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24 further demonstrated that aligning PNR vibrational modes by a poling electric field can enhance the phonon softening. 23 proposed that the softening of the transverse acoustic mode was due to the existence of PNRs, while Manley et al. Therefore, there have been numerous studies to understand the relationship between the high piezoelectric properties and PNRs in relaxor–PT systems 23, 24, 25, 26, 27. These factors lead to a unique characteristic of relaxor-based ferroelectrics in contrast to classical ferroelectrics, that is, the presence of polar nanoregions (PNRs) 17, 18, 19, 20, 21, which are believed to be responsible for the high dielectric properties of relaxors 17, 18, 22. Relaxors, for example, PMN, are characterized by cation disorder on the nanoscale 17, 18, 19, 20, leading to random fields 19, 20 and local phase fluctuations 17, 18. To further clarify the origin of ultrahigh piezoelectricity in relaxor–PT crystals, the microstructural characteristics of relaxor ferroelectrics should be surveyed first. However, these mechanisms fail to explain why relaxor-ferroelectric solid solutions exhibit significantly higher piezoelectricity when compared with non-relaxor-based MPB ferroelectrics, for example, Pb(Zr xTi 1− x)O 3 crystals (500-1,000 pC N −1 for x around 0.5). The corresponding mechanisms include ‘electric field-induced phase transition’ 6, ‘ease of polarization rotation via a monoclinic phase’ 13, 14, ‘giant electromechanical response as a critical phenomenon’ 15, ‘adaptive domain structure’ 16 and so on. One of the most remarkable breakthroughs in perovskite ferroelectrics was the discovery of ultrahigh piezoelectricity ( d 33*=1,500-2,500 pC N −1) and electromechanical coupling factors ( k 33*>0.9) in domain-engineered relaxor–PbTiO 3 (PT) solid solution crystals with MPB compositions, for example, Pb(Mg 1/3Nb 2/3)O 3–PT (PMN–PT) and Pb(Zn 1/3Nb 2/3)O 3–PT (PZN–PT) crystals 6, 7, 8, 9, 10, 11, 12.īy considering the diversity and instability of ferroelectric phases at MPBs, a number of qualitative mechanisms have been proposed to elucidate the high piezoelectric activity in relaxor–PT crystals. In proximity of an MPB, different ferroelectric phases possess similar energies, leading to facilitated variation of polarization and strain under an external stimulus. The enhanced piezoelectric response of perovskite ferroelectrics are generally associated with the long-range cooperative phenomena near morphotropic phase boundaries (MPBs) 1, 2, 3, 4, 5. Perovskite ferroelectrics (general formula, ABO 3) exhibit the highest electromechanical activity among all known piezoelectrics. This mechanism emphasizes the critical role of local structure on the macroscopic properties of ferroelectric materials. A mesoscale mechanism is proposed to reveal the origin of the high piezoelectricity in relaxor ferroelectrics, where the polar nanoregions aligned in a ferroelectric matrix can facilitate polarization rotation. The contribution of polar nanoregions to the room-temperature dielectric and piezoelectric properties is in the range of 50–80%. Here we quantitatively characterize the contribution of polar nanoregions to the dielectric/piezoelectric responses of relaxor-ferroelectric crystals using a combination of cryogenic experiments and phase-field simulations.
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Despite two decades of extensive studies, the contribution of polar nanoregions to the underlying piezoelectric properties of relaxor ferroelectrics has yet to be established. A key signature of relaxor-ferroelectric solid solutions is the existence of polar nanoregions, a nanoscale inhomogeneity, that coexist with normal ferroelectric domains. The discovery of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution single crystals is a breakthrough in ferroelectric materials.