Landau Parameters For Relaxor Ferroelectric PMN-PT And Their Behaviors Near Phase Transition Points

Relaxor ferroelectric single crystals always show higher performance of the piezoelectric responses than that of the classical ferroelectrics,and have wide application in structural health monitoring in civil engineering.The nano-polar regions are considered as the main reason for the ultrahigh properties of the relaxor ferroelectrics,however the mechanism of the excellent performance is still unknown.Further more,due to the lack of Landau parameters for relaxor ferroelectrics,the theoretical calculation for predicting the critical ferroelectric performances under the external field can not be carried on.Therefore,obtaining the Landau parameters for the relaxor ferroelectrics will help understand the mechanism of the ultrahigh piezoelectricity and explore the new-generation piezoelectric single crystal especial the lead-free ferroelectrics.According to the above problems,the paper made the following work:We analyze the effect of the polar nano-regions?PNRs?in the relaxors,and determine the properties of the PNRs in different temperature ranges.Then we condiser the nature of the PNRs into the relaxor ferroelectrics to study the phase structures in the relaxor ferroelectrics.Based on the composition-temperatute phase diagram of Pb(Mg_1/3Nb_2/3)O₃-x Pb Ti O₃?PMN-PT?,we sort out three different phase structres,the rhombohedral phase,the monoclinic phasse and tetragonal phase.Then we find out the different methods of obtaining the Landau pasrameters for PMN-PT with different composition.For the PMN-0.30 PT,which is close to the left boundary of morphotropic phase boundary,we get the Landau parameters by fitting the temperature-dependent spontaneous polarization and dielectric and the phase transition temperature from rhombohedral phase to tetragonal phase.Applying the landau parameters,we calculated the ferroelectric properties of PMN-0.30 PT,and compare the theoretical results with the experiments.For the coexistence phases appeared in the experiments,we used the canonical distribution to determine the phase fraction of the phases by judging the stability of the phases.Then the ex perimental results can be well revised.Since the singal domain state is demanded for fitting the Landau parameters,we observe the domain state at the temperatures around the phase transition by using the polarized light microscope.To apply the different direction of the electric field in our experiments,we cut the crystals with different orientation.By observing the optical properties of the crystal,the condition of forming the single domain state can be determined.In addition,we obtain different domain structures by poling the crystal at different temperatures and electric field.For the PMN-0.36 PT,which has major tetragonal phase at room temperature,we obtain the Landau parameters by fitting the hysteresis loops,dielectric constant,and the electric field-induced rhombehral phase.By using the parameters,we firstly calculated the temperature-dependent ferroelectric properties and compared results with the experiments.By using the obtained parameters,we calculated the phase diagram of film of PMN-0.30 PT and PMN-0.36 PT,and compare the differences between them.