Multiple melting of orientationally disordered mixed crystals of the two-component system (CH3)2CCl2–(CH3)CCl3

Abstract

Orientationally disordered (OD) stable and metastable mixed crystals of the two-component system 2,2-dichloropropane ((CH₃)₂CCl₂) + methylchloroform ((CH₃)CCl₃) have been characterized from crystallographic and thermodynamic points of view. Continuous series of mixed crystals in the stable rhombohedral (R) phase points out an isomorphism relationship. The monotropic behavior of the non-isomorphous OD metastable phases in the pure components (simple cubic, SC, and face-centered cubic, FCC, respectively) with respect to the R stable phase is partially retained by the mixed crystals. In particular, SC mixed crystals were found persistent enough as metastable crystals in the whole composition range with the exception of the pure compound (CH₃)CCl₃, for which only the FCC metastable phase was experimentally available. On the contrary, samples with compositions greater than X = 0.70 in (CH₃)CCl₃ crystallized into either the FCC or SC, both being metastable with respect to the R phase. Thus, three kinds of OD mixed crystals appear which give rise to three independent OD + Liquid equilibria. The thermodynamic analysis of such non-interfering equilibria, [R + L], [SC + L] and [FCC + L], enables us to infer the stability relationships between the whole of OD phases.