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Two-component system CCl4 + (CH3)3CBr: extrema in equilibria involving orientationally disordered phases
M Barrio1, L C Pardo, J Ll Tamarit
1Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Catalonia, Spain.
The Journal of Physical Chemistry. B
|June 28, 2006
Summary
This study reveals an isomorphism between orientationally disordered (OD) phases of carbon tetrachloride (CCl4) and 2-methyl-2-bromomethane ((CH3)3CBr). Phase equilibria were mapped, confirming a continuous series of OD rhombohedral mixed crystals.
Area of Science:
- Materials Science
- Crystallography
- Physical Chemistry
Background:
- Understanding phase equilibria in molecular systems is crucial for materials design.
- Orientationally disordered (OD) phases exhibit unique structural properties.
- The system of carbon tetrachloride (CCl4) and 2-methyl-2-bromomethane ((CH3)3CBr) presents complex phase behavior.
Purpose of the Study:
- To determine the phase equilibria of the CCl4 and (CH3)3CBr system across temperature and composition ranges.
- To investigate the isomorphism between the orientationally disordered (OD) face-centered cubic (FCC) phase of (CH3)3CBr and the metastable FCC phase of CCl4.
- To clarify the crystallographic symmetry of the OD phase II of (CH3)3CBr and its solid-solid equilibrium.
Main Methods:
- X-ray powder diffraction was employed to analyze crystal structures and phase transitions.
- Thermal analysis techniques were utilized to identify phase boundaries and transition temperatures.
- Thermodynamic assessment was performed to model the observed phase diagram.
Main Results:
- An isomorphism was demonstrated between the OD FCC phase of (CH3)3CBr and the metastable FCC phase of CCl4, evidenced by evolving lattice parameters.
- A continuous series of OD rhombohedral (R) mixed crystals was identified, confirming the R lattice symmetry for OD phase II of (CH3)3CBr.
- The study revealed a rare solid-solid equilibrium between R and FCC OD phases with two extreme compositions.
Conclusions:
- The phase diagram of the CCl4-(CH3)3CBr system was coherently reproduced, including stable and metastable equilibria.
- The findings resolve long-standing misinterpretations of the crystallographic data for OD phase II of (CH3)3CBr.
- The research provides valuable thermodynamic data for phase transitions not accessible through direct experimentation.
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