Phase And Domain Structure In Relaxor-based Ferroelectric Lead Magnesium Niobate-lead Titanate Single Crystals

High-performance piezoelectric materials are vital to modern technology and scientific research.In the history of piezoelectric material investigations,the most outstanding piezoelectric property was obtained in relaxor-based ferroelectric single crystals with ABO3 perovskite structure.In the widely used relaxor-based ferroelectric,Pb(Mg1/3Nb2/3)O3-x Pb Ti O3(PMN-x PT)single crystal,the best piezoelectric property appears in the morphotropic phase boundary(MPB)composition,with d33 > 2800 p C/N and k33 > 94%.For the goal of designing the high performance and environmentally friendly ferroelectric materials as substitute of Pb-based ferroelectrics,the underlying structural origin of the ultrahigh piezoelectric property in the widely used PMN-x PT single crystal should be clarified.Although the underlying origin of the ultrahigh piezoelectric properties is still not fully understood,the structure is well recognized to be a critical factor.The investigation of the crystal and domain structure in PMN-x PT single crystal is of fundamentally significance.However,until now,the understanding about the structure of PMN-x PT single crystal is far from enough,and there are still some debates about the phases existed in MPB region.Therefore,more visualized and convincing data about the structure in the MPB region is urgently needed.In this thesis,we have investigated the structure of PMN-x PT in the following aspects.In the first part,the composition enforced structure evolution in a composition gradient PMN-x PT single crystal,which with PT content continuously varies from x=0.30 to x=0.37,have been investigated.The high energy synchrotron X ray diffraction(SXRD)measurement was continuously performed from the PT-deficient end to the PT-rich end of the sample,and the sample was rotated to obtain the diffraction patterns of H0 L,0KL and HHL reciprocal planes.Based on the pattern of(400)high energy SXRD spots on H0 L reciprocal plane,the complementary SXRD-3D-reciprocal space mapping(RSM)and piezoresponse force microcopy(PFM)measurement results of typical sample points,the composition enforced phase and domain evolution process was visually and systematically investigated.Besides,the structure evolution in MPB and phase transition regions have been intensively studied,the existence of monoclinic(M)phase was verified in this sample,and the structure evolution between different phases have been visually clarified.Additionally,the lattice parameters of typical sample point were calculated,the rest of high energy SXRD patterns were qualitatively simulated,and the structure analysis result was confirmed by the comparison between experimental and simulation patterns.Then,the structure and domain switching under PFM tip DC voltage(Vdc)of PMN-0.34 PT single crystal,which located at the boundary between MPB region and tetragonal(T)phase,have been intensively investigated.Based on the SXRD-3D-RSM results,the coexistence of T and MC phases in PMN-0.34 PT single crystal was confirmed.The affiliation of each diffraction spot in the 3D-RSM was identified with the assistance of qualitative simulation.Most importantly,based on the tails between different diffraction spots,the twinning structure between different domains in such a mixed phase PMN-PT crystal is firmly clarified.And then,the local twin domains and tip-voltage-induced domain switching of MC phase have been intensively investigated by PFM.By theoretically analyzing the experimental patterns of domain walls on(001)C face,the specific MC twin domains in the initial annealed state of a selected area have been clarified,and the polarization orientation of the MC phase in this sample have been derived.In addition,based on the evolution of domains and the motion of domain walls under the step-increased Vdc,the switching process and features of different types of MC domain variants were visually revealed.Phase and domain structure evolution induced by temperature and electric-field in [001]C oriented PMN-0.30 PT single crystal have been intensively studied.The temperature induced phase transition and domain switching process in zero-field-heating(ZFH)process was visually studied by polarization light microscopy(PLM)and PFM.By the complementary measurements of polarization-electric(P-E)loops and current-electric(I-E)curves,a temperature range in which the electric-field induced phase transitions would occur more efficiency was determined.The electric-field induced phase transition and domain switching process at 142 °C and in the field-cooling(FC)process was studied.Besides,the local domain switching under the step-increased PFM tip DC voltage(Vdc)is investigated.The time-dependent relax process of the poled area is closely observed,and the temperature stability of the domain structure in the poled area was also investigated.