Isomorphous phase transition and orientational freezing in hexagonal mixed crystal C_(70(1-x))C_(60x). A pseudospin model

摘要

A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2→hcp-1 in pure C_(70) and in C_(70)-rich mixed crystal C_(70(1-x))C_(60x). With the specific anisotropic pseudospin interactions adapted to the C_(70) crystal the model is equivalent to a two-state Ising model in a temperature-dependent field. Replica symmetric state of the model is shown to approach the critical point when the width of distribution of random fields and/or of random bonds increases. The temperature of the phase transition and the phase equilibrium temperature then are practically constant, whereas the experiment shows their strong decrease with x. The main effect of dilution resides in an x-dependence of the model parameters. Dilatometric data on the hexagonal C_(70(1-x))C_(60x) are used to fit these parameters. A metastable disordered phase subsisting below the phase transition is discovered in a range of the model parameters and is shown to be responsible for the macroscopic behaviour of the system. A good agreement with experimental data is obtained for the spontaneous strain and for the x-dependence of the hysteresis.