Phase And Domain Structure Of Pb(Mg_1/3Nb_2/3)O₃-0.34PbTiO₃ Ferroelectric Single Crystals

Relaxor-based ferroelectric single crystals have excellent piezoelectric property,for example,the piezoelectric coefficient and electromechanical coupling coefficient of ferroelectric single crystals Pb(Mg_1/3Nb_2/3)O₃-0.34PbTiO₃ can reach 2820 pC/N and 0.94,which are significantly higher than the traditional ferroelectric materials,and this excellent piezoelectric properties is called giant piezoelectric.In order to further improve the piezoelectric properties of relaxor-based ferroelectric single crystal as well as the preparation of a new-type ferroelectric material of high performance,scientists at home and abroad have adopted a variety of methods to explore the source of its giant piezoelectric.Nowadays,the idea that crystal structures are the most important factors which affect its performance has been widely accepted by researchers.But,until now,there are still many deficiencies for the crystal structures of relaxor-based ferroelectric single crystal PMN-xPT,especially for the samples in associate with the structures in the MPB region.Therefore,we selected the PMN-0.34 PT whose phase existed in MPB region,and investigated its phases and domain structures in the following aspects.Firstly,we have measured the dielectric temperature spectrum of the sample,and verified the composition by the empirical formula.Then,the sample was cut and polished to insure the surface achieve optical level.Behind,using the polarizing microscope（PLM）and piezoresponse force microscope（PFM）respectively studied the extinction angles,the macroscopic domain structure and microscopic domain structure of the（001）C surface in the PMN-0.34 PT ferroelectric single crystal.The analysis shows that the phase structure of the sample is coexist of T phase and MC phase,and the T phase accounts for a major role at room temperature.Afterwards,we have determined the variable temperature range,heating rate,cooling rate and appropriate temperature observation points by analyzing the dielectric spectrum.The extinction state and domain wall orientation of the sample at different temperatures were observed in situ by PLM.It was found that the phase transition of the sample is T and MC phases→T phase→C phase occurred during the process of increasing from room temperature to 200 oC.It can also be found that there is no formation of the mesophase structure over the entire temperature range,but only the transition between the MC phase and the T phase domain structure occurs.Then,the PMM-0.34 PT ferroelectric single crystal（001）C surface was heated by PFM heating ceramic module,and the evolution of microstructure structure with temperature was studied.With the increase of temperature,the 180o domain structure began to decompose into a regular band-like nanostructure with a smaller domain wall orientation of 0o,and many polar nanotubes（PNRs）appeared.When the temperature increased to180 oC,the domain wall disappears and becomes a single domain structure,and the piezoelectric response intensity becomes zero.That is,the ferroelectric phase transforms into the electric phase structure.Finally,the microstructure of the（001）C surface was partially polarized by the PFM probe tip,and the polarized region was scanned.The evolution of the process of the microstructure of PMN-0.34 PT ferroelectric single crystal was studied under different voltage.The results show that the flip-flop path of the 180o domain under the action of voltage is T-a/T-b,which is directly connected to the T-c domain.The inversion process is a mutation process without the formation of the mesophase structure;rather than the area of the flipping path of the 180o band domain structure under voltage is not the same.A part of the region is directly from the T-a / T-b twin structure to the T-c domain structure;and a part of the region is formed by the 90o domain wall formed by the T-a/T-b twin structure to the 60odomain wall formed by the MC-a/MC-b twin structure to the T-c domain structure,the inversion process is a gradual process,which produced the mesophase.That is,the phase transition process are the T phase→MC phase→T phase.When the voltage increases to 40 V,the entire observation area becomes a single domain structure,and the spontaneous polarization orientation is all turned to the applied voltage direction.