CONSTRICTION OF THE POLARIZATION BY INCOHERENT OXYGEN OCTAHEDRAL TILTING IN RHOMBOHEDRAL-STRUCTURED LEAD-ZIRCONATE-TITANATE

Double hysteresis loops have been observed in ferroelectric lead zirconate titanate ceramics with a Zr/Ti ratio of 65/35 (PZT 65/35). In addition, dielectric dispersion was found near 50°C, corresponding to the high-temperature to low-temperature rhombohedral ferroelectric [FE R(HT)-FER(LT)] transformation. Structurally this transformation is an oxygen octahedra tilting along the 〈111〉. It is proposed that an inability of the oxygen octahedra to coherently rotate within a rigid lattice generates random internal stresses which constrain the polarization. Small amounts of impurities on either the A site (La) or B site (Nb) were found to result in normal square-loop hysteresis behavior. It is believed that aliovalent impurities relax the lattice, resulting in increased coherent of the 〈111〉 oxygen rotations. Electron diffraction studies revealed a strong increase of the reflection intensity of 1/2〈111〉 superlattice spots after doping, supporting this possibility. In addition, thermal expansion measurements revealed a lattice contraction after La-modification, possibly reflecting enhanced octahedral tilting. © 1995 American Institute of Physics.