Non-stable mixtures of molecular alloys The p-bromoiodobenzene/p-diiodobenzene phase diagram

Abstract The p-bromoiodobenzene/p-diiodobenzene phase diagram has been determined from samples prepared by quenching from the melt, from room temperature to the melting. The solid state miscibility between the monoclinic p-bromoiodobenzene and the orthorhombic p-diiodobenzene is extensive and it is only interrupted by a narrow region of demixing derived from a peritectic invariant. In the course of this investigation non-stable mixtures of two molecular alloys have been identified from X-ray diffraction experiments on samples of the central compositions prepared by precipitation from solution. The phenomenon can be easily confused with a region of demixing when observed by X-ray diffraction, particularly in the case of two pure compounds with very close crystalline unit cells. This finding could be the key for the interpretation of the contradictions found in the 10 binary systems’ group of the monoclinic p-dihalobenzenes. In five of these systems, demixing phenomena not predicted by thermodynamic analysis were deduced from X-ray diffraction experiments. The thermodynamic analysis of the experimental p-bromoiodobenzene/p-diiodobenzene phase diagram confirms the absence of any demixing phenomenon for the central compositions where the non-stable mixtures were found. The coefficient of crystalline isomorphism e M is more indicative of the observed solid state miscibility than Kitaigorodsky's coefficient e K of molecular shape similarity. A detailed formalism is presented for extracting the characteristic temperatures of the DSC signals by means of two shape-factors.