Properties Of Surface Acoustic Waves In Lead Magnesium Niobate And Lead Zinc Niobate-based Ferroelectric Single Crystals

Relaxor-based ferroelectric single crystals lead magnesium niobate-leadtitanate （1-x）Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃（PMN-PT） and lead zinc niobate-leadtitanat （1-x）Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃（PZN-PT） have attracted considerable attentionin the past several years due to theirs excellent piezoelectric, dielectric, optical andacoustical properties compared to traditional piezoelectric materials. Hence, thesecrystals are favored in the next generation electromechanical devices, includingultrasonic transducers, actuators, ultrasonic motors, etc. The PMN-PT and PZN-PTsingle crystals have tremendous potential applied in surface acoustic wave （SAW）devices. SAW velocities of PMN-33%PT have been measured by Choi KH et al.However, there is no report on the literature on the detailed theoretical analysis ofSAW propagation properties for PMN-PT and PZN-PT single crystals. For meeting theplenteously potential application requirements in the future, surface acoustic wavepropagation characteristics of PMN-PT, PZN-PT and lead indium niobate-leadmagnesium niobate-lead titanat （PIN-PMN-PT） crystals are studied in this paper,and results could contribute to the design of SAW devices.Firstly, using the measured material parameters of PMN-PT relaxor-basedferroelectric single crystals, the SAW propagation characteristics in PMN-PT singlecrystals at rhombohedral, tetragonal and morphotropic phase boundary be governed bythe Christoffel equation with semi-infinite boundary conditions. Reasons whyPMN-PT single crystals have superior SAW performance were analyzed from thecrystal structure. The theoretical results indicate that, the PMN-PT single crystals allhave superior SAW performance at rhombohedral, tetragonal and morphotropic phaseboundary. We found that the PMN-PT have lower phase velocity and higherelectromechanical coupling coefficient compared and small power flow angle （PFA）to traditional piezoelectric materials, which could drastically improve the performanceof SAW devices. The SAW propagation properties of PMN-33%PT single domainsingle crystals poled along [001]care even better than that of the other componentscrystals. For example; the maximumk2value could reach as high as33.46%in thedirection5o canted, SAW phase velocity and power flow angle be less than1800m/sand1.5o, respectively. Components near MPB are the reasons why PMN-PT singlecrystals have superior SAW performance. After poling, rhombohedral phase andtetragonal phase are coexisting in PMN-33%PT single crystals, and engineereddomain configuration are formed, change of the crystals symmetry brings the largertrain. Moreover, the motion of domain wall was inhibited, and strain delay is small. By comparison, SAW properties of tetragonal phase PMN-38%PT single crystal bejust shy of PMN-33%PT. Moreover, the anisotropy of tetragonal phase PMN-38%PTsingle crystal was more significant than rhombohedral phase crystal. AlthoughPMN-PT single crystal has excellent properties, but poor temperature stability limitsits application.In addition, we also studied influence of polarization direction and ingredientchange on SAW propagation properties in PMN-PT ferroelectric single crystals. Ourresults showed that the same component but different polarization direction, PMN-PTsingle crystals have different SAW performance. SAW propagation properties inPMN-33%PT ferroelectric single crystals poled along [001]cdiffer from those of[111]c. It shows, the SAW properties of ferroelectric single crystals can be optimizedthrough changing crystals structure. The appropriate doping can effectively improvethe SAW properties of relaxor-based ferroelectric single crystals. In this paper theeffects of doping on SAW properties of single crystals were preliminary studied. TheSAW properties of0.26PIN-0.46PMN-0.28PT single crystals were studied bycontrast with0.71PMN-0.29PT single crystals. The study found lead indium niobatedoping can be helpful to enhance the temperature stability, which depresses the SAWperformance of0.26PIN-0.46PMN-0.28PT single crystals. But overall, the SAWpropagation properties of0.26PIN-0.46PMN-0.28PT single crystals poled along [011]care even better than that of the traditional piezoelectric materials.Because of the PZN-PT single crystals have better temperature stability thanPMN-PT, the we discussed the SAW propagation characteristics in PZN-PT singlecrystals at rhombohedral, tetragonal and near morphotropic phase boundary. We foundthat the SAW propagation properties of PZN-7%PT single crystal are even better thanthat of the other components crystals. The SAW propagation properties of PZN-8%PTsingle crystal are approximate to those of PZN-7%PT. The PZN-7%PT andPZN-8%PT single crystals have superior SAW performance, because spontaneouspolarization direction of rhombohedral PZN-PT single crystal is [111]c, when thePZN-PT single crystals are polarized along [001]c, they have engineered domainconfiguration with better piezoelectric properties than single domain structure.Moreover, the spontaneous polarization direction of tetragonal PZN-PT single crystalis [001]c. After poling along [001]c, the PZN-PT single crystals have single domainstructure, therefore SAW propagation properties of PZN-12%PT single crystal arelower than that of the rhombohedral crystals poled along [001]c. For example; themaximumk2value of PZN-12%PT single crystal only is about6.97%, far below thatof PZN-7%PT, PZN-8%PT and PZN-4.5%PT. In addition, when the same componentbut different polarization direction, PZN-PT single crystals have different SAW performance. The PZN-7%PT ferroelectric single crystals poled along [011]cwithmm2symmetry have better SAW propagation properties than crystals poled along[001]c, such as the maximumk2value of PZN-7%PT poled along [011]ccould reachas high as24.16%, this is far higher that of PZN-7%PT poled along [001]c.