Photoluminescence properties of Sr3Y2(BO3)4: Eu3+ phosphor codoped with Gd3+ and La3+ under ultraviolet excitation

Sr3Y2 (BO3)4: Eu3+ phosphor codoped with Gd3+ and La3+ was generated using the solid state method, and XRD and fluorescence spectrum researched the structure and luminescence properties. The strongest excitation peak of Sr3Y2 (BO3)4:Eu3+ is in the near ultraviolet region of 394 nm, which comes from the 7F0 → 5L6 transition of Eu3+. With the stimulation of 394 nm ultraviolet-light, the strongest emission of Sr3Y2 (BO3)4:Eu3+ phosphor is a red light with a wavelength of 618 nm. It is produced by the transition of 5D0 → 7F2 of Eu3+. The optimum preparation temperature is 1000°C, and the doping amount of Eu3+ is 5% (mol). The doping of Gd3+ and La3+ greatly enhanced the luminescent intensity of Sr3Y2 (BO3)4:Eu3+ phosphor, and the doping of 20% (mol) was suitable.


Introduction
White LED has attracted great attention for its excellence in high efficiency, low energy consumption, and environmental protection [1,2] .Using a near-ultraviolet InGaN chip (380-410 nm) to excite three primary colours phosphor to achieve white LED has become one of the international research hotspots.At present, the performance of the red powder in terms of stability, luminous efficiency, and color rendering properties needs to be further improved.There are many types of oxysalt that can be used as a phosphor matrix, including aluminate, borate, vanadate, tungstate, molybdate, etc.Among them, the borate matrix has been generally considered by researchers because of its characteristics of high luminescence efficiency, low preparation temperature, and good chemical stability [3] .Li et al. [4] prepared the series of phosphors Eu 3+ and Dy 3+ doped alkaline earth borate.Li et al. [5] investigated the preparation of Eu 3+ doped LiBaB 9 O 15 phosphor and the energy transfer process by high temperature solid-phase method.Ce 3+ , Tb 3+ co-doped Sr 2 B 2 O 5 matrix phosphor was prepared by Sun et al. [6] .All the research on phosphor structure and luminescence performance proved that the alkaline earth borate system can be explored in white LED applications.
This paper applies a high-temperature solid phase method to generate Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ phosphor and digs into the influence of codoped Gd 3+ and La 3+ on the structure and luminescent properties.

Experimental section
The stoichiometric amounts of Eu 2 O 3 , Y 2 O 3 , La 2 O 3 , SrCO 3, and H 3 BO 3 (20% excess of H 3 BO 3 to make up for the loss in the high temperature calcining process) were fully mixed and ground in a quartz mortar, then placed in an alumina crucible, and calcined in a high temperature oven at a given environment for 5 h.
With the utilization of Cu Kα radiation of XRD (Rigaku D/max-2200), the structure of the phosphor was depicted.The fluorescence spectrophotometer (Hitachi F-7000) measured the luminescent properties under UV excitation.The emission spectra of Sr 3 Y 2 (BO 3 ) 4 : Eu 3+ phosphors.Figure 1 portrays the excitation spectra of Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ generated at a calcination temperature of 1000°C and Eu 3+ doping of 5% (mol), measured at a monitoring wavelength of 618 nm.It portrays the formation of an excitation spectrum containing a broadband around 270 nm and several narrow peaks between 360-400 nm.The transition of the 2p electron of O 2-in the matrix to the f-orbitals of Eu 3+ generated the broadband around 270 nm.Eu 3+ is strongly coupled to the lattice in the process, which results in the form of broadband peaks in the excitation spectra.The rest of the peaks originate from the f→f transition of Eu 3+ .The highest excitation peak at 394 nm originates from Eu 3+ of 7 F 0 → 5 L 6 transition.Therefore, Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ can be applied as a luminescence material for near-ultraviolet-excited LEDs.

Luminescent properties of
Figure 2 demonstrates the emission spectra of phosphor prepared at different calcining temperatures from 800°C to 1100°C.The phosphors generated under different calcining temperatures have two strong emission peaks at 596 nm and 618 nm.The 596 nm emission peak is the magnetic dipole transition of 5 D 0 → 7 F 1 of Eu 3+ , and the stronger one at 618 nm is the electric dipole transition of 5 D 0 → 7 F 2 of Eu 3+ .It indicates that the doping of Eu 3+ ions mainly occupies the asymmetric center lattice position in the structure.The other main emission peaks, 656 nm, and 709 nm, belong to the energy level transition 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 of Eu 3+ ions.From the intensity of the peaks, it is revealed that the luminescent intensity of the prepared phosphor enhances with the rising of the calcining temperature between 800°C and 1000°C and reaches the maximum at 1000°C.The luminescence intensity of the material starts to decrease when it continues to rise to 1100°C, which is caused by the temperature quenching effect.Figure 3 shows the relationship between luminescence intensity and Eu 3+ ion doping amount of Sr 3 Y 2-x (BO 3 ) 4 :xEu 3+ (x≤0.25)phosphors stimulated by UV light (394 nm).It is observed that from Figure 3, when the Eu 3+ doping amount is less than 15%, the luminescent intensity of Sr 3 Y 2-x (BO 3 ) 4 :xEu 3+ phosphor increasingly enhances with the gradual rise in the doping amount of Eu 3+ .The intensities reached a maximum when it was 15% (mol), and then increased the europium-doped amount of luminescence intensity instead of weakening, which is caused by the non-radiative ionic transition, that is, there is the effect of concentration quenching.
What can be observed from Figure 4(a) is the affection of Gd 3+ doping amount on the structure of Sr 3 Y 1.95-y Gd y (BO 3 ) 4 :0.05Eu3+ phosphor.When the gadolinium content y value is less than or equal to 0.6, the position of Sr 3 Y 1.95-y Gd y (BO 3 ) 4 :0.05Eu3+ diffraction peak is the same as that of Sr 3 Y 2 (BO 3 ) 4 standard card PDF No.54-1120, indicating that the small doping amount of Gd 3+ will not affect the crystal structure of Sr 3 Y 2 (BO 3 ) 4 .This is because the three ions of Gd 3+ , Eu 3+ , and Y 3+ have a similar ionic radius, and the entry of Gd 3+ ions and Eu 3+ ions into Sr 3 Y 2 (BO 3 ) 4 crystals to replace part of the Y 3+ will not change its crystal structure.phosphor is significantly higher than that of phosphor Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ , the doping of Gd 3+ and La 3+ improves the luminescence intensity of the Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ phosphor greatly, but the position of the emission peaks is not changed.In Sr 3 Y 2-y M y (BO 3 ) 4 :Eu 3+ (M=Gd, La) phosphor, Gd 3+ and La 3+ ions acted as sensitizers to improve their luminescence efficiency.The luminescence intensity of phosphor Sr 3 Y 1.95-y Gd y (BO 3 ) 4 :0.05Eu3+ showed a tendency of enhancement initially and then diminution with the increase in the number of Gd 3+ ions.When the Gd 3+ concentration is less than or equal to 20% (mol), the luminescence intensity is enhanced with the enlargement of the Gd 3+ doping amount, and the luminescence strength reaches the maximum value when the Gd 3+ doping amount is 20% (mol).The luminescence intensity increased by 40% under the optimal preparation conditions.With the rise of Gd 3+ ion concentration, the luminous intensity decreases correspondingly.It can be explained that because of the growth of Gd 3+ ion concentration, the distance between the Gd 3+ ion and Eu 3+ ion in the lattice decreases, resulting in the increase of non-radiative transfer of Eu 3+ ion and the weakening of luminous intensity.
The same can be discovered.The luminous intensity of doped La 3+ ions is obviously better than that of undoped phosphors, and the optimal lanthanum content is also 20% (mol).Under the best preparation conditions, doped La 3+ ions can increase the luminous intensity by 42%.
Table 1 shows the CIE color coordinates of phosphors prepared from Sr 3 Y 1.45 Gd 0.4 (BO 3 ) 4 :0.15Eu3+ and Sr 3 Y 1.45 La 0.4 (BO 3 ) 4 :0.15Eu3+ calcined at 1000°C for 5 h.It is observed that it is close to NTSC red (0.67, 0.33), indicating that the prepared phosphor meets the red phosphor standard for white LEDs.

Conclusion
This research has generated Eu 3+ and Gd 3+ , La 3+ co-doped Sr 3 Y 2 (BO 3 ) 4 series phosphors using the solid phase method and investigated luminescence properties.The results show that the strongest emission peaks of the phosphors are the electric dipole transition of Eu 3+ ions 5 D 0 → 7 F 2 under near-ultraviolet excitation at 394 nm.Doping a small amount of Eu 3+ , Gd 3+ , and La 3+ will not change the substrate structure of Sr

Figure 2 .
Figure 2. The emission spectra ofSr 3 Y 2 (BO 3 ) 4 : Eu 3+ phosphors.Figure1portrays the excitation spectra of Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ generated at a calcination temperature of 1000°C and Eu 3+ doping of 5% (mol), measured at a monitoring wavelength of 618 nm.It portrays the formation of an excitation spectrum containing a broadband around 270 nm and several narrow peaks between 360-400 nm.The transition of the 2p electron of O 2-in the matrix to the f-orbitals of Eu 3+ generated the broadband around 270 nm.Eu 3+ is strongly coupled to the lattice in the process, which results in the form of broadband peaks in the excitation spectra.The rest of the peaks originate from the f→f transition of Eu 3+ .The highest excitation peak at 394 nm originates from Eu 3+ of 7 F 0 → 5 L 6 transition.Therefore, Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ can be applied as a luminescence material for near-ultraviolet-excited LEDs.Figure2demonstrates the emission spectra of phosphor prepared at different calcining temperatures from 800°C to 1100°C.The phosphors generated under different calcining temperatures have two strong emission peaks at 596 nm and 618 nm.The 596 nm emission peak is the magnetic dipole transition of 5 D 0 → 7 F 1 of Eu 3+ , and the stronger one at 618 nm is the electric dipole transition of 5 D 0 → 7 F 2 of Eu 3+ .It indicates that the doping of Eu 3+ ions mainly occupies the asymmetric center lattice position in the structure.The other main emission peaks, 656 nm, and 709 nm, belong to the energy level transition 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 of Eu 3+ ions.From the intensity of the peaks, it is revealed that the luminescent intensity of the prepared phosphor enhances with the rising of the calcining temperature between 800°C and 1000°C and reaches the maximum at 1000°C.The luminescence intensity of the material starts to decrease when it continues to rise to 1100°C, which is caused by the temperature quenching effect.

Figure 3 .
Figure 3.The emission intensity of Sr 3 Y 2-x (BO 3 ) 4 :xEu 3+ with different Eu 3+ doping molar ratios.Figure3shows the relationship between luminescence intensity and Eu 3+ ion doping amount of Sr 3 Y 2-x (BO 3 ) 4 :xEu 3+ (x≤0.25)phosphors stimulated by UV light (394 nm).It is observed that from Figure3, when the Eu 3+ doping amount is less than 15%, the luminescent intensity of Sr 3 Y 2-x (BO 3 ) 4 :xEu 3+ phosphor increasingly enhances with the gradual rise in the doping amount of Eu 3+ .The intensities reached a maximum when it was 15% (mol), and then increased the europium-doped amount of luminescence intensity instead of weakening, which is caused by the non-radiative ionic transition, that is, there is the effect of concentration quenching.

Figure 4 .Figure 5 .
Figure 4. XRD patterns of Sr 3 Y 2-y M y (BO 3 ) 4 :Eu 3+ (M=Gd, La) phosphors.Figure 4(b) displays the XRD patterns of phosphor Sr 3 Y 2-y La y (BO 3 ) 4 :Eu 3+ prepared by La 3+ ion doping.It can be seen that the relative strength and location of the diffraction peaks are the same as those of the standard card when the amount of La 3+ doping is ≤ 25%, which indicates that the small doping amount of La 3+ cannot change the crystal structure of the substrate.When the La 3+ doping

3 Y 2 (
BO 3 ) 4 .The doping of Gd 3+ and La 3+ increases the luminous intensity of phosphors Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ by about 40%, and the color coordinates have a very close distance from the standard red coordinates specified by the NTSC standard.Sr 3 Y 2-y Gd y (BO 3 ) 4 :Eu 3+ and Sr 3 Y 2-y La y (BO 3 ) 4 :Eu 3+ series of red phosphors have the potential application prospect in the white light LED.

Table 1 .
CIE (Chrominance coordinates) of phosphor.The data listed in Table1depicts the CIE color coordinate diagram, as shown in Figure6.The coordinate points are close to the red light area, indicating that the prepared phosphor powder has good color purity.