Structure and luminescence properties of a novel broadband green-emitting oxyapatite-type phosphor

Abstract

In recent years, synthetic apatite-doped rare-earth luminescent materials and their optical properties have attracted extensive worldwide attention. In this study, a series of novel green phosphors Sr₂Y₃(SiO₄)₂(PO₄)O:Eu²⁺ with apatite structure was fabricated via a high temperature solid-state reaction. X-ray diffraction, structure refinement, photoluminescence excitation, emission spectra, and temperature-dependent emission intensity were employed to describe the phase and property of the samples. Under the excitation of 365 nm, the phosphors emit strong green emission in the broad band range from 400 nm to 700 nm, which almost covers the visible light spectrum. The quenching concentration of Eu²⁺ in Sr₂Y₃(SiO₄)₂(PO₄)O was about 0.05, which was attributed to the dipole–dipole interactions. The evidence that the as-prepared phosphor can be successfully excited by near ultraviolet light indicates that it can be potentially used as a near UV-convertible phosphor for white light-emitting diodes.

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1. Introduction

It is well known that energy efficiency is one of the key parameters in illumination sources and other pervasive products. White light-emitting diodes (wLEDs) have bright future in traffic lights, displays, general lighting, and decorative lighting owing to their high efficiency and environment friendly nature.^1–3 Currently, phosphor-converted wLEDs combining tricolor phosphors and near ultraviolet (n-UV) or blue chips have been extensively applied as solid-state lighting sources owing to their excellent performance and properties, e.g., long life, conversion efficiency, high color-rending index, and lower power consumption.^4–6

In general, a phosphor is composed of hosts and activators (sometimes sensitizers are also included).⁷ Rare-earth (RE) ions (Ce³⁺, Eu²⁺, and Tb³⁺) can be used as activators for white light-emitting diodes (wLEDs). The emission color changes when the RE ions are doped into different hosts derived from their 4f–4f or 4f–5d transitions.^8,9 Eu²⁺ with a wide emission color range is a popular activator with intensive absorption in the UV and visible regions.^10–14 Transitions between the ground state (4f energy level) and the excited 5d-state of Eu²⁺ can be influenced by the crystal lattice pattern, e.g., crystal-field strength, atom coordination, and covalence.^15–17 Moreover, the careful selection of the host material is also critical since the host structure will affect the crystal field environment of the RE ions, which will change the energy transitions of the RE ions.¹⁸ A deeper understanding of the crystal structure of the targeted compound is necessary for choosing the host material because there are at least two or three suitable non-equivalent crystallographic sites for the Eu²⁺ occupation that can regulate the crystal field environment of Eu²⁺.¹⁹ Accordingly, efforts have been devoted for the development of novel host materials, containing abundant cationic crystallographic sites doped with Eu²⁺, which can be typically excited by n-UV light.^20–23

The apatite-type structured compounds have two nonequivalent cationic sites for the Eu²⁺ occupation, which enables them to be one of the most potential phosphor host materials.^24,25 In general, compounds with the apatite structure are characterized by a general chemical formula of M₁₀[XO₄]₆Y₂, where M is defined as a divalent cation (Mg²⁺, Ca²⁺, Ba²⁺, etc.), whereas rare-earth ions (Y³⁺, La³⁺, Eu²⁺ etc.), alkali metal ions (Na⁺, K⁺, Ag⁺, etc.) can also enter the M-site, thus forming a coupling isomorphic substitution. [XO₄] is defined as a complex anionic group ([PO₄], [VO₄], [AsO₄], etc.), and Y commonly represents the anions such as F⁻, Br⁻, or OH⁻.²⁶ In the past few decades, Eu²⁺-doped apatite phosphors have attracted considerable attention worldwide, e.g., Ca₅(PO₄)₃F,²⁷ Sr₅(PO₄)₃Cl,²⁸ Ca₈Gd₂(PO₄)₆O₂,²⁹ and Ca₂Ba₃(PO₄)₃Cl.³⁰ However, no report is available on the Eu²⁺-doped Sr₂Y₃(SiO₄)₂(PO₄)O₂ phosphor with the apatite structure. On the other hand, developing novel apatite phosphors doped with Eu²⁺ that can be excited by n-UV light (350–420 nm) has a significant practical value.

Herein, a novel apatite-type phosphor Sr₂Y₃(SiO₄)₂(PO₄)O:Eu²⁺, emitting green fluorescence, was successfully prepared via a high-temperature solid-state reaction,. The crystal structure of the as-prepared sample was identified by the Rietveld refinement method in detail, which was used to study how the crystal structure and composition affected the luminescence property of the sample. In addition, the thermal luminescence property of the phosphor has also been revealed in this study. Under 365 nm excitation, the phosphor exhibits a strong green emission at 505 nm with an obvious band ranging from 400 nm to 700 nm, which almost covers the visible light spectrum. The results indicate that the Sr₂Y₃(SiO₄)₂(PO₄)O:Eu²⁺ phosphor can act as a potential green phosphor for n-UV phosphor-converted wLEDs.

2. Experimental procedure

2.1. Materials and synthesis

The Sr₂Y₃(SiO₄)₂(PO₄)O:Eu²⁺ phosphor was synthesized using the traditional high-temperature solid-phase synthesis method with starting materials, including SrCO₃ (Aldrich, 99.9%), Y₂O₃ (Aldrich, 99.99%), NH₄H₂PO₄ (Aldrich, 99.9%), SiO₂ (Aldrich, 99.99%), and Eu₂O₃ (Aldrich, 99.999%). First, they were weighed in an electronic balance according to the stoichiometric ratio. Then, they were mixed and ground thoroughly in an agate mortar. Further, the raw materials were preheated at 500 °C for 3 h in a furnace and moved into alumina crucibles after being cooled down to room temperature. In addition, the preliminary products were annealed at 1400 °C for 5 h under a reducing atmosphere (10% H₂ + 90% N₂). When the furnace was cooled down to room temperature again, the final product was smashed and re-ground for subsequent tests.

2.2. Characterization

The phase structure of each sample was evaluated using an X-ray powder diffractometer (RigakuDmax 12 kW, Japan) with Cu Kα radiation (λ = 0.15418 nm) from 10° to 70° (2θ). The Rietveld refinement of Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ was carried out by the TOPAS software. Photoluminescence and excitation spectra of the sample were recorded using a fluorescence spectrofluorometer (FL-4600, HITACHI, Japan) fitted with a 700 V and 150 W Xe lamp source. Lifetime of the samples was tested using a spectrometer (HORIBA JOBIN YVON FL3-21) with excitation by a pulsed radiation laser at 370 nm.

3. Results and discussions

Fig. 1 presents the XRD patterns of Sr_(2−x)Y₃(SiO₄)₂(PO₄)O:xEu²⁺ (x = 0.01 and 0.05) and the referred standard data for Ca₈Nd₂(PO₄)₆O₂ (PDF-32-175). The diffraction peaks of the Sr_(2−x)Y₃(SiO₄)₂(PO₄)O:xEu²⁺ phosphors are consistent with those of the Ca₈Nd₂(PO₄)₆O₂ (Fig. 1), where a minor deviation of the dominant diffraction peak at higher 2θ can be found due to the replacement of Sr by Eu, indicating that a small amount of Eu²⁺ doping does not lead to a variation in the structure.

Fig. 1 The XRD patterns of Sr_(2−x)Y₃(SiO₄)₂(PO₄)O:xEu²⁺ (x = 0.01 and 0.05), and the standard card (PDF-32-175-Ca₈Nd₂(PO₄)₆O₂) is shown as a reference.

To further investigate the phase purity and occupation of Eu²⁺, TOPAS 4.2 (ref. 31) was used to analyze the Rietveld refinement. The structure of Ca₈Nd₂(PO₄)₆O₂, possessing a hexagonal structure with the space group P6₃/m, was employed as the initial model.³² Silicon oxygen tetrahedron and phosphorus oxygen tetrahedron is independent with each other Sr1 and Sr2 occupy two independent sites for Sr²⁺ in the crystal lattice. Sr1 is in a 9-fold coordination state, and Sr2 is in a 7-fold coordination state. Similar to Sr²⁺, the radii of Eu²⁺ in the 7-fold coordination and 9-fold coordination sites are 0.120 and 0.130 nm, respectively. Eu²⁺ can occupy the two positions of Sr²⁺. The Rietveld analysis patterns for the X-ray power diffraction data of Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ is shown in Fig. 2. The occupation of the cations was set to a fixed value according the chemical formula. The fractional atomic coordinates, isotropic displacement parameters, and the cell parameters were refined. The atomic coordinates, occupancies and displacement parameters (Ų) are listed in Table 1, and the cell parameters and reliability factors are listed in Table 2. The cell parameters are a = b = 9.477(10) Å, c = 6.954(80) Å, α = 90°, β = 90°, γ = 120°, and V = 540.53(16) Å. The refinement was stable and the fit is convincing with low R-factors, which verify the phase purity of the as-prepared sample.

Fig. 2 XRD refinement result of the Sr_3.95Eu_0.05Y₆(SiO₄)₄(PO₄)₂O₂ sample: experimental results (×), calculated results (|), and their difference (bottom). The brown solid lines represent the Bragg reflection positions, and the inset shows a crystal structure diagram of the host.

Table 1 Crystal structural data and lattice parameters for Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺

Formula	Sr3.95Y6(SiO4)4(PO4)2O2:0.05Eu^2+
Space group	P63/m – hexagonal
Cell parameters	a = b = 9.477(10) Å, c = 6.954(80) Å
	α = 90°, β = 90°, γ = 120°
	V = 540.53(16) Å, Z = 2
Reliability factors	Rwp = 8.89%
	Rp = 6.66%
	χ^2 = 3.64

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Table 2 Fractional atomic coordinates and isotropic displacement parameters (Ų) of Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺

Atom	x	y	z	Occupancy	Uiso/Å^2
Sr1(Eu1)/Y1	1/3	2/3	0	0.35(0.05)/0.6	0.0292(7)
Sr2(Eu2)/Y2	0.24632(0)	0.23413(5)	1/4	0.35(0.05)/0.6	0.0250(2)
P1/Si1	0.03324(1)	0.40456(0)	1/4	0.33/0.67	0.0239(3)
O1	0.48621(8)	0.15352(7)	1/4	1	0.0431(0)
O2	0.12523(5)	0.59325(1)	1/4	1	0.0341(2)
O3	0.11200(4)	0.34721(7)	0.07382(0)	1	0.0181(6)
O4	0	0	1/4	1	0.0197(3)

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The photoluminescence excitation and emission spectra of the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ are given in Fig. 3(a). It is clear that the PLE spectrum has an absorption band at 274 nm, which resulted from the 4f–5d transitions in Eu²⁺. The PL spectrum exhibits a green emission peak at 505 nm, which can be decomposed into two sub-bands with Gauss imitating decomposition at 498 nm and 555 nm, respectively (Fig. 3(b)). Therefore, Eu²⁺ can occupy two cationic lattices, which is consistent with the refined results. The lattice site centers of Eu²⁺ can be determined using the van Uitert empirical equation:³³

where E is the position of the low-energy of the 5f excited-state (cm⁻¹) or the emission peak of Eu²⁺, Q is 34 000 cm⁻¹, V is 2, n is the anion number around Eu²⁺, E_a is a constant, r is the effective radius of Sr²⁺ (nm). Therefore, E is negatively correlated with n. Two different luminescent centers, having nine and seven coordinations, contribute to the fluorescence emission peaks at 498 and 555 nm, respectively.

Fig. 3 (a) Excitation and emission spectra of the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ sample, (b) PL spectrum of Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ excited at 365 nm and Gaussian fitting result. Two Gaussian components are peaked at 498 nm and 555 nm.

Fig. 4(a) shows PL spectra of Sr_(4−x)Y₆(SiO₄)₄(PO₄)₂O₂:xEu²⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09, and 0.12). It can be seen that the luminescence intensity increases first and then decreases. The luminescence properties of phosphors mainly depend on the rare-earth ion content, e.g., the luminous intensity is the highest when Eu²⁺ is 0.05.

Fig. 4 (a) PL spectra of Sr_(4−x)Y₆(SiO₄)₄(PO₄)₂O₂:xEu²⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09, and 0.12) under the excitation of 365 nm, (b) intensities of Eu²⁺ as a function of Eu²⁺ doping concentration, and (c) linear fitting of log(x) versus log(I/x) in the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ samples.

As shown in Fig. 4(b), the luminescence intensity of Eu²⁺ first increases with the increase in the Eu²⁺ concentration, and reaches the maximum when x = 0.05; further, the emission intensity decreases caused by the concentration quenching effect. The concentration quenching is caused by the energy transfer of Eu²⁺. In order to establish the concentration quenching mechanism, the critical energy transfer distance R_C can be calculated first:³⁴

For the host with x_C = 0.05, V = 540.53 ų, N = 10, the obtained critical distance R_C is 12.74 Å. According to Dexter theory, when the R_C value is >5, a non-radiative transition occurs between the Eu²⁺ ions; therefore, the type of energy transfer can be described as follows:^35,36

where I is the emission intensity and x is the activator concentration of Eu²⁺, θ = 6, 8, and 10 for dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q), respectively.^37,38 Fig. 4(c) shows the relational curve between log(I/x) and log(x) in the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ phosphors under the 365 nm light irradiation with line slope of −2.085, and the calculated θ value based on eqn (3) is 6.255, which means that the concentration quenching mechanism of Eu²⁺ in the sample is d–d.

The PL decay curve of the Eu²⁺ phosphor was detected at 365 nm, which can be used to determine the fluorescence lifetime. Fig. 5 describes the decay curves of the phosphor, which can be fitted by a double-exponential equation:^39,40

where I₀ is the initial emission intensity, t represents time, A₁ and A₂ are invariant constants after fitting, and τ₁ and τ₂ are two different decay lives. The calculated Eu²⁺ lifetime is 0.613 μs.

Fig. 5 Decay curves of Eu²⁺ in the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ phosphor.

The variable temperature fluorescence test was carried out to show the thermal stability of the sample. Fig. 6(a) presents the relationship between the emitted light color and the ambient temperature in the wavelength range from 400 to 700 nm. As shown in Fig. 6(b), the emission intensity of the sample is 63% of that at room temperature with the temperature rising to 150 °C, indicating an excellent thermal stability. In addition, the strongest emission peak moves from 502 to 490 nm. The emission band has slight blue shift with an increase in temperature. The activation energy (E) can be obtained by the Arrhenius equation:^41,42

where I(0) is the initial emission intensity of the phosphor at 25 °C and I(T) is the emission intensity at different temperatures, c is constant, E is the activation energy for thermal quenching, and k is the Boltzmann constant (8.617 × 10⁻⁵ eV). The plot of ln[(I₀/I)⁻¹] vs. 1/kT in Fig. 6(c) follows a line trend, obtaining 0.210 eV as the activation energy E. Fig. 6(d) shows that the calculated CIE chromaticity coordinates of Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ is (0.233, 0.446), and the green emission can be observed clearly from the image of Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ at ultraviolet excitation.

Fig. 6 (a) Temperature-dependent emission intensity of the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ sample in the wavelength range of 400–700 nm, (b) temperature dependence of the emission intensity, (c) the plot of ln[(I₀/I_T) − 1] varied as a temperature function, (d) the CIE chromaticity coordinate diagram for Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ and the inset shows a digital photograph of the green-emitting phosphor.

4. Conclusions

A series of green phosphors Sr₂Y₃(SiO₄)₂(PO₄)O₂:Eu²⁺ with apatite structure were successfully synthesized via a traditional high-temperature solid-phase reaction and their luminescent properties were investigated. The phosphors generated a strong green emission band at 505 nm with an obvious broad band ranging from 400 to 700 nm. The determined optimal Eu²⁺ concentration of Sr₂Y₃(SiO₄)₂(PO₄)O:Eu²⁺ is 0.05. The experimental results and theoretical calculations suggest that the concentration quenching mechanism of Eu²⁺ in the Sr_3.95Y₆(SiO₄)₄(PO₄)₂O₂:0.05Eu²⁺ phosphor is d–d and the decay time of it was around 0.613 μs. All the investigations show that the as-prepared phosphor can be a green phosphor having potential research value.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The current work has been supported by the National Natural Science Foundation of China project (Grant No. 41672044 and 41802040), the Fundamental Research Funds for the Central Universities (Grant No. 2652017091), and China Postdoctoral International Exchange Program.

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