Origin of the Superlattice Reflections in Pb(Fe2/3W1/3)O3

Abstract

Two types of superlattice reflections have been observed in pressureless-sintered Pb(Fe 2/3 W 1/3 )O 3 perovskite ceramics from selected-area electron diffraction patterns (SADPs). The presence of commensurate superlattice reflections of the {1/2 1/2 1/2} type found in as-sintered samples persisted after prolonged postsintering annealing at 800°C for 300 h. These reflections, conventionally termed the F-spots, represent the existence of the nanoscale cation-ordered domains of cubic perovskite Pb 1-x (Fe 1/2 W 1/2 )O 3 . However, the satellite-type incommensurate superlattice reflections, appearing as sideband spots to the fundamental reflections and occurring only along , (110), and (100), have gradually reduced their intensities and finally disappeared on postsintering annealing at 800°C. Observations via transmission electron microscopy (TEM) suggested that the black dots revealed in bright-field (BF) images have given rise to the incommensurate superlattice sideband spots. These dots of ∼5-10 nm are in fact a different type of nanocrystalline domain whose structure (and chemical composition) differs from those of the disordered Pb(Fe 2/3 W 1/3 )O 3 matrix and the ordered nanoscale Pb 1-x (Fe 1/2 W 1/2 )O 3 domains. The nanocrystalline precipitates containing the paracrystalline phase are pyrochlore (Pb 2 (Fe,W)O 6.5 ) formed during powder preparation via the solid-state mixed-oxide route. They become dispersed in the Pb(Fe 2/3 W 1/3 )O 3 perovskite matrix grain and have also undergone phase transformation to perovskite progressively during sintering and postsintering annealing. It is suggested that the nanocrystalline pyrochlore precipitates are metastable and disappear when chemical composition within the perovskite grains becomes homogenized from solid-state diffusion on postsintering annealing.