Growth And Characterization Of Rare Earth Modified Ferroelectric Crystals

RelaxorferroelectricsinglecrystalsPb(Mg_1/3Nb_2/3)O₃-PbTiO₃?PMN-PT?and Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃?PZN-PT?have aroused great interest in the research of ferroelectric and piezoelectric materials due to excellent electrical properties.However,the low rhombohedral-tetragonal phase temperature,the weak optical properties and the lower coercive field limit the applications of PMN-PT and PZN-PT single crystals in high temperature piezoelectric devices,optical devices,and high-power piezoelectric devices.In the present work,rare earth ions Ho³⁺?Er³⁺?Eu³⁺and Yb³⁺were modified PMN-PT and PZN-PT crystals to further improve the rhombohedral-tetragonal phase temperature,coercive field and optical properties.Meanwhile,the phase structure,microstructure and relaxation charateristics of the crystals were studied.The Ho³⁺modified PMN-32PT and Ho/Yb,Er/Yb,Eu/Yb modified PZN-9PT crystals were grown by using the flux method.The largest sizes of PMN-32PT:Ho,PZN-9PT:Ho/Yb,PZN-9PT:Er/Yb and PZN-9PT:Eu/Yb crystals are 5 mm×4 mm×3 mm,5 mm×5 mm×3 mm,4mm×3 mm×2 mm and 5 mm×4 mm×4 mm,respectively.The results of XRD study show that all the crystals are pure perovskite structure without the presence of the second phase,indicating that the rare earth ions enter the crystal lattices to form the solid solution.The results of SEM study show that the growth mode of the crystals is mainly stepped growth.The electrical properties study indicates that the E_C of crystals has been significantly improved through introducing rare earth ions.The E_C of PMN-32PT:Ho crystals is 4.31 kV/cm comparing to the pure PMN-32PT crystals?2.5 kV/cm?.The E_C of PZN-9PT:Ho/Yb and PZN-9PT:Er/Yb crystals is increased to 9.26 and 11.08 kV/cm,respectively,comparing to the pure PZN-9PT crystals?E_C<3.0 kV/cm?.For the PZN-9PT:Eu/Yb crystals,it is even higher,12.56 kV/cm.The T_R-T-T of the PZN-9PT:Er/Yb crystals is 380.22 K,which is obviously higher than that of pure PZN-9PT crystals(T_R-T=363 K).At the same time,the relaxation characteristics of the crystals are studied by the Curie-Weiss law,the modified Curie-Weiss law and the Lorentz formula.The PMN-32PT:Ho,PZN-9PT:Ho/Yb,PZN-9PT:Er/Yb and PZN-9PT:Eu/Yb crystals obtained the parameter ?_A are 40.87,36.48,46.43 and 43.96 K for the100 kHz,respectively.It is shown that all crystals have the typical characteristic of diffused phase transition and frequency dispersion.The introduction of rare earth ions makes the crystals have the upconversion and down conversion luminescence properties.Under the excitation of 465 nm wavelength,the PMN-32PT crystals realized down conversion luminescence of 550 nm?green light?and 590⁶74 nm?red light?.In the range of 1860³500 cm^-1,the transmittance of the crystals reaches60%.Meanwhile,the four obvious concave valleys are observed,which is located 2900,2345,1650 and 1419 cm^-1,indicating strong absorption at these locations.Under the excitation of 650nm,the crystal achieves upconversion luminescence of 480-520 nm?blue and green?,540-570nm?green?and 580-620 nm?yellow and orange?in the range of 400-630 nm.