Preparation of Sr₈Mg_1−mZn_mY(PO₄)₇:Eu²⁺ solid solutions and their luminescence properties

Abstract

A series of orthophosphate solid solutions with a general formula of Sr₈Mg_1−mZn_mY(PO₄)₇: Eu²⁺ (0 ≤ m ≤ 1) were synthesized by the combustion-assisted synthesis technique. The XRD patterns confirm the formation of a solid solution of Sr₈Mg_1−mZn_mY(PO₄)₇ and lattice parameters of the solid solutions are linearly dependent on m. The excitation spectra of these phosphors show broad band excitation and absorption in the 250–470 nm near ultraviolet (UV) region, which is ascribed to the 4f⁷–4f⁶5d¹ transitions of Eu²⁺. With an increase in m, the emission color shifts from yellow to green under near-UV excitation, which is rationalized in terms of two competing factors of the crystal filed strength and bond covalence. All the results indicate that the solid solution Sr₈Mg_1−mZn_mY(PO₄)₇: Eu²⁺ can be a good candidate of phosphor applicable to near-UV LEDs for solid-state lighting.

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1. Introduction

Light-emitting diodes (LEDs) are considered as a promising technology for next generation solid-state lighting systems because of their high efficiency, small size, compactness, safety, high material stability, durability, and resultant energy-saving capacity.¹ White LEDs fabricated with multi-color phosphors and near ultraviolet (n-UV) LEDs are considered to be a very promising route to realize a high-quality light source.² For LED phosphors, they are generally required to have high conversion efficiency, excellent chemical stability and high thermal quenching temperature. In addition, they are also expected to show a tunable ability in emission color, allowing the white LED designers to have greater flexibility in choosing emission wavelength for the improvement either in correlated color temperature or color rendering index of white LEDs for the phosphor applications.³ Therefore, it has a great impact on research on luminescent materials (phosphors).

Eu²⁺ doped phosphors have been widely investigated in the past years because of their promising applications in LEDs. The divalent rare-earth Eu²⁺ ion, which has 4f⁷ ground state and the excited state of 4f⁶5d, usually give broad emission due to parity allowed 4f⁷–4f⁶5d¹ transitions.⁴ The peak positions in the emission spectra depend strongly on the nature of the Eu²⁺ surroundings. Therefore Eu²⁺ can be efficiently excited in a broad spectral range depending on the host lattices in which it is incorporated. The absorption and emission bands of activators Eu²⁺ can also be well controlled by varying the crystal field or the bond covalence depending on site size, site symmetry and coordination environment of activator ions. In order to realize desirable luminescent properties, one promising way is to modify the composition of the host lattice through a solid solution with similar crystal structure.⁵

Phosphate compounds are a kind of important luminescence host, which can produce plenty of crystal field environments imposed on emission centers. Recently, a variety of phosphates with the formula Sr₈MgRe(PO₄)₇ have been reported in the literature, such as Sr₈MgSc(PO₄)₇,⁶ Sr₈MgLu(PO₄)₇,⁷ Sr₈MgGd(PO₄)₇,⁸ and Sr₈MgLa(PO₄)₇.⁹ Sr₈MgRe(PO₄)₇ hosts are good candidates as host structures due to several merits, such as high chemical stability and good thermal quenching, and exhibit intense luminescence for white LEDs application when activated with Eu²⁺. Additionally, the variety of the structures in these compounds makes it possible to finely tune the physical/chemical properties to design new materials. Recently, Huang et al. reported⁹ that the Eu²⁺-activated Sr₈MgY(PO₄)₇ exhibited a broad yellow emission upon excitation at near ultraviolet (near-UV) light and a warm white-light near-UV LED was fabricated using a near-UV 400 nm chip pumped by a phosphor blend of blue-emitting BaMgAl₁₀O₁₇: Eu²⁺ and yellow-emitting Sr₈MgY(PO₄)₇: Eu²⁺.

In this paper, we present a strategy to tune the emission color of Sr₈MgY(PO₄)₇: Eu²⁺ phosphor by introducing foreign cations, Zn²⁺, to modify the crystal field strength acting on Eu²⁺ activators. The emitting color can be tuned from yellow to green using this solid solution strategy. The modification of crystal field is discussed in details in connection with crystal structure changes and photoluminescence excitation. The solid solution strategy suggested here greatly improves certain key properties of these promising phosphate phosphors.

2. Experimental

2.1 Materials and methods

In this study, polycrystalline phosphors with the compositions of (Sr_0.99Eu_0.01)₈(Mg_1−mZn_m)Y(PO₄)₇ (hereafter abbreviated as Zn_m-SMYP: Eu²⁺, 0 ≤ m ≤ 1) were synthesized by a combustion-assisted synthesis method.¹⁰ Stoichiometric amount of raw materials NH₄H₂PO₄ (analytical reagent, AR), Mg(NO₃)₂·6H₂O (AR), Sr(NO₃)₂ (AR), and Zn(NO₃)₂·6H₂O (AR) were thoroughly mixed, and an appropriate amount of CO(NH₂)₂ (AR) was added as fuel. Y₂O₃ (99.99%) and Eu₂O₃ (99.99%) were dissolved in HNO₃ to convert into the corresponding nitrate completely. These reagents were dissolved in water and introduced into a muffle furnace maintained at 600 °C for 5 min. The obtained precursors were fired at 1300 °C for 3 h under a 5% H₂/95% N₂ mixture gas to form crystalline phosphates.

2.2 Characterization

The phase purity of the as-prepared samples was checked using a Bruker D8 diffractometer (Bruker Co., Karlsruhe, Germany) in the 2θ range from 10° to 80°, with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm) operating at 40 kV and 40 mA. The lattice constants were calculated by cell refinement fitting with MDI Jade 5.0 on the basis of the XRD data. The excitation (PLE) and emission (PL) spectra were recorded on a Jasco FP-6500 fluorescence spectrophotometer (Jasco Co., Tokyo, Japan) equipped with a 150 W xenon lamp as the excitation light source. Scanning electron micrographs (SEM) were taken on a Hitachi SU8000 scanning electron microscopy (Hitachi, Japan). The chemical composition of the particles was carried out using the SEM equipped with energy dispersive X-ray spectroscopy (EDX). For comparison, all measurements were performed at room temperature with the identical instrumental parameters.

3. Results and discussion

3.1 Phase identification and crystal structure

The crystal structure studies of Sr₈MgY(PO₄)₇ have been carried out by Huang et al., which reported that the compound Sr₈MgY(PO₄)₇ is isostructural with Sr₉In(PO₄)₇. In the structure of Sr₈MgY(PO₄)₇, the Sr²⁺ ions have five different coordination environments: Sr(1) is defined as being eight-coordinated; Sr(2), Sr(3), and Sr(4) are defined as being nine-coordinated; and Sr(5) is defined as being ten-coordinated. The crystal structures of Sr₈Mg_1−mZn_mY(PO₄)₇ were evolved from the Sr₈MgY(PO₄)₇ phase by the iso-structural replacement of Mg/Zn atoms. Fig. 1 shows the XRD patterns of the synthesized Sr₈Mg_1−mZn_mY(PO₄)₇ powders at various concentrations (0 ≤ m ≤ 1) of Zn²⁺. No peaks of the raw materials or other allotropic forms are detected up to m = 1. The Sr₈MgY(PO₄)₇ (m = 0, hereafter abbreviated as SMYP) powders show the characteristic pattern of the monoclinic structure (space group I2/a), matches well with Inorganic Crystal Structure Database (ICSD) file no. 59722. All the diffraction peaks of the prepared sample SMYP agrees well with those reported previously.⁹ There is no obvious difference among these XRD patterns when 0 ≤ m ≤ 1, which are all consistent with ICSD file no. 59722. There is no alien diffraction peaks. Thus the samples with 0 ≤ m ≤ 1 can be indexed to single monoclinic phase with I2/a space group. This indicates that doping of Zn²⁺ does not significantly change the phase structure of samples. However, the diffraction peaks shifts slightly to the lower angle side with concentration increase of Zn²⁺ ions, as shown in Fig. 1b. This observation indicates that substitution of larger Zn²⁺ (r = 0.90 Å, CN = 8) for Mg²⁺ (r = 0.89 Å, CN = 8) causes the changes in the lattice constant of host lattice, which can be calculated by indexing XRD data. Powder XRD analyses were performed to determine the chemical purity and phase homogeneity of the Sr₈Mg_1−mZn_mY(PO₄)₇ powders. The lattice parameters were calculated based on the experimental XRD profiles using the cell refinement software. The structural parameters reported on Sr₉In(PO₄)₇ were used as initial parameters in the Rietveld analysis. Structural refinements of Sr₈Mg_1−mZn_mY(PO₄)₇ were all performed by a monoclinic space group of I2/a. The refined unit cell parameters for m = 0 and 1 are summarized in Table 1. It is noticeable that in the ICSD there are no powder diffraction data or reference data that could be found for the end compound of Sr₈ZnY(PO₄)₇ (hereafter abbreviated as SZYP, m = 1). The results indicate that the as-prepared SZYP sample is also obtained as a single phase compound that matches well with ICSD file no. 59722.

Fig. 1 (a) Powder XRD patterns of Zn_m-SMYP phosphor as a function of Zn concentration (m), and ICSD file no. 59722 as a reference; and (b) magnified XRD patterns in the region between 25° and 35° for Zn_m-SMLP phosphors (m = 0, 0.2, 0.5, 0.8, 1.0).

Table 1 Rietveld refinement and crystal data of Sr₈MgY(PO₄)₇ and Sr₈ZnY(PO₄)₇

Formula	Sr9In(PO4)7	Sr8MgY(PO4)7	Sr8ZnY(PO4)7
a (Å)	18.0425(2)	18.47387(5)	18.4903(2)
b (Å)	10.66307(4)	10.10571(5)	10.07955(1)
c (Å)	18.3714(2)	18.14809(3)	18.32116(3)
β (°)	132.9263(5)	131.9509(2)	132.17(1)
V (Å^3)	2588.02	2519.79	2530.71
Z	4	4	4

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As seen in Fig. 1 (right part), by substituting Mg²⁺ with Zn²⁺ ions, the diffraction peaks of these samples shift slightly towards the lower 2θ angle. This can be attributed to the lattice expansion due to the substitution of Mg²⁺ by the larger ions of Zn²⁺. The change of lattice constant of host lattice can be calculated by indexing XRD data. As shown in Fig. 2, lattice constant c increase linearly as Zn²⁺ concentration increases, while the lattice constants a and b have little changes. This observation suggests that introduction of Zn²⁺ enlarges and distorts the SMYP lattice. The distortion of host lattice will have a strong impact on the luminescence properties of Eu²⁺ ions with 5d energy levels, which will be discussed in section of photoluminescence. The ionic radii of the eight- and nine-coordinated Sr²⁺ ions are 1.26 and 1.31 Å, respectively. Meanwhile, the ionic radii of the eight- and nine-coordinated Eu²⁺ ions are 1.25 and 1.3 Å, respectively. On account of the matching of ionic radii, the Eu²⁺ ions are expected to randomly occupy the Sr²⁺ or Mg²⁺ or Zn²⁺ or Y³⁺ ions sites in the Zn_m-SMYP host.

Fig. 2 Unit cell parameters of Zn_m-SMLP samples (m = 0, 0.2, 0.5, 0.8, 1.0).

In our previous study,¹¹ the crystal structure of Sr₈(Mg,Zn)La(PO₄)₇ was investigated by changing the content of Zn substitution. Although the crystal structures of Sr₈(Mg,Zn)La(PO₄)₇ were evolved from the Sr₈MgLa(PO₄)₇ phase by the iso-structural replacement of Mg/Zn atoms, Sr₈(Mg,Zn)La(PO₄)₇ formed a eutectic mixture containing two distinct phases, Sr₈MgLa(PO₄)₇ and Sr₈ZnLa(PO₄)₇. On the contrary, the XRD patterns of Zn_m-SMYP in Fig. 1 show no obvious difference among the five XRD patterns when 0 ≤ m ≤ 1. There is no alien diffraction peaks. Thus the samples with 0 ≤ m ≤ 1 can be indexed to single monoclinic phase. This indicates that doping of Zn²⁺ does not significantly change the phase structure of samples. Fig. 2 displays the variation of the lattice parameters with m. The lattice constants either linearly increase of decrease with increasing m, indicating Vegard's law is followed,¹² proving complete solid solution between the two distinct end-member compounds, SMYP and SZYP.

In order to investigate the precursor, a series of other measurements were performed on the samples. The elements detected by EDX are largely comprised of Sr, Mg, Zn, Y, P, and O. The calculated value of stoichiometry from the EDX data was found to be consistent with the composition of the precursor used in the experiment and is shown in Table 2. The analyzed Zn to Y ratio in the Sr₈(Mg,Zn)Y(PO₄)₇ phase is appreciably lower compared to that of the reaction system, indicating that a part of Zn was eliminated from the surface of phosphor powders by evaporation during the high temperature post-treatment.

Table 2 Calculated value of stoichiometry of Sr₈Mg_1−mZn_mY(PO₄)₇ from the EDX data

m	0	0.2	0.5	0.8	1
Y	1	1	1	1	1
Sr	8.06	7.81	7.65	8.15	7.91
P	6.86	7.13	6.83	6.55	6.78
Mg	1.05	0.82	0.52	0.19	—
Zn	—	0.08	0.35	0.53	0.72

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3.2 SEM images of Zn_m-SMYP: Eu²⁺ phosphor

Fig. 3 shows the representative SEM micrograph of Zn_m-SMYP: Eu²⁺, m = 0, 0.5 and 1. The phosphors have an irregular elliptical shape, and the particle sizes of the obtained powders are no more than 100 μm. There was an aggregation of the particles. The microstructural features of the specimens are generally similar except for the slightly different grain sizes. This indicated that the addition of zinc could accelerate the growth of the grains. These particles show a slight tendency to agglomerate, especially for the specimen with the addition of zinc. Though the uniform morphology is not obtained by this route, it is acceptable for the white LED application as the particle size requirement is not as strict as that in the plasma display panel (PDP) field.

Fig. 3 SEM images of Zn_m-SMYP: Eu²⁺, m = 0 (a), 0.5 (b) and 1 (c).

3.3 Photoluminescence properties

The PL/PLE spectra of SMYP: Eu²⁺ and the deconvoluted Gaussian components are shown in Fig. 4. The observed PLE spectrum monitoring by 513 nm is composed of one strong absorption peak at 377 nm and two weak absorption peaks located around 318 and 433 nm in the spectral range from 250 to 470 nm, which is attributed to 4f⁷(⁸S_7/2)–⁴f⁶5d transitions of the doped Eu²⁺ ions. The asymmetric shape of the PLE spectrum is due to the complexity of the 5d excited state of Eu²⁺. The broad band excitation perfectly fulfils the requirements for a conversion phosphor excited by n-UV to blue light. Since phosphate hosts exhibit absorption in the vacuum ultraviolet region (100–200 nm),¹³ these excitation bands in the near-UV range mainly arise from the 4f⁶5d¹ multiplets of Eu²⁺ excitation states. An asymmetric yellow emitting broad band was observed in the wavelength range of 380–680 nm, which corresponds to the allowed 4f⁶5d¹ → 4f⁷ electronic transitions of Eu²⁺. The emission spectrum shows the strongest emission around 513 nm and two weak shoulders in the high energy region at 432 nm and in the low energy region at 575 nm, respectively. The asymmetric emission spectrum of SMYP: Eu²⁺ indicates that the Eu²⁺ ions have more than one emission centre in the SMYP lattice. In the SMYP crystal structure, an 8f site corresponding to the Ca4 and Ca6 sites in the β-Ca₃(PO₄)₂ type structure is vacant and Sr/Mg has five different coordination numbers: Sr/Mg2 is defined to be eight-coordinated; Sr/Mg1, Sr/Mg3, Sr/Mg4, and Sr/Mg5 are nine-coordinated. Huang et al. reported⁶ that the PL spectra of the SMYP: Eu²⁺ phosphor could be decomposed into five Gaussian profiles in the wavelength range of 450–800 nm. However, due to the serious overlap, the PL spectrum of the SMYP: Eu²⁺ phosphor obtained in this paper can be decomposed into only three Gaussian profiles with peaks centered at 432 nm (Eu1, 23 148 cm⁻¹), 504 nm (Eu2, 19 841 cm⁻¹), and 575 nm (Eu3, 17 391 cm⁻¹) in the wavelength range of 380–680 nm. These peaks can be ascribed to at least three different emission sites, which could be identified as the different coordination environments of the Sr²⁺ ions being occupied by Eu²⁺ ions. No other characteristic emission peaks from Eu³⁺ are observed in the PL spectrum, indicating that Eu³⁺ have been reduced to Eu²⁺ completely in our experiments.

Fig. 4 PLE and PL spectra of SMYP: Eu²⁺ (a) and SZYP: Eu²⁺ (b) phosphors. The dashed lines are Gaussian deconvolution curves.

It is well-known that the 5d wave function has large spatial extension, and it depends on the surroundings of Eu²⁺ ions. Thus, the emission position of the Eu²⁺ ion is strongly dependent on its local environment. In order to further verify the occupancy ascription of the three emission bands, the well-known experiential equation given by Van Uitert has been used to qualitatively analyse the present experimental result.¹⁴ According to the report of Van Uitert, for Eu²⁺ in suitable matrices, the following experiential equation, provides a good fit to the emission peak and excitation edge data. On the basis of this, the problem about which kind of crystallographic site substituted by Eu²⁺ in SMYP can be investigated theoretically.

where E is the position for the Eu²⁺ ion emission peak, Q is the position in energy for the lower d-band edge for the free Eu²⁺ ion (34 000 cm⁻¹); V is the valence of the Eu²⁺ ion (V = 2); n is the number of anions in the immediate shell around the Eu²⁺ ion; r is the effective radius of the host cation replaced by the Eu²⁺ ion (in Å); and “E_a” is the electron affinity of the anion atoms (in eV) dependent on the anion complex type. Here, E_a is approximately determined as 2.19 eV for the phosphate host.¹⁵ Table 3 show the experimental and calculated emission wavelengths of Eu²⁺ ions occupying different cation ions and coordination environments in the SMYP host. The result indicate that the experimental values agree well with the calculated values and the broad asymmetric yellow-emitting band is due to Eu²⁺ ions occupying eight-coordinated Sr²⁺ and Mg²⁺, and six-coordinated Y³⁺ site. Therefore, we can draw a conclusion that the first band centered at 432 nm (Eu1) is attributed to the 5d–4f transitions of Eu²⁺ occupying Sr²⁺ and Mg²⁺ sites with nine-coordination, and the second band centered at 504 nm is due to the 5d–4f transitions of Eu²⁺ occupying Sr²⁺ and Mg²⁺ sites with eight-coordination, and the third emission band with peak at 575 nm can be assigned to the 5d–4f transitions of Eu²⁺ occupying Y³⁺ sites with six-coordination, respectively.

Table 3 Experimental and calculated emission wavelengths of Eu²⁺ ions occupying different cation ions and coordination environments in the SMYP and SZYP hosts

n	r (Å)	Ecalcd (cm^−1)	λcalcd (nm)	λexp (nm)
8	rSr/1.26	21 264	470
9	rSr/1.31	22 566	443
8	rMg/0.89	18 652	536	432, 504, 575
8	rZn/0.9	18 729	534	413, 504, 558
6	rLa/0.9	16 894	592

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Variations in the PLE and PL spectra of the Zn_m-SMYP: Eu²⁺ (0 ≤ m ≤ 1) samples depending on the m values are shown in Fig. 5. The results illustrate that all of the PLE spectra of the materials show characteristic excitation bands of Eu²⁺ ions, which are broadly in the range from 250 nm to 470 nm. Besides, the PLE spectra consist of three bands originating from the 4f–5d transition of Eu²⁺. The shoulders of the excitation spectra around 318 nm and 433 nm showed shifts toward shorter wavelength with increasing value of m. Upon the excitation of 375 nm, all of the samples exhibit the yellow emission bands, which are attributed to the 4f⁶5d–4f⁷ transition of the Eu²⁺ ion. The emission peak position is almost constant, while the emission edge shifts toward a shorter wavelength as the Zn²⁺ concentration increases from m = 0 to m = 1.0. The experimental and calculated emission wavelengths of Eu²⁺ ions occupying different cation ions and coordination environments in the SZYP host are also shown in Table 3. At the same time, the emission intensities were gradually enhanced and maximized at the Zn²⁺ concentration of 0.8. Therefore, Zn_0.8-SMYP: Eu²⁺ would be selected as the studied composition of the potential yellow-emitting phosphors for white LED applications in the present series.

Fig. 5 PL/PLE spectra of Zn_m-SMYP: Eu²⁺ (0 ≤ m ≤ 1).

With substitution of Mg²⁺ for Zn²⁺ (i.e., from SMYP: Eu²⁺ to SZYP: Eu²⁺), the emission edges of longer and shorter wavelength show obvious blue-shifting, whereas the emission peak do not show any obvious shifting. This observation could be explained in terms of two competing factors: the crystal field and the bond covalence.^4,16

Continuous change of the emission edge wavelengths in PL spectra is observed with increasing Zn²⁺ concentration. This blue shift is partly ascribed to the changes in crystal field strength acting on the Eu²⁺ because the incorporation of Zn²⁺ ions into the lattice causes the distortion and changes the symmetry of host lattice, which results in a decrease of crystal field spitting for the 5d energy levels. According to reports by Robertson et al.¹⁷ and Jang et al.,¹⁸ crystal field splitting (D_q) can be determined by the following equation:¹⁹

where D_q is a measure of the energy level separation, Z is the anion charge, e is the electron charge, r is the radius of the d wave function, and R is the bond length. When Mg²⁺ is substituted by a larger Zn²⁺ ion, the crystal lattice produces slight expansion. The distance between Eu²⁺ and O²⁻ becomes longer and the magnitude of the crystal field decreases, which induces the crystal fields strength surrounding the Eu²⁺ to change and decreases the splitting of the 5d state. This leads to the slight blue-shift of the emission band. As shown in Table 4, the crystal-field splitting, roughly estimated from the energy difference between highest and lowest observed 5d excitation levels of Eu²⁺ decreases from 8352 cm⁻¹ for m = 0 to 7539 cm⁻¹ for m = 1. For the variations of the crystal field strength, one of the phenomenological characteristics is the change in the position and shape of the excitation band.²⁰ As shown in Fig. 5, with increasing the Zn²⁺ concentration, different excitation spectra for lower energy emission bands testify the increase of crystal field strength acting on Eu²⁺.

Table 4 Crystal filed splitting (D_q), normalized PL intensity as well as spectral half-width (FWHM) of Zn_m-SMYP: Eu²⁺, depending on the partial substitution (m) of Mg by Zn

m	Dq (cm^−1)	PL intensity (%)	FWHM (nm)
0	8352	68.9	150
0.2	8235	75.3	147
0.5	7961	76.8	145
0.8	7580	100	117
1	7538	90.7	104

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In addition, the observed blue-shift could be explained in terms of the decreasing nephelauxetic effect.²¹ The nephelauxetic effect is a measure of the degree of covalency in the bonds between a metal ion and its surrounding ligands. Covalency is an important feature of the local structure.²² The observed emission from Eu²⁺ arises from the transition from the lower 4f⁶5d state to the 4f⁷ (⁸S_7/2) ground state. For the 4f⁶5d configuration, the nephelauxetic effect markedly reduces the 4f–5d electrostatic interaction;²³ covalency effects are primarily responsible for affecting the energy of the 4f⁶5d¹ excited level.²⁴ Because the f-electrons are well shielded, the nephelauxetic effect on the 5d orbital of Eu²⁺ is expected to be very strong. As the Zn²⁺ concentration (m) increases, the interaction between the 5d electron of Eu²⁺ and the ligand decreases and shrinks the electron cloud of the 5d orbital of Eu²⁺.²⁵ With the decrease in the degree of covalence of Eu–O bonds in the order of Mg and Zn, less negative charge transfer to Eu²⁺ ions and thus increase the energy difference between the 4f and the 5d levels.²⁶ Thus, the degree of covalence in the Eu–O bonds was decreased with substitution of Mg²⁺ with smaller Zn²⁺ cations, and consequently can result in the blue shifting of the Eu²⁺ emission band. The strong nephelanxetic effect is also supported by the observed FWHMs for Zn_m-SMYP: Eu²⁺ phosphors, resulting in a decrease of the energy gravity between 4f and 5d energy levels.

As shown in Fig. 5, the emission peak position is almost constant within the range of 0 ≤ m ≤ 1. In contract, the shoulder position is shifted to shorter wavelength by increasing Zn occupancy. The chromaticity coordinates on commission international de I'Eclairage (CIE) 1931 of SMYP partial replaced by different concentrations of Zn²⁺ ions were calculated according to the emission curve as shown in Fig. 5. The change in the PL spectra in the range of m = 0–1.0 caused a marked change in emission color, as indicated by the shift in CIE color coordinates in Fig. 6. The chromaticity coordinates (x, y) shift from (0.355, 0.469) to (0.251, 0.451), when the Zn²⁺ concentration (m) varies from 0 to 1, corresponding to the hue converted from yellow to green, which was attributed to the emission intensity and peak position change under ultraviolet (UV) light of 375 nm. The results demonstrate that the Zn_m-SMYP: Eu²⁺ is a color tunable phosphor by changing the Zn/Mg ratio, which could be more flexible for the generation of white light by combination of other phosphors and LED chips.

Fig. 6 CIE chromaticity coordinates of Zn_m-SMYP: Eu²⁺ phosphors. (1) m = 0, (2) m = 0.2, (3) m = 0.5, (4) m = 0.8, and (5) m = 1.0.

4. Conclusions

The ability to tune the color point of photoluminescence materials is particularly pertinent for practical applications. Here we have successfully synthesized the solid solution Sr₈Mg_1−mZn_mY(PO₄)₇ (0 ≤ m ≤ 1) doped with 1 atom% Eu²⁺ and investigated the structural and luminescence properties. By partially substituting Mg²⁺ with Zn²⁺, a regular variation was found among the XRD patterns. With Zn²⁺ content increasing, the crystal field splitting between two lowest Eu²⁺ 5d levels decreased. As a result, the emission color alters from yellow to green. The decrease of the crystal field strength and the covalence results in a blue shift reflected in the luminescence spectra with the doping of Zn. The excitation spectra show broad band excitation in the 250–470 nm n-UV regions, which matches well with n-UV chips. The emission spectra exhibit strong tunable emissions color. All these results indicate that the Eu²⁺-doped Sr₈(Mg,Zn)La(PO₄)₇ phosphor can be applied as a color-tunable phosphor for green or greenish white light emitting diode based on ultraviolet chip/phosphor technology.

Acknowledgements

This research was financially supported by the National Pharmacy Experimental Teaching Centre of South-Central University for Nationalities (no. 406702).

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