Luminescence of Eu 2+in alkali earth chlorosilicate phosphor
and their color-tunable properties
Zhiguo Xia,Jiayue Sun *,Haiyan Du,Wei Zhou
College of Chemical and Environmental Engineering,Beijing Technology and Business University,11Fucheng Road,Beijing 100037,PR China
Received 2October 2004;accepted 8January 2005
Available online 2June 2005
Abstract
Alkali earth chlorosilicate of the types Sr 4Àx Mg x Si 3O 8Cl 4:Eu 2+and Sr 4Àx Ca x Si 3O 8Cl 4:Eu 2+have been synthesized from mixtures of SrCO 3,CaCO 3,SrCl 2Æ6H 2O,SiO 2,MgO,Eu 2O 3and proper fluxing agent lected from NH 4Cl and H 3BO 3,which were fired at 900–1000°C for 2–4h,and carbon powder was ud as a reducing agent.According to the Van Uitert experimental equation,the color-tunable photoluminescence properties and crystal-lattice environment of Eu 2+in t
he host lattice have been studied.Depending on the ratio of Sr 2+and other alkali earth ions (Mg 2+or Ca 2+)in the host,the position and intensity of different luminescence cen-ters are influenced by Eu 2+located at different chemical environmental sites.Owing to this,it shows from blue-violet (411nm)to greenish-yellow (565nm)luminescence under 365nm ultraviolet radiation.Ó2005Elvier B.V.All rights rerved.
Keywords:Optical materials;Chlorosilicate;Color-tunable;Spectra;Luminescence
1.Introduction
Chlorosilicate crystal material is a suitable host lat-tice for luminescence materials.Alkali earth halide and silicate compound are both effective host as the Eu 2+luminescence matrices.As the multiplex compound,al-kali earth chlorosilicate crystal has lower synthesis tem-perature and higher physical chemistry stability [1–3].In the chlorosilicate host,the luminescence of Eu 2+consists of a 4f 65d 1–4f 7(8S 7/2)broad-band emission,which be-longs to electric-dipole allowed transition and has the properties of large absorption of UV light and broad emission range from ultraviolet to infrared light depend-ing on different crystal-lattice environment [4].
In this paper,a ries of color-tunable photolumines-cence materials having the chemical formula as
Sr 4Àx Mg x Si 3O 8Cl 4:Eu 2+(x =0,0.5,1.0,1.5)and Sr 4Àx -Ca x Si 3O 8Cl 4:Eu 2+(x =0.2,0.4,0.6,1.0,1.5,2.0)are syn-thesized through the high temperature solid state method.By analyzing the excitation and emission spec-tra,the luminescence mechanism of Eu 2+in this kind of host lattice are discusd.And the relationship of mate-rial structure with the luminescence properties is also studied.This ries of materials are promising to be ud for light emitting diodes (LEDs)and will promote the development of the application in luminescence and dis-play fields.
2.Experimental
Through the high-temperature solid state synthesis method,a ries of powder samples of (Sr,Mg)4Si 3O 8-Cl 4:Eu 2+and (Sr,Ca)4Si 3O 8Cl 4:Eu 2+were prepared respectively by firing intimate mixtures of high-purity SrCO 3(A.R.),CaCO 3(A.R.),SrCl 2Æ6H 2O (A.R),
0925-3467/$-e front matter Ó2005Elvier B.V.All rights rerved.doi:10.1016/j.optmat.2005.01.029
*
Corresponding author.Tel.:+861068985467;fax:+861068986936.
E-mail address: (J.Sun).
/locate/optmat
Optical Materials 28(2006)
524–529
SiO 2(A.R.),MgO(A.R.),Eu 2O 3(99.999%)and proper flux lected from NH 4Cl(A.R)and H 3BO 3(A.R).All starting materials were weight in the proper stoichiome-tries,and mixed in an agate mortar,then placed in corundum crucibles.The samples were fired at 900–1000°C for 2–4h,and carbon powder was ud as a reducing agent,by which the samples were covered dur-ing firing (Table 1).
The X-ray diffraction patterns of the samples were re-corded by using SHIMADZU model XRD-6000X-ray powder diffractometer (Cu K a radiation,40kV,30mA and a scanning speed 2.0°/min).All excitation and emis-sion spectra were recorded by using HITACHI model F-4500fluorescence spectrophotometer with a 400V photomultiplier tube and a 150W Xe lamp.3.Results and discussion
3.1.Pha composition of the obtained phosphors JCPDS file 40-0074supplies the X-ray diffraction data for Sr 4Si 3O 8Cl 4,and XRD pattern of the Sr 4Si 3O 8-Cl 4:Eu 2+sample is shown in Fig.1.With careful com-parison between file 40-0074of Sr 4Si 3O 8Cl 4and the sample of Sr 4Si 3O 8Cl 4:Eu 2+,the position and intensity of the main peaks are nearly the same.It also indicates that the crystal lattice and basic structure of the Sr 4Si 3O 8Cl 4host can obtained successfully by the tradi-tional high temperature solid state method.By adding Mg 2+or Ca 2+to the host lattice,Sr 4Àx Mg x Si 3O 8-Cl 4:Eu 2+and Sr 4Àx Ca x Si 3O 8Cl 4:Eu 2+phosphors are synthesized with the incre
a of amount of the magne-sium or calcium which replaced the strontium.3.2.Luminescence of Eu 2+in Sr 4Si 3O 8Cl 4host lattice The excitation spectra of Sr 4Si 3O 8Cl 4:Eu 2+show characteristic excitation spectra band of Eu 2+ion,which is broad in the range from 250nm to 400nm.As en in
Table 1
Chemical composition and synthesis technology of the alkali earth chlorosilicate phosphor Nominal composition SrCO 3(wt.%)MgO (wt.%)CaCO 3(wt.%)SrCl 2Æ6H 2O (wt.%)SiO 2(wt.%)Eu 2O 3(wt.%)NH 4Cl (wt.%)H 3BO 3(wt.%)Synthesis Sr 4Si 3O 8Cl 4:0.05Eu 2+
28.4340051.39717.3570.8480 1.965900°C,4h Sr 3.5Mg 0.5Si 3O 8Cl 4:0.05Eu 2+22.301 2.015053.74718.1510.8860.967 1.934950°C,4h Sr 3.0MgSi 3O 8Cl 4:0.05Eu 2+15.739 4.265056.89919.2150.9380.981 1.962950°C,4h Sr 2.5Mg 1.5Si 3O 8Cl 4:0.05Eu 2+8.365 6.801060.48520.4260.9980.975 1.950950°C,4h Sr 3.8Ca 0.2Si 3O 8Cl 4:0.1Eu 2+25.3670 1.90950.92617.198 1.680 2.9200920°C,3h Sr 3.6Ca 0.4Si 3O 8Cl 4:0.1Eu 2+22.6770 3.84051.22917.300 1.690 3.2640920°C,3h Sr 3.4Ca 0.6Si 3O 8Cl 4:0.1Eu 2+20.0970 5.83751.90717.529 1.712 2.9180920°C,3h Sr 3CaSi 3O 8Cl 4:0.1Eu 2+
14.63709.91752.91617.870 1.745 2.9160920°C,3h Sr 2.5Ca 1.5Si 3O 8Cl 4:0.1Eu 2+7.500015.264
54.22818.313 1.789 2.9070920°C,3h Sr 2Ca 2Si 3O 8Cl 4:0.1Eu 2+
21.275
56.706
19.150
1.870
0.999
900°C,
日本高知工科大学
2h
Z.Xia et al./Optical Materials 28(2006)524–529525
Fig.2,it gives the excitation and emission spectra of Sr 4Si 3O 8Cl 4:Eu 2+.Under 365nm UV light,Sr 4Si 3O 8-Cl 4:Eu 2+shows broad emission,related to the 5d–4f transition,and gives a strong blue-green luminescence with peak wavelength at 496nm and FWHM 70nm.But as shown in Fig.2,the emission spectra has some appreciable asymmetry,the possible reason is that the excitation and emission spectra overlap each other.So it results in the lf-absorption of the sample Õs short wavelength side.Nevertheless,the sample possibly has only one emission center,that is to say that the coordi-nating environments for all Eu 2+are the same,thus,the luminescent emission shows only one broad peak [2].According to the report of Van Uitert [5],for most divalent and some trivalent rare earth ions in suitable matrices,such as sulfide,oxide,halide and aluminates,the following experiential equation provides a good fit to the emission peak and excitation edge data for Eu 2+and Ce 3+.Bad on this,the problem about which kind of crystallographic site substituted by Eu 2+in the Sr 4Si 3O 8Cl 4host lattice is discusd and testified.
E ¼Q 1ÀðV =4Þ1=V 10Àðnear Þ=80
h i
ð1ÞIn the above Eq.(1),E reprents the position of the d-band edge in energy for rare earth ions (cm À1),Q is the position in energy for the lower d-band edge for the free ions,value of Q is 34,000cm À1for Eu 2+,V is the valence of the ‘‘active’’cation,here V =2.Ôea Õis the electron affinity of the atoms that form anions,ea is regarded as 2.19eV in Sr 4Si 3O 8Cl 4host.n is the number of anions in the immediate shell about this ion,and r is the radius of the host cation replaced by the ‘‘active’’cation.It is reported that Sr is coordinated by four O atoms and four Cl atoms in the Sr 4Si 3O 8Cl 4host and the sites of Sr 2+will substitute by Eu 2+in the Sr 4Si 3O 8Cl 4:Eu 2+phosphor,whichever factor like radius and charge are considered [2].So the value of coordination number n of Eu 2+is eight,and the radius of Sr 2+under this coor-dination environment is 0.126nm.Through the above data,it can be calculated that there is only one octa-coordination Sr 2+sites in the host lattice and the Eu 2+center that shows blue-green luminescence really comes from octa-coordination Sr 2+site [6].
垃圾的危害3.3.Luminescence properties of (Sr,Mg)4Si 3O 8Cl 4:Eu 2+phosphor
By changing the dopant amount of Mg 2+ions,a ries of powder materials reprented by the chemical formula Sr 4Si 3O 8Cl 4:Eu 2+,Sr 3.5Mg 0.5Si 3O 8Cl 4:Eu 2+,Sr 3.0-MgSi 3O 8Cl 4:Eu 2+and Sr 2.5Mg 1.5Si 3O 8Cl 4:Eu 2+were successfully synthesized under the condition of invari-abl
e dopant concentration of Eu 2+in the host lattice.As mentioned above,all the excitation spectra of the materials show characteristic excitation spectra band
of Eu 2+ion,which is broad in the range from 250nm to 400nm.The emission spectra of them are shown in Fig.3.And Table 2gives the respective center position of different Sr 4Àx Mg x Si 3O 8Cl 4:Eu 2+phosphors.
According to the experimental equation of Van Uit-ert,the theoretical value of Eu 2+luminescence center that substituted the octa-coordination Sr 2+site is 471nm.So we can conclude that the Eu 2+center that shows blue-green luminescence really comes from octa-coordination Sr 2+site,and signed it as Sr (A)site.While considering the emission peak that shows blue-violet luminescence,it aris from the dopant of Mg 2+in the host lattice,and its emission intensity increas remark-ably with the concentration of Mg 2+increasing as en from the Fig.3.This indicates that the Eu 2+center that shows blue-violet luminescence here originates from a completely different site substitution from the former,and we can consider it as Sr (B)site.The correlative data can be en form the emission peak (B)in the Table 2,which is obviously different from theoretical calculated value of Eu 2+center occupied octa-coordination Sr 2+site.From the above spectroscopic analysis,we can con-clude that there are two different property cation sites in the host lattice becau of the dopant of Mg 2+.The po
s-sible reason is that the large size difference between
暗保
Table 2
Respective figure position for different compositions of Sr 4Àx Ca x -Si 3O 8Cl 4:Eu 2+phosphor Compound
Emission peak A (nm)Emission peak B (nm)Sr 4Si 3O 8Cl 4:Eu 2+
496None Sr 3.5Mg 0.5Si 3O 8Cl 4:Eu 2+478415Sr 3.0MgSi 3O 8Cl 4:Eu 2+475414Sr 2.5Mg 1.5Si 3O 8Cl 4:Eu 2+
467
411
526Z.Xia et al./Optical Materials 28(2006)524–529
Mg2+and Sr2+,and it results in the distortion of host lattice to some certain extent.But simultaneously we no-tice that the framework of Sr4Si3O8Cl4can hardly change completely by the introduction of impurity ions owing to cation replacement.Therefore,the coordina-tion numbers of cation ions whatever Sr(A)site or Sr (B)site are invariably eight as likely as not,and the sub-stitution process can hardly produce other new coordi-
nation numbers in the host lattice becau of similar physical chemistry properties between Sr2+and Mg2+. So we consider that the different luminescence peaks are not aroud by the luminescence centers of Eu2+lo-cated in the sites posssing different coordination num-bers,and the two different luminescence centers of Eu2+ should be ascribed to their respective crystal-lattice envi-ronment[6].
朴恩惠
3.4.Luminescence properties of(Sr,Ca)4Si3O8Cl4:Eu2+ phosphor
Different from the Eu2+luminescence in Sr4Àx Mg x-Si3O8Cl4host,the following experimental conclusion indicate that the emission peak of Sr4Àx Ca x Si3O8-Cl4:Eu2+change irregularly depending on the ratio of Sr2+and Ca2+in the host lattice.When the doping amount of calcium(x)ranged from0to0.5,the sample shows blue-green emission as en in Fig.4.And the respective center position for the different compositions of Sr4Àx Ca x Si3O8Cl4:Eu2+is listed in Table3.
Obviously,the spectra properties of Sr4Àx Ca x Si3O8-Cl4:Eu2+does not differ largely from Sr4Si3O8Cl4:Eu2+. Its emission peak does not change to some extent except for the decrea of the luminescence intensity.Consider-ing the small dopant amount of Ca2+,similar radius dif-ference and physical chemical property between Ca2+and Sr2+,it is possible that Ca2+ions are embedded in the host lattice of Sr4Si3O8Cl4and substitute the Sr2+ sites partly;finally they form solid dissolved system among definite component range.So Eu2+will mainly substitute the octa-coordination Sr2+sites,and show blue-green luminescence.It is obvious that the lumines-cence of Sr4Àx Ca x Si3O8Cl4:Eu2+that x ranged from0to 0.5is similar to the above discussion about Sr4Àx-Mg x Si3O8Cl4:Eu2+.
On the other hand,Sr4Àx Ca x Si3O8Cl4:Eu2+shows greenish-yellow luminescence when the range of x is be-tween0.5and2.The emission spectra of them are shown in Fig.5.The relationships of the main emission peaks and compositions are given in Table4.As shown in Fig.5and Table4,wefind that Sr4Àx Ca x Si3O8Cl4:Eu2+ (x>0.5)phosphor show obvious emission peak in the
Table3丁谓
中国古代名将
Respectivefigure position for different compositions of Sr4Àx Ca x-
Si3O8Cl4:Eu2+phosphor
Compound Emission peak(nm)
Sr4Si3O8Cl4:Eu2+496
Sr3.8Ca0.2Si3O8Cl4:Eu2+497
Sr3.6Ca0.4Si3O8Cl4:Eu2+498
Table4
Respectivefigure position for different compositions of Sr4Àx Ca x-
Si3O8Cl4:Eu2+phosphor
Compound Main emission peak(nm)
Sr3.4Ca0.6Si3O8Cl4:Eu2+547
Sr3CaSi3O8Cl4:Eu2+556
Sr2.5Ca1.5Si3O8Cl4:Eu2+561
Sr2Ca2Si3O8Cl4:Eu2+565
Z.Xia et al./Optical Materials28(2006)524–529527
long wavelength side,while the emission peak in the short wavelength side become even invisible.But on an energy scale,the emission bands of Sr4Àx Ca x Si3O8-Cl4:Eu2+ranging from0.5to2.0appear to be asymmet-ric,and each of them can be divided into two Gaussian bands.Fo
r example,as en in Fig.6,the emission spec-trum of Sr2Ca2Si3O8Cl4:Eu2+is divided into two Gauss-ian bands with maxima at467and565nm.We propod that there are two different emission center in the host lattice of Sr4Àx Ca x Si3O8Cl4:Eu2+.The emis-sion center in the short wavelength side is signed as Eu (A),and Eu(B)is the center in the long wavelength side. Table5shows the respective position of Eu(A)and Eu (B)emission centers that obtained from Gaussianfit of different compositive samples of Sr4Àx Ca x Si3O8Cl4:Eu2+ (x>0.5).
Even as the above discussion,the theoretical value of Eu2+luminescence center that substituted the octa-coor-dination Sr2+site is471nm.And Eu(A)is mostly probably ascribed to Eu2+ions that substituted the octa-coordination Sr2+sites.Obviously,the measured position of emission peak is the same as the calculated values by Van Uitert experimental equation.Owing to the same reason as the former,the forming of Eu(B) emission canÕt be explained by the existing of other coor-dination number cations in the host lattice.Eu(B)emis-sion is at long wavelengths,which is similar to what has been obrved in BaF2:Eu2+[7].This is due to the fact that the Eu2+emission originated from the formation of an impurity-trapped exciton state.Eu(B)emission center is located in the impurity-trapped exciton state, which is the lowest excited state of this system.The rea-son is that chlorosilicate compounds have not only the properties of silicate compound,but also that of alkali ear
th halide.Along with the substantive adding of Ca2+,it gradually shortens the distance of M2+(Sr2+ or Ca2+)and halide ions,also including other anion ions in the system,becau of the smaller radius of Ca2+.At the same time,it also accelerates the formation of impurity-trapped exciton state as the dopant amount of Ca2+is beyond0.5.
A model for the luminescence of Sr4Àx Ca x Si3O8-Cl4:Eu2+(x>0.5)phosphors can be given in terms of configuration coordinate diagrams as shown in Fig.7. From it,curve G reprents the ground state;and relative to the original localized excited state L,the impurity-trapped exciton state E has lower energy.By the direct UV radiation,the photon located at the ground state G is excited to the localized excited state L,and then relaxed to the impurity-trapped exciton state E in terms of the crossing F.When the optical tran-sition happens between curve E and G,Eu2+emission is at long wavelengths and a large Stokes shift is obrved [7].
4.Conclusion
Sr4Àx Mg x Si3O8Cl4(x=0,0.5,1.0,1.5)and Sr4Àx Ca x-Si3O8Cl4(x=0.2,0.4,0.6,1.0,1.5,2.0)samples were pre-pared by the high-temperature solid-state reaction.
Table5
Respectivefigure position of different Sr4Àx Ca x Si3O8Cl4:Eu2+emission spectra obtained from Gaussianfit
Sr4Àx Ca x Si3O8Cl4:Eu2+ sample Eu(A)emission
figure k/nm
Eu(B)emission
figure k/nm
x=0.6481547 x=1.0477556 x=1.5471561 x=2.0467565
528Z.Xia et al./Optical Materials28(2006)524–529
Bad on its excitation and emission spectra of Sr4Àx Mg x Si3O8Cl4:Eu2+phosphors,the possibilities of two Eu2+emission centers is found and the emission peak shifts from blue-green to blue-violet with the in-crea of amount of the magnesium which replaces the strontium.Depending on the ratio of Sr2+and Ca2+in the host lattice of Sr4Àx Ca x Si3O8Cl4:Eu2+,the intensity of two Eu2+emission centers,viz,Eu(A)and Eu(B), changes irregularly and the corresponding relationships with spectra are also discusd.Eu(A)emission center, which has emission peak located at496nm,is ascribed to Eu2+ions that substituted the octa-coordination Sr2+sites,and Eu(B)emission is in the long wavelength range,which is due to the fact that the Eu2+emission originated from the formation of an impurity-trapped exciton state.Owing to their color-tunable photo-luminescence properties,this insight can be extremely important for understanding the optical properties of rare-earth materials and for guiding the development of materials for applications.Acknowledgements
The authors gratefully acknowledge thefinancial sup-port of the National Natural Science Foundation of China(Grant No.20476002),the Beijing Natural Sci-ence Foundation(Grant No.2042007),and Science and Technology Development key foundation of Beijing Education Committee(Grant No.KZ20041001107). References
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