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

Abstract

The experimental phase diagram of the CBrCl3+CBr4 system has been determined by means of X-ray powder diffraction and thermal analysis techniques from 200 K to the liquid state. Before melting, the two components have the same orientationally disordered (OD) face-centered cubic phase, and solid-liquid equilibrium is explained by simple isomorphism. The application of multiple crossed isopolymorphism formalism to the low-temperature solid-solid equilibria has enabled the inference of an OD rhombohedral metastable (at normal pressure) phase for CBr4. Experimental determination of the pressure-volume-temperature and construction of the pressure-temperature phase diagrams for CBr4 reveal the existence of a high-pressure phase, the rhombohedral symmetry of which is inferred by means of the thermodynamic assessment of the experimental phase diagram and demonstrated by means of high-pressure neutron diffraction measurements. The procedure used in this work confirms the connection between the appearance of metastable phases at normal pressure and their existence at high-pressure.

Comparison of the atomic level structure of the plastic crystalline and liquid phases of CBr2Cl2: neutron diffraction and reverse Monte Carlo modelling

“…In this special phase the molecular centres are located in a highsymmetry lattice (face centered cubic, body centered cubic or rhombohedral), but the molecules can rotate around their equilibrium position. The three characteristic phases (ordered, plastic crystalline and liquid) have been studied extensively for each halomethane mentioned above [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

Melting of orientational degrees of freedom

“…However, to 57 date most high-pressure theoretical and experimental studies are rather limited and 58 have focused mainly in the determination of the T -p diagram whereas high-pressure 59 energetic and volume data are scarce, basically due to the technical difficulties and 60 lack of standard commercial high-pressure equipment [11][12][13][14][15][16][17][18][19]. Clausius-Clapeyron is 61 then evaluated at normal pressure.…”

Relationship between the two-component system 1-Br-adamantane + 1-Cl-adamantane and the high-pressure properties of the pure components

“…The thermodynamic analysis has been based on the crossed isopolymorphism [3,6,7,11,13] and the Equal-Gibbs-Curve concepts [39]. We briefly describe both concepts for the sake of completeness.…”

“…Even more, the field of phase diagrams involving at least two chemical species, the temperature-composition (T-X) phase diagrams, is generally decoupled from the field of the polymorphic behavior at high pressure. Nevertheless, thermodynamics imposes an univocal relation between the controlled pressure and temperature conditions at which a polymorph should exist and the physical properties derived at normal pressure and, hence, the properties that can be extracted from a temperature-composition phase diagram [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The other way around also holds, properties derived for stable or metastable phases emerging in the temperature-composition phase diagrams can provide information about the nature of high-pressure phases even when they do not exist as stable phases at normal pressure.…”