Phase Manipulation And Related Luminescent Properties Of Rare Earth Doped Phosphate Materials

Rare earth doped phosphate luminescent materials have been widely used in display,solid-state emission devices,solar cells,and so on.Considering that the luminescent properties of some specific crystal phases/structure in the materials are much better than the others,it is of great significance to study the preparation and the relationship between the crystal phase and the luminescent properties of the rare earth doped phosphate luminescent materials.In this thesis,the yttrium phosphate(YPO4)and bismuth phosphate(BiPO4)doped with the europium ions(Eu3+)are synthesized by hydrothermal method.Based on controllable parameters of the crystal phases formation,a series of the powder samples are prepared by using three methods,i.e.adjusting the molar concentration of PO43-,changing the pH value and introducing additives Na2CO3,respectively.The crystal phases,morphologies and luminescent properties of the samples are characterized by XRD,SEM,PL and FT-IR.The crystal phase formation process and the main factors affecting the luminescent properties of YPO4:Eu and BiPO4:Eu are studied,and the rule between the preparation condition of YPO4:Eu and BiPO4:Eu and the factors regulating the crystal phases are discussed.The excitation spectra,emission spectra,lifetime,quantum efficiency and non-radiative transition probability of the samples with different crystal phases are analyzed in detail.The main results in this thesis are as follows:(1)It is found that for YPO4:Eu system,there is a common phase regulation rule that the low free energy in reaction solution of PO43-and RE3+(Y3++RE3+)help to form the hexagonal phase(HP),and improving the free energy turns toward the formation of the tetragonal phase(TP).(2)It is found that the crystal phase of YPO4:Eu changes gradually from HP/TP mixture to TP as the molar ratio increases from 1:1 to 9:1,and the crystal phase of the YPO4:Eu gradually transforms from HP into TP by improving the pH value from about 0.5 to 6.All of them follow the regulation rule in(1).The luminescent properties(excitation and emission intensity,lifetime,and quantum efficiency)of TP YPO4:Eu are much better than that of the HP one,It is due to that 1)the higher non-radiative transition probability caused by the high frequency vibration relaxation of water molecules in HP YPO4:Eu quenches the intensity and 2)the asymmetry of the TP is higher than that of the HP so as to improve electric dipole transition possibility of 5D0? 7F2.In addition,the morphology of HP YPO4:Eu is rod-like,while the TP ones show granular-like morphology.The more uniform the particle size is,the higher the luminescent properties become.(3)Based on this rule,the introduction of Na2CO3 can manipulate the crystal phase of YPO4:Eu.When the molar concentration of CO32-is increased in reaction solution,the crystal phase first changes from HP/TP mixture to TP,then back to HP.The first stage is due to the CO32-giving more PO43-,resulting in the formation of the TP.The second stage happens,mainly due to that the excessive CO32-is combined with RE3+ to form CO32--RE3+ complex,sharply decreasing the concentration of RE3+.Meanwhile,the free energy of the system decreases,which leads to the phase transition to HP.(4)It is found that for BiPO4:Eu system,the acidity and alkalinity in the reaction solution are the key factors affecting the formation and change of crystalline phase.When the reaction solution is in acid condition,the crystal phase of BiPO4:Eu gradually transforms to low temperature monoclinic phase(LTMP)from HP by increasing the concentration of PO43-,due to the improvement of free energy in reaction solution.The LTMP BiPO4:Eu is rhombohedron-like and anhydrous.Its luminescent intensity,lifetime and quantum efficiency are much higher than that of HP.When the reaction solution is in neutral and alkaline condition,respectively,the crystal phase of BiPO4:Eu gradually changes to the unknown one from HP with increase in the concentration of PO43-.The unknown phase products have column-like morphology.Their luminescent intensity,lifetime and quantum efficiency are higher than that of HP but lower than that of LTMP,closely related to the content of the O-H bond.(5)It is found that only increasing pH value leads to phase change of BiPO4:Eu from LTMP to HP.In the case of higher pH value,strong collision between OH-and M3+(Bi3++Eu3+)generates the M3+-O-H complex,which participates in the crystallization of BiPO4:Eu and in turn forms the hydrated HP.