In-situ domain structure characterization of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ crystals under alternating current electric field poling

Alternating current (AC) electric field poling (AC-poling) has been actively studied for tailoring the domain configuration of relaxor ferroelectric crystals to further improve their piezoelectric and electro-optical properties. Understanding the change and redistribution of ferroelectric domains under AC electric field is essential for exploring the mechanism of the enhanced piezoelectricity in AC-electric-field-poled (AC-poled) crystals. Despite extensive investigations over recent years, no consensus has yet been reached on this topic. Here, we performed a three-dimensional synchrotron X-ray diffraction study on [001]_C-oriented rhombohedral Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) ferroelectric crystals to characterize the domain structure variation during the AC-poling process. Our study clearly reveals that the AC electric field has an ability to efficiently merge the ferroelectric domains on both sides of 71^o domain walls, leading to a considerable increase of domain size and thus a significant enhancement of electromechanical properties. In addition, this work indicates that, for the AC-poled [001]_C-oriented rhombohedral PMN-PT crystal, the most stable state should be the lamellar domain structure with only 109^o domain walls and the same volume fractions of the two types of ferroelectric domains on both sides of domain walls, which is expected to benefit the design of high-performance ferroelectric via domain or domain wall engineering.