Phase diagram, optical, nonlinear optical, and physicochemical studies of the organic monotectic system: Pentachloropyridine–succinonitrile

The two immiscible liquid phases in equilibrium with a single liquid phase have been observed during the phase diagram study of an organic analog of a metal–nonmetal system involving pentachloropyridine (PCP)–succinonitrile (SCN). The phase equilibrium shows the formation of a monotectic and a eutectic, with large miscibility gap in the system, containing 0.0456 and 0.9658 mole fractions of SCN, respectively, and the consolute temperature being 99.0 °C above the monotectic horizontal line. The heat of mixing, entropy of fusion, roughness parameter, interfacial energy, and excess thermodynamic functions were calculated based on enthalpy of fusion data determined via the differential scanning calorimetry method. The effects of solid–liquid interfacial energy on morphology of monotectic structure as well as the variation of interfacial energies with temperature have been discussed. The microstructures of monotectic and eutectic show peculiar characteristic features. The material properties of PCP and PCP doped with SCN crystals, grown by the Bridgman–Stockbarger method, have been studied via studying second harmonic generation efficiency, transparency range, and mechanical hardness.