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Recently, a series of reports showing ultra-high electrostrain (> 1 %) haveappeared in several Pb-free piezoceramics. The ultrahigh electrostrain has beenattributed exclusively to the defect dipoles created in these systems. Weexamine these claims based on another report arXiv:2208.07134 whichdemonstrated that the measured electric field driven strain increaseddramatically simply by reducing the thickness of the ceramic discs. We preparedsome representative Pb-free compositions reported to exhibit ultrahigh strainand performed electrostrain measurements. We found that these compositions donot 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 strainvalues were obtained when the thickness was below 0.3 mm. We compare theelectrostrain obtained from specimens designed to exhibit defect dipoles withspecimens 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 higherstrain levels (4- 5 %) in the defect dipole free piezoceramics in the smallthickness regime. Our results do not favor the defect dipole theory as theexclusive factor for causing ultrahigh strain in piezoceramics. A new approachis called for to understand the phenomenon of ultrahigh electrostrain caused bythe thickness reduction of piezoceramic discs.

Ultrahigh electrostrain > 1% in lead-free piezoceramics: A critical review