In the simple perovskite oxide $\mathrm{Pb}{\mathrm{Zr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ (PZT), an excellent piezoelectric response was obtained in the vicinity of a morphotropic phase boundary (MPB) between the ferroelectric monoclinic $\left(F_M\right)$ and rhombohedral $\left(F_R\right)$ phases. In order to understand the origin of this exceptional response, we have used transmission electron microscopy to investigate the detailed features of the ferroelectric domain structures near the MPB in PZT. Two types of domain structures, domains I and II, existed at room temperature in the $F_M$ side of the MPB, while the room-temperature structure in the $F_R$ side was confirmed to be a usual structure consisting of the 109° and 180° domain boundaries. The notable feature of these domain structures is that each domain in both the domain II and the $F_R$ domain structure near the MPB can be identified as an aggregation of nanometer domains with an average size of about $10\mathrm{nm}$. In order to clarify the formation of these domain structures, we conducted in situ observations from room temperature to about $800\mathrm{K}$. The results revealed that the usual $F_R$ domain structure at room temperature was produced by a conversion from a nanometer-sized domain structure consisting of nanometer ferroelectric domains, which formed just below the Curie temperature. The characteristic feature of the nanometer-sized structure is that nanometer regions with the [001] or $\left[00\overline{1}\right]$ polarization component were uniformly distributed in a large [110]-component area. Because spatial average of ⟨001⟩ components must be zero in a large [110]-component area, the nanometer-sized domain structure may have an average [110] polarization with average orthorhombic symmetry. In the $F_M$ side, on the other hand, a new banded domain structure appeared in the interior of each domain of domain II on heating at a temperature $T_T$, but basically disappeared at $T_T$ on subsequent cooling. This reversible change in the banded structure indicates that a phase transition occurs at $T_T$. Because the banded domain structure appeared in the heating process, the higher- and lower-temperature phases may have triclinic and monoclinic symmetries, respectively. In addition, a similar banded domain structure was observed in a poled sample. On the basis of the existence of this feature, we believe that the presence of the triclinic phase near the MPB may be the crucial factor responsible for the excellent piezoelectric response in PZT.

• Received 20 July 2005

DOI:https://doi.org/10.1103/PhysRevB.75.214111