Self-polarized high piezoelectricity and its memory effect in ferroelectric single crystals

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In ferroelectric materials, significant piezoelectric response only occurs after poling by an external electric field applied along a certain crystallographic direction, and the piezoelectric effect generally disappears upon heating to the Curie temperature (TC) where the poled state vanishes. This thermally induced depoling often restricts the temperature range for the applications of piezoelectric devices. It has been a challenge to develop materials that exhibit high piezoelectric performance that is not affected by electric poling, nor thermal depoling. Here we report an unusual piezoelectric effect found in Pb(Mg1/3Nb2/3)O3-PbSnO3-PbTiO3 ternary ferroelectric crystals which exhibit a relatively high piezoelectricity (with the piezoelectric coefficient d33 = 1350–1400 pC/N) without undergoing any poling process. Moreover, this high piezoelectric performance is retained after repeated thermal annealing at temperatures above TC, showing a piezoelectric memory effect. This uncommon piezoelectric effect is explained by a self-polarization mechanism based on the defect-dipoles formed in the crystals due to the presence of mixed valence states of Sn2+/4+. The defect-dipoles generate internal bias electric fields which, in turn, provide the stabilizing and restoring forces leading to preferred orientation states and the retention of spontaneous polarization. This mechanism offers a new perspective for designing novel piezoelectric materials with high performance and a wide temperature range of operation.