Domain variance and superstructure across the antiferroelectric/ferroelectric phase boundary in Pb_1−1.5xLa_x(Zr_0.9TiM_0.1)O₃

Transmission electron microscopy, x-ray diffraction, relative permittivity as a function of temperature, and polarization versus field loops were used to study the antiferroelectric/ferroelectric (AFE/FE) phase boundary in Pb_1–1.5xLa_xZr_0.9Ti_0.1O₃ (PLZT, 100x/90/10) ceramics. X-ray diffraction and electrical measurements indicated a FE rhombohedral (R) to AFE tetragonal (T) phase transition between PLZT 2/90/10 and 4/90/10. Both phases exhibited superstructure reflections in electron-diffraction patterns at ½{hkl} positions consistent with rotations of the octahedra in antiphase. Previously, neutron diffraction suggested that the FE_R has an a⁻a⁻a⁻ tilt system (Glazer notation), in agreement with its macroscopic symmetry. By analogy, it is proposed that the AFE_T phase has an a⁰a⁰c⁻ tilt system. The AFE phase was also characterized by incommensurate superstructure along pseudocubic 〈110〉_p directions, whereas the FE phase had extra commensurate superlattice reflections at 1/2{hk0}^p positions. 1/2{hk0}_p reflections are forbidden in both tilt systems, but their presence is explained by Pb ion displacements averaged along 〈111〉 but with short coherence antiparallel components along 〈110〉 directions. The antiparallel Pb displacements are coupled to an a⁻b⁻b⁻ (a ≈ b) monoclinic tilt system in the vicinity of the AFE/FE boundary.