The investigation of the plastic phase transition and the critical behaviour of the dielectric constant of a binary mixture

Two types of phase transition were investigated. The stability of the plastic phase in spite of a large increase in volume which is associated with its formation was studied. Mossbauer effect measurements were performed on an impurity of 1% SnMe4 in neopentane in the vicinity of the transition temperature. The conclusion is that the stability of the plastic phase cannot be explained by the non-weakening of the binding forces - as was formerly sugaested - since the latter are indeed reduced at the transition. A thermodynamic description of the plastic transition based on the expansion of the free energy in the translational and rotational order parameters (o.p.) was consequently developed. The description explains both the plastic phase stability and the occurrence of the transition close to the expected melting temperature. The behaviour of the dielectric constant (EPSILON) near the critical point (c.p.) of the binary mixture - nitrobenzene -hexane was also examined. An experimental setup by which the cooling down of the homogeneous mixture to the separation point could be obtained in a very slow temperature scan (2 mdegk/hr) was constructed. The behaviour of EPSILON as a function of the relative concentration near the transition temperature was also investigated. To explain the finite peak found in EPSILON, a model was developed. The Step Effect is explained as a geometrical effect caused by the presence of free volume of mixture outside the capacitor-plates region. (B.G.)