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Page 2 of 11 Zhang et al. Microstructures 2023;3:2023046 https://dx.doi.org/10.20517/microstructures.2023.57

INTRODUCTION
Relaxor ferroelectric solid solutions of (1-x)Pb(Mg Nb )O -xPbTiO (PMN-xPT) have long been the
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research focus over the last few decades by virtue of their superb piezoelectric and electromechanical
performance that is closely related to their complex microstructure . In pursuit of high piezoelectric
[1-6]
properties, normally two approaches are adopted: the intrinsic one through composition tuning of the solid
solution or doping [5,7,8] and the extrinsic one through domain/domain wall engineering [9-11] . To achieve high
electromechanical properties in the PMN-xPT system, regarding the intrinsic approach, the composition x
is usually tuned to a composition of x~0.3 near the morphotropic phase boundary (MPB), which separates
the rhombohedral and tetragonal phases. The improved piezoelectric performance has been attributed to
the intermediate monoclinic phase bridging the adjacent rhombohedral and tetragonal phases, thus
allowing easy polarization rotation [2,8,12] . The resulting domain structures at the MPB region in various
systems have also demonstrated beyond-binary multilevel switching and memristive behavior, which is
promising for the new paradigms of non-volatile memory applications [13,14] . When it comes to the extrinsic
approach, both direct current (DC) poling and an emerging alternating current (AC) poling approach,
along with various other approaches, have been utilized to enhance the piezoelectricity through domain wall
engineering [1,3,15-28] .

Therefore, PMN-30PT is of particular research interest for revealing its structure-property relationship.
This interest stems not only from its status as an MPB composition but also from its rich domain structures.
From the composition perspective, the hierarchy of domain structures has been known to evolve with
different Ti concentrations. Pb(Mg Nb )O (PMN) and PbTiO (PT) are typical relaxors and classic
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ferroelectric systems featuring short-range-ordered polar nanoregions and long-range-ordered domains,
respectively. The increased concentration of PT leads to an increased off-center polar displacement within
the unit cell accompanied by mechanical stress [29,30] . To accommodate the elastic strain, smaller domains are
self-assembled into larger ones with orientations along the polarization variants. Bian et al. have
investigated the domain structures of PMN-xPT systems at diverse compositions and their associated local
piezoresponse . From the domain engineering perspective, significant effort has been made to examine the
[31]
relationship between the domain structure and the electromechanical performance in PMN-xPT systems.
The piezoelectric response of PMN-PT has been enhanced more than three times by increasing the domain
wall density through DC poling [1,16] . Recently, it has been found that three varied domain structures can be
[32]
induced by the DC poling on the patterned electrodes . Under AC poling, it is suggested that the field-
[22]
induced M phase contributes to the enhanced piezoelectricity in PMN-PT . It has also been discovered
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that with increasing AC poling cycles, the domain structure can evolve from 4R to 2R with a concomitant
[23]
increasing length of 109° domain walls . To note, 4R/2R denotes that there are four/two possible
spontaneous polarization directions along the polar axis in the rhombohedral phase . Additionally, the
[23]
elimination of light-scattering 71° domain walls has been used to fabricate transparent PMN-PT single
crystals under AC poling [3,33] .

In this work, three distinctive domain structures have been revealed in different regions of PMN-30PT
single crystals using piezoresponse force microscopy (PFM). X-ray diffraction (XRD) reciprocal space
mappings (RSM), a series of rotational in-plane (IP) PFM, and trailing field experiments have been used to
confirm the M structure of the crystal, and three-dimensional (3D) polar domain variants have been
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reconstructed. The correlation between the domain structures and the local electrical switching properties
has been revealed using switching spectroscopy PFM (SSPFM).