Crystallographic features of ferroelectric states around the tetragonal/rhombohedral phase boundary in Pb_1-xLa_x(Zr _1-yTi_y)O₃

To understand dielectric properties around the ferroelectric tetragonal (F_T)/rhombohedral (F_R) phase boundary in Pb _1-xLa_x(Zr_1-yTi_y)O₃, the crystallographic features of ferroelectric states around the boundary have been investigated by transmission electron microscopy. It was found that, when the Ti content approached to the phase boundary in the F_T-phase area, a dielectric property became the relaxor behavior. The corresponding change in the crystallographic features is that the usual F_T state is converted into the nanometer-sized coexisting state of the ferroelectric monoclinic (F_M) and F_R phases. Because the crystal structure of F_R-phase regions in the coexisting state involves the R ₂₅-type rotational displacement of oxygen octahedra, the F _R phase can be identified as the low-temperature ferroelectric rhombohedral phase that is present in Pb(Zr_1-yTi_y)O ₃. It is thus understood that the relaxor behavior found in Pb _1-xLa_x(Zr_1-yTi_y)O₃ should be associated with the presence of the nanometer-sized coexisting state that consists of two ferroelectric phases.