POSSIBLE ORIGIN OF FERROELECTRICITY IN YBA2CU3O7 COMPOUND

A two-sublattice pseudospin model in which the apical oxygens in the Y BaCuO compound have two inequivalent positions in an asymmetric double -well potential is proposed. The free energy of the model is evaluated in the mean field approximation and the phase diagram is studied. It is shown that there exists a temperature range ( 90 K < T < 250 K) whe re a ferroelectric phase is favoured i.e. in this region the two subla ttice polarizations become different resulting in a nonzero macroscopi c polarization. The present model implies that the potential of the IR active mode involving the in-phase motion of O4 ions along the c-axis is a symmetric double-well potential while for A(g) Raman mode involv ing anti-phase O4 ion motion is of a single-well type.