Luminescent properties of Sr4V2O9：Eu3＋（5%），Ba2＋（x%）phosphors tuned by Ba doping and sintering temperature

WANG Wei－wei，DAI Zhen－xiang，ZHENG Gan－hong，LI Yong－qiang，MA Yong－qing

（Anhui Key Laboratory of Information Materials and Devices，School of Physics and Materials Science，Anhui University，Hefei 230039，China）

0 Introduction

Recently，the white light－emitting diode（w－LED）has been widely investigated due to its advantages such as high reliability，high luminescent efficiency，long lifetime，low energy consumption，safety and its environment－friendly characteristic，which will become the fourth generation lightly sources to replace the incandescent and fluorescent lamps［1－3］.

The optical properties of phosphors depend intimately on the local structure and bonding of dopant luminescence ions.The study of local environments around luminescence ions will greatly contribute to the modification of luminescent phosphors since electronic transitions of dopant ions depend intimately on the located sites.So tunable luminescence properties of phosphors can be realized by means of structure evolution which can be induced through varying synthesis procedure.Recently，Tian and Mho［4］found that the photoluminescence of YVO4∶Eu phosphors can be greatly increased by co－doping Ba2＋ions.Jia et al.used a solvo－thermal method to synthesize YVO4：Ln3＋（Ln＝Eu，Dy）and reported that a large enhancement in the luminescence by co－doping Ba2＋［5］.At the same time，there are also many investigations performed on barium and strontium vanadates（Ba，Sr）3（VO4）2.Some Ln3＋such as Y3＋，La3＋，and Gd3＋，is to form solid solutions of（Ba，Sr）3－xLn2x（VO4）2without the structure change［6－7］.It has been found that（Ba，Sr）3（VO4）2can be served as a good host materials for Eu3＋to emit light.For example，the photoluminescence properties for Ba3（1－x）Eu2x－（VO4）2and Sr3（1－x）Eu2x（VO4）2have been reported and their emission spectrum only show the peaks corresponding to the5D0→7Fj（j＝1，2，…）transitions of the Eu3＋ion without the emission of VO43－，resulting from the complete energy transfer of the vanadates host materials［6－8］.

In this paper，we will describe the properties of Eu，Ba co－doped Sr4V2O9phosphors and focus on the photoluminescence influence of Ba－doping content and sintering temperature.

1 Experiment

1.1 Sample preparation

The staring materials included strontium nitrate Sr（NO3）2，ammonium metavanadate NH4VO3，europium nitrate Eu（NO3）3·6H2O，and barium nitrate Ba（NO3）2，sodium hydroxide NaOH，and deionized water.At first，Sr（NO3）2（99.5%），NH4VO3（99%），Eu（NO3）3·6H2O（99.99%），and Ba（NO3）2（99%）were dissolved in deionized water with stirring for 30min to form separated solutions.Stoichiometric amounts of Sr（NO3）2，NH4VO3，Eu（NO3）3·6H2O，and Ba（NO3）2solutions were mixed with continuous stirring.Then，an appropriated amount of NaOH solution was added to adjust their pH value to 8.Finally，the precursor solution was placed in a 50mL capacity，Teflon－lined stainless－steel autoclave and heated at 180℃for 10h.After the autoclave was cooled to room temperature naturally，the product was separated by centrifugation，washed with ethanol and deionized water several times，and dried at 80℃to obtain white powders.

1.2 Characterization of samples

The crystal structure of the products was characterized by X－ray diffraction using an X－ray diffractometer（XRD dx－2000SSC）with CuKαradiation（λ＝1.541 8Å）over a scanning range of 20－80Åwith a step of 0.02 Å.The excitation and emission spectra were measured on a FL fluorescence spectrophotometer（F－4500，Hitachi）.All the measurements were carried out at room temperature.

2 Results and discussion

2.1 Crystal structure of Sr4V2O9：Eu（5%），Ba（x%）

Fig.1 shows the XRD patterns of Sr4V2O9：Eu（5%），Ba（x%）as prepared at 180℃and annealed at 300℃and 500℃.The XRD patterns indicate that all of samples have similar patterns and the diffraction peak positions are all matched with those of JCPDS（No.＃765409）patterns of Sr4V2O9.The results suggest that a little amount of the substitution of Eu／Ba at Sr sites does not change the crystal structure.Additionally，the strongest diffraction peaks obviously shift to smaller 2θangles with Ba doping，due to the larger ionic radius of Ba2＋than that of Sr2＋.In addition，it can be seen that XRD patterns intensity is strengthened after sintered，which suggests well－crystallized.

2.2 Photoluminescence properties of Sr4V2O9∶Eu3＋（5%），Ba2＋（x%）

Fig.2 shows the excitation and emission spectra of the non－sintered Sr4V2O9：Eu3＋（5%），Ba2＋（x%）（x＝0，5）samples.The excitation spectra monitored by 619nm also display three peaks at 417，468，and 538nm，which are attributed to the direct absorption of the Eu3＋ion assigned to transitions of7F0→5L6，7F0→5D2，and7F1→5D1，respectively.The intensity of the7F1→5L6and7F0，1→5D1，2after 5%Ba2＋co－doping is increased by about 39%and 53%，respectively.This means that Ba2＋dopants can improve the optical absorption rates of the Eu3＋ions.

The emission spectra of the Sr4V2O9：Eu3＋（5%），Ba2＋（x%）（x＝0，5）phosphors under 398nm excitation is presented in Fig.2 b.The transitions from the metastable orbital singlet state5D0to the spin－orbital states7FJ（J＝1，2，3，4）of Eu3＋result in the four emission peaks locating at 596，619，658，and 709nm.5D0→7F1and5D0→7F3are magnetic diploe transition and insensitive to a local symmetry，whereas5D0→7F2and5D0→7F4are electric diploe transition and hypersensitive to a local symmetry.The significant strong intensity of the5D0→7F2and5D0→7F4transition in the Sr4V2O9：Eu3＋（5%）revealed that Eu3＋ions are located in the asymmetry sites.Comparing with no－doping Ba，the emission intensity is enhanced with 5%Ba2＋doping.Specifically，the emission intensity of Eu3＋at 619nm（5D0→7F2）was increased by about 40%due to 5%Ba－doping.It is well known that the5D0→7F2transition of Eu3＋is very sensitive to crystal factors such as the lattice parameter and the site symmetry around the Eu3＋ions.And the lower symmetry around the Eu3＋site increases the probability of radiative transitions within the 4f－electron shell and therefore enhances the photoluminescent intensity.In our samples，the lattice parameter increases with the increasing Ba2＋content because of the larger ionic radius of Ba2＋（0.135nm，coordination number（CN）＝6）compared with Sr2＋（0.118nm，CN＝6），resulting in the lattice distortion.On the other hand，to keep the electro－neutrality of the compound，two Eu3＋ions would substitute for three Sr2＋ions.Therefore，the charge compensation leads to the generation of two positive defects of［EuSr］and one negative Sr2＋vacancy of［VSr］″，which also distorts the lattice［9］.In addition，as reported in reference［10］，Ba2＋ions can conspicuously enhance the energy transfer from the excited states of O2－（the charge transfer states）to Eu3＋，not merely from VO43－to Eu3＋.On the other hand，the charge－to－radius ratio of the Ba2＋ions is smaller than that of the Sr2＋ions.When the Ba2＋ion is substituted into the Sr2＋site in the lattice，a tight binding of the oxygen electrons toward the Sr2＋ions is expected to be released.Hence，the charge transfer state excitation band of Eu3＋is enhanced by Ba2＋doping into the Sr4V2O9lattice，leading to the enhanced emission of Eu3＋ions.Hence，the luminescent intensity of Sr4V2O9：Eu3＋is increased by the Ba2＋incorporation into the lattices.

Fig.3 shows the emission spectra of Sr4V2O9：Eu3＋（5%），Ba2＋（x%）（x＝0，5）samples after sintered 300，500，and 750℃.Under 398nm excitation，three emission peaks locating at 619，709，and 776nm are observed，which can be attributed to the optical transitions from the excited state5D0→7FJ（J＝2，3，4）of the 4f6configuration of Eu3＋ions.Comparing with non－sintered samples，it can be seen that after sintered，the emission line at 776nm appears，which is corresponding to the forced electric dipole5D0→7F4transition of Eu3＋.And its intensity is so strong that the peaks at 596 nm and 658nm observed for non－sintered samples are covered up as shown in Fig.3 .In addition，after sintered，the emission intensity is obviously lowered for both samples.When the sintered temperatures increase from 300to 750℃，the emission intensity is lowered by 23%and 28%forx＝0，5sample.Generally speaking，with increasing temperature，the emission intensity is enhanced.However，here，for our samples，the phenomenon is otherwise.It may be ascribed to with increasing sintering temperatures，small particles will conglomerate to larger particles and the cross section for receiving excitation light becomes smaller，consequently resulting in the luminescence quenching.

In addition，it is noted that，after sintering，magnetic dipole transitions of5D0→7F1，3disappear，there only exist electric dipole transitions of5D0→7F2，4，as shown in Fig.3 .The electric dipole transitions of5D0→7F2（619nm）and5D0→7F4（709nm and 776nm）are hypersensitive transitions particularly sensitive to the chemical environments［11－12］.This demonstrated that the crystal field surrounding Eu3＋ions is highly changed due to the distinct ionic radii and sintered，resulting in the local distortion and lower symmetry.Eu3＋ions can be applied as a structural probe to luminescent materials.

3 Conclusion

We have synthesized Sr4V2O9：Eu（5%），Ba（x%）（x＝0，5）samples by a solvo－thermal method and sintered.The partial substitution of Sr2＋by Eu3＋and Ba2＋does not change the crystal structure.Ba2＋substitution enhances the emission intensity for Sr4V2O9：Eu（5%），Ba（x%）sample.The reason may be ascribed to the crystal factors such as the lattice parameter and the site symmetry around the Eu3＋ions due to Ba2＋doping.In addition，the smaller charge－to－radius ratio of the Ba2＋ions than that of the Sr2＋ions may be another influencing factor.The dependence of emission behavior on sintering temperature is also clearly observed.With increasing sintering temperature，the emission intensity is lowered due to small particles conglomerating to larger particles.The magnetic dipole transition disappears after sintered，demonstrating the local distortion and lower symmetry of Eu3＋ions.

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