Transmission electron microscopy study of high-Zr-content lead zirconate titanate

Transmission electron microscopy studies have been performed on a series of Pb(${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ti}}_{\mathit{x}}$)${\mathrm{O}}_3$ compositions for 0≤x≤0.2. These studies revealed several important features of the phase transformational sequence. Locally ordered R-type and M-type oxygen octahedral rotations were found in the paraelectric state. Near the Curie temperature, a longer-range ordering of oxygen octahedral rotations was observed to develop. For x≤0.05, bright-field imaging and convergent beam electron-diffraction patterns revealed an intermediate ferroelectric phase with rhombohedral symmetry between low-temperature antiferroelectric orthorhombic and high-temperature paraelectric cubic states. This intermediate ferroelectric state was characterized by the presence of 1/2〈110〉 superlattice reflections, associated with ordered M-type oxygen octahedral rotations. Lattice imaging revealed nanosized domains which contained 〈110〉 structural modulations. With increasing Ti content, the intensity of the 1/2〈110〉 reflections was found to decrease. For 0.10<x≤0.20, only weak diffuse 1/2〈110〉 reflections were observed; however, on cooling strong 1/2〈111〉 reflections were found to develop. It is believed that a hierarchy of symmetries exists over a wide region of the rhombohedral ferroelectric phase field. A modification of the high-Zr-content end of the Pb(${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ti}}_{\mathit{x}}$)${\mathrm{O}}_3$ phase diagram is then proposed to acount for this hierarchy.