Symmetry-bridging phase as the mechanism for the large strains in relaxor-PbTiO3 single crystals

Relaxor-PbTiO3 piezoelectric single crystals have been of a great interest, since the discovery of ultrahigh piezoresponse demonstrated in < 001 > -oriented crystals of the composition at the rhombohedral side of morphotropic phase boundary. It has been proposed that the exceptionally large piezoelectric properties should originate from an electric-field-induced polarization rotation that involves a reversible phase transformation between rhombohedral and tetragonal via monoclinic symmetry. However, this commonly accepted polarization rotation mechanism has its limit in explaining still the excellent piezoelectricity even at a small excitation field far below the coercive field. Here, we show by a comparative study using single crystals from two distinct processing techniques, the polarization rotation has, if ever, little influence on the strain properties of < 001 > -oriented rhombohedral relaxor-PbTiO3 single crystals. Instead, they may come from a reversible shear-mode piezoelectric contribution from electric-field-susceptible 'symmetry-bridging' unit-cell-level phases, the polarization direction of which spans monoclinic symmetry.