Eur. Phys. J. B 24, 155-165 (2001)
https://doi.org/10.1007/s100510170003

Isomorphous phase transition and orientational freezing in hexagonal mixed crystal C_70(1-x)C_60x. A pseudospin model

¹ Institute of Nuclear Physics, ul. Radzikowskiego 152, 31-342 Kraków, Poland
² Institute for Experimental Physics, University of Vienna, Strudlhofgasse 4, 1090 Wien, Austria
³ Institute for Computer Science, Technical University of Czestochowa, Al. Armii Krajowej 17, 42-200 Czestochowa, Poland

Corresponding author: ^a Piotr.Zielinski@ifj.edu.pl

Received: 20 April 2001
Revised: 26 September 2001
Published online: 15 November 2001

Abstract

A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2->hcp-1 in pure C₇₀ and in C₇₀-rich mixed crystal CC. With the specific anisotropic pseudospin interactions adapted to the C₇₀ 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 CC 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.