FERROELASTICITY IN Y-BA-CU-O

A spring-defect model has been developed by us earlier for describing the connection between elastic distortion and oxygen ordering and the concomitant structural phase transition (from tetragonal to orthorhomb ic at T-s similar to 900 K) in the high T-c (similar to 90 K) supercon ductor YBa2Cu3O7-delta (Y-Ba-Cu-O). This model is used to discuss ferr oelasticity, exhibited in the orthorhombic phase of Y-Ba-Cu-O. Ferroel asticity is shown to be a consequence of the interaction between elast ic (Zener) dipoles, formed around each oxygen defect. The effective Is ing interaction between the defects mediated by host springs, derived earlier, can be recast in the equivalent form of a strain-strain inter action. This form is eminently suitable for discussing the paraelastic to ferroelastic transition, temperature dependence of the ferroelasti c order parameter and the static compliance. Results for these quantit ies, obtained in mean field theory, are presented. The notable feature of this work is not only a derivation of the defect-defect interactio n, but also an explicit calculation of the strain dipole tensor associ ated with each defect, from a microscopic model.