Excellent Electromechanical Properties And Mechanism Of Ternary System Relaxor Ferroelectric Single Crystals

Relaxor-based ferroelectric single crystals have become a hot topic due to their superior piezoelectric and electromechanical coupling properties.Among all the relaxor ferroelectrics,the ternary system Pb(In_1/2Nb_1/2)O)3-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃（PIN-PMN-PT）and manganese doped PIN-PMN-PT（PIN-PMN-PT:Mn）are the most concerned systems for their improved phase transition temperature,coercive field and mechanical quality factors.In recent years,the mechanism of ultra-high piezoelectricity in relaxor ferroelectric single crystals has been investigated extensively.Most of the studies focus on the structure instability,i.e.lattice deformation.The properties of crystals are not only related to lattice deformation,but also to the domain wall motions under electric bias.However,few works investigated dielectric/piezoelectric properties from the point of domain wall motions.In addition,after the introduction of Mn,the mechanical quality factor Qm is greatly enhanced.It is generally believed that Qm can be improved by acceptor doping,while the pinning effect of acceptor ions on anisotropic Qm enhancement is not clear,which lead to ambiguity of the understanding of doping effect,thus affecting the design of single crystals with lower mechanical loss.Moreover,it is found that Pb(Sc_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃（PSN-PMN-PT）ceramics and PSN-PT single crystals exhibit excellent temperature and electric field characteristics,so we design and grow the ternary PSN-PMN-PT crystals.The electromechanical properties and mechanism of ternary PIN-PMN-PT,PIN-PMN-PT:Mn and PSN-PMN-PT single crystals are extensively studied in the following aspects.The domain wall dynamics and electric properties of PIN-PMN-PT and PIN-PMN-PT:Mn single crystals were investigated.The reversible and irreversible dielectric responses and their temperature dependence were quantitatively evaluated by Rayleigh analysis.Both reversible and irreversible dielectric responses elevated with temperature increasing.The contribution of irreversible dielectric to the total dielectric response increases from about 1% at room temperature to 34% and 13% at around TR–T for PIN-PMN-PT and PIN-PMN-PT:Mn,respectively.That is to say,the polymorphic phase transition has a great influence on reversible and irreversible dielectric response.In addition,the Rayleigh analysis was carried out in the frequency range of 0.1-1000 Hz for PIN-PMN-PT and PIN-PMN-PT:Mn.The dielectric responses from lattice deformation,reversible and irreversible domain wall motions at 2 Hz were 65%,23%,12% for PIN-PMN-PT and 91%,6%,3% for PIN-PMN-PT:Mn,respectively.The influence of acceptor ions on dielectric response has been evaluated quantitatively.Results show that the intrinsic lattice deformation is slightly inhibited by Mn,while both reversible and irreversible domain wall motions are greatly suppressed to below 20% of the non-Mn doped values.The dielectric contribution of domain wall dynamics is described systematically,the source of high piezoelectric/dielectric property is revealed and the influence of acceptor ions on macroscopic property is clarified,which provides important information for the basic research and proves a method for the optimization of the functional properties.The mechanism of high mechanical quality factor in Mn doped PIN-PMN-PT single crystal was studied.the polarization rotation and domain wall motion in various modes（electric field parallel to poled direction E_∥ and perpendicular to poled direction E_⊥）for Mn-doped PIN-PMN-PT single crystals have been investigated comprehensively.The results demonstrate that the pinning effect on domain wall motion and polarization rotation is anisotropic.Combined with the domain switch characteristics under high electric field and the observation of domains by polarization light microscope（PLM）and piezoresponse force microscope（PFM）,it is concluded that the anisotropy result from the internal bias and charged domain wall.The polarization rotation is suppressed by the internal bias,whereas the domain wall motions are not only clamped by the internal bias,but are strongly pinned by charged domain walls.By comparing to undoped Mn single crystals,we also confirmed that the enhancement of longitudinal mechanical quality factor Q₃₃ is mainly attributed to the restricted polarization rotation,whereas the enhancement of shear mechanical quality factor Q₁₅ is ascribed to both restricted polarization rotation and clamped domain wall motions.Our results revealed the fundamental mechanism of the acceptor doping effects in relaxor-based ferroelectric materials,and provided important design principles to achieve high Qm piezoelectric materials to satisfy the demands for high-power electromechanical devices.The electromechanical properties and domain configuration of ternary PSN-PMN-0.33 PT relaxor ferroelectric single crystal have been investigated.The dielectric,piezoelectric and ferroelectric properties and their thermal behaviour have been investigated comprehensively.The results exhibit that the crystal has ultra-high coercive field（EC=8.2 k V/cm）,while the piezoelectric properties(d₃₃ = 2210 p C/N)and electromechanical coupling properties(k₃₃ = 91%)are maintained,which is much better than that of PMN-PT system（2.8 k V/cm）and PIN-PMN-PT system（5.5 k V/cm）.At the same time,the crystal shows excellent shear piezoelectric performances,the shear piezoelectric coefficient and electromechanical coupling coefficient are d₁₅=490 p C/N and k₁₅=55%,respectively.Moreover,the microscopic domain configurations and evolution with increasing temperature were observed by polarizing light microscope and piezo-response force microscope.It is proved that the higher piezoelectric properties,coercive field,phase transition temperature and shear properties originate from the smaller domain configuration and the bigger domain wall motion contribution（up to 27%）.