  Recently, a series of reports showing ultra-high electrostrain (> 1 %) have
appeared in several Pb-free piezoceramics. The ultrahigh electrostrain has been
attributed exclusively to the defect dipoles created in these systems. We
examine these claims based on another report arXiv:2208.07134 which
demonstrated that the measured electric field driven strain increased
dramatically simply by reducing the thickness of the ceramic discs. We prepared
some representative Pb-free compositions reported to exhibit ultrahigh strain
and performed electrostrain measurements. We found that these compositions do
not show ultrahigh electrostrain if the thickness of the discs is above 0.3 mm
(the disc diameters were in the range 10- 12 mm diameter). The ultrahigh strain
values were obtained when the thickness was below 0.3 mm. We compare the
electrostrain obtained from specimens designed to exhibit defect dipoles with
specimens that were not designed to have defect dipoles in Na0.5Bi0.5TiO3 (NBT)
and K0.5Na0.5NbO3 (KNN) -based lead-free systems and could obtain much higher
strain levels (4- 5 %) in the defect dipole free piezoceramics in the small
thickness regime. Our results do not favor the defect dipole theory as the
exclusive factor for causing ultrahigh strain in piezoceramics. A new approach
is called for to understand the phenomenon of ultrahigh electrostrain caused by
the thickness reduction of piezoceramic discs.
