Study On The Luminescence Properties And Mechanism Of Bi³⁺ In Ca₅（BO₃）₃F By Host Composition

Currently,color-tunable and high emission brightness phosphors are urgently needed materials for LED conversion devices.Controlling the host structure is an effective means to realize the regulation of material properties.6s electron and 6p electron of Bi³⁺ions are located in its outermost layer;thus,luminescence properties of Bi³⁺are strongly associated with the environment around Bi³⁺,so Bi³⁺is a luminescent material that is well dependent on the host system.The luminescent property of Bi³⁺will change when the coordination environment or the anionic group around Bi³⁺is changed.Based on the Bi³⁺luminescence properties,aseriesofCa₅（BO₃）₃F:Bi³⁺,Ca₅（BO₃）_3-x（PO₄）_xF:Bi³⁺and Ca_5-xM_x（BO₃）₃F:Bi³⁺(x=Na⁺,Sr²⁺,Ba²⁺)phosphors are synthesized using a high-temperature solid-state method.The coordination environment and anionic group around Bi³⁺is changed by changing the doping concentration of Bi³⁺and introducting new anionic group respectively.meanwhile,the coordination environment around Bi³⁺can be greater tuned by substituting the cationic components of host.The relationship between luminescence properities of Bi³⁺and changes of surrounding environment is studied to realize the regulation of Bi³⁺luminescence properities in Ca₅（BO₃）₃F.The specific research results are as follows:（1）In order to explore the relationship between luminescence properities of Bi³⁺and the coordination environment around Bi³⁺,a series of Bi³⁺doped Ca₅（BO₃）₃F species are synthesized.There are three types of Ca²⁺sites in the host,which could be substituted by Bi³⁺,as the doping concentration of Bi³⁺increases,Bi³⁺ions occupy different cationic positions in hosts leading to changes in coordination environment around Bi³⁺,which causes the movement of emission spectrum.The emission color can gradually change from blue to cyan,which realizes the regulation of Bi³⁺luminescence color in Ca₅（BO₃）₃F.It may result from Bi³⁺occupying different positions of Ca²⁺in the host,which can give rise to different degrees of a nephelauxetic effect and crystal field splitting.In order to investigate relationship between the luminescence properties of Bi³⁺and the nepelauxetic effect,the value of centroid shift is calculated.Centroid shift is related to the covalence and average bond length of octahedron in which the influence of covalence is primary.The relationship between the luminescence properties of Bi³⁺and the crystal field splitting is discussed.The crystal field splitting is related to the interaction between the Bi³⁺species,the crystal field splitting energy and the distortion of crystal.Emission spectra of Ca₅（BO₃）₃F:Bi³⁺are asymmetric;meanwhile,the emission spectra have remarkable change at various excitation wavelengths.It proves that the broadband emission band consists of at last two emission centers.In order to determine this hypothesis,the decay curves are measured.It confirms that there are three luminescence centers in host.On the one hand,considering the effect of centroid shift and crystal field splitting,the source of three luminescence centers are confirmed by calculating the total shift（D（A））of 6s6p level of Bi³⁺in host.On the other hand,the source of three luminescence centers is determined by the changing trend of average bond length of octahedron.In addition,the luminescent color of phosphor can be tuned in a wider range by the efficient energy transfer from Bi³⁺to Eu³⁺ions.the emission color can gradually change from cyan to red,and the energy transfer mechanism from Bi³⁺to Eu³⁺is studied.（2）a series of Ca_5-y（BO₃）_3-x（PO₄）_xF（CBP_xF）:yBi³⁺（y=0.05,0.15,x=0-3）,Ca_5-y（PO₄）_3-X（BO₃）_XF（CPB_XF）:yBi³⁺（y=0.05,0.15;X=0-1）,Ca_4.9（PO₄）₃F（CPF）:0.1Eu³⁺,and Ca_4.95（PO₄）₃F（CPF）:0.05Bi³⁺,nCaF₂/CaCl₂（n=0-0.1）are synthesized to explore the variation of anionic groups on Bi³⁺luminescence properties.In CBP_x F:0.15Bi³⁺（x=0-3）,（PO₄）^3-is doped to substitute for（BO₃）^3-,the position of emission spectra remains unchanged and the emission intensity decreases rapidly with increasing x.The underlying mainly reason for that is the formation of triangular plane（PO₄）^3-,which have been verified by performing a series of verification experiments of CPB_XF:yBi³⁺（y=0.5,0.15;X=0-1）.In CPB_XF:yBi³⁺（y=0.5,0.15;X=0-1）,（BO₃）^3-is doped to substitute for（PO₄）^3-,P-O2 bond breaks and the coordination of（PO₄）^3-varies from four to three when 0.5<X<1,meanwhile,the crystal structure transforms from Ca₅（PO₄）₃F（ICSD-9444）to Ca₅（PO₄）₃F（ISCD-30261）,which impedes the abnormal reduction from Bi³⁺to Bi²⁺.Furthermore,Bi³⁺should non-luminance in plane triangular（PO₄）^3-,but luminescence in（BO₃）^3-.Therefore,the emission intensity starts to increase and the emission position suddenly changes from 553nm to 474nm in CPB_XF:yBi³⁺（y=0.05,0.15;0.5<X<1）.From this,the crystal structures of CBP_xF:yBi³⁺（y=0.05,0.15;x=0-3）has been inferred to transform from Ca₅（BO₃）₃F（ISCD-65763）to Ca₅（PO₄）₃F（ISCD-30261）,and then to Ca₅（PO₄）₃F（ISCD-9444）with x increasing.Emission position remains unchanged and emission intensity decreases rapidly in CBP_xF:yBi³⁺（y=0.05,0.15;x=0-3）due to the formation of triangular plane（PO₄）^3-.In addition,the rate of abnormal reduction from Bi³⁺to Bi²⁺can be improved by reducing the electronegativity of environment around the activator or increasing the ionization energy of activator,which has been confirmed by verification experiments of CPF:0.05Bi³⁺,nCaF₂/CaCl₂（n=0-0.1）and CPF:0.1Eu³⁺（3）a series of Ca_4.85-xM_x（BO₃）₃F:0.15Bi³⁺are synthesized to further explore the variation of coordination environments on Bi³⁺luminescence properties.The coordination environment around Bi³⁺is changed to a greater extent by controlling host cationic components.Ca₅（BO₃）F can produce more Frenkel defects by introducing Na⁺,Sr²⁺and Ba²⁺,which causes the enhancement of the emission intensity of Ca_4.85（BO₃）₃F:0.15Bi³⁺.It is found that the number of Frenkel defects are relate to volume and covalence of crystal,which mainly is affected by covalence of crystal.Thus,despite the volume of Ca_4.85-xSr_x（BO₃）₃F:0.15Bi³⁺is larger than that of Ca_4.85-xNa_x（BO₃）₃F:0.15Bi³⁺,both have basically the same trap depth（）and defect density（）in the quenching concentration because of the covalence of Na⁺is basicallylinewithSr²⁺.Furthermore,Ca_4.85-xSr_x（BO₃）₃F:0.15Bi³⁺and Ca_4.85-xNa_x（BO₃）₃F:0.15Bi³⁺have the same distortion at quenching concentration,thus,the emissionintensityofCa_4.85-xNa_x（BO₃）₃F:0.15Bi³⁺（x=0.07）keepswith Ca_4.85-xSr_x（BO₃）₃F:0.15Bi³⁺（x=0.08）.Ca_4.85-xBa_x（BO₃）₃F:0.15Bi³⁺has larger the volume and stronger covalence,meanwhile,it has deeper trap depth（）and larger defect density（）at the quenching content,comparing with Ca_4.85-xSr_x（BO₃）₃F:0.15Bi³⁺and Ca_4.85-xNa_x（BO₃）₃F:0.15Bi³⁺.However,the distortion of Ca_4.85-x.85-x Ba_x（BO₃）₃F:0.15Bi³⁺is in agrees with the Ca_4.85-x.85-x Na_x（BO₃）₃F:0.15Bi³⁺and Ca_4.85-xSr_x（BO₃）₃F:0.15Bi³⁺,which leads to the emission intensity of Ca_4.85-xBa_x（BO₃）₃F:0.15Bi³⁺basically same as that of Ca_4.85-xNa_x（BO₃）₃F:0.15Bi³⁺and Ca_4.85-x.85-x Sr_x（BO₃）₃F:0.15Bi³⁺in quenching concentration.And the different rates of distortion result in the different quenching concentration of Ca_4.85-xNa_x（BO₃）₃F:0.15Bi³⁺,Ca_4.85-xSr_x（BO₃）₃F:0.15Bi³⁺and Ca_4.85-xBa_x（BO₃）₃F:0.15Bi³⁺.Similarly,more Frenkel defects can be generated in host by introducing anionic Cl^-,which also gives rise to the enhance of emission intensity of Ca_4.85（BO₃）₃F:0.15Bi³⁺phosphors.