The effect of addition of boron on the long afterglow property of SrAl₂O₄: Eu²⁺, Dy³⁺ by preparing the phosphors at various amount and types of boron compounds as flux was investigated. The addition of boron compounds as flux improved the long afterglow property and the sample with addition of H₃BO₃ showed higher afterglow intensity than that with addition of B₂O₃. The investigation of long afterglow property of the samples with various amounts of addition of H₃BO₃ revealed that there is a distinct correlation between afterglow intensity and boron content in the samples. Molar ratio of Sr₄Al₁₄O₂₅ phase increased and unit cell volume of SrAl₂O₄ crystal decreased with an increase in the amount of addition of H₃BO₃. The result indicated that the substitution of BO₄ for AlO₄ in SrAl₂O₄ crystal phase promoted both incorporation of Dy³⁺ ions, which could act as electron traps, into Sr²⁺ sites and formation of strontium vacancies, which could act as hole traps, and then afterglow property of the phosphors was enhanced.

Eu²⁺, Dy³⁺ co-doped SrAl₂O₄ phosphor was developed as long lasting phosphor in the early 1990’s.¹⁾ SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor has good chemical stability, high quantum efficiency, and high brightness compared with sulfide phosphor. This phosphor attracts attention as one of the excellent phosphorescent materials and it is mainly used for fluorescent pigment of luminous clocks and safety marks. Recently, several studies have been reported about new application such as long-lasting phosphorescent pigments,¹⁾ a spectral downshifter for photovoltaic devices,²⁾ luminescent fingerprinting powders,³⁾ and nanophosphors as cancer cell tagging.⁴⁾

The base material of this phosphor is strontium aluminate and Eu²⁺ ion and Dy³⁺ ion are added as activators. The emission mechanism depends on the recombination of electrons and holes.⁵⁾ Under UV irradiation, electrons are promoted from the occupied 4f levels of Eu²⁺ to the empty 5d levels and the holes are created in 4f levels of Eu²⁺ at the same time. Excited electrons on 5d levels emit external energy as heat and return to 4f levels. Then electrons and holes recombine at 4f levels of Eu²⁺. The recombination make electrons stable and they emit excessive energy as light. This is the mechanism of fluorescence of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor. 5d levels are strongly affected by ligand fields, so the emission spectrum depends on a type of base materials. The emission peak of green-emitting SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor is 520 nm. Theoretical studies about photo luminescence spectra of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor have been reported so far. According to the density functional theory (DFT), the hosts band gap calculated for both available strontium positons to be at about 4.5–5 eV and above the top of the valence band.⁶⁾ The other researchers reported that the band gap of SrAl₂O₄ was estimated to be about 7 eV from and MBJ-LDA calculation.¹⁾ The fluorescent from this phosphor is thought to be lost rapidly because 4f-5d transition is the allowed transition. Then, it was assumed that the assistance activator, Dy³⁺ ions, enhanced long afterglow property.^7,8) Many studies on the mechanism of long afterglow of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor have been reported so far as following:

In certain condition, Dy³⁺ has been shown to act as a hole trap.⁹⁾ Long afterglow luminescence was yield when [Al]/[Sr] ratio was slightly higher than the stoichiometric value of 2.¹⁰⁾ Dy³⁺ ion, as a hole trap,²⁾ pay on important role in the long afterglow property of SrAl₂O₄: Eu²⁺, Dy³⁺. In the SrAl₂O₄: Eu²⁺, Dy³⁺ materials, electron/hole pairs may be produced simultaneously under UV light irradiation and trapped by electron traps ($\text{Dy}_{\text{Sr}}{}^{ \boldsymbol{\cdot} }$) and hole traps (V_sr′′), respectively.¹¹⁾ Trapping of the electron delocalization to the conduction band.¹²⁾ A higher concentration of codopants, i.e., Dy³⁺ alone in combination with Nd³⁺, results in more hole trap levels, leading to an enhancement of the photo luminescence properties.¹³⁾ The maximal long afterglow luminescence was not only governed by the quantity of the luminescent host but also by the quality of the host (the concentration of the Eu²⁺ and especially the Dy³⁺ in the luminescent phases).¹⁴⁾ The perfect SrAl₂O₄ in an insulator with its bandgap of 7.61 eV which was larger than the estimated value. V_sr′′ introduce shallow acceptor levels which are very close to valence band. Such shallow hole traps indicate that can easily capture holes for valence band of the host.¹⁾ Oxygen vacancies introduced shallow electron traps can capture electron from conduction band. The afterglow luminescence center of these phosphors is determined by the deep electron trap depth of $\text{Vo}^{ \boldsymbol{\cdot} \boldsymbol{\cdot} }$ in the band gap of SrAl₂O₄.¹⁵⁾

Recently, the investigation of the long afterglow properties of SrAl₂O₄ phosphor without Eu²⁺ indicated that luminescence center, Eu²⁺, is not prerequisite for the afterglow of the phosphors.¹⁶⁾ In that paper, it was reported that the shallow electron trap, $\text{Dy}_{\text{Sr}}{}^{ \boldsymbol{\cdot} }$, is located at somewhere below the conduction band and oxygen vacancies introduced shallow electron trap below the conduction band, the hole trap levels created by V_sr′′ and V_Al′′′ above valence band, the deep electron trap level of oxygen vacancies above valence band.¹⁶⁾

However, the mechanism of afterglow properties of SrAl₂O₄: Eu²⁺, Dy³⁺ is very complicated and still an open debate.

There are some ways to prepare SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor such as combustion synthesis,^2,4,17–20) sol-gel method^15,16,21,22) and co-precipitation method,²³⁾ but the most common method is thought to be solid state reaction.^{6,10,11,24–26)} In this method, raw materials are mixed together and reacted at a high temperature to get samples. The experimental procedure of this method is easy, but it is essential for preparing samples to sinter at a high temperature for a long time. Therefore, a flux is added to lower reaction temperature and accelerate a diffusion of raw materials.²⁷⁾ Various fluxes are used such as oxide, fluoride, and sulfide.

Thus far the role of flux is just to improve the crystallinity and control the grain size of sample, and flux is a nonreactive medium to raw materials. However, it was reported that B₂O₃ affects the optical property of SrAl₂O₄ phosphor in the past.²⁸⁾ There have been a few researches about the effect of addition of boron compounds until now, however the exact role of boron has not been revealed yet. The flux facilities the binding of the standing materials, the material transport during the synthesis process, and the bonding of the activator with the host crystal. It also lowers the temperature of the synthesis.¹⁴⁾ The flux influences the concentration of the doped Eu²⁺ and Dy³⁺ in the host crystal and eventually influences the long afterglow luminescence.¹⁰⁾ In the combustion method, boric acid reduces the overall combustion temperature and when it melts, the surface of the liquid helps particles to coagulate, and to initiate the combustion process.³⁾ H₃BO₃ introduced lattice contractions in SrAl₂O₄ and photo luminescence intensity depended on the amount of H₃BO₃.¹⁶⁾

Also, a type of boron compounds, H₃BO₃ or B₂O₃, used as flux depends on the researchers. That is because B₂O₃ is formed when H₃BO₃ is heated at a high temperature and decomposed. It is thought that the effect on the various properties of SrAl₂O₄ phosphor is the same either H₃BO₃ or B₂O₃ are used as flux. However, it was found that the various properties of SrAl₂O₄ phosphor changed depended on the boron compounds in the present study. Then, it is suggested the effect of addition of boron is different with H₃BO₃ and B₂O₃, and the purpose of this study is to reveal the exact role of boron about each flux compounds. The application of SrAl₂O₄ phosphor is limited like fluorescent pigments although it has a great long afterglow property. Other applications will develop if the afterglow mechanism is revealed. In addition, B₂O₃ levels of a commercial SrAl₂O₄ phosphors could not determine by XRF.²⁹⁾ There are no reports on the effect of the amount of boron in the SrAl₂O₄ phosphors on the afterglow properties in the phosphors.

In this study, boron content measuring of fired samples carried out in addition of the other characterization of the phosphors. The present work was motivated to find effect of addition of boron on the long afterglow property of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor.

All samples were prepared by solid state reactions. Starting materials used in this study were pure powders of such as SrCO₃ (Wako Pure Chemical, 99.9%), Al₂O₃ (Sumitomo Chemical, 99.9%), Eu₂O₃ (Mitsuwa Chemicals, 99.9%), Dy₂O₃ (Wako Pure Chemical, 99.5%), H₃BO₃ (Nacalai Tesque, 99.5%), B₂O₃ (Wako Pure Chemical, 90.0%). The powders were weighed according to the nominal composition of (Sr_0.94Eu_0.03Dy_0.03)Al₂O₄. B₂O₃ and H₃BO₃ were added as a flux. The amount of boron compounds was set as molar ratio of boron to strontium of 0.9 in case of B₂O₃ and as molar ratio of boron to strontium of 0.9, 1.5 and 1.8 in the case of H₃BO₃. Hereafter, molar ratio of boron to strontium is represented as B/Sr and sample with addition of boron of B/Sr = 1.5 is represented as sample of B/Sr = 1.5, for example.

Preweighed powders were mixed by ball milling with ethanol for 24 h and dried at 90°C for 24 h. The dried powders were put into alumina crucible and calcined at 1000°C for 5 h in air. The calcined powders were pulverized by using an alumina mortar and shifted to alumina boat, followed by heat treatment in an alumina tube furnace at 1350°C for 5 h in hydrogen atmosphere.

The crystal phase of the synthesized phosphors was identified by using an X-ray diffractometer (RINT2000, Rigaku, Japan) with Cu Kα radiation. For the quantitative analysis of the crystal phases, a Rietveld refinement of the obtained XRD patterns was performed by using the RIETAN-FP software package.³⁰⁾ For investigating the excitation and the emission properties, the photoluminescence (PL) spectra were measured in the wavelength range from 400 to 650 nm by using a PL spectrometer (FP-6500, JASCO, Japan). The wavelength of exciting light was 360 nm. For the evaluation of the afterglow luminescence, decay curves with continuous time were measured by using a luminescence meter (LC-5, Tsubosaka Electric, Japan). A UV lamp (365 nm) was used as a light source and irradiated samples for 10 min before the measurement of the decay curves. ESR spectra were measured by X-band ESR spectrometer (JES-TE300, JEOL, Japan) at room temperature. Concentration of boron in fired samples was measured by spectrophotometric analysis with 8-hydroxy-1-(salicylideneamino)-3,6-naphthalene-disulfonic acid, sodium salt.³¹⁾

Figure 1 shows the XRD patterns of the samples with or without boron compounds as flux. Focusing on the peak at around 2θ = 35°, separated diffraction peaks clearly in the samples with addition of flux. On the other hand, there were overlapped diffraction peaks in the sample without addition of flux. Therefore, it was clear that the crystallinity of SrAl₂O₄ was improved by the addition of flux. As shown in Fig. 1, SrAl₂O₄ as the main crystal phase and DyAlO₃ as the secondary phase were observed in the samples with B₂O₃ and without flux. On the other hand, SrAl₂O₄ as the main phase and Sr₄Al₁₂O₂₅ as the secondary phase were observed^32,33) in the sample with H₃BO₃. This result indicated that incorporation of Dy³⁺ ions into Sr sites in SrAl₂O₄ crystal was promoted by addition of H₃BO₃ as a flux.

Fig. 1

XRD patterns of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors. (a): without flux; (b): with addition of B₂O₃; (c): with addition of H₃BO₃.

Figure 2 shows the emission spectra of the samples with or without boron compounds as flux. The broad peak at around 520 nm is a typical peak of SrAl₂O₄: Eu²⁺. Although the emission peak was observed at almost the same position in all samples, the intensity was largely different depending on presence or absent of flux.

Fig. 2

Emission spectra of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors. (a): without flux; (b): with addition of B₂O₃; (c): with addition of H₃BO₃.

Figure 3 shows the ESR spectra of the samples with or without boron compounds as flux. Very weak signals given by Eu²⁺ ions were observed for the sample without flux. It was assumed that the amount of Eu²⁺ ions incorporated into SrAl₂O₄ phases in the sample without flux was very small. It was assumed that boron promoted incorporation of Eu²⁺ ions into SrAl₂O₄ phases. It was considered that incorporation of Eu²⁺ ions into SrAl₂O₄ crystal was difficult for lack of flux. The low intensity of emission spectra of the sample without flux seems to be for the similar reason. XRD results revealed that Sr₄Al₁₄O₂₅ phase was formed in the sample with addition of H₃BO₃ and was not formed in the sample with addition of B₂O₃. However, the ESR spectra of the sample were similar to those of the sample with addition of B₂O₃. Therefore, it was indicated that Eu²⁺ ions are incorporated mainly into SrAl₂O₄ phases of the sample with addition of H₃BO₃.^34–36)

Fig. 3

ESR spectra of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors. (a): without flux; (b): with addition of B₂O₃; (c): with addition of H₃BO₃.

Figure 4 shows the decay curves of the samples with or without boron compounds as flux. The initial intensities of the samples without flux, with addition of B₂O₃, and with addition of H₃BO₃ were 0.01, 2.93 and 11.59 cd m⁻², respectively. The highest initial intensity and afterglow intensity at 600 sec after stopping irradiation were observed for the sample with addition of H₃BO₃ and the lowest of them were observed for the sample without flux. As a result, the long afterglow property of the samples was different depending on presence or absence of flux and types of boron compounds.

Fig. 4

Decay curves of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors. (a): without flux; (b): with addition of B₂O₃; (c): with addition of H₃BO₃.

The reason why the results were different between the case of using H₃BO₃ and B₂O₃ as a flux, is considered as following.

Because H₃BO₃ change into B₂O₃ during heating at high temperatures, the behaviors of two types of flux would be different before heating at high temperatures, namely, in the wet milling or drying process. It is likely that B(OH)₄⁻ ions are formed by the reaction between H₃BO₃ and H₂O in the wet milling process.^37,38) B(OH)₄⁻ ions would absorb on the surface of SrCO₃ particles easily because the surface has a positive electric charge. After drying mixtures, relatively large amount of boron would exist on the surface of SrCO₃ particles in case of H₃BO₃.^39–41) SrB₂O₄ would form easily from SrCO₃ containing boron during heating at high temperatures.^16,29,42,43) Therefore, SrAl₂O₄ contained larger amount of boron in the case of H₃BO₃ (boron: 0.50 mass%) compared to in the case of B₂O₃ (boron: 0.07 mass%). It was considered that the deference of the amount of boron in the fired samples leaded to the deference of the properties of the samples. In addition, the relationship between boron content and luminescence properties will be discussed in detail in the next section.

Effect of the amount of H₃BO₃ on the luminescence property was investigated because the sample with addition of H₃BO₃ had the highest afterglow intensity.

Figure 5 shows the XRD patterns of samples with various amount of H₃BO₃ (B/Sr = 0.9, 1.5 and 1.8). SrAl₂O₄ as the main phase and Sr₄Al₁₄O₂₅ as the secondary phase were observed in all samples. As the value of B/Sr increased, the peaks of Sr₄Al₁₄O₂₅ phase became high and sharp while the peaks of SrAl₂O₄ phase became lower and broader. Thus, it was revealed that the crystallinity of SrAl₂O₄ phase became lower and the amount of Sr₄Al₁₄O₂₅ phase increased as the amount of addition of H₃BO₃ increased. DyAlO₅ phase was not observed in these samples. This result was indicated that the addition of H₃BO₃ promoted the incorporation of Dy³⁺ ions into SrAl₂O₄ crystal.

Fig. 5

XRD patterns of samples with addition of various amount of H₃BO₃. (a): B/Sr = 0.9; (b): B/Sr = 1.5; (c): B/Sr = 1.8.

Figure 6 shows the emission spectra of samples with various amount of H₃BO₃ (B/Sr = 0.9, 1.5 and 1.8). The intensity of the sample of B/Sr = 0.9 was the highest in three types of samples. In general, it is known that the low crystallinity leads to decrease in the quantum efficiency of the sample. As a result of XRD analysis (Fig. 5), it was considered that the low crystallinity of samples of B/Sr = 1.5 and 1.8 caused a decrease in emission intensity of them.

Fig. 6

Emission spectra of samples with addition of various amount of H₃BO₃. (a): B/Sr = 0.9; (b): B/Sr = 1.5; (c): B/Sr = 1.8.

Figure 7 shows the ESR spectra of samples with various amount of H₃BO₃ (B/Sr = 0.9, 1.5 and 1.8). The overlapped ESR signals were observed at the magnetic field of around 160, 320 and 420 mT in case of the sample of B/Sr = 1.8. Comparing with ESR spectra reported by Nag and Kutty,³⁶⁾ it was revealed that the signals of Eu²⁺ in SrAl₂O₄ phase and Eu²⁺ in Sr₄Al₁₄O₂₅ phase were overlapped in the sample of B/Sr = 1.8.

Fig. 7

ESR spectra of samples with addition of various amount of H₃BO₃. (a): B/Sr = 0.9; (b): B/Sr = 1.5; (c): B/Sr = 1.8.

Figure 8 shows the decay curves of samples with various amount of H₃BO₃ (B/Sr = 0.9, 1.5 and 1.8). The initial intensities of the samples of B/Sr = 0.9, 1.5 and 1.8 were 11.59, 15.11 and 10.96 cd m⁻², respectively. The highest initial intensity and afterglow intensity at 600 sec after stopping irradiation were observed for the sample of B/Sr = 1.5.

Fig. 8

Decay curves of samples with addition of various amount of H₃BO₃. (a): B/Sr = 0.9; (b): B/Sr = 1.5; (c): B/Sr = 1.8.

In general, the eq. (1) of the degradation of emission from long afterglow phosphors is given as follows.⁴⁴⁾   

Fig. 9

Plot of logarithm of afterglow intensity (I) versus logarithm of decay time for samples with addition of various amount of H₃BO₃. (a): B/Sr = 0.9; (b): B/Sr = 1.5; (c): B/Sr = 1.8.

The n-value of each sample was obtained from the slope of a straight line shown in Fig. 9. Table 1 shows afterglow intensity at 3600 sec after stopping irradiation and n-value of each sample. The sample of B/Sr = 1.8 had the highest afterglow intensity and the smallest n-value in three types of samples as shown in Table 1. The sample of B/Sr = 1.8 had the most excellent long afterglow property in this work.

Table 1 Afterglow intensity at 1 h after stopping irradiation and n-value of samples with addition of with addition of various amount of H₃BO₃ (B/Sr = 0.9, 1.5 and 1.8).

Figure 10 shows the comparison of boron contents with afterglow intensity for the samples of B/Sr = 0.9, 1.5 and 1.8. It is clear that there is a distinct correlation between boron content and afterglow intensity as shown in this figure. This result indicated boron in samples affected the afterglow property.

Fig. 10

Boron content and afterglow intensity at 1 h after stopping irradiation as a function of amount of H₃BO₃ added.

It was revealed that there is a correlation between boron content in the samples and long afterglow property of phosphor as mentioned above. Hereafter, the role of boron to improve the long afterglow property of SrAl₂O₄ phosphor is discussed. Moreover, the long afterglow mechanism of (Sr_0.94Eu_0.03Dy_0.03)Al₂O₄ phosphor is also suggested.

It was assumed that the boron substituted as BO₄ into AlO₄ framework of SrAl₂O₄ host lattice. The structure of SrAl₂O₄ is the stuffed tridymite style which is a family of nepheline, AlO₄ frameworks are sharing their tops with others.⁴⁵⁾ The ion radius⁴⁶⁾ (four coordinated sites) of B³⁺ (0.11 Å) is smaller than Al³⁺ (0.39 Å), and boron can form oxygen polyhedrons like BO₄. Therefore, it is likely for boron to substitute for AlO₄ as BO₄.^47–49) If BO₄ substitute for AlO₄, the amount of aluminum will be excess stoichiometrically and acceleration of the formation of aluminum-rich Sr₄Al₁₄O₂₅ is expected.

The quantitative analysis was conducted for the samples of B/Sr = 0.9, 1.5 and 1.8 by Rietveld method to confirm this expectation. As shown in Fig. 11, the molar ratio of Sr₄Al₁₄O₂₅ increased with an increase in the amount of addition of H₃BO₃. This result suggests the substitution of BO₄ for AlO₄. On the other hand, the crystallinity of the samples became lower with an increase in the amount of addition of H₃BO₃ as shown in Fig. 5. The result indicates the formation of amorphous phase of the sample with large amount of addition of H₃BO₃. It needs to investigate whether a large amount of boron exists in the amorphous phase or not.

Fig. 11

Molar fraction of phases of SrAl₂O₄ and Sr₄Al₁₄O₂₅ for samples with addition of various amount of H₃BO₃.

Therefore, we tried to measure boron contents in amorphous phases as follows. The samples of B/Sr = 0.9, 1.5 and 1.8 were heated in boiling water for 30 min for leaching amorphous phase. The leached solution was filtered and diluted for appropriate concentration and analyze by ICP-AES (ICPE 9800, Shimadzu, Japan). The percentage of boron content in amorphous phase was less than 2% in relation to the entire boron in the fired samples. Therefore, it was confirmed that BO₄ substituted for AlO₄ in SrAl₂O₄ crystal phase.

Eu²⁺ ions and Dy³⁺ ions can incorporate into Sr²⁺ sites in crystal structure formed by AlO₄ framework. Since the electronegativity of boron is higher than aluminum (e.n. of B = 2.0, Al = 1.6 Pauling’s scale), Sr²⁺ sites near BO₄ is more electronegative than those near AlO₄. Therefore, Dy³⁺ ions are likely to incorporate into Sr²⁺ sites near BO₄ because the charge density of Dy³⁺ is higher than that of Eu²⁺.

The unit cell volume of the samples of B/Sr = 0.9, 1.5 and 1.8 was obtained using Rietveld analysis as shown in Fig. 12. The unit cell volume of sample without flux was also analyzed to compare with samples with addition of H₃BO₃. It was found that the unit cell volume of sample with flux was smaller than that of the sample without flux. Since the ionic radius¹⁸⁾ (nine coordinated sites) of Sr²⁺ ion, Eu²⁺ ions, and Dy³⁺ ion is 1.31 Å, 1.30 Å, and 1.08 Å respectively, it is expected that the unit cell volume becomes smaller due to the incorporation of Dy³⁺ ions into Sr²⁺ sites. It was suggested that the substitution of BO₄ for AlO₄ occurred due to the addition of H₃BO₃ as flux, and then the incorporation of Dy³⁺ ions, which were likely to incorporate into Sr²⁺ sites near BO₄, into SrAl₂O₄ crystal was accelerated.^10,14) The acceleration of incorporation of Dy³⁺ ions into Sr²⁺ sites leads to the enhancement of long afterglow SrAl₂O₄ phosphor because Dy³⁺ ions in the crystal acts as electron traps.^9,11,16)

Fig. 12

Unit cell volume of SrAl₂O₄ crystal in samples with addition of various amount of H₃BO₃.

Moreover, strontium vacancies form due to charge compensation when Dy³⁺ ions incorporate into Sr²⁺ sites according to eq. (3).   

In this work, we have investigated the effect of addition of boron on the long afterglow property of SrAl₂O₄: Ed²⁺, Dy³⁺ by preparing the phosphors at various amount and types of boron compounds as flux.

The composition of crystal phases and the behavior of incorporation of Eu²⁺ ions into SrAl₂O₄ crystal structure were different depending on the presence or absence of flux and the types of boron compounds. Furthermore, the addition of boron compounds as flux improved the long afterglow property and the highest afterglow intensity was observed for the sample with addition of H₃BO₃.

The long afterglow property of the samples with various amounts of addition of H₃BO₃ was investigated. The sample of B/Sr = 1.8 showed the highest afterglow intensity, the highest intensity of diffraction peak of Sr₄Al₁₄O₂₅ phase and the largest boron content in the sample. Form the results, it is clear that there is a distinct correlation between boron content and afterglow intensity.

Molar ratio of Sr₄Al₁₄O₂₅ phase increased and unit cell volume of SrAl₂O₄ crystal decreased with an increase in the amount of addition of H₃BO₃. This result suggested that substitution of BO₄ for AlO₄ in SrAl₂O₄ crystal phase promoted both incorporation of Dy³⁺ ions, which could act as electron traps, into Sr²⁺ sites and formation of strontium vacancies, which could act as hole traps, and then the long afterglow property of the phosphors was enhanced.

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