Structural and luminescence properties of red-emitting Cs_1−xMgPO₄:xEu²⁺ phosphors for near-UV-pumped light emitting diodes

Abstract

A series of luminescent phosphate phosphors Cs_1−xMgPO₄:xEu²⁺ were synthesized via a high temperature solid-state reaction. The phase structure and photoluminescence (PL) properties, as well as the PL thermal stability of Cs_0.96MgPO₄:0.04Eu²⁺ were investigated to characterize the resulting sample. The crystal structure and chemical composition of the Cs_0.96MgPO₄:0.04Eu²⁺ phosphor were analyzed based on the Rietveld refinements and the crystal chemistry rules, respectively. The optimum concentration of Eu²⁺ in the CsMgPO₄ phosphor was about 4 mol% and the concentration quenching effect can be attributed to the dipole–dipole interaction. This phosphor shows a broad red emission band ranging from 500 to 800 nm under the 410 nm light excitation. The above results indicate that Cs_1−xMgPO₄:xEu²⁺ phosphors have potential applications for near UV-excited w-LEDs.

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Introduction

Recently, white light-emitting diodes (w-LEDs) have attracted increasing attention owing to the great advantages over the traditional fluorescent lamps and incandescent bulbs, such as high efficiency, low power consumption, and good stability of chemical and physical properties, as well as environmentally-friendly characteristics.^1–4 w-LEDs fabricated using a blue-emitting InGaN chip and a yellow Y₃Al₅O₁₂:Ce³⁺ phosphor show great potential in the next-generation general lighting due to their advantages, such as long life time, energy saving, and high efficiency.^5–7 However, they have some disadvantages such as poor color-rendering index (CRI) and high correlated color temperature because of weak red emission. In order to improve the CRI and the weak red emission, red-emitting Eu²⁺ activated phosphors have gained increasing, which have broad band emission in the red light wavelength range and significantly enhances the color rendering index of white LEDs. So far, the researches and applications of red-emitting phosphors are mainly consisted of three types: alkaline earth metal sulphide, aluminate and nitride. Among them, the chemical stability of alkaline earth metal sulphide is instable and deliquescent, which restricts its application. The research of aluminate is in its infancy, the properties of which needed to be improved. It is acknowledged Eu²⁺ activated red-emitting nitride phosphors have been a hot research topic. Although the nitride phosphors have been commercialized, the synthesis processes of the nitride phosphors are usually rather complex and highly energy consuming (e.g. high temperature, high pressure of nitrogen atmosphere, raw materials are very sensitive to oxygen and moisture, high costs, and low yields).^8–11 Therefore, it is important and necessary to find new host for Eu²⁺ doped, which have broad band emission in the red light.

As is known to us, due to excellent chemical stability, charge stabilization and easy to synthesis, compounds with the double phosphates structure crystallizing in the orthorhombic system are suitable host lattices for various luminescent ions.^12–16 In general, double phosphates compounds can be represented with the general formula A^IB^IIPO₄ (A^I = Li, Na, K, Rb; B^II = Ca, Sr, Ba, Zn, Cd, Pb).^17–19 To date, some Eu²⁺ activated A^IB^IIPO₄ have been reported,^20–26 such as LiMgPO₄:Eu²⁺; NaCaPO₄:Eu²⁺; KSrPO₄:Eu²⁺, ABaPO₄:Eu²⁺ (A = Na, K), KCaPO₄:Eu²⁺, KMgPO₄:Eu²⁺, but among which emission spectra do not cover the red range. In addition, the relative size of the A and B ions would determine the structure of A^IB^IIPO₄ and few luminescence properties of the Eu²⁺ doped orthophosphate CsMgPO₄ have been reported.

In this work, we successfully synthesized Cs_1−xMgPO₄:xEu²⁺ phosphors with red emission. The crystal structure and the luminescence properties of Eu²⁺ in this CsMgPO₄ host were also investigated in detail. In general, the Cs_1−xMgPO₄:xEu²⁺ phosphors can be potentially applied as the blue-emitting component in w-LEDs.

Experimental

Materials and synthesis

Cs_1−xMgPO₄:xEu²⁺ phosphors were prepared via a traditional high temperature solid-state method. The stoichiometric amounts of Cs₂CO₃ (A.R.), (MgCO₃)₄·Mg(OH)₂·5H₂O (A.R.), (NH₄)₂HPO₄ (A.R.) and Eu₂O₃ (99.99%) were employed as the raw materials, which were weighed and mixed by grinding in an agate mortar. The mixture was firstly pre-heated at 500 °C for 3 h in air atmosphere in alumina crucibles with covers. After cooling to room temperature, the preliminary products were ground thoroughly in an agate mortar, and they were placed into alumina crucibles and annealed at 1150 °C in a CO reducing atmosphere for 3 h with highly pure carbon particles as a reducing agent. After firing, the samples were gradually cooled to room temperature in the furnace. The products were crushed and finally obtained for measurements.

Characterization methods

The composition and phase purity of the as-prepared products were measured by the X-ray diffractometer (XD-3, PGENERAL, China) with Cu-Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA. The continuous scanning XRD data were collected in a 2θ range from 10° to 70°. The continuous scanning rate (2θ ranging from 10° to 120°) used as phase formation determination was 4° (2θ) per min and step scanning rate used for Rietveld analysis was 3 s per step with a step size of 0.02. Powder diffraction data were obtained using a computer software General Structure Analysis System (GSAS) program. The morphology of the as-prepared samples was characterized by a field emission scanning electron microscopy (FE-SEM, JSM-7001F). Room-temperature photoluminescence (PL) spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer PL system equipped with a xenon lamp (400 V, 150 W) as an excitation source. A 400 nm cutoff filter was used in the measurement to eliminate the second-order emission of source radiation. The temperature-dependence luminescence properties were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace. The quantum yield value was measured by a fluorescence spectrophotometer (FluoroMax-4, HORIBA) with an integrating sphere at room temperature.

Results and discussions

Fig. 1 illustrates the structure of CsMgPO₄ compound viewed along the b axis. As given in Fig. 1, the P and Mg atoms are tetrahedral coordinated forming [PO₄] and [MgO₄] groups, which are connecting with each other. CsMgPO₄ is built up from MgO₄ and PO₄ tetrahedral (both with m symmetry) linked together by corners, which forms a three-dimensional framework. The Cs atoms are 11-coordinated with a cut-off distance of 3.7 Å, and Cs atoms locate in hexagonal channels running along the a- and b-axis directions with have m site symmetry. The bond lengths of Cs–O and Mg–O are 3.1951–3.6968 Å and 1.8847–1.9228 Å.²⁷ The phase purities of Cs_1−xMgPO₄:xEu²⁺ samples were studied by the powder X-ray diffraction. Fig. 2 shows the XRD patterns of the as-prepared Cs_1−xMgPO₄:xEu²⁺ (x = 0.005, 0.04, and 0.10) samples and the standard data for CsMgPO₄ (JCPDS card no. 45-275) is shown as a reference. It can be seen that relative intensities and the positions of the main diffraction peaks for all the samples are consistent with the standard card data for CsMgPO₄ (JCPDS card no. 45-275). These results indicate that doping of Eu²⁺ does not make any appreciable changes in the host structure.

Fig. 1 Crystal Structure of CsMgPO₄ compound view along the b axis.

Fig. 2 The XRD patterns of Cs_1−xMgPO₄:xEu²⁺ (0.005, 0.04, and 0.10), and the standard data for CsMgPO4 (JCPDS card no. 45-275) is shown as a reference.

In order to further understand the crystallographic sites of Eu²⁺ in CsMgPO₄, the Rietveld structural refinement for Cs_0.96MgPO₄:0.04Eu²⁺ was performed using GSAS software. The observed, calculated, and difference results for the Rietveld refinement XRD patterns of Cs_0.96MgPO₄:0.04Eu²⁺ are shown in Fig. 3. The structural parameters of CsMgPO₄ were used as an initial model in the Rietveld analysis. Moreover, the final refined residual factors, crystallographic data and fractional atomic coordinates are summarized in Table 1. The reliability parameters of refinement are R_wp = 8.50%, R_p = 5.96%, and GOF = 5.732, which can verify the phase purity of the as-prepared sample. The refined lattice constants are a = 8.9294 Å, b = 5.5141 Å, c = 9.6315 Å, and V = 474.236 ų, and all the constant values are smaller than that of the standard CsMgPO₄,²⁷ respectively (a = 8.9327 Å, b = 5.5277 Å, c = 9.6487 Å, and V = 476.4207 ų). Cs in 11-coordination and Mg in 7-coordination also have been observed, which are same to the previous reported data.²⁷ According to the ionic radii of rMg²⁺ = 0.75 Å, rEu²⁺ = 1.2 Å for coordination number (CN) = 7, the lattice constants of Eu²⁺ doped CsMgPO₄ would increase if Eu²⁺ occupied the Mg²⁺ site. However, the observed small lattice constants demonstrate Eu²⁺ must substituted the Cs⁺ site because of the larger radii of Cs⁺ (the effective ionic radii of rEu²⁺ = 1.35 Å, for CN = 10, rEu²⁺, = 1.37 Å, for CN = 12, rCs⁺ = 1.85 Å for CN = 11).²⁸ Simultaneously, the data (Table 1) of the Rietveld analysis also proved this. Thus the activators Eu²⁺ ions occupied the Cs⁺ sites randomly in the CsMgPO₄ host. However, because of the difference between the valences of Cs⁺ and Eu²⁺, charge compensation is required. The possible mechanism as followed: Cs⁺ → Eu²⁺ + , where Eu²⁺ ions substitute for Cs⁺ and combine with Cs⁺ vacancy and forming the dipole complexes of .

Fig. 3 Experimental (crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for Rietveld structure analysis of the selected Cs_0.96MgPO₄:0.04Eu²⁺ compound.

Table 1 Crystallographic data for Cs_0.96MgPO₄:0.04Eu²⁺ as the results of Rietveld refinement^a

Atom	x	y	z	Occupancy	Uiso/Å^2
a Orthorhombic a = 8.9294 Å, b = 5.5141 Å, c = 9.6315 Å, and V = 474.236 Å^3 2θ-interval = 10–120°; Rwp = 8.50%, Rp = 5.96%, GOF = 5.732.
Cs1	0.4971	0.2500	0.7032	0.9600	0.0352
Eu1	0.4971	0.2500	0.7032	0.0400	0.0352
Mg1	0.3213	0.2500	0.0813	1.0000	0.0214
P1	0.2031	0.2500	0.4148	1.0000	0.0304
O1	0.2632	0.2500	0.2678	1.0000	0.0569
O2	0.2627	0.0262	0.4882	1.0000	0.0320
O3	0.0331	0.2500	0.4156	1.0000	0.0294

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The morphology of Cs_0.96MgPO₄:0.04Eu²⁺ sample prepared via solid-state reaction is depicted in Fig. 4. SEM shows that the as-prepared sample has smooth surface and good crystalline with an average diameter of about 5–12 μm.

Fig. 4 SEM images of Cs_0.96MgPO₄:0.04Eu²⁺ phosphor particles.

Fig. 5 presents the excitation [photoluminescence excitation (PLE); λ_em = 630 nm] and emission (PL; λ_ex = 410 nm) spectra of the Cs_0.96MgPO₄:0.04Eu²⁺ phosphor. Generally, Eu²⁺ ions doped in inorganic materials have the characteristics of broad band emission and excitation spectra. As the result shown, the PLE spectra have a broad band between 500 and 800 nm in UV and n-UV range attributed to 4f⁷(⁸S_7/2) → 4f⁶5d transitions of the doped Eu²⁺ ions.^29–31 Upon the excitation of 410 nm, the phosphor presents a broad red emission band peaked at 630 nm extending from 500 to 800 nm, which is attributed to the 4f⁶5d → 4f⁷ transition of Eu²⁺.

Fig. 5 The excitation [photoluminescence excitation (PLE); λ_em = 630 nm] and emission (PL; λ_ex = 410 nm) spectra of Cs_0.96MgPO₄:0.04Eu²⁺ phosphor.

Fig. 6(a) illustrates the PL spectra of Cs_1−xMgPO₄:xEu²⁺ (x = 0.005, 0.01, 0.04, 0.07, and 0.10) phosphors monitored by 365 nm, and Fig. 6(b) lists the Eu²⁺ content dependent emission intensity corresponding to the peaks at 630 nm. It can be easily found that the emission intensities of Eu²⁺ at 630 nm increased firstly with its concentration increasing, and maximized at x = 0.04, and then decreased with increasing Eu²⁺ content caused by the concentration quenching effect. Furthermore, the critical energy transfer distance R_C between Eu²⁺ ions can be estimated by using concentration quench eqn (1) proposed by Blasse:³²

where V is the volume of the unit cell, x_c is the atom fraction of activator at which the quenching occurs, and N is the number of host cations in the unit cell. For the CsMgPO₄ host, it exists two number of host cations in one unit cell, that N was determined as 2. The result of Rietveld structural refinement for Cs_0.96MgPO₄:0.04Eu²⁺ showed V is 474.236 ų, x_c is the critical concentration (quenching concentration), as shown in Fig. 6(a), x_c equals to 0.04. According to the above eqn (1), the critical distances of energy transfer are turned out to be about 22.45 Å. Therefore, the electric multipolar interaction will take place for energy transfer between Eu²⁺ ions in CsMgPO₄ host with distance more than 5 Å.

Fig. 6 The PL spectra of Cs_1−xMgPO4:xEu²⁺ (x = 0.005, 0.01, 0.04, 0.07, and 0.10) phosphors monitored by 365 nm (a), and the Eu²⁺ content dependent emission intensity corresponding to the peaks at 630 nm (b).

The interaction type between sensitizers or between sensitizer and activator can be calculated by the following eqn (2)^33,34 to further confirm the process of energy transfer between Eu²⁺ ions in the CsMgPO₄ host lattice:

where k and β are constants for each type of interaction for a given host lattice. In the equation, I is the emission intensity, x is the concentration of the activator ions, and θ is an indication of electric multipolar character. In general, θ = 6, 8, and 10 corresponds dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively.^35,36 To get a correct θ value, the dependence of lg(I/x) on lg(x) is plotted, and it yield a straight line with a slope equal to −θ/3. Fig. 7 shows the fitting lines of lg(I/x) vs. lg(x) in Cs_1−xMgPO₄:xEu²⁺ phosphors for the emission peak of 630 nm beyond the quenching concentration. It is found that lg(I/x) showed a relatively linear dependence on lg(x), and the slopes were determined to be −1.419. The values of θ can be calculated as 4.257 close to 6. Therefore, the interaction type is the dipole–dipole (d–d) interaction for Eu²⁺ in the CsMgPO₄ host lattice.

Fig. 7 The fitting line of lg(I/x) vs. lg(x) in Cs_0.96MgPO₄:0.04Eu²⁺ phosphors beyond the quenching concentration.

The temperature-dependent photoluminescence of the Cs_0.96MgPO₄:0.04Eu²⁺ has been studied and measured. Fig. 8 shows the temperature dependence of the PL spectra of Cs_0.96MgPO₄:0.04Eu²⁺ monitored at 365 nm. It can be found that the relative emission intensity of Cs_0.96MgPO₄:0.04Eu²⁺ phosphor decreases with the increase of the temperature ranging from 298 K to 573 K. At the same time, the emission wavelength shows strong blue shift behavior with increasing temperature, which is ascribed to the thermally active phonon-assisted excitation from the lower-energy emission band to higher-energy emission band in the excited states of Eu²⁺.

Fig. 8 The PL spectra (λ_ex = 365 nm) of Cs_0.96MgPO₄:0.04Eu²⁺ phosphor under different temperatures in the range of 298–573 K.

CIE chromaticity coordinates (x, y) of the Cs_1−xMgPO₄:xEu²⁺ samples upon 365 nm excitation calculated through their PL spectrum and the digital images of the as-prepared samples under 365 nm UV lamp excitation are shown in Fig. 9. It is found that the color coordinates for Cs_1−xMgPO₄:xEu²⁺ (0.005, 0.04, and 0.10) are calculated to be (0.601, 0.302), (0.592, 0.298), (0.598, 0.291). The correlated color temperatures is 1806 K, 1523 K, 1630 K for Cs_1−xMgPO₄:xEu²⁺ (x = 0.005, 0.04, and 0.10), respectively. All the samples show intense red emission, which indicates that the Cs_1−xMgPO₄:xEu²⁺ samples exhibit great potential to serve as a red emitting phosphor for n-UV w-LEDs. The measured quantum yield values of Cs_0.96MgPO₄:0.04Eu²⁺ phosphor was determined to be about 9.58% under 410 nm excitation.

Fig. 9 Color coordinates of Cs_1−xMgPO₄:xEu²⁺ (x = 0.005, 0.04, and 0.10) in the CIE chromaticity diagram and the digital images of the red-emitting.

Conclusions

In summary, we have successfully synthesized a series of single-phase Cs_1−xMgPO₄:xEu²⁺ phosphors via a solid-state reaction, and the photoluminescence spectrum shows a broad red emission band peaks centered at 630 nm. The critical distances R_C of energy transfer were calculated by concentration quenching and turned out to be about 22.45 Å (x_c = 0.04). It is found that the interaction type is the dipole–dipole (d–d) interaction for Eu²⁺ in the CsMgPO₄ host lattice. The temperature dependent luminescence properties, CIE values, the correlated color temperatures, quantum yield values and typical morphology of the selected Cs_0.96MgPO₄:0.04Eu²⁺ phosphor has been investigated in detail. The present results indicate that Cs_1−xMgPO₄:xEu²⁺ could be used as a red emitting phosphor for n-UV w-LEDs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 51032007, 51472222, 51372232).

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