Study On The Growth Technology Of Rare Earth Doped Relaxation Ferroelectric Single Crystal PSN-PMN-PT

Rare-earth ions doped Pb(Sc_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-Pb Ti O₃（PSN-PMN-PT）ferroelectric single crystals show a great application prospect in acoustic-optical-electrical coupling devices due to their excellent ferroelectric,piezoelectric,pyroelectric,electrostriction and optical properties.In recent years,most researchers have paid close attention to this system.However,the systematic exploration on the growing technology of these crystals is rare,which makes the success rate of growing crystal is low,limiting the study on electrical and optical properties of the rare-earth ions doped crystals.In this thesis,the effect of process parameters adjustment in high temperature solution method on the crystal quality of rare earth ion doped PSN-PMN-PT is analyzed.The optimized technology for growing rare-earth ions doped PSN-PMN-PT crystals were proposed.On this basis,the rare earth ions Dy³⁺,Sm³⁺,Gd³⁺,Tb³⁺doped PSN-PMN-PT crystals were grown.The applicability of the optimized growth process for the growth of different rare-earth ions doped crystals was investigated,and the structure and properties of the grown crystals were characterized.（1）Taking the growth of Dy-doped PSN-PMN-PT crystals as a case,we investigat the effects of the cosolvent ratio,holding time,and cooling rate on the macro-morphology and size of the crystal systematically.It is found that the increase of the cosolvent ratio in the selected range is beneficial to obtaining cubic crystal in shape,but has no obvious influence on the dimension and defects of the crystal.The size of the crystal is significantly affected by the cooling rate,and the crystal with a larger size can be achieved when the cooling rate is lower.The holding time has almost no effect on the size and macro-morphology of the crystal.In the light of the macro-morphology and size of the as-grown crystal,the optimized technological parameters for rare-earth ions doped PSN-PMN-PT crystals are determined:the ratio of cosolvent is 80%,the cooling rate is 2.64℃/h,and the holding time is 10 h.（2）The maximum size of as-grown Dy-doped PSN-PMN-PT crystal is about 12 mm×8mm×5 mm,but its shape is irregular rather than cubic and there are many internal defects.Meanwhile,some obvious growth defects can be observed in the crystal.The XRD result shows that Dy-doped PSN-PMN-PT crystal possesses a pure perovskite structure.The Curie temperature T_c,coercivity field E_c,and piezoelectric constant d₃₃ of grown Dy-doped PSN-PMN-PT crystal are 166℃,20.9 k V/cm,and 870 p C/N,respectively.The emission spectrum of Dy-doped PSN-PMN-PT crystal excited by 453 nm laser shows two obvious emission peaks at 490nm（blue light）and 580 nm（yellow light）corresponding to the energy level transition of 4f electron from ~4F_9/2 to ~6H_15/2and ~6H_15/2,respectively.（3）Sm-doped PSN-PMN-PT crystal was grown using the same technological parameters as the Dy-doped crystal.The maximum size of the crystal is about 5 mm×4 mm×3 mm.It is found that rare-earth Sm-doping is beneficial to achieving the crystal with a cubic shape and reducing the defects in the crystal.The XRD result shows that Sm-doped PSN-PMN-PT crystal is pure perovskite structure.The remanent polarization P_r and coercivity field E_c are 38μC/cm~2 and 11.85k V/cm,respectively.The dielectric constant is 2610,the piezoelectric constant d₃₃ is 878 p C/N,and the Curie temperature is 146℃.Excited by 408nm laser,the emission peaks located at 563nm（green light）,600 nm（orange light）,649 nm,and 709 nm（red light）are detected,corresponding to the energy level transition of 4f electron:~6G_5/2→~6H_5/2,~6G_5/2→~6H_7/2,~6G_9/2→~6H_5/2 and ~6G_5/2→~6H_11/2,respectively.（4）Gd-doped PSN-PMN-PT crystals are grown using the same technological parameters as the Dy-doped crystal.The maximum size of the crystal is about 8 mm×5 mm×3 mm.It is found that the introduction of the rare-earth Gd is beneficial to achieving the crystal with a cubic shape and reducing the defects and impurities in the crystal.The XRD result shows that Gd-doped PSN-PMN-PT crystal is a pure perovskite structure.The remanent polarization intensity P_r is 16.1μC/cm~2,the coercivity field E_c is 3.51 k V/cm,the dielectric constant is 4476,and the piezoelectric constant d₃₃ is 934 p C/N.The phase transition temperature is obtained by studying the temperature dependence of the dielectric constant.The Curie temperature of crystal is 155℃and the rhombohedral to tetragonal phase transition temperature is 133℃.（5）Tb-doped PSN-PMN-PT crystal is grown using the same technological parameters as the Dy-doped crystal.The maximum size of the crystal is 4 mm×3 mm×3 mm.It is found that the introduction of the rare-earth Tb is helpful to achieve the crystal with a cubic shape,and decreases the defects of the as-grown crystal.However,the size of the crystal is also reduced.The XRD result shows that the crystal is a pure perovskite structure.The residual polarization intensity P_ris 8.94μC/cm~2,the coercivity field E_c is 3.46 k V/cm,the Curie temperature is 144℃,the rhombohedral to tetragonal phase transition temperature is 97℃,the dielectric constant of the crystal is 2329,and the piezoelectric constant d₃₃ is 1666 p C/N.The crystal exhibits five emission peaks located at 505 nm,518 nm,534 nm,566 nm（green light）,and 602 nm（red light）,respectively,under excitation of 245 nm UV light.These five emission peaks correspond to energy level transition of 4f electron of Tb³⁺ion,respectively:~5D₃→~7F₆,~5D₃→~7F₅,~5D₃→~7F₄,~5D₄→~7F₆,and ~5D₄→~7F₅.