(CH 3 ) 3 system [28]. The analysis of the excess properties of the OD R and FCC phases and the ubiquitous relations with the dipole moment of the components will be discussed in a next work together with some related systems sharing compounds of the methyl-chloromethane and halogenomethane series.…”

From High-Temperature Orientationally Disordered Mixed Crystals to Low-Temperature Complex Formation in the Two-Component System (CH3)3CBr + Cl3CBr

“…45 Similar behavior was recently found for the system CCl 4 þ (CH 3 ) 3 CBr. 40 The analysis of the assessed excess properties of the OD R and FCC phases evidences the role played by the entropic term in the excess Gibbs energy, which counterbalances the positive excess enthalpy.…”

“…They involved MHM derivatives and thus bring into play crossed interactions between methyl and halogen. 40,41 Unfortunately, the low tem perature region of these phase diagrams has not been reported, and thus the influence of such intermolecular interaction within the fully ordered state is unknown.…”