LI Han－bo，ZHENG Gan－h(huán)ong ，LI Yong－qiang，DAI Zhen－xiang，NIE Xiao－xiao

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

0 Introduction

The white light－emitting diode（w－LED）has been extensively investigated due to its advantages such as high reliability，high luminescent efficiency，long lifetime，low energy consumption，safety and its environment－friendly characteristic.It is expected to be the fourth generation light sources replacing the incandescent and fluorescent lamps［1－3］.Recently，Sr3V2O8and rare－earth（Re＝Eu，Sm，Nb，Yb，etc）doped orthovanadates materials have attracted increasing interests due to their great potential application in luminescent［4－5］.Furthermore，Eu3＋ion is an especially important activator for red phosphors，which has been extensively studied for years，because the visible emission of Eu3＋ion is 4fshell is insensitive to the influence of the surroundings due to the shielding effect of 5sand 5p electron.More importantly，as reported in our previous work the luminescence of Eu3＋ions can be well sensitized by the isolated VO43－group in Sr3V2O8matrix［6－7］.These advantages have impelled more and more researchers to study experimental synthesis process and characterization of rare earth－doped Sr3V2O8phosphor.At the same time，Bi3＋ions can be used as the sensitizer of Eu3＋luminescence through energy transfer from Bi to Eu in these systems such as YVO4：Bi3＋，Eu3＋［8］，Y2O3：Bi3＋，Eu3＋［9］，CaWO4：Bi3＋，Eu3＋［10］，Y（Gd）BO3：Bi3＋，Eu3＋［11］，which can also strengthen and broaden ultraviolet excitation bands［12］.In the current work，the effect of Bi on the Sr1.8Eu0.8V1.2P0.8O8phosphors has been investigated.Our results show that the incorporation of some Bi3＋ions is favorable for the photoluminescence of Sr1.8Eu0.8V1.2P0.8O8.

1 Experimental

1.1 Sample preparation

Sr1.8Eu0.8－xBixV1.2P0.8O8（x＝0，0.1，0.2，0.3，0.4，0.5，0.6，and 0.7）were prepared by the traditional solid state reaction method.The initiative materials were SrCO3（99.99%），Eu2O3（99.99%），Bi2O3（99.0%），NH4VO3（99.0%），（NH4）2HPO4（99.0%）.The initial materials were mixed together in an agate mortar and finely ground.The obtained mixture was heated in an alumina crucible（in air）at 680℃for 10h.After being naturally cooled and carefully ground，these mixtures were pressed into tablets.Then the obtained mixtures were heated in an alumina crucible（in air）at 850℃for 10h.Finally，the obtained mixtures were heated in an alumina crucible（in air）at 1 000℃for 12h.

1.2 Characterization of Samples

The crystal structure of the phosphor powders was characterized by X－ray diffraction（XRD）analysis.In the process of XRD analysis，one X－ray diffractometer DX－2000SSC with CuKαirradiation（λ＝0.154 06 nm）is used，with operating voltage being 36kV and the operating current being 25mA.The activation and emission spectra were measured on a FL fluorescence spectrophotometer（F－4500）.The weight of every sample was equal.All these operations were carried out at room temperature.

2 Results and Discussion

2.1 The crystal structure of Sr1.8Eu0.8－xBixV1.2P0.8O8（0≤x≤0.7）

Fig.1 shows the XRD patterns of the Sr1.8Eu0.8－xBixV1.2P0.8O8（0≤x≤0.7）phosphors annealed at 1 000 ℃.From this figure，it is observed that these Sr1.8Eu0.8－xBixV1.2P0.8O8samples mainly exhibit similar phase as Sr3V2O8.The diffraction peak positions and the relative intensities are matched with those of the PCPDF（＃811844）of Sr3V2O8，indicating that the samples have a palmierite－type structure with the space groupmH.These results indicate that the host crystal structure varies little with a small amount of Eu／Bi substitution at Sr sites.However，some weak extra diffraction peaks（marked with arrow）between 25and 30degree correspond to the major lines of the PCPDF（＃130194）for Sr3P2O7（not shown here）.

2.2 The photoluminescence properties of the Sr1.8Eu0.8－xBixV1.2P0.8O8（0≤x≤0.7）

Fig.2 apresents that the emission spectra in case ofx＝0under excitation at 365nm.The four emission peaks locating at 594，617，656，and 702nm are assigned to the transitions from the metastable orbital singlet state5D0to the spin－orbital states7Fj（j＝1，2，3，4）of Eu3＋.The5D0→7F1transition is magnetic－dipole－allowed and its intensity is almost independent of the local environment around Eu3＋ions.The5D0→7F3transition exhibits a mixed magnetic dipole and electric dipole character［13］.The5D0→7F4is electric dipole transition.The5D0→7F2transition is electric－dipoleallowed due to an admixture of opposite parity 4fn－15dstates by an odd parity crystal－field component［14－15］.Therefore，its intensity is sensitive to the local structure around Eu3＋ions.When the Eu3＋activator ion is located at a low－symmetry local site（without an inversion center），the electric dipole transition is often dominated in the emission spectra［16］.In addition，the results provide apiece of information as following：Sr3P2O7has not optical activity in our studying wavelength region，because we could not observe a distinct emission band.

The emission spectra of the Sr1.8Eu0.8－xBixV1.2P0.8O8samples withx＝0.1，0.2，0.3，0.4，0.5，0.6and 0.7are exhibited in Fig.2 band 2c.From this figure，it is found that the spectra are similar with that in the case ofx＝0.However，the relative emission intensity of the four characteristic peaks of Eu3＋is varied with the doped Bi3＋content，especially for 617nm.It is also found that，when Bi doped content increases fromx＝0to 0.3，the maximum emission intensity（at 617nm）is enhanced by 33times.The intensity of5D0→7F2（617nm）transitions is enhanced with Bi doping，which proves that there exists an efficient energy transfer process between Bi3＋and Eu3＋ion.In our previous studies，it has reported that in Sr3－3xEu2xV1.2P0.8O8（0＜x≤0.3）［6］samples，the emission intensity first increases with increasing the Eu3＋concentration tillx＝0.2，and then it decreases with increasingxbecause the energy can transfer from Eu3＋to Eu3＋or to a quenching centers non－radiatively at high Eu3＋concentration，leading to the decrease of emission intensity with increasing the Eu3＋concentration.Therefore，in our sample Sr1.8Eu0.8V1.2P0.8O8，Eu3＋ions may be aggregated，and this kind of aggregation acts as trapping centers and dissipate absorbed energy non－radiatively.When Bi doped in Sr1.8Eu0.8－xBixV1.2P0.8O8，Eu content decreases，the distance between Eu－Eu ions may be larger，and the emission intensity of Eu3＋is enhanced.

On the other hand，as we all know，acting as one sensitizer，Bi3＋absorbs the excitation energy and then transfers the energy to an activator（a luminescent center）.As a result，the emission intensity increases.There are electric mulitpole－multipole interaction and exchange interaction between sensitizer（S）and activator（A）.Postulating that the dipole－dipole interaction plays an important role in the energy between Bi3＋and Eu3＋ion，the probability of energy transfer is given by the following equation［6］

wherePSAis the probability of energy transfer，QAis the absorption cross section ofA，Ris the distance betweenSandA，nis the refraction index of host lattice，τsis the radiative decay time ofS.On the other hand，（ε／x1／2εc）4is the local electric field，xis the dielectric constant.The integral represents the overlap between the normalizedSemission andAexcitation band.The distance in which the energy transfer betweenSandAcan occur is defined asRc.Considering structural factors，Rccan be described by the following equation

whereXcis the Bi3＋ion concentration andNis the number of Bi3＋ions in the unit cell.

In this case，as Bi3＋concentration increases，the distance between Bi3＋and Eu3＋ion decreases，subsequently the energy transition probability increases and consequently red light by Eu3＋increases.However，when Bi3＋ion content is more than 0.30，the emission intensity decreases，in other words，the optimal Bi3＋doped content in our samples is 0.30.This can be understood as follows.One is the decrease of Eu3＋ions with Bi3＋addition in Sr1.8Eu0.8－xBixV1.2P0.8O8，which directly results in the luminescent intensity lower.On the other hand，when Bi content is more than 0.30，the distance between Bi and Bi would be near enough to cause one efficient energy transfer process between Bi ions.Such an energy transfer process may reduce the probability of energy transfer from Bi to Eu.As a result，the sensitized effectiveness of Bi on the Eu emission intensity is reduced，which leads to the strengthening of emission intensity limits.In particular，in case ofx＝0.6and 0.7the emission intensity is even much lower than that of thex＝0sample as shown in Fig.2 c.Additionally，the effect of structural change on a radiation transition of Eu3＋ion by Bi3＋addition due to the difference between Bi3＋and Eu3＋ions may be considered.

For clarity，the emission intensity at 617nm vs Bi concentration of Sr1.8Eu0.8－xBixV1.2P0.8O8sample is shown in Fig.3 .With increasing the Bi concentrationx，the Eu3＋emission intensity is found to increase gradually and reach the maximum atx＝0.3.As discussed above，the energy transfer between Bi and Eu plays an important role in the emission of the Sr1.8Eu0.8－xBixV1.2P0.8O8（0≤x≤0.7）systems.In our present samples，with Bi concentration increasing，the distance between Bi and Eu ion decreases correspondingly，the energy transition probability increases subsequently，and thus red light by Eu is enhanced.However，in the case of the amount of Bi being bigger than 0.3，the distance between Bi and Bi ions may be close enough to cause the direct Bi－Bi energy transfer.Such a direct energy transfer process results in the lower energy transfer between Bi－Eu.Therefore，the emission intensity decreases as shown in Fig.3 .

Fig.4 apresents the excitation spectra of Sr1.8Eu0.8V1.2P0.8O8sintered at 1 000 ℃ for monitoring the5D0→7F2transition of Eu3＋.In the short wavelength region from 200to 300nm，it exhibits a shoulder peak at about 272nm（marked as arrow）.This is assigned to the excitation of the Sr3V2O8host crystal due to O2－－V5＋charge transfer，followed by energy to Eu3＋.This peak is also overlapped by the excitation peak due to O2－－Eu3＋charge transfer around 260nm［17］.Another peak located at 318 nm（7F0→5H6）is likely to the f－f transitions within Eu3＋4f6configuration.In the longer wavelength region，the excitation band exhibits some weaker narrow bands，resulting from the f－f transitions within Eu3＋4f6electron configuration.In this region，the main peaks at 318，385，398，467，and 538 nm correspond to the electron transitions from the7F1ground state to5L7，5L6，5D2，and5D1，consistent with spectral characterizations observed in previous work［18－19］.Fig.4 band 4cpresent the excitation spectra of the samples Sr1.8Eu0.8－xBixV1.2P0.8O8withx＝0.1，0.2，0.3，0.4，0.5，0.6and 0.7.They show the same excitation profiles as thex＝0sample.With Bi doping，the Eu content decreases，the shoulder at 272nm is weaker and gradually disappears.At the same time，the excitation peak at 318nm becomes stronger，even for 0.1≤x≤0.7samples，only one larger peak near 315nm is observed.As discussed before，the peak located at 318nm comes from the f－f transition within Eu3＋4f6configuration.The excitation intensity at 315nm is enhanced with Bi doping shows the sensitization effect of the Bi3＋ion on the Eu3＋emission plays an important role in the excitation region of the V－O components.On the other hand，the phenomenon is ascribed to the appearance of the charge transition from Bi3＋to V5＋with Bi doping.With increasing Bi concentration from 0.1to 0.3，this charge transition becomes stronger and brings about the increase of excitation intensity，shown in Fig.4 a.With increasing the Bi concentrationx，the excitation intensity at 318nm increases gradually and then decreases.The strongest excitation intensity is achieved in case ofx＝0.3sample.This phenomenon is attributed to the decrease of Eu content with Bi doping.In addition，forx＝0.6and 0.7samples，another broad excitation band with a peak at 341nm is also found.Generally speaking，this broad excitation is attributed to the1A1→1T1transition of VO43－and the1S0→3P1transition of Bi3＋in YVO4：Eu，Bi［8］.In this case，40%VO43－h(huán)as been replaced by PO43－，therefore，the contribution of1A1→1T1transition of VO43－may be not obvious.And only when the Bi content reaches some certain contentx＝0.6and 0.7，this peak at 341nm appears.The result is corresponding with the disc ussion in emission part.That is to say，with Bi doping，the distance between Bi and Bi ions may be close enough to cause the direct Bi－Bi energy transfer.Such a direct energy transfer process results in the lower energy transfer between Bi－Eu.

3 Conclusions

We have synthesized the Sr1.8Eu0.8－xBixV1.2P0.8O8（0≤x≤0.7）phosphors by the conventional solid－state reaction and investigated their ultraviolet activation and emission spectra.The host crystal structure is found to vary little with Bi doping.The emission spectra of the phosphors under excitation at 365nm exhibit four emission peaks located at 594，617，656，and 702nm.The emission intensity enhances with increasing Bi.It is ascribed to the two following reasons，one is the decrease of Eu content with Bi doping and make Eu－Eu aggregation destroy.The other is the energy transfer between Eu3＋and Bi3＋.However，with increasing Bi concentration further，the decrease of Eu content makes the emission intensity lower.At the same time，the direct Bi－Bi energy transfer occurs and makes the energy transfer between Bi－Eu lower with Bi doping.As a result，the emission intensity is lower correspondingly.The peak at 318nm in the excitation spectra is assigned to the f－f transition within Eu3＋4f6configuration.The f－f transitions within Eu3＋4f6electron configuration also lead to the other three peaks in the longer wavelength region.The excitation intensity increases with Bi doping，however，with Bi concentration increasing further，the Eu concentration decreases，Eu excitation intensity is lowered in Sr1.8Eu0.8－xBixV1.2P0.8O3phosphors.Additionally，with Bi content reaches 0.6 and 0.7，the broad excitation band with a peak at 341nm appears which is attributed to1S0→3P1transition of Bi3＋.

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