DEFECT-DIPOLE ALIGNMENT AND TETRAGONAL STRAIN IN FERROELECTRICS

We show the alignment of defect dipoles along the direction of the spo ntaneous polarization in polycrystalline Pb(Zr,Ti)O-3 and BaTiO3 ferro electric ceramics using electron paramagnetic resonance (EPR). The ali gnment is demonstrated:via orientation dependent paramagnetic centers in the polycrystalline materials and computer modeling of the EPR line shapes. It is shown that defect dipoles can become aligned by oxygen vacancy motion in the octahedron about a negatively charged center for the oxygen vacancy-related dipole complexes or by defect displacement and domain realignment in the lattice for isolated defect centers. We find that the alignment is not observed in nonferroelectric materials such as SrTiO3, and is destroyed in ferroelectric materials by heatin g above the Curie temperature. These observations suggest an interplay between distortion in the unit cell and the ability to align defect d ipoles, as is the case more generally for ferroelectric dipole alignme nt. We also directly observe aligned intrinsic Ti and Pb ion displacem ents in the ferroelectric Pb(Zr,Ti)O-3 perovskite unit cell. (C) 1996 American Institute of Physics.