Relation between structure conversion and spectra-tuning properties of Eu²⁺-doped strontium aluminate phosphor

Abstract

Phosphors based on strontium aluminates activated by Eu²⁺ ions with various Al/Sr ratios were synthesized by a traditional high-temperature solid-state method. The influences of Al/Sr ratio, sintering temperature, the doping concentration of europium ions on structural transformation and luminescent properties of the phosphors were studied. The quenching and luminescent mechanisms were also discussed. The optimum synthetic temperature and time are about 1350 °C and 3.5 h derived from the analysis and experiment. At the range of Al/Sr ratio from 1.5 to 4.0, there are only three crystal structures, Sr₃Al₂O₆ (1.5) cubic structure, SrAl₂O₄ (2.0–3.0) monoclinic crystal system and Sr₄Al₁₄O₂₅ (3.0–4.0) orthorhombic crystal system. Under UV light excitation, the emission peaks gradually generate blueshift from 510 nm of Sr₃Al₂O₆:Eu²⁺ phosphor to 483 nm of Sr₄Al₁₄O₂₅:Eu²⁺ phosphor and the two emission peaks are originated from 4f ⁶5d ¹ to 4f ⁷ intrinsic transition of Eu²⁺ ions. The best green and blue luminescent samples are SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺, and their color purity is 0.6667 and 0.8229, respectively. The best doping amount of Eu²⁺ is 0.5 at.%, and the concentration quenching mechanism of Eu²⁺ ions in SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors can be ascribed to the dipole–dipole interaction. There are three types of 6, 7 and 10 coordination numbers around Sr²⁺(Eu²⁺) ion, which forms three types of polyhedras, resulting in generating three excitation peaks at 334, 368 and 430 nm corresponding to the 6, 7 and 10 coordination polyhedral. The relation between energy level splitting and coordination number can be expressed as E ₆ (6-coordination) > E ₇ (7-coordination) > E ₁₀ (10-coordination) in aluminate phosphor, and the splitting width of 6, 7 and 10 coordination numbers is 1.29, 0.78 and 0.31 eV, respectively. The coordination field only affects the energy top of crystal field splitting, and the larger the coordination number is, the lower the energy level top of crystal field splitting in Sr₄Al₁₄O₂₅:Eu²⁺ phosphor is.

Introduction

Luminescent materials have been extensively investigated in recent years because of their manifold range of potential application, such as color display technologies, high-efficiency solid-state lighting, information storage, biological sensing, and labeling, photoelectronic and photonic devices [1]. Rare-earth-ions-doped phosphors as a stable and high-efficient luminescent materials have become the mainstream orientation in luminescent material. The Eu²⁺ ions-doped strontium aluminate luminescent materials, in particular, have attracted increasing attention due to its superior luminescent properties, abundant sorts, lower cost, stable chemical performance, etc. [2]. Palilla et al. [3] firstly found SrAl₂O₄:Eu²⁺ green luminescent materials. Strontium aluminate (SrAl₂O₄) phosphors have been paid more attention because they have the high quantum efficiency, considerable thermal and chemical stability [4]. Matsuzawa et al. [5] improved the luminescent intensity of phosphor by Eu²⁺ and Dy²⁺ codoping. Then, all kinds of alkaline earth metal aluminate systems based on MAl₂O₄:Eu²⁺ (M = Ca, Sr, Ba) have aroused wide concern [6, 7]. Sahu et al. [8] pointed the crystal structures of the prepared phosphors were consistent with standard monoclinic phase with a space group P2₁ and the SrAl₂O₄:Eu²⁺ phosphor would emit green light, belonging to the broad emission band ascribed to the 4f ⁶5d ¹–4f ⁷ transition. Subsequently, various matrix strontium aluminate phosphors, such as Sr₃Al₂O₆ [ 9,10,11 ] and Sr₄Al₁₄O₂₅ [12,13,14], began to appear and they also have been investigated extensively. Chang et al. [15] studied the photoluminescence of Eu²⁺, Dy³⁺ and Eu³⁺, Dy³⁺-codoped Sr₃Al₂O₆ phosphor and pointed out the emission peaks of Eu²⁺, Dy³⁺-codoped Sr₃Al₂O₆ phosphor is at 512 nm. Inan Akmehmet et al. [16] studied the Eu²⁺ and Dy³⁺-codoped Sr₄Al₁₄O₂₅ phosphors, discussed the influence of B-doping on structure, found the microstructural evolution, promoted more uniform distribution of the optically active of Eu²⁺ ion in the Sr²⁺ cation sublattice, and improved the phosphors luminescent intensity. Although all kinds of strontium aluminate matrix phosphors have been investigated, the relation between structural transition and spectral properties is scarcely reported in same synthetic conditions. In view of this, the Sr₃Al₂O₆:Eu²⁺, SrAl₂O₄:xEu²⁺ and Sr₄Al₁₄O₂₅:xEu²⁺ phosphors were synthesized by the traditional high-temperature solid-state reaction method via tuning Al/Sr ratio. The relation between structural transition and spectral property was established. The influence of Eu²⁺ ions content on the luminescent properties and quenching mechanism was also discussed.

Experimental

The phosphors with different Al/Sr ratios were synthesized via the traditional high-temperature solid-state reaction method. The starting materials are SrCO₃(A.R.), Al₂O₃(A.R.), H₃BO₃(A.R.), Eu₂O₃ (99.99%) whose manufacturer is Sinopharm Chemical Reagent Co., Ltd (Shanghai). Firstly, the Al/Sr ratios of 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 were designed. Then, according to the ratios, weigh some quantity of Al₂O₃ and SrCO₃, add 1.0 at.% H₃BO₃ of the product weight as fluxing agent and weigh 0.2, 0.5, 0.8, 1.0, 1.4, 2.0, 4.0, 6.0, 8.0 at.% Eu₂O₃ as activator. Thirdly, these raw materials were mixed and ground thoroughly in an agate mortar for half an hour. Lastly, the mixture was placed into an alumina crucible and sintered at 1100 °C for 45 min, which makes SrCO₃ decompose enough, and then sintered at 1350 °C for 3.5 h in the reducing atmosphere. When finally cooling to room temperature, the phosphors were obtained.

The thermogravimetric (TG)/different scanning calorimeter (DSC) was monitored to determine the synthetic temperature by Netzsch STA449C at a heating rate of 10 K/min. X-ray power diffractions (XRD) were measured by Japan Rigaku D/max-IVB diffractometer, CuK_α, 40.0 kV, 20.0 mA, λ = 0.15406 nm, in order to establish the crystallization and phase transformation behavior of these phosphors. The emission and excitation spectra were characterized by Japan SHIMADZU RF-5301 fluorescence spectrophotometer, Xenon lamp, 150 W. All the measurements except TG/DSC were performed at room temperature.

Results and discussion

Thermal analysis

Figure 1 presents TG/DSC curves of raw materials synthesized SrAl₂O₄:Eu²⁺ phosphor. It is obvious in the TG curve that the weight loss mainly takes place in the range of room temperature to 1100 °C, and there is a significant weight loss occurred at 900–1000 °C for a decomposition process of SrCO₃ which produces carbon dioxide. There are one endothermic peak at 920.77 °C and one apparent exothermic peak at 1348.12 °C in the DSC curve, which are caused by the decomposition of SrCO₃ and the product crystallization of Sr _x Al _y O _z :Eu²⁺ phosphor, respectively [17, 18]. In Fig. 1, it shows DSC and TG curves of the raw mixture. Obviously, from the DSC curve, there is an endothermic peak at 920.77 °C, which is identical with the temperature of the decomposition of SrCO₃. Meanwhile, from the TG curve, there is a dramatic decline of weight at about 920 °C, because the decomposition of SrCO₃(SrCO₃(s) → SrO(s) + CO₂ $\left( {\text{g}} \right) \uparrow$) releases the carbon dioxide and leads to the mass reduction. To sum up, the endothermic at 920.77 °C is caused by the decomposition of SrCO₃. Combining TG/DSC curves and a great deal of experimental data, the calcination temperature of 1350 °C was adopted to ensure the prepared phosphor with various Al/Sr ratios. In order to determine the best calcination time, the various holding times were carried out for SrAl₂O₄:Eu²⁺ phosphor and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor. The XRD patterns of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors calcined at 3, 3.5 and 4 h are displayed in Figs. S1 and S2 in supplemental file, respectively. The spectra of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors calcined at 3, 3.5 and 4 h are displayed in Figs. S3 and S4 in supplemental file, respectively. When the sintering time is 3 h, the XRD patterns are chaotic and luminous intensity of phosphor is weak. When prolonging the sintering time, crystallized completeness of crystal structure is increasing, grains can grow up enough, and Eu²⁺ ions have enough time to enter the lattice to form more illuminant center. But, when it reaches 4 h, the phosphors over-burn and weaken the luminous intensity. Based on the data, the sintering time of 3.5 h was chosen for preparation phosphors.

Figure 1

TG/DSC curves of raw materials for synthesis SrAl₂O₄:Eu²⁺ phosphor

The effects of Al/Sr ratio on structural and luminescent properties

Al/Sr ratio has significant effects on luminescent properties and phase component of Sr _x Al _y O _z :Eu²⁺ phosphors. Figure 2 illustrates the XRD patterns of Sr _x Al _y O _z :Eu²⁺ phosphors with different Al/Sr ratios (Al/Sr = 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0). When Al/Sr ratio is 1.5, the diffraction peaks are accordant with that of Sr₃Al₂O₆ (PDF No. 24-1187), which indicates that the product is cubic structural Sr₃Al₂O₆ phase, and the cell parameter is 1.5844 nm with a Pa-3 space group. When Al/Sr ratio is 2.0, 2.5 and 3.0, by comparing with standard PDF card No. 34-0379, it is clear that the products belong to monoclinic crystal system of SrAl₂O₄ phase. And the sample of Al/Sr ratio is 2.0 and purer, indicating the crystallization is more complete. With further increase in Al/Sr ratio, the impure phase of Sr₄Al₁₄O₂₅ begins to appear. When Al/Sr ratio approaches 3.5 and 4.0, the main phase turns into Sr₄Al₁₄O₂₅ crystal structure, and the diffraction peaks are identical with the diffraction of PDF No. 52-1876, indicating that the crystal structure of products is orthorhombic crystal system with a Pmma space group, and the cell parameters are 2.4791, 0.8486 and 0.4886 nm. The pure phase of Sr₄Al₁₄O₂₅ can be obtained while the Al/Sr ratio is 3.5. There are no any diffraction peaks corresponding to Eu²⁺ ions for all samples, indicating the doping of Eu²⁺ ions has no effects on lattice of phosphors [19, 20]. It is true that the composition and structure of products are not completely same with original design (that is stoichiometric ratio). At the range of Al/Sr ratio from 1.5 to 4.0, there are only three structures, Sr₃Al₂O₆ (1.5) cubic structure, SrAl₂O₄ (2.0–3.0) monoclinic crystal system and Sr₄Al₁₄O₂₅ (3.0–4.0) orthorhombic crystal system. In view of the rule of crystal structure conversion, the orthorhombic structure could convert to monoclinic structure and also translate to cubic structure, so that the structure of Sr₄Al₁₄O₂₅ is unstable and difficult to get high-purity phase.

Figure 2

XRD patterns of Sr _x Al _y O _z :Eu²⁺ phosphors with various Al/Sr ratio

Figure 3 is the excitation (a) and emission (b) spectra of Sr _x Al _y O _z :Eu²⁺ phosphors with Al/Sr ratio of 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0. Excitation wavelength is 368 nm, and the monitored wavelength measured in excitation spectra is the wavelength of samples luminescent peak value. It is obvious that there are only two types of spectra properties in spite of three types of products. With Al/Sr increasing from 1.5 to 3.5, the luminous intensity of material increased gradually. When Al/Sr ratio equals to 3.5, the luminous intensity of the phosphor is the highest. However, when Al/Sr continuously increases from 3.5 to 4.0, the luminous intensity of the phosphor reversely decreases obviously. It is the fact that the optimum luminescent sample is Sr₄Al₁₄O₂₅:Eu²⁺ phosphor. For the green light peaking at 510 nm, the SrAl₂O₄:Eu²⁺ sample is superior to the Sr₃Al₂O₆:Eu²⁺ sample, the luminescent intensity of Sr₃Al₂O₆:Eu²⁺ phosphor is lowest, and the shape and the position of peak have no differences with SrAl₂O₄:Eu²⁺ phosphor. In addition, the emission peak gradually shifts to the short wavelength (blueshift) from 510 to 483 nm with increasing Al/Sr ratio and the blueshift is 27 nm. In view of this, we will discuss the Sr₄Al₁₄O₂₅:Eu²⁺ and the SrAl₂O₄:Eu²⁺ phosphors by the following [21,22,23]. When Al/Sr ratio is <2.5, the phosphors emit green light and the emission spectra of phosphors are broadband spectra peaking approximately at 510 nm caused by 4f ⁶5d ¹–4f ⁷ transition, the excitation spectra are also a wideband spectra from 300 to 410 nm, only one excitation peak near at 345 nm caused by the transition of 4f ⁷ ground state to 4f ⁶5d ¹ excitation state of Eu²⁺ ions, and the width of the excitation band approaches 110 nm. While Al/Sr ratio is more than 3.0, the phosphors emit Cambridge blue light, and the emission spectra are also wide band from 430 to 540 nm peaking at 483 nm ascribed to 4f ⁶5d ¹–4f ⁷ intrinsic transition of Eu²⁺ ions, but the excitation spectra have two excitation peaks near at 368 and 430 nm, respectively, and one excitation band <321 nm [24,25,26]. It related to crystal structure and luminescent mechanism and the reason will be explained below.

Figure 3

Excitation (a) and emission (b) spectra of Sr _x Al _y O _z :Eu²⁺ phosphor with various Al/Sr ratio

From Fig. 3, we could know the emission wavelength of these materials is controlled in 483–510 nm with increasing Al/Sr ratio and the luminescent color changes from green to blue. In order to prove again this result, the color coordinate pictures of the SrAl₂O₄:Eu²⁺ (a) and Sr₄Al₁₄O₂₅:Eu²⁺ (b) phosphors are shown in Fig. 4. The color coordinate of (x, y) is (0.1261, 0.6282), (0.1057, 0.2681) of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors, respectively [5, 27, 28]. The real photographs and the emission spectra of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors under excitation of UV light are shown in insets. From the insets, it is proved again that SrAl₂O₄:Eu²⁺ phosphor and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor emit bright green and blue light, respectively.

Figure 4

Color coordinate of samples when Al/Sr = 2 (a) and Al/Sr = 3.5 (b). Insets are the real photographs of phosphor and emission spectra under UV excitation

The purity of color reflects the quality of phosphor. The higher its value is, the higher the quality is. Therefore, it is necessary to measure the purity of color of phosphor and improve its value as much as possible. The plotting method was taken to calculate the purity of color of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor, and the formulas are as follows:

$$ P_{a} = \frac{{\overline{aE} }}{{\overline{{EP_{1} }} }} $$

(1)

$$ P_{b} = \frac{{\overline{bE} }}{{\overline{{EP_{2} }} }}.$$

(2)

Through the formulas and the coordinates of the white point E(0.333, 0.333), the chrominance point of phosphor a(0.1261, 0.6282) and b(0.1057, 0.2681) and the dominant wavelength point P ₁(0.0236, 0.7732) and P ₂(0.0554, 0.2658), the highest color purity of SrAl₂O₄:Eu²⁺and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor is 0.6667 and 0.8229, respectively [29,30,31], which are pretty high and could satisfy the demand of production and living.

Effect of the concentration of Eu²⁺ ions on luminescent properties

Figure 5 shows the excitation (a) and emission (b) spectra of SrAl₂O₄:xEu²⁺ phosphors. The doping concentration of Eu²⁺ ions was 0.2, 0.5, 1.0, 2.0, 4.0, 6.0 at.%. The shapes of these excitation and emission spectra are basically identical for different Eu²⁺ ions concentration. The excitation range of these phosphors is from 280 to 440 nm, which has good absorption to UV and blue light, indicating these phosphors could be excited by UV and visible light. The excitation intensity firstly increases and then decreases with the increase in doping Eu²⁺ ions concentration, when the doping Eu²⁺ ions concentration approaches 0.5 at.%, the excitation intensity is the strongest and the excitation peak value wavelength is the shortest at 334 nm. However, while the doping concentration of Eu²⁺ ions is more than 0.5 at.%, the excitation peak begins redshift from 334 to 356 nm. From the emission spectra, the emission peaks position of phosphors with different doping Eu²⁺ ions concentration are at about 510 nm. The doping of Eu²⁺ ions only effected on luminescent intensity. The emission intensity also firstly increases and then decreases with the increase in the Eu²⁺ ions concentration, and when the doping concentration of Eu²⁺ ions is 0.5 at.%, the emission intensity is the strongest. This is because a lot of Eu²⁺ ions enter crystal lattice at beginning, and they form more luminescent center, so the luminescent intensity is enhanced. When the doping concentration of Eu²⁺ ions is 0.5 at.%, the luminescent center reaches saturation and the luminescent intensity is significantly higher than the other proportion. Continuing increasing the concentration of Eu²⁺ ions, because there are dislocation and crossed relaxation interaction, the energy will release as heat or other form, the luminescent intensity of the phosphors will be weakened [32,33,34]. Otherwise, because of the excessive luminescence center density, energy transfer rate is greater than the rate of fluorescent emission. The energy transfer appears between adjacent two Eu²⁺ ions through lattice vibration, which will be discussed on the quenching mechanism.

Figure 5

Excitation (a) and emission (b) spectra of SrAl₂O₄:Eu²⁺ phosphor with different Eu²⁺ ions doping

Figure 6 displays the excitation (a) and emission (b) spectra of Sr₄Al₁₄O₂₅:xEu²⁺ phosphors with the doping concentration of Eu²⁺ ions of 0.2, 0.5, 1.0, 2.0, 4.0, 6.0 at.%. The excitation spectra are composed of three areas, and there are two excitation peaks at 368 and 430 nm at area 2 and area 3, respectively. Combined with crystal structure, we thought that the excitation at area 1 gives rise to the electric charge-transfer transition of phosphor matrix, since the band gap of Sr₄Al₁₄O₂₅ matrix is 6.3 eV [35, 36], which is larger than the excitation band, the excitation peaks at 368 and 430 nm are caused by the transition of ground state to excitation state of Eu²⁺ ions located at Sr1 and Sr2 lattice position, respectively. The excitation intensity firstly increases and then decreases with the increase in the doping concentration of Eu²⁺ ions. When the Eu²⁺ ions concentration is 0.2 at.%, the main excitation peaks are at 368 nm, but while the Eu²⁺ ions concentration is more than 0.2 at.%, the excitation peak at 430 nm is oppositely increased more than that of 368 nm. Especially, when the doping content of Eu²⁺ ions is 1.0 and 2.0 at.%, the excitation intensity of 430 nm is identical with that of 368 nm, indicating that these doping concentrations are benefit to the visible light excitation. There are no any changes in the shape of emission spectra, but the emission intensity increases firstly and then decreases with the increase in Eu²⁺ ions concentration. The spectral intensity is the lowest when the concentration of Eu²⁺ ions was 0.2 at.%. Continuing increasing concentration of Eu²⁺ ions, the spectral intensity increases somewhat. When the concentration of Eu²⁺ ions increases to 0.5 at.%, the emission intensity achieves largest. When the concentration of Eu²⁺ ions continues to increase, the emission intensity decreased gradually. The Eu²⁺ ions concentration is 0.5 at.% for the sample of the highest excitation and emission intensity.

Figure 6

Excitation (a) and emission (b) spectra of Sr₄Al₁₄O₂₅:Eu²⁺ phosphor with different Eu²⁺ doping

When Eu²⁺ ions concentration is more than 0.5 at.%, the luminescent intensity begins to decrease. Some analysis indicates there are some luminous centers in luminescent material, and the energy transfer will proceed in these luminescent center and crystal lattice. When the concentration of Eu²⁺ ions reaches certain value, the energy will transfer to in lattice. The following will discuss the relation between the energy transfer and fluorescent quenching. According to the theory of Dexter, the relation between luminescent intensity and the concentration of luminescent center of Eu²⁺ ions is shown in formula (3):

$$ I/x = K\left[ {1 + \beta \left( \chi \right)^{Q/3} } \right]^{ - 1}.$$

(3)

In the formula, I represents light intensity, x represents luminescent center concentration; K and β are constant (under the condition of the same excitation). According to the interaction between the ions of luminescent center, Q value should be for 6, 8, 10 corresponding to the quenching mechanism of dipole–dipole interaction, dipole-quadrupole interaction, quadrupole–quadrupole interaction, respectively [37,38,39].

The formula (3) can be simplified to (4):

$$ { \lg }(I/x) = c - (Q/3){ \lg }\chi .$$

(4)

Based on the data of Figs. 5 and 6, the relation of lg(I/x) and lg(x) of SrAl₂O₄:Eu²⁺ phosphors (a) and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors (b) is shown in Fig. 7 while x is more than 0.5. By means of liner fitting, the slope of curve is −1.4566 and −1.1998 of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor, respectively, so that the Q values of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor are 4.3698 and 3.5994, which is more near to 6. Therefore, the concentration quenching mechanism of Eu²⁺ ions in SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors can only belong to the dipole–dipole interaction.

Figure 7

Relation between lg(x) and lg(I/x) of SrAl₂O₄:Eu²⁺ (a) and Sr₄Al₁₄O₂₅:Eu²⁺ (b) phosphors

The luminescence mechanism of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors

In order to better explain the spectral properties of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors, the crystal structure and coordination geometry of SrAl₂O₄ and Sr₄Al₁₄O₂₅ are constructed and are shown in Fig. 8. Figure 8a is the unit cell and local coordination geometry environment of Sr²⁺ in SrAl₂O₄ crystal, and Fig. 8b is that in Sr₄Al₁₄O₂₅ crystal. SrAl₂O₄ belongs to monoclinic system with a space group of P2₁, the lattice parameters are a = 0.8442 nm, b = 0.8822 nm and c = 0.5161 nm, and the angle of cell is 93.415°. In unit cell of SrAl₂O₄, the structure consists of AlO₄ tetrahedra and strontium ions are filled in the interstice formed by the AlO₄ tetrahedra. Every unit cell contains two species strontium ionic coordination polyhedron with the same coordination number and similar average Sr–O distances. Every Sr²⁺ ion forms asymmetric octahedron with surrounding six adjacent oxygen ions. Despite all that, there are two lattice position of Sr²⁺ ion in an unit cell, but only one luminescent property when Eu²⁺ ions substitute different Sr²⁺ ions. Sr₄Al₁₄O₂₅ is orthorhombic crystal structure with a Pmma space group, and the lattice parameters are a = 2.4785 nm, b = 0.8487 nm and c = 0.4886 nm. In unit cell of Sr₄Al₁₄O₂₅, the structure consists of AlO₆ octahedra which separated by a double layer of AlO₄ tetrahedra. There are two species of strontium ionic coordination polyhedron with coordination numbers 10 and 7 in every unit cell. Therefore, when Eu²⁺ ions replace Sr²⁺ ions, there are two different types of luminescent Eu²⁺ centers. The average Sr–O bond length of tenfold coordinated strontium marked Sr1 (2.77 Å) is longer than that of the sevenfold coordinated strontium marked Sr2 (2.62 Å), and the interactions between oxygen and strontium ions at the tenfold coordinated site will be weaker than for the sevenfold coordinated site. Because of the different local environments (based on the coordination number) of Eu²⁺ ions substituting different Sr²⁺ ions, there are two Eu²⁺ luminescence centers named Eu1 and Eu2 which occupy Sr1 and Sr2 lattice position, respectively. Due to the shorter Sr–O distances, there are the higher covalency and the larger crystal field splitting for Eu2 in the sevenfold coordinated site than for Eu1 in the tenfold coordinated site, and the band gap of Eu²⁺ from ground state to excitation state is narrow [40,41,42]. In order to confirm the result, the emission spectra of Sr₄Al₁₄O₂₅:Eu²⁺ phosphor doped with 0.5 and 1.0 at.% Eu²⁺ ion were analyzed again through Guess fitting. The results are shown in supplement Figs. S5 and S6. It is concluded that there are two excitation peaks except the excitation band excitation wavelength which is shorter than 320 nm. While doping 0.5 at.% Eu²⁺ ion, the excitation peak is at 360 and 431 nm; however, while doping 1.0 at.% Eu²⁺ ion, the excitation peak is at 376 and 432 nm, respectively. In addition, the long wavelength of 432 nm excitation apparently enhances and the short wavelength excitation extends from 360 to 376 nm, which indicates that more Eu²⁺ ion occupies Sr2 lattice site; simultaneously, the Sr2 lattice site is stretched since the ionic radius of Eu²⁺ (0.947 Å) is less than that of Sr²⁺ (1.12 Å).

Figure 8

Unit cell and local coordination geometry environment of Sr²⁺ of SrAl₂O₄ (a) and Sr₄Al₁₄O₂₅ (b)

Figure 9 shows the excitation and emission spectra and wave numbers of SrAl₂O₄:Eu²⁺ (a) and Sr₄Al₁₄O₂₅:Eu²⁺ (b) phosphor. The Stokes frequency shift between the excitation and emission spectra of SrAl₂O₄:Eu²⁺ is 10333 cm⁻¹ (176 nm). However, in the Sr₄Al₁₄O₂₅:Eu²⁺ phosphor, the shortest Stokes frequency shift is 2553 cm⁻¹ (53 nm) and the longest is only 6470 cm⁻¹ (115 nm) corresponding to Eu2 and Eu1, respectively, which indicates that the frequency shift decreases with the increase in Al/Sr ratio and the non-radiation transition descends originated from the compressing of 5d energy level splitting. In order to best explain the changeable rule of phosphor spectrum, combining established crystal structure of SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor, the energy band and transition of Eu²⁺ ion is built and shown in Fig. 10. The electron configuration of Eu²⁺ ion is Xe atomic configuration plus 4f ⁷5s ²5p ⁶. There are seven f shell electrons in the ground state of Eu²⁺ ion, and the 4f ⁷ electrons organize themselves into 4f ⁷ configurations, and the ground-state level is decided by Hund rule. First of all, the electron spin in the lowest energy state should be parallel as much as possible , that is, the total spin angular momentum quantum number S should be the biggest. Secondly, on the premise of S for the biggest, the total orbital angular momentum L should be minimum. There are seven electrons in one Eu²⁺ ion, and all the electron spin is parallel, so S = 7/2, L = 0. Therefore, we get its ground-state spectral term of ⁸S_7/2, defining the spectral term overlaps with valence top of SrAl₂O₄ and Sr₄Al₁₄O₂₅ crystal. The band gap of SrAl₂O₄ and Sr₄Al₁₄O₂₅ crystal is 6.6 and 6.3 eV, which is far higher than the excitation state of Eu²⁺ ion; therefore, the band gap transition is in existence. For Eu²⁺ ion, the 4f ⁶5d ¹ electronic configuration forms when it is excited due to the shielding effect of 5s ²5p ⁶ electrons. Affected by crystal field, the low energy level of 4f ⁶5d ¹ generates splitting, and the energy level splitting becomes large with the decrease in coordination number and symmetry. When coordination number is 6, the energy level splitting arrives to 1.29 eV, when the coordination number is 7, the energy level splitting is 0.78 eV; however, when the coordination number is 10, the energy level splitting is only 0.31 eV. Therefore, the relation of energy level splitting and coordination number can be expressed as E ₆ (6-coordination) > E ₇ (7-coordination) > E ₁₀ (10-coordination) in aluminate phosphor. For this reason, the different crystal field environment of Eu²⁺ ion brings the different forms of electron transition [43, 44]. In the Sr₄Al₁₄O₂₅:Eu²⁺ phosphor, only one emission peak for the two Eu²⁺ ion lattice positions indicates there is the same energy band bottom of Eu1 and Eu2 luminescent centers. However, there are two excitation peaks caused by the transition from identical ground state to the excitation tops of Eu1 and Eu2 coming from the crystal field splitting, indicating coordination field only affects the energy top of crystal field splitting. Larger coordination number is, lower the energy level top of crystal field splitting in Sr₄Al₁₄O₂₅:Eu²⁺ phosphor is. When excitation wavelength is <320 nm, the Al–O group transfers one electron to centric cation forming the charge-transfer-band transition, and the lowest charge-transfer energy is 3.88 eV.

Figure 9

Excitation and emission spectra and wave numbers of SrAl₂O₄:Eu²⁺ (a) and Sr₄Al₁₄O₂₅:Eu²⁺ (b) phosphor

Figure 10

Energy level models of SrAl₂O₄:Eu²⁺and Sr₄Al₁₄O₂₅:Eu²⁺ phosphor

Conclusions

Phosphors based on strontium aluminates activated by Eu²⁺ ions with various Al/Sr ratios were synthesized by the high-temperature solid-state method. The optimum synthetic temperature and time are about 1350 °C and 3.5 h. At the range of Al/Sr ratio from 1.5 to 4.0, there are only three crystal structures, cubic (1.5), monoclinic (2.0–3.0) and orthorhombic (3.0–4.0) phases. Under UV light excitation, the emission peaks gradually generate blueshift from 510 nm of Sr₃Al₂O₆:Eu²⁺ phosphor to 483 nm of Sr₄Al₁₄O₂₅:Eu²⁺ phosphor and the two emission peaks are originated from 4f ⁶5d ¹ to 4f ⁷ intrinsic transition of Eu²⁺ ions. The best green and blue luminescent samples are SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺, and their color purity is 0.6667 and 0.8229, respectively. The best doping amount of Eu²⁺ is 0.5 at.%, and the concentration quenching mechanism of Eu²⁺ ions in SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors can be ascribed to the dipole–dipole interaction. There are three types of 6, 7 and 10 coordination numbers around Sr²⁺ (Eu²⁺) ion, which forms three types of polyhedras, resulting in generating three excitation peaks at 334, 368 and 430 nm corresponding to the 6, 7 and 10 coordination polyhedral. The relation of energy level splitting and coordination number can be expresses as E ₆ (6-coordination) > E ₇ (7-coordination) > E ₁₀ (10-coordination) in aluminate phosphor, and the splitting width of 6, 7 and 10 coordination numbers is 1.29, 0.78 and 0.31 eV, respectively. The coordination field only affects the energy top of crystal field splitting, and the larger the coordination number is, the lower the energy level top of crystal field splitting in Sr₄Al₁₄O₂₅:Eu²⁺ phosphor is. These results can be a fundament of wavelength tuning of strontium aluminate phosphor.