  The study of grain size effects in ferroelectric ceramics has attracted great
research interest over the last 50 years. Although different theoretical models
have been proposed to account for the variation in structure and properties
with grain size, the underlying mechanisms are still under debate, creating a
significant level of uncertainty in the field. Here, we report the results of a
study on the influence of grain size on the structural and physical properties
of Ba(Ti0.96Sn0.04)O3, which represents a model perovskite system, where the
effects of point defects, stoichiometry imbalance and phase transitions are
minimized by Sn substitution. It was found that different microscopic
mechanisms are responsible for the various grain size dependences observed. In
fine-grained ceramics, high permittivity is due to high domain wall density and
polar nanoregions; high d33 in coarse-grained ceramics results from a high
degree of domain alignment during poling; large electric field-induced strain
in intermediate-grained ceramics is an outcome of a favourable interplay
between constraints from grain boundaries and reversible reorientation of
non-180 degree domains and polar nanoregions. These paradigms can be regarded
as general guidelines for the optimization of specific properties through grain
size control.
