From high-temperature orientationally disordered mixed crystals to low-temperature complex formation in the...

Area of Science:

• Physical Chemistry
• Materials Science
• Crystallography

Background:

• Understanding phase diagrams of molecular systems is crucial for predicting material properties.
• Orientationally disordered (OD) phases, like face-centered cubic (FCC) and rhombohedral (R), are common in molecular crystals.
• Intermolecular interactions dictate the formation of complexes and influence phase behavior.

Purpose of the Study:

• To experimentally determine the phase diagram of the (CH3)3CBr + Cl3CBr two-component system.
• To investigate the thermodynamic properties and structural characteristics of the system's phases and complexes.
• To elucidate the role of intermolecular interactions in complex formation.

Main Methods:

• Differential scanning calorimetry (DSC) for thermal analysis.
• X-ray powder diffraction (XRD) for structural determination.
• Thermodynamic assessment of phase equilibria.

Main Results:

• The phase diagram was mapped from low-temperature ordered phases to the liquid state.
• Isomorphic relationships between OD phases (FCC and R) were identified.
• Two new complexes, (CH3)3CBr:Cl3CBr (1:1) and (CH3)3CBr:2Cl3CBr (1:2), were discovered with determined crystal structures.
• Complex structures exhibit disorder in Cl3CBr entities, similar to the pure Cl3CBr low-temperature phase.
• Excess properties of OD phases were calculated, explaining phase equilibria features.

Conclusions:

• The study provides a comprehensive phase diagram for the (CH3)3CBr + Cl3CBr system.
• The formation of 1:1 and 1:2 complexes is driven by specific methyl-halogen intermolecular interactions.
• Thermodynamic analysis rationalizes the observed phase equilibria, including maxima and minima.