LATTICE GAS MODEL FOR THE PROTON SUBSYSTEM IN SUPERIONIC CRYSTALS

Ihor Stasyuk, Natalija Pavlenko

Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine

It is common knowledge that the application of the Ising model goes far beyond the phenomena of magnetism. In particular, using the simple transformation from pseudospin variables to the occupation numbers this model can be resresented in the form of the so called lattice gas model. In this case the chemical potential which is entered here for the description of systems with a varying number of particles plays the role of external field.
Comparing the results of MFA and CEM, it can be noted that the main features of thermodynamical properties of considered system may be described within the mean-field approximation allowing for the longrange proton interactions. In this case the obtained results agree with the experimental data, but the better quantitative agreement could be obtained by taking into account the molecular field fluctuations. The short-range proton correlations considered in CEM don't change qualitatively the topology of phase diagram but lead to a change in transition temperature and favour the transformation of the II-III transition order from second to first one.
Besides the description of the thermodynamics the proton ordering model can be basic for the further investigations of proton transport processes in this system.
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