CORE-SHELL STRUCTURES IN PRESSURELESS -SINTERED UNDOPED Pb(Fe_(2/3)W_(1/3))O_3 CERAMICS

摘要

Core-shell structures have been found by both scanning and transmission electron microscopy in undoped Pb(Fe_(2/3)W_(1/3))O_3 ceramics when a solid-state-reacted powder of stoichiometric composition was sintered at 800 deg C. The presence of superlattice reflections of the {1/2 1/2 1/2}-type, often termed F-spots, indicates that the existence of nanoscale ordered domains of 2-5 nm size in both core and shell is due to chemical ordering of the B-site Cations with alternating Fe~(3+) and W~(6+) along the direction. The ceramic is densified by the assistance of a low-temperature liquid eutectic at approx 690 deg C. A formation mechanism of core-shell structures in Pb(Fe_(2/3)W_(1/3))O_3 ceramic is proposed. It can be described by solution-reprecipitation and phase transformation subsequently to perovskite Pb(Fe_(2/3)W_(1/3))O_3, involving at least two intermediate phases which are formed at temperatures below and above the low-temperature liquid eutectic point and related to pyrochlore (Pb_2FeWO_(6.5)). The Cores are Characterised by "black dots" and Contain relatively Fe-excess. The shells exhibiting featureless Contrast are free of "black dots" but Contain W-excess relative to the cores. Long term sintering at 800 deg C has facilitated the inward diffusion of W~(6+) to the grain interior when the distribution of the B-site Cations is improved. As a result, the shell increases its size at the expense of the core.