Domain engineering via AC poling in relaxor-based monoclinic Pb(Mg1/3Nb2/3)O3-PbTiO3 piezoelectric crystals

Poling of relaxor-based piezo-/ferroelectric crystals with a rhombohedral perovskite structure with an alternating current (AC) electric field is an effective domain engineering technique, which allows to significantly improve the piezoelectric coefficients and dielectric constant compared to the conventional direct current (DC) poling method. However, the mechanisms responsible for the efficacy of the AC poling technique remain a subject of ongoing discussion. Among the suggested mechanisms are the changes in the density of ferroelectric domain walls and the field-driven transformation from the rhombohedral to a monoclinic phase. To investigate the poling mechanisms, we apply a combination of techniques such as polarized light microscopy, X-ray diffraction, and piezoelectric and dielectric measurements, and compare the effects of DC and AC poling of [001]-oriented 0.68Pb(Mg 1/3 Nb 2/3 )O 3 – 0.32PbTiO 3 single crystals, which have the monoclinic M B symmetry even before poling. The same monoclinic M B phase is observed after poling. The AC-poled crystals demonstrate a higher transparency, a larger dielectric constant and a piezoelectric constant d 33 = 2630 pC/N that is ∼ 50 % higher than in the DC-poled crystals. This increase is attributed to two reasons: the absence of domain walls inclined toward the surface electrodes (as a decreased local intrinsic piezoresponse is otherwise expected in the vicinity of inclined walls) and an intermediate switching of most domains. In these domains the angle between the poling field and the spontaneous polarization is not minimized. Our results underline the value of domain engineering strategy and demonstrate the effectiveness of the AC poling method in optimising the performance of monoclinic piezoelectric materials.