Condensed Matter Physics, 1997, No 9, p. 89-106, English
Authors: I.V.Stasyuk, K.O.Trachenko (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii St., 290011 Lviv-11, Ukraine)

In the context of description of the structural phase transition in solids, the anharmonic potential effecting on the ion in the crystal lattice is selected in the form that in addition to quadric includes cubic anharmonicity, thus possessing more general non-symmetrical form, and in the symmetrical form with the Gaussian barrier. The approach based on the numerical diagonalization of the one-site Hamiltonian matrix is applied. The peculiarity of the energy spectrum of the system with the Gaussian barrier potential, particularly the existence of the parts of the spectrum with the quasidoublets or quasi-equally distanced levels structure which crossover takes place in the narrow energy interval, is established. The dependencies of the order parameter, free energy and dielectric susceptibility on the externally applied field and the order parameter and free energy on the temperature are derived for various parameters of both models. The phase diagram (temperature-field) is derived and the influence of the anharmonicity on the form of the phase coexistence curve and location of the critical point is studied. It is shown that the asymmetry of the local potential results in the deviation of the phase coexistence curve from the vertical line in the plane (temperature-field) and in the appearing of the first order phase transition occurring as the temperature varies.
Comments: Figs. 16, Refs. 15, Tabs. 2.

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