Possible origin of ferroelectricity in YBa 2Cu 3O 7 compound

Abstract

A two-sublattice pseudospin model in which the apical oxygens in the YBaCuO 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) where a ferroelectric phase is favoured i.e. in this region the two sublattice polarizations become different resulting in a nonzero macroscopic 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 involving anti-phase O4 ion motion is of a single well type.