Polymorphism of CBr₂Cl₂

Abstract

The polymorphism of dibromodichloromehtane (CBr 2 Cl 2 ) has been studied by means of a wide set of experimental techniques as a function of temperature and pressure. From the p-V-T diagram and the derived p-T diagram the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray and neutron powder diffraction. By combining the experimental techniques, it has been demonstrated the existence of a highpressure orientationally disordered rhombohedral phase (a R E 14.6 AE 0.3 A ˚and a E 89.2 AE 0.21). The existence of a glass transition within the monoclinic (C2/c, Z = 32) low-temperature ordered phase associated with the freezing of exchange positions between Cl and Br atoms is analyzed considering the asymmetry of the intermolecular interactions by means of the study of the thermal expansion tensor. The change of the derivative of the aspherism index, previously reported as a possible ''fingerprint'' for this kind of glass transitions, is found.

The Pressure-Temperature Phase Diagram of Metacetamol and Its Comparison to the Phase Diagram of Paracetamol

“…For α V,I , it leads to 2.50×10 -4 K -1 and for α V,II 1.98×10 -4 K -1 can be found. Both expansivity coefficients are close to the average value of about 2×10 -4 K -1 found for molecular solids [10][11][12][13][14][15][16][17][18][19] and active pharmaceutical ingredients. [20][21][22][23] However, a closer study of the two expressions 2 and 3 and the data in Table 1 demonstrates that although form I appears to be denser at low temperature, it exhibits the largest thermal expansion (α V,I > α V,II ) and at room temperature its density is actually less than that of form II.…”

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

“…A common feature that makes these systems attractive is that an orientationally disordered crystalline phase -also called 'plastic crystal' -appears between the low-temperature ordered phase and the liquid phase [5,6,10]. 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.…”

“…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].…”

Monoclinic mixed crystals of halogenomethanes CBr4−nCln (n=0,…,4)

“…Data corresponding to mixed crystals between CBr 2 Cl 2 and CCl 4 prove the same conclusion and make then evident the isostructural character of all the compounds of this family. These data will be published soon [26]. It should be stressed that substitutional mixed crystals correspond to a disordered structure, from a compositional distribution point of view, in which molecules of the two components are distributed regularly over the sites of the lattice.…”