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

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 (T_C) 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(Mg_1/3Nb_2/3)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 T_C, 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 Sn^2+/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.