INCOMMENSURATELY MODULATED POLAR STRUCTURES IN ANTIFERROELECTRIC SN-MODIFIED LEAD-ZIRCONATE-TITANATE - THE MODULATED STRUCTURE AND ITS INFLUENCES ON ELECTRICALLY-INDUCED POLARIZATIONS AND STRAINS

Tin-modified lead zirconate titanate ceramics, Pb-(0.99)Nb-0.02-[(Zr0.55Sn0.45)(0.98)Ti-y]O-3 (PZST 45/y/2), have been studied by hot-stage transmission electron microscopy (TEM), dielectric spectroscopy. Sawyer-Tower polarization (P-E) measurements, and electrically induced strain (epsilon-E) techniques. The compositional range chosen for investigation was 0 < y < 0.09. Electron diffraction confirmed the existence of 1/x[110] superlattice reflections in the previously designated antiferroelectric tetragonal state. For PZST 45/0/2, the room-temperature modulated structure was found to be commensurate with the lattice and isomorphous with the antiferroelectric orthorhombic phase of PbZrO3. With increasing Ti content, the modulation wavelength (lambda) increased and became incommensurate. At high temperatures, lambda was found to be incommensurate for all compositions. A model is discussed which suggests that the origin of incommensuration is due to a competition between antiferroelectric and ferroelectric tendencies. P-E studies revealed antiferroelectric-ferroelectric (AFE-FE) polarization curves which possessed no hysteresis in the incommensurate region, indicating a thermally reversible AFE-FE transformation. On cooling, strong hysteresis developed as the incommensurate structure became metastably locked. Investigations then revealed that the electrically induced strain associated with the AFE-FE transformation was not realized until field strengths were attained significantly above that required for polarization saturation. It is believed that the electrically induced strain is decoupled from the polarization due to AFE-FE switching by the field modulation of the incommensurately modulated polar structure.