Core-shell Grain Structures and Dielectric Properties of Na0.5K0.5NbO3-LiTaO3-BiScO3 Piezoelectric Ceramics.

The origins of distinctive compositional dependence of relative permittivity, ɛr, in the Pb-free piezoelectric system (1-x)Na0.5K0.5NbO3-xLiTaO3, x≤ 10 mol.% modified with BiScO3 have been revealed using transmission electron microscopy with energy dispersive X-ray analysis (TEM-EDX). As the LiTaO3 content increased the Curie peak at ~370°C in ɛr –T plots became more diffuse, and at x= 5 mol.% an additional higher temperature peak occurred. TEM-EDX analysis showed the change in dielectric properties at x= 5 mol.% was due to a change in microstructure: micron-scale grains were replaced by submicron grains exhibiting core-shell chemical segregation. The outer shell was similar to the target solid solution composition, slightly enriched in Bi, Sc and Ta, whilst the core approximated to (Na, K, Li)NbO3 and was responsible for the additional dielectric peak. Examples of a novel three-tier metastable grain structure were observed for certain compositions.