Combination cation substitution tuning of yellow-orange emitting phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺

Abstract

Cation substitution is a valuable approach to tune the crystal field splitting of garnet phosphor and hence enhances the red shift of the Ce³⁺ emission. A novel aluminate silicate garnet phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ is designed with the aim of creating a large distortion of the dodecahedron and an external pressure from the large octahedron through the combination of the substitution of both dodecahedral Y³⁺ and octahedral Al³⁺ sites with Mg²⁺. The garnet structure and elemental composition of this phosphor were confirmed by XRD, SEM and TEM. The particular coordination environment of each element of the phosphor was illustrated by XPS and Rietveld refinement. Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ exhibits a strong and broad yellow-orange emission band with a CIE coordinate of (x = 0.519, y = 0.472) and shifts to the red side of 57 nm, compared with the commercial phosphor YAG:Ce³⁺. This red-shift could be mainly ascribed to the distortion and shrinking of the dodecahedron, and subsequently, the larger crystal field splitting of the Ce³⁺ 5d levels. A white LED was fabricated and showed a high colour rending index of up to 84. These results reveal that Ce³⁺-doped Mg₂Y₂Al₂Si₂O₁₂ phosphor is a promising blue light converted yellow-orange light emitting phosphor for white LED.

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1 Introduction

White LEDs possess the advantages of a long working life, high luminous efficiency, energy savings and they are environmentally friendly, which makes them a potential candidate to replace traditional incandescent and fluorescent lamps.^1–3 Since Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) can absorb blue light and emit yellow light at a high quantum efficiency (>85%), Ce³⁺-activated YAG has become the most important commercial yellow phosphor for blue LED chips.^4,5 However, the relative weak red component of the YAG:Ce³⁺ emission spectrum results in low colour rending index (R_a < 80) and high correlated colour temperature (CCT > 5000 K), which seriously limit their indoor lighting applications.⁶ Therefore, more and more attention has been paid on the development of new phosphors which can emit longer wavelength light compared to YAG:Ce³⁺.

Besides the co-doping of other activator ions, including the Pr³⁺,⁷ Sm³⁺,⁸ Cr³⁺,⁹ and Mn²⁺–Si⁴⁺ pairs,¹⁰ cation substitution is a useful approach to tune the crystal field splitting of the 5d orbit of Ce³⁺, and hence makes the emission considerably shift to the red side. These modifications involve the direct substitution of the dodecahedral activator ion sites with lager ion substituted Tb₃Al₅O₁₂:Ce^3+ 11 and Gd₃Al₅O₁₂:Ce³⁺.¹² The octahedral sites could also be substituted by the widely accepted strategy of replacing Al³⁺ (0.51 Å) by the Mg²⁺ (0.66 Å)–Si⁴⁺ (0.42 Å) pair, which results in a stronger crystal field splitting and a further shift of the Ce³⁺ emission to the red side.^13,14 Recently, a double substitution of Mg²⁺–Si⁴⁺/Ge⁴⁺ to replace Al(1)³⁺–Al(2)³⁺ on the octahedral and tetrahedral sites was reported and the red shift of the emission was interpreted via the bond distance increasing in the octahedron.¹⁵ However, the relationship between the luminescence shifts with the crystallography of these substitutions is still unclear, except for the phenomenological design rules of the cation size effects.¹⁴ On the other hand, besides this cation size effect on the redshift, Cheetham et al. preferred the distortion factor of the dodecahedral doping site.^16,17 Furthermore, Seshadri's group revealed that the compression of the Ce³⁺ local environments could be ascribed to the unusually large crystal-field splitting of garnet, giving insights into the local environment of the activator ions.¹⁸

Considering the factors mentioned above, one could hypothesize that the part substitution of the dodecahedral site with small ion radii might directly result in a compression environment for the activator ion Ce³⁺ and hence a large distortion of the dodecahedron, and hence should result in strong crystal field splitting. Meanwhile, the large octahedron may provide external pressure on the activator centre dodecahedron and could, furthermore, enhance the crystal field splitting. Based on the above design rule, the combination cation substitution tuning garnet phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ is proposed. Herein, Mg²⁺ is predicted to incorporate both dodecahedral Y³⁺ and octahedral Al³⁺ sites. Subsequently, a large distortion dodecahedron and a large octahedron could be built up, and hence a longer wavelength emission could be expected.

In this article, a novel Ce³⁺-doped garnet phosphor Mg₂Y₂Al₂Si₂O₁₂ was synthesized by a traditional solid state reaction at a relatively low temperature. The crystal structure and the particular coordination environment of each element were illustrated by XRD, TEM and XPS. An intense yellow-orange emission was observed for the synthesized phosphor, and the effect of the crystal structure parameters of Mg₂Y₂Al₂Si₂O₁₂ on such a red shift is discussed. A white LED (WLED) was fabricated and the colorimetric properties were evaluated.

2 Experimental section

A series of samples of Mg₂Y_2−xAl₂Si₂O₁₂:xCe³⁺ with different Ce³⁺ concentrations (x = 0, 0.01, 0.03, 0.06, 0.09 and 0.12) were synthesized by the traditional solid state reaction method. Y₂O₃ (99.99%), MgO (99.5%), SiO₂ (99.99%), Al₂O₃ (99%) and CeO₂ (99.9%) were used as the raw materials, and mixed uniformly in an agate mortar. Then, the precursor mixture was fired at 900 °C for 8 h in air in an alumina crucible. After the samples cooled down to room temperature and ground again into powder, the as-obtained precursor was sintered under a reducing atmosphere of 5% H₂: 95% N₂ at 1350 °C for 10 h. The products of Mg₂Y_2−xAl₂Si₂O₁₂:xCe³⁺ were allowed to cool down to room temperature in the furnace in a reducing atmosphere, and were then ground and pulverized for further measurement.

The phase composition of the obtained powder samples were characterized by X-ray diffraction (XRD) analysis on a PNAlytical X'Pert Pro diffractometer with Cu Kα radiation at 40 mA and 40 kV in the 2θ range from 5° to 125°, with a scanning step of 0.0167°. A Rietveld structural refinement was carried out by General Structure Analysis System (GSAS) software package. The particle size, morphology and the content of each element were observed with a scanning electron microscope (SEM, Hitachi S-4700II) with an accelerating voltage of 15 kV, which also had an energy-dispersive X-ray spectrometer (EDX) attachment. High-resolution and elemental mapping analyses of the samples were operated under a high-angle annular dark field mode by scanning transmission electron microscope (STEM, FEI Tecnai G2 F30 S-TWIN) with an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Axis Ultra DLD spectrometer (Kratos Analytical). The instrument was equipped with a monochromatized aluminium X-ray source powered at 15 kV and 3 mA, which delivered an X-ray beam of 300 μm × 700 μm. Charge compensation was obtained with the built-in charge neutralization system. The pass energy was set to 160 eV for the survey spectra and for the high-resolution spectra, and the pass energy of Al 2p, Si 2p and Y 3d are 40 eV, 40 eV and 20 eV, respectively. The binding energies were calculated with respect to the C component (BE = 284.8 eV) of the C 1s peak, and a Shirley background subtraction was used. For the spectrum fitting, an XPS analysis program XPS peak 4.1 was used, and a Lorentz–Gauss algorithm was applied. The fitted peak positions were compared with those reported in the literature and the full width half maximum (FWHM) of all the peaks was also used to evaluate the reasonableness of the fitting results. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on a Fluoromax-4 (JOBIN YVON) equipped with a 150 W xenon lamp as the excitation light source. A cut-off filter was used in the measurements to eliminate the second-order bands of the source radiation. The temperature-dependent emission spectra were recorded using a Fluoromax-4 spectrophotometer equipped with a heater (TAP-02, Orient Koji Scientific) in the sample compartment. The PL spectra of all the samples were recorded three times and averaged to reduce the error. The quantum efficiency was measured using the same fluorescent spectrometer (Fluoromax-4) with an integrating sphere (F3018) accessory. The electroluminescence spectra of phosphor-converted WLED were measured using a UV-VIS-near IR Spectrophoto Colormeter (PMS-80, EVERFINE PHOTO). All the measurements were performed at room temperature. The commercial phosphor YAG:Ce³⁺ was provided by HangZhou Ying-he Optoelectronic Technology Co., LTD (YH-Y552M). And the Blue LED chip (455 nm) was purchased from JiangXi Yuan-en Optoelectronic Technology Co., LTD. The chip size was 45 mil × 45 mil and the nominal voltage range was 3.4–3.6 V.

3 Results and discussion

3.1 Crystal structure of the host Mg₂Y₂Al₂Si₂O₁₂

The X-ray diffraction pattern of the host Mg₂Y₂Al₂Si₂O₁₂ is shown in Fig. 1. The main diffraction peaks of the sample are in accordance with the standard diffraction peaks of Y₃Al₅O₁₂ (no. 088-2048), demonstrating that Mg₂Y₂Al₂Si₂O₁₂ is cubic with the space group Ia3̄d. The strong diffraction peaks at 2θ = 33.4° and 18.2° are attributed to the (420) and (210) crystal planes, respectively. The peak marked with an asterisk belongs to the competition second phase of Y_4.67(SiO₄)₃O (no. 30-1457), which is also observed as the second phase for the phosphor Lu₂CaMg₂(Si,Ge)₃O₁₂:Ce³⁺.¹⁴ Similar to the conclusions of Setlur,¹⁴ the phase of Y_4.67(SiO₄)₃O with an apatite structure does not influence the photoluminescence properties of the phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺. In addition, the diffraction peaks of the host Mg₂Y₂Al₂Si₂O₁₂ are very sharp and strong, suggesting that a good crystallinity of the phosphor can be obtained through this method. Also, this is very important for phosphor, because better crystallinity means less traps and a stronger luminescence.

Fig. 1 XRD patterns of the host Mg₂Y₂Al₂Si₂O₁₂, standard card no. 088-2048 (Y₃Al₅O₁₂) and no. 30-1457(Y_4.67(SiO₄)₃O).

The morphology was characterized by SEM, and it was found that the phosphors consist of many irregular granular microcrystals with particle sizes of 10–20 μm, as shown in Fig. S1.† EDX was used to determine the stoichiometric ratio of each element of the synthesized samples. The measurement results are shown in Table 1. The sample is mainly composed of Mg, Y, Al, Si and O, and the atomic ratio was in accordance with the theoretical composition 1 : 1 : 1 : 1 : 6 of the sample. A similar composition was detected for the different surface sites of the powder, indicating that each element is distributed uniformly in the sample. From the picture of the high resolution transmission electron microscopy (HRTEM), the interplanar distance between the adjacent lattice fringes were determined to be 0.27 nm and 0.5 nm (Fig. 2a). The corresponding 2θ value could be calculated by the Bragg's equation nλ = 2d sin θ, where λ is the X-ray wavelength, d is the distance between the two crystal planes, and θ is the diffraction angle of an observed peak. According to the equation, the calculated 2θ results were 33.2° and 17.7°, respectively, which are close to the 2θ value of the (420) and (210) crystal planes (2θ = 33.4° and 18.2°) obtained by XRD. These indicate that the lattice fringes shown in Fig. 2a belong to the (420) and (210) crystal planes. The energy dispersive X-ray mapping performed by the scanning transmission electron microscope was carried out to further confirm the elemental composition and the uniform distribution of the elements. As shown in Fig. 2b, the phosphor is composed of Mg, Y, Al, Si and O, and the distribution of Mg, Y, Al, Si and O elements is homogeneous in the phosphor, both of which agree well with the results measured by EDX.

Table 1 EDX results of the elemental contents in the host Mg₂Y₂Al₂Si₂O₁₂

Element	Wt%	At%
Mg (K)	6.75	7.56
Al (K)	10.03	10.13
Si (K)	11.06	10.73
Y (L)	34.32	10.51
O (K)	35.61	61.63

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Fig. 2 HRTEM image (a) and element distribution mapping (b) of the host Mg₂Y₂Al₂Si₂O₁₂.

To illustrate the particular coordination sites of each element occupied in the garnet structure for the present novel host, XPS was used to confirm the chemical states of each element. The XPS survey scan is shown in Fig. 3a, suggesting that the host contains Mg, Y, Al, Si and O elements. Fig. 3b shows the high resolution XPS analysis of the variation in the Al 2p peak. The Al 2p peak is broad and can be roughly de-convoluted into two peaks, one at a lower binding energy, 73.7 eV, and the other at a slightly higher binding energy, 75.3 eV, which belong to [AlO₄] tetrahedral and [AlO₆] octahedral coordination, respectively.^19–21 The Si 2p XPS peak is displayed in Fig. 3c with two peaks, which could be assigned to the Si 2p_1/2 and Si 2p_3/2 of [SiO₄], proving that Si exists in the form of [SiO₄] in the structure of Mg₂Y₂Al₂Si₂O₁₂.^22,23 In addition, the two peaks of Y 3d can be well fitted to the 3d orbit of the Y splitting of 3d_3/2 and 3d_5/2, indicating that the Y exists as one type of dodecahedral coordination in Mg₂Y₂Al₂Si₂O₁₂.²⁴ The XPS data of Mg²⁺ could be fitted into two peaks, i.e., two kinds of the chemical states, which occupy the left dodecahedral and the octahedral sites of the garnet structure. From the aforementioned, Mg²⁺ was incorporated into both dodecahedral Y³⁺ and octahedral Al³⁺ sites successfully, and the chemical formula of Mg₂Y₂Al₂Si₂O₁₂ could be represented as {Y₂Mg}[MgAl](AlSi₂)O₁₂, where the {}, [] and () denote dodecahedral, octahedral and tetrahedral coordination, respectively. In conclusion, the XPS results provide additional evidence for the formation of the garnet structure Mg₂Y₂Al₂Si₂O₁₂.

Fig. 3 XPS survey scan (a) and high-resolution Al 2p (b), Si 2p (c), Y 3d (d) and Mg 2p (e) XPS spectra of the host Mg₂Y₂Al₂Si₂O₁₂.

Crystal structure refinement is a powerful tool to obtain detailed crystal structure information on Mg₂Y₂Al₂Si₂O₁₂ and to ensure the phase purity of the sample. Based on the XPS results, the atom positions are set as following: Y³⁺ and half of Mg²⁺ occupy the dodecahedral sites, while the octahedral sites are occupied by the rest of Mg²⁺ and half of Al³⁺, and the tetrahedral sites are occupied by Si⁴⁺ and the rest of Al³⁺. Moreover, as no structural detail of Y_4.67(SiO₄)₃O was reported, the crystallographic data of Gd_4.67(SiO₄)₃O, which has the same structure as Y_4.67(SiO₄)₃O, was used as the starting model.²⁵

Fig. 4 shows the Rietveld refinement result of Mg₂Y₂Al₂Si₂O₁₂, with a final converged weighted-profile of R_wp = 4.00%, by indexing to two phases of YAG and Gd_4.67(SiO₄)₃O. The refinement results show that the main phase of the prepared sample is garnet, with a phase content of 95.6%. The crystallographic data, atomic coordinates and the isotropic parameters of the final refinement of Mg₂Y₂Al₂Si₂O₁₂ and Y_4.67(SiO₄)₃O are summarized in Table 2. The well-converged Rietveld refinement is further proof of the designed element occupancy and the formation of the garnet structure {Y₂Mg}[MgAl](AlSi₂)O₁₂.

Fig. 4 XRD profiles for the Rietveld refinement result of Mg₂Y₂Al₂Si₂O₁₂ and Y_4.67(SiO₄)₃O. The observed intensities (cross), calculated patterns (red line), Bragg positions (tick mark), and the differences plot (blue line) are presented.

Table 2 Crystallographic data, atomic coordinates and isotropic parameters of Mg₂Y₂Al₂Si₂O₁₂ and Y_4.67(SiO₄)O by Rietveld refinement

Atom	x/a	y/b	z/c	S.O.F.	U (Å^2)
a Cubic; space group: Ia3̄d; lattice parameters: a = 11.972 Å; V = 1715.76 Å^3; α = β = γ = 90°; T = 298 K; Z = 8.b Hexagonal; space group: P63/m; lattice parameters: a = b = 9.3294 Å, c = 6.661 Å, V = 579.51 Å^3; α = β = 90°, γ = 120°; T = 298 K; Z = 2. Cu Kα, λ = 1.5418; total reflections = 7171.
Mg2Y2Al2Si2O12^a
Al	0	0	0	0.50	0.03418
Mg	0.25	0.125	0	0.33	0.01919
Y	0.25	0.125	0	0.67	0.01919
Si	0.25	0.375	0	0.67	0.03619
O	0.032	0.053	0.65	1.00	0.03563
Al	0.25	0.375	0	0.33	0.03619
Mg	0	0	0	0.5	0.03418
Y4.67(SiO4)O^b
Y	0.2270	0.2289	0.75	1	0.03233
Y	0.6667	0.3333	0	1	0.03458
Si	0.4714	0.3884	0.25	1	0.06273
O	0.3178	0.4872	0.25	1	0.06742
O	0.6002	0.4740	0.25	1	0.02195
O	0.3418	0.2497	0.0575	1	0.07056
O	0	0	0.25	1	0.04281

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The garnet crystal structure of Mg₂Y₂Al₂Si₂O₁₂ is shown in Fig. 5, which can be depicted as a network of octahedrons and tetrahedrons connected by sharing oxygen ions at the corners of the polyhedral. These polyhedrons are arranged in chains along the three crystallographic directions and form dodecahedral cavities, which are occupied by the Y³⁺ and Mg²⁺ ions. On the basis of the ionic radii in the Mg₂Y₂Al₂Si₂O₁₂ lattice, the Ce³⁺ ions are considered to mainly occupy the distorted dodecahedral sites by substituting the Y³⁺ ions and as being coordinated by eight O²⁻ ions. The interatomic distances in Mg₂Y₂Al₂Si₂O₁₂ are listed in Table 3, and the changes versus that in YAG are in accordance with the ionic radii of the substitute element in each polyhedron. The decreasing Y–O bond distance could be attributing to the substitution of the dodecahedral sites with the smaller Mg²⁺ ion, as well as the compression effect from the bigger neighbouring octahedron. Furthermore, according to the interatomic distance, the difference of the bond length between the two kinds of Y–O in the host Mg₂Y₂Al₂Si₂O₁₂ is greater than that in YAG, suggesting that the dodecahedron becomes more distorted and a red-shift of the Ce³⁺ emission hence is expected.

Fig. 5 Crystal structure of the host Mg₂Y₂Al₂Si₂O₁₂ along the z-axis direction.

Table 3 Selected interatomic bond distances of Mg₂Y₂Al₂Si₂O₁₂^a

	Mg2Y2Al2Si2O12	Y3Al5O12
a Bond distances in Å.
Y–O	2.294	2.317
Y–O	2.417	2.437
Al–O(6)	1.992	1.938
Al–O(4)	1.715	1.754
Mg–O(8)	2.294
Mg–O(8)	2.417
Mg–O(6)	1.992
Si–O	1.715

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Fig. 6 illustrates the XRD pattern of the phosphor Mg₂Y₂Al₂Si₂O₁₂ with different doping concentrations of Ce³⁺ ion. It is obvious that the diffraction peaks of the samples concur with the peaks of Y₃Al₅O₁₂. However, all the diffraction peaks slightly shift to low angles, manifesting that Ce³⁺ ion has been effectively doped into the matrix lattice of Mg₂Y₂Al₂Si₂O₁₂. Because the replacement of Y³⁺ by the lager radius Ce³⁺ results in a bigger d-spacing, the diffraction peaks shift to the low angle according to Bragg's equation. Meanwhile, when the doping concentration of the Ce³⁺ ion increases, the diffraction peaks consistently shift to low angles (see the inset of Fig. 6), demonstrating that the more the Ce³⁺ ion is doped into the matrix lattice, the larger the cell parameters will become. Table 4 reveals such a relationship, as the lattice parameters and cell volumes increase gradually with increasing Ce³⁺ ions for all the samples, which were calculated by cell refinement fitting with GSAS on the basis of the XRD data.²⁶

Fig. 6 XRD patterns of the phosphor Mg₂Y_2−xAl₂Si₂O₁₂:xCe³⁺ (x = 0.01, 0.03, 0.06, 0.09 and 0.12). The inset displays an enlargement of the XRD patterns, showing the shift of the XRD peak (420) on the Ce³⁺ doping concentration.

Table 4 Calculated unit cell parameters of the phosphors Mg₂Y_2−xAl₂Si₂O₁₂:xCe^3+a

	Z = 0.01	Z = 0.03	Z = 0.06	Z = 0.09	Z = 0.12
a Crystal system: cubic, space group: Ia3̄d (no. 088-2048).
a = b = c (Å)	11.973	11.975	11.976	11.980	11.982
Cell volume (Å^3)	1716.36	1717.22	1717.65	1719.37	1720.24
χ^2	6.23	6.45	4.69	6.58	6.76
Rwp (%)	5.19	5.33	4.75	5.53	5.68
Rp (%)	3.47	3.61	3.30	3.70	3.81

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3.2 Photoluminescence properties of phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺

Fig. 7 displays the PLE (λ_em = 599 nm) and PL spectra (λ_ex = 460 nm) of phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺. The emission spectrum of this phosphor has a broad emission peak at 599 nm and shifts to the red side by 57 nm, in comparison with the peak at 542 nm for YAG:Ce³⁺. The position of the Ce³⁺ emission band depends on the covalency (i.e. the nephelauxetic effect), the crystal field strength and the Stokes shift.²⁷ This red-shift could be mainly ascribed to the larger crystal field splitting of the Ce³⁺ 5d levels in the silicate garnet. In the as-synthesized phosphor, [Mg]–[Si] doping leads to a shrinking of the tetrahedral and dodecahedral volumes, and an increase in the octahedral volume. As shown in Table 3, in comparison with the cell volume and bond distance of YAG (V = 1731.85 ų, Y–O bond length: 2.317 Å, 2.437 Å), Mg₂Y₂Al₂Si₂O₁₂ possesses a relatively smaller volume of the unit cell (V = 1715.76 ų) and a smaller dodecahedral [YO₈] (Y–O bond length: 2.294 Å, 2.417 Å). Thus, the shrinking of the dodecahedral volumes will result in a stronger crystal field splitting and a redder emission, i.e. when Ce³⁺ is doped into the Y³⁺ site of phosphor Mg₂Y₂Al₂Si₂O₁₂. Meanwhile, because of the energy transfer between the high energy and low energy parts of the Ce³⁺ ions emission, the Stokes shift of the Ce³⁺ emission in the garnet host may increase. The Stokes shift of phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ is 5100 cm⁻¹, which is larger than that of YAG:Ce³⁺ (3220 cm⁻¹). The large Stokes shift of phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ is another factor accounting for the red shift of the yellow-orange light emission. Moreover, the emission bands in these samples are broader (FWHM = 159 nm) than that of YAG (FWHM = 116 nm), which is due to the inhomogeneous broadening. That is to say, different Ce³⁺ site environments occur in the garnet structure, arising from the dodecahedral site disorder between Y³⁺ and Mg²⁺ in Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺. The larger FWHM means a larger spectral coverage, which leads to higher R_a. In addition, the emission spectrum can be decomposed into two overlapping Gaussian bands, indicating that the ground state energy level of 4f is split by the spin–orbit interaction into the ²F_5/2 and ²F_7/2 levels, where the splitting value in the host Mg₂Y₂Al₂Si₂O₁₂ is about 2300 cm⁻¹. This splitting value is larger than that of YAG, which is also due to the inhomogeneous broadening causing by the dodecahedral site disorder between Y³⁺ and Mg²⁺. The excitation spectrum consists of two broad bands, with peaks at 360 nm and 460 nm, which are assigned to the 4f–5d electronic transitions of the Ce³⁺ ion. From the excitation spectrum, the phosphor can be seen to possess a strong broad absorption band from 440 nm to 480 nm, indicating that the phosphor can be effectively excited by the blue light emitting diode chip.

Fig. 7 PLE and PL spectra of phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺.

The dependence of the emission intensities on the Ce³⁺ doping concentration is shown in the Fig. 8. The emission intensity increases with the increasing Ce³⁺ content, until it reaches a maximum at x = 0.06, and then the emission intensity declines when the concentration of Ce³⁺ goes beyond 0.06 (also see the inset of Fig. 8), because of concentration quenching. Thus, the optimal doping concentration of Ce³⁺ is 0.06. Furthermore, the inset of Fig. 8 also shows the relationship of the peak wavelength with the Ce³⁺ doping concentration. With the increasing of the doping concentration of Ce³⁺, the emission spectra gradually shift to the red side, with the wavelength shift from 584 nm for x = 0.01 to 617 nm for x = 0.12. This indicates that the crystal field splitting of the 5d energy level of the Ce³⁺ ions increases with the increasing Ce³⁺ concentration, which can be supported by the unit cell parameters in Table 4.

Fig. 8 Emission spectra of phosphors with different doping concentration. The inset displays the dependence of wavelength and the intensity of the emission on the concentration of Ce³⁺.

The concentration quenching could be ascribed to the energy transfer via one Ce³⁺ ion to another Ce³⁺ ion, which often occurs as a result of a radiation reabsorption, exchange interaction, or electric multipolar interaction. The mechanism of the radiation reabsorption comes into effect only when there is a broad overlap of the spectra of the excitation and emission. As shown in Fig. 7, the overlap between the spectra of the excitation and emission is less than 2%, suggesting that the radiation reabsorption can be neglected. In this phosphor, when in low concentration, the average distance between the identical Ce³⁺ ions is so large that energy migration does not occur. This critical distance is of great importance, and can be calculated with the following equation:

where N is the number of cations in the unit cell, V is the volume of the unit cell and x_c is the critical concentration. According to the formula of Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺, the critical distance was determined to be 19.0 Å. The exchange interaction is generally responsible for the energy transfer of forbidden transitions and the typical critical distance is less than 5 Å. Therefore, the process of energy transfer between Ce³⁺ ions in phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ should be controlled by the electric multipolar interaction according to Dexter's theory.^28,29

3.3 Temperature-dependent photoluminescence, CIE and quantum efficiency of phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺

For application in high power LEDs, the thermal quenching property of phosphor is an important parameter to be considered. The thermal quenching of phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ was evaluated by measuring the temperature-dependent emission from 25 °C up to 250 °C. Temperature-dependent emission spectra for this phosphor under excitation at 460 nm were thus investigated, and are shown in Fig. 9. With the increasing temperature, the emission intensity remains relatively stable at first, but then declines to 84.7% efficiency at 160 °C, compared with the T = 25 °C emission, as shown in the inset of Fig. 9. The peak positions of the emission spectra slightly red shift and the peaks show a thermal broadening phenomenon.³⁰

Fig. 9 Temperature-dependent photoluminescence of Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ under excitation of 460 nm. The inset displays the dependence of the emission intensity on the increasing temperature.

Also, the emission intensity of YAG:Ce³⁺ declines to 89% when the temperature increases to 160 °C. The thermal stability of YAG:Ce³⁺ is better than that of Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺. The quenching activation energy (E_a – energy barrier for thermal quenching) reduces when the Stokes shifts between 4f–5d absorption and then the 5d–4f emission gradually increases.¹⁵ Because the Stokes shift of Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ is larger than that of YAG:Ce³⁺, thermal quenching is more likely to occur.

The CIE chromaticity diagram for the sample Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ under excitation at 460 nm is shown in Fig. 10. The colour coordinate of the present phosphor is (x = 0.519, y = 0.472), which is located in the yellow-orange region, indicating that warm light with low CCT could be expected when coupling this yellow-orange light emitting phosphor with a blue LED chip. The quantum efficiency (QE) of phosphor is considered to be another important parameter for practical application, and the main two factors leading to low quantum efficiency are concentration quenching and thermal quenching. The quantum efficiency data were measured for the phosphors with different Ce³⁺ doping concentrations. Also, the quantum efficiency of phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ was measured to be 57% at room temperature. As a reference, the quantum efficiency value of the commercial yellow phosphor YAG:Ce³⁺ was measured with the same instruments, and the value was found to be 73% under 460 nm excitation. Many factors will affect the QE, such as the excitation wavelength, the synthesis conditions and particle size, and also further optimizing the synthesis conditions is necessary to improve the QE.

Fig. 10 CIE coordinates of the phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ and the commercial phosphor YAG:Ce³⁺.

3.4 Electroluminescence properties of phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺

To study the practical application of Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ for a phosphor-converted white LED, LED lamps were fabricated using phosphor Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ with blue InGaN chips (455 nm). The electroluminescence spectra of the obtained LED lamp are shown in Fig. 11. A bright white light was observed with the naked eye, and the inset of Fig. 11 shows the photograph of the lamp with a driven forward bias DC current of 300 mA. The colour rendering index, chromaticity coordinates, correlated colour temperature and luminous efficacy of Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ were found to be 82.4, (0.3389, 0.3001), 5091 K and 18.30 lm W⁻¹, respectively, indicating that the Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ phosphor-converted white LED had a high CRI and low CCT. However, the luminous efficacy was low, which was due to the relatively low quantum efficiency and the irregular shape of the particles with large size distribution. A further improvement of conversion efficiency would be possible by optimizing the lamp fabrication and phosphor synthesis. For application in high power LEDs, the effect of current stability on the light output and the CRI of the phosphor should be considered. The current was tested from 100 mA to 500 mA to power the LED device. The electroluminescence properties, including the colour rendering index, chromaticity coordinates and correlated colour temperature, are listed in Table 5. From Fig. 11, it can be seen that the shift of emission peak is negligible and the intensity does not become saturated. Meanwhile, the colour rendering index, chromaticity coordinates and correlated colour temperature change slightly as the current increases. These results reveal that phosphor-converted white LEDs would be stable for use in applications.

Fig. 11 Electroluminescent spectra of Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ phosphor-converted WLEDs under various drive currents from 100 mA to 500 mA.

Table 5 Colour rendering index (R_a), colour coordinates and correlated colour temperature of the Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ phosphor-converted WLED under various currents

Current (mA)	Coordinate	CCT (K)	CRI (Ra)	Luminous efficacy (lm W^−1)
100	(0.3493, 0.3129)	4620	81.0	27.54
150	(0.3466, 0.3097)	4730	81.2	26.31
200	(0.3436, 0.3061)	4860	81.6	23.42
250	(0.3414, 0.3035)	4967	82.0	20.47
300	(0.3389, 0.3001)	5091	82.4	18.30
350	(0.3368, 0.2975)	5206	82.7	17.68
400	(0.3343, 0.2946)	5356	83.3	15.87
450	(0.3306, 0.2896)	5599	84.2	15.20
500	(0.3279, 0.2863)	5796	84.7	13.56

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4 Conclusions

In conclusion, a novel Ce³⁺-activated yellow-orange light emitting phosphor Mg₂Y₂Al₂Si₂O₁₂ was prepared by the combination of the substitution of both dodecahedral Y³⁺ and octahedral Al³⁺ sites with Mg²⁺. The crystal structure of the host Mg₂Y₂Al₂Si₂O₁₂ was illustrated to be {Y₂Mg}[MgAl](AlSi₂)O₁₂, where { }, [ ] and ( ) denote the dodecahedral, octahedral and tetrahedral coordinations, respectively. Moreover, the large distortion of the dodecahedron and an external pressure from the large octahedron lead to a large crystal field splitting, and hence, Mg₂Y_1.94Al₂Si₂O₁₂:0.06Ce³⁺ shows a broad emission band with CIE chromaticity coordinates of x = 0.519, y = 0.472. Owing to the larger FWHM and the redshift of the emission band, a high colour rendering index white LED lamp was achieved by combining the phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ with a blue LED chip. Our results demonstrate that phosphor Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ is a promising candidate for WLED.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Project no. 10804099), the Natural Science Foundation of Zhejiang Province (Project no. Y4110536), the Key Science and Technology Innovation Team of Zhejiang Province (2009R50002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14425b

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