Phase Structure And Phase Transition In PMN-PT Single Crystals Near The MPB

Relaxor-based ferroelectric single crystals have attracted extensive attention in recent years as high-performance materials for ultrasonic transducers and piezoelectric actuators,owing to their ultrahigh electromechanical coupling and piezoelectric coefficients,far out-performing state-of-the-art polycrystalline lead zirconate titanate(PZT)ceramics.The structural origin of the high performance is a vital topic for scientists and the intrinsic structure around the morphotropic phase boundary(MPB)region still remains in dispute till now.In this work,we focus on this issue and systematically investigate phase structure,phase transition and phase stability in relaxor-based lead magnesium niobate-lead titanate(PMN-PT)single crystals near the MPB via domain structure observations combined with electrical characterizations.The phase structure of the PMN-PT single crystals near the MPB before and after poling is identified.The unpoled single crystals are either rhombohedral(R)or tetragonal(T)according to the extinction positions of the domains under the polarized light microscope.Poling at room temperature,a bridging phase appears near the rhombohedral-tetragonal(R-T)phase boundary.By domain structure and electrical property analysis,the bridging phase is identified to be orthorhombic rather than the reported monoclinic or adaptive regardless of the poling directions.Based on the temperature induced phase transition behavior,the temperature-composition phase diagrams are constructed for both the poled and unpoled PMN-PT single crystals.A domain memory effect is discovered during the temperature induced phase transitions in the PMN-PT single crystals near the MPB.The domain walls result in large-scale distortion of the lattice aside the domain walls due to the MPB driven structural instability in the crystals.Near the MPB,the domain size is small and can couple with the distortion regions so that the polarization rotation paths of the domains are directed by these distortion regions.Therefore the domain memory effect emerges based on this mechanism.The phase stability is also discussed based on the poling induced phase transitions and the poling effect on local structure in the PMN-PT single crystals near the MPB.The crystals experience irreversible phase transitions induced by the poling field near the R-T phase boundary,which is responsible for the appearance of the orthorhombic bridging phase.Due to the poor phase stability,the domain switch and phase transition compete with each other during the poling process near the MPB region.Meanwhile,the long-range ferroelectric order and the short range disorder are supposed to have a cooperative effect on the phase instability.The ordering of the local structure after the poling process,which gives rise to the free energy of the locally disordered rhombohedral and tetragonal phases,is responsible for the emergence of the hidden orthorhombic phase near the MPB.