Condensed Matter Physics, 1996, No. 7, p. 163-177, English
The two-sublattice pseudospin model of Mitsui type supplemented by the interaction with electron subsystem is considered. Such a model can be used for the description of dielectric properties of YBaCuO-type superconductors along c-axis (the pseudospins represent anharmonic motions of apical oxygen O4). The behaviour of the the transverse dielectric susceptibility \chi_\perp is considered in the noninteracting clusters approximation. Possibility of different temperature dependencies of \chi_\perp is established. The free energy of the model is evaluated in the mean field approximation. It is established that the long-range interaction between pseudospins can lead to symmetry breaking and to appearance of the ferroelectric component of pseudospin ordering. The phase transitions into the state with decompensated dipole moments can be both of second and first order. The corresponding (Tc, h) diagrams (h is the asymmetry parameter of the field acting on pseudospins) at different values of long-range interaction constants in the regimes of fixed values of electron concentration (n=const) or chemical potential (\mu=const) are obtained. The anomalies of susceptibility \chi_\perp in the vicinity of phase transitions points are investigated.Comments: Refs. 11, Figs. 8, Tabs. 0.
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