Domain variance and superstructure across the antiferroelectric/ferroelectric phase boundary in Pb1-1.5xLax(Zr0.9Ti0.1)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 Pb1-1.5xLaxZr0.9Ti0.1O3 (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 1/2{hkl} positions consistent with rotations of the octahedra in antiphase. Previously, neutron diffraction suggested that the FER 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 a(0)a(0)c(-) 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 approximate to b) monoclinic tilt system in the vicinity of the AFE/FE boundary.