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

The phase diagram of the two-component systems (CH(3))(3)CBr + Cl(3)CBr has been experimentally determined by means of differential scanning calorimetry and X-ray powder diffraction techniques from the low-temperature ordered phases to the liquid state. Before melting, both components have the same orientationally disordered (OD) face-centered cubic (FCC) and rhombohedral (R) phases, and the two-phase equilibria [FCC + L] and [R + FCC] are accounted for by means of the existence of an isomorphic relationship between the OD phases of pure compounds. The thermodynamic assessment of such equilibria enables us to get the excess properties of the involved OD phases and to rationalize the existence of a maximum and a minimum in the [R + FCC] equilibrium on the basis of the contribution of the entropic term in the excess Gibbs energy function. At low temperature, two complexes, (CH(3))(3)CBr:Cl(3)CBr (1:1) and (CH(3))(3)CBr:2Cl(3)CBr (1:2), appear. The structures of 1:1 and 1:2 complexes have been determined to be monoclinic (P2(1)/n, c, Z = 4) and hexagonal (P6(3), Z = 6). Within both "ordered" structures, the Cl(3)CBr entities of the asymmetric unit were found to be disordered so that sites have fractional occupancies of 0.75 and 0.25 for Cl and Br atoms, in the same way that it occurs for the low-temperature monoclinic (C2/c, Z = 32) phase of Cl(3)CBr. Finally, the existence of complexes is connected with the special intermolecular interactions appearing between a methyl group and a halogen, as previously inferred by Calvet et al. [T. Calvet et al. J. Chem. Phys. 1999, 110, 4841].