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

The p-bromoiodobenzene/p-diiodobenzene phase diagram has been determined fr om samples prepared by quenching from the melt, from room temperature to th e melting. The solid state miscibility between the monoclinic p-bromoiodobe nzene and the orthorhombic p-diiodobenzene is extensive and it is only inte rrupted by a narrow region of demixing derived from a peritectic invariant. In the course of this investigation non-stable mixtures of two molecular a lloys have been identified from Xray diffraction experiments on samples of the central compositions prepared by precipitation from solution. The pheno menon can be easily confused with a region of demixing when observed by X-r ay diffraction, particularly in the case of two pure compounds with very cl ose crystalline unit cells. This finding could be the key for the interpret ation of the contradictions found in the 10 binary systems' group of the mo noclinic p-dihalobenzenes. In five of these systems, demixing phenomena not predicted by thermodynamic analysis were deduced from X-ray diffraction ex periments. The thermodynamic analysis of the experimental p-bromoiodobenzen e/p-diiodobenzene phase diagram confirms the absence of any demixing phenom enon for the central compositions where the non-stable mixtures were found. The coefficient of crystalline isomorphism epsilon (M) is more indicative of the observed solid state miscibility than Kitaigorodsky's coefficient ep silon (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. (C) 2001 Elsevier Science Ltd. All rights reserved.