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

Abstract

Transmission electron microscopy studies have been performed on a series of Pb(${\mathrm{Zr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ti}}_{\mathit{x}}$)${\mathrm{O}}_{3}$ compositions for 0\ensuremath{\le}x\ensuremath{\le}0.2. These studies revealed several important features of the phase transformational sequence. Locally ordered R-type and M-type oxygen octahedral rotations were found in the paraelectric state. Near the Curie temperature, a longer-range ordering of oxygen octahedral rotations was observed to develop. For x\ensuremath{\le}0.05, bright-field imaging and convergent beam electron-diffraction patterns revealed an intermediate ferroelectric phase with rhombohedral symmetry between low-temperature antiferroelectric orthorhombic and high-temperature paraelectric cubic states. This intermediate ferroelectric state was characterized by the presence of 1/2〈110〉 superlattice reflections, associated with ordered M-type oxygen octahedral rotations. Lattice imaging revealed nanosized domains which contained 〈110〉 structural modulations. With increasing Ti content, the intensity of the 1/2〈110〉 reflections was found to decrease. For 0.10x\ensuremath{\le}0.20, only weak diffuse 1/2〈110〉 reflections were observed; however, on cooling strong 1/2〈111〉 reflections were found to develop. It is believed that a hierarchy of symmetries exists over a wide region of the rhombohedral ferroelectric phase field. A modification of the high-Zr-content end of the Pb(${\mathrm{Zr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ti}}_{\mathit{x}}$)${\mathrm{O}}_{3}$ phase diagram is then proposed to acount for this hierarchy.