A broad-band orange-yellow-emitting Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor for application in warm white light-emitting diodes

Abstract

A new garnet-type Lu₂Mg₂Al₂Si₂O₁₂ compound was successfully synthesized via high-temperature solid–state reaction. The crystal structure was confirmed by the Rietveld refinement method and belongs to a cubic system with a space group of Ia3̄d (230) and cell parameters of a = b = c = 11.88310(1) Å. There are two different crystallographic positions for Mg²⁺ with coordination numbers of eight and six, respectively. A relatively large band gap (4.83 eV) was estimated by the diffuse reflectance spectrum, indicating that Lu₂Mg₂Al₂Si₂O₁₂ may be a promising host matrix for luminescent materials. The synthetic Ce³⁺-doped Lu₂Mg₂Al₂Si₂O₁₂ phosphor shows intense and broad-band orange-yellow emission peaking at about 575 nm under near-ultraviolet (n-UV) and blue light excitation. Additionally, the quantum efficiency, thermal stability, and packing performance were investigated to evaluate the practical use in white LEDs. The results indicate that Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ may be a promising orange-yellow phosphor for warm white LEDs.

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1. Introduction

White light-emitting diodes (w-LEDs) have recently received intensive attention because of their superior advantages, such as high efficiency, low power consumption, great stability, and eco-friendliness.^1–3 Combination of the yellow-emitting phosphor Y₃Al₅O₁₂: Ce³⁺ (YAG: Ce³⁺) with a blue InGaN chip is considered to be one of the most common and efficient ways to generate w-LEDs.⁴ However, this combination usually generates cool white light with a modest color-rendering index (CRI, R_a) of ∼75 due to the lack of red emission.^5–8 Thus, there is a drive to develop new orange-yellow-emitting phosphors that show a broad-band emission with an intensified red component to obtain warm w-LEDs.^9,10

As an important part of w-LEDs, phosphors are required to exhibit a high quantum efficiency and small thermal quenching. According to the work of Seshadri and co-workers, a large Debye temperature (Θ_D) could be a predictor of high quantum efficiency (QE), and a wide band gap (E_g) of host is necessary to limit thermal quenching.¹¹ Typically, the well-known yttrium aluminium garnet (YAG) matrix owns a large Debye temperature (726 K) and a wide band gap (6.43 eV); and correspondingly, YAG: Ce³⁺ phosphor shows a high QE (∼90) and small thermal quenching.^11,12 Furthermore, Ce³⁺-doped compositional derivatives of YAG with these outstanding characteristics have also been reported and lead to a wide variety of applications, such as the commercial green Lu₃Al₅O₁₂: Ce³⁺ and Ca₃Sc₂Si₃O₁₂: Ce³⁺ phosphors.^13,14 Therefore, it is preferable for many researchers to focus on the garnet structure when searching for new phosphor hosts.

Garnet structure belongs to the cubic system (space group of Ia3̄d) with a general formula {C}₃[A]₂(D)₃O₁₂. Wherein, {C}, [A], and (D) represent the cations that occupy 24c, 16a, and 24d Wychoff positions and are surrounded by 8, 6, and 4 oxygen atoms to form dodecahedron, octahedron, and tetrahedron, respectively.^15–17 The solid solution design for the host compound through cation/anion substitution, cosubstitution, or chemical-unit substitution is well employed for the photoluminescence tuning.^18–20 Especially, garnet phosphors exhibit unique tunability of luminescence properties by chemical substitutions on the {C}, [A], and (D) cation sublattices.^21,22 On that basis, a large amount of Ce³⁺-doped garnet hosts with various emission colors have been developed and reported, such as cyan Ca₂LuHf₂(AlO₄)₃: Ce³⁺ and Ca₃Zr₂SiGa₂O₁₂: Ce³⁺ phosphors, green CaLu₂Al₄SiO₁₂: Ce³⁺ and Mg_1.5Lu_1.5Al_3.5Si_1.5O₁₂: Ce³⁺ phosphors, yellow Y₃Mg_xSi_xAl_5−2xO₁₂: Ce³⁺ and Lu_3−xY_xMgAl₃SiO₁₂: Ce³⁺ solid solution phosphors, and orange Lu₂CaMg₂(Si,Ge)₃O₁₂: Ce³⁺ phosphor.^23–29 It is worth noticing that Mg²⁺ can not only be incorporated into [A] sites, such as Y₃Mg_xSi_xAl_5−2xO₁₂: Ce³⁺, but also be introduced in {C} sites, such as Mg_1.5Lu_1.5Al_3.5Si_1.5O₁₂: Ce³⁺. However, Mg²⁺–Si⁴⁺ incorporation into the [A] and (D) sites, respectively, leads to the red shift of Ce³⁺ emission, but Mg²⁺–Si⁴⁺ substitution on the {C} and (D) sites, respectively, can shift the Ce³⁺ emission to the short wavelength.^26,27 The above results indicate that Mg²⁺ incorporation into dodecahedral or octahedral sites has totally different effects on the coordination environment for Ce³⁺. However, there is few research on the incorporation of Mg²⁺ into both {C} and [A] sites and its effect on luminescence properties.

Motivated by the above knowledge, we designed a new Lu₂Mg₂Al₂Si₂O₁₂ composition based on the typical Lu₃Al₅O₁₂ matrix, where a Lu³⁺ ({C} site) as well as an Al³⁺ ([A] site) are substituted by two Mg²⁺, and for charge neutrality two other Al³⁺ ((D) site) are simultaneously replaced by two Si⁴⁺. The Rietveld refinement results confirmed that Mg²⁺ ions were incorporated into both dodecahedral and octahedral sites as designed. And the luminescence properties of Ce³⁺-doped Lu₂Mg₂Al₂Si₂O₁₂ were investigated systematically. The obtained Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor shows intense orange-yellow emission containing more red component than that of YAG: Ce³⁺ under blue light excitation. Furthermore, the temperature-dependent photoluminescence behaviours as well as the fabrication properties of the phosphor were studied to evaluate the applicability as a color converter for w-LEDs.

2. Experimental

2.1. Materials and synthesis

Polycrystalline samples of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) were synthesized by high-temperature solid–state reaction. The starting materials (Lu₂O₃, 99.99%; MgO, 99.95%; Al₂O₃, 99.99%; SiO₂, 99.99%; CeO₂, 99.995%) were all weighed out in the desired stoichiometry and thoroughly mixed. The mixtures were put in an alumina crucible and continually fired at 1400 °C in a reducing atmosphere of H₂/N₂ (3/1) for 6 h. Then the samples were furnace-cooled continuously to room temperature, and ground well for the subsequent measurements. The commercial YAG: Ce³⁺ phosphor used for comparison was manufactured by Grirem Advanced Materials Co., Ltd. The blue InGaN chips (λ_ex = 455 nm) were purchased from Hang Zhou Silan Azure Co., Ltd.

2.2. Characterization methods

The phase structure determination of the as-prepared samples were carried out by a powder X-ray diffractometer (Rigaku, Japan) with Co-Kα radiation (λ = 0.178752 nm), and the date were recorded in the 2θ range of 10–80° with scan step of 6° min⁻¹. The XRD data, for structural refinement, of Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺ were collected on a powder X-ray diffractometer (PANalytical, Holland) with Cu-Kα radiation (λ = 0.154056 nm) in the 10–120° range (2θ) with scan speed of 0.0170° s⁻¹. Diffuse reflection (DR) spectra were detected on an UV-Vis-NIR spectrophotometer (SHIMADZU, Japan) with an integrated sphere attachment, and BaSO₄ powder sample served as a standard. The room-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra, as well as the temperature-dependent photoluminescence properties, were measured by a Fluoromax-4 spectrofluorometer (Edison, USA) equipped with a 200 W xenon lamp, a photomultiplier tube operating, and a heating attachment. The fluorescent decay curves were measured on a TemPro-01 time-resolved fluorescence spectrofluorometer (Horbia Jobin Yvon, France). The quantum efficiency (QE) value was measured by an intensified multichannel QE-2100 spectrometer (Otsuka Electronics, Japan) equipped with a Xe lamp. The Commission International de I'Eclairage (CIE) chromaticity coordinates of samples were determined by a DT-101 colour analyser with a CCD detector (Laiko, Japan).

3. Results and discussion

3.1. Phase and crystal analysis

Fig. 1 presents the XRD patterns for the series of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ samples with different doped concentrations of Ce³⁺ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12). The diffraction peaks of all these samples could be indexed to the Ia3̄d space group of the cubic system, and no diffraction peak of impurities was observed. It indicates that the single-phased Lu₂Mg₂Al₂Si₂O₁₂ compound can be attained by the given synthesis conditions and the dopant concentration of Ce³⁺ in the designated range didn't incur any impurities or cause significant structure change in the host. Moreover, the diffraction peaks slightly shift to the low-angle side with the increasing Ce³⁺ concentration (as shown in the right panel of Fig. 1), meaning the expansion of the lattice caused by the substitution of Lu³⁺ (r^VIII = 97.7 pm) by Ce³⁺ (r^VIII = 114.3 pm).³⁰

Fig. 1 XRD patterns of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ samples with different Ce³⁺ (0 ≤ x ≤ 0.12). Right panel: the magnified patterns in the 2-theta range of 31.5–33.5°.

To confirm the phase structure and obtain detailed crystallographic information of the as-prepared samples, Rietveld structural refinement of XRD patterns for the selected Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺ sample was performed (as shown in Fig. 2) by GSAS software package.^31,32 In the process, the single-crystal structure data of Lu₃Al₅O₁₂ (ICSD no. 23846) were used as an initial model. The refinement process was stably convergent to χ² = 6.30, R_wp = 4.04%, R_p = 5.55%, which makes sure the formation of a single garnet phase. The final refined structure data and residual factors are listed in Table 1. Wherein, the unit cell parameters were a = b = c = 11.88310(1) Å and V = 1677.991(3) ų.

Fig. 2 Rietveld refinement XRD patterns of the selected Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺ sample.

Table 1 Main results of Rietveld refinement of the Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺ sample

Crystal system	cubic
Space group	Ia3̄d
a = b = c (Å)	11.88310(1)
α = β = γ (deg)	90
V (Å^3)	1677.991(3)
Z	8
Rwp (%)	4.04
Rp (%)	5.55
χ^2	6.30

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Atom	Site	Occu.	x	y	z
Lu	24c	0.6467	0.12500	0.000000	0.25000
Ce	24c	0.02000	0.12500	0.000000	0.25000
Mg1	24c	0.3333	0.12500	0.000000	0.25000
Mg2	16a	0.5000	0.00000	0.000000	0.00000
Al1	16a	0.5000	0.00000	0.000000	0.00000
Al2	24d	0.3333	0.37500	0.000000	0.25000
Si	24d	0.6667	0.37500	0.000000	0.25000
O	96h	1.0000	−0.03863	0.052202	0.15270

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From the refinement results, Fig. 3(a) displays a schematic diagram of the Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺ garnet structure. The dodecahedral, octahedral, and tetrahedral sites are randomly occupied by Lu/Mg/Ce (1.94/1/0.06), Mg/Al (1/1) and Al/Si (1/2), coordinated by eight, six and four oxygen ions, respectively. In addition, the [(Lu/Mg/Ce)O₈] dodecahedron connects with neighbour [(Mg/Al)O₆] octahedrons or other dodecahedrons by sharing edge, but connects with part of neighbour [(Al/Si)O₄] tetrahedrons by sharing edge and the other by sharing one O point, as shown in Fig. 3(b). Actually, each [(Lu/Mg/Ce)O₈] dodecahedron is surrounded by four [(Lu/Mg/Ce)O₈] dodecahedrons, four [(Mg/Al)O₆] octahedrons and six [(Al/Si)O₄] tetrahedrons, indicating that the photoluminescence of Ce³⁺ would be affected by the floating ratio of Lu/Mg/Ce, Mg/Al and Al/Si in surrounding dodecahedrons, octahedrons and tetrahedrons, respectively.

Fig. 3 (a) Structure of Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺ along a axis and (b) the connection between polyhedrons.

3.2. Photoluminescence properties

Fig. 4 depicts the diffuse reflectance (DR) spectra of the as-prepared Lu₂Mg₂Al₂Si₂O₁₂ host and Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor. For the Lu₂Mg₂Al₂Si₂O₁₂ host, the DR spectrum shows an intense reflection in the visible range (380–780 nm), and a drastic decrease in the ultraviolet range (200–350 nm) originating from the valence-to-conduction transitions. The band gap value (E_g) of the Lu₂Mg₂Al₂Si₂O₁₂ host can be estimated by the following equation:^33,34

[F(R_∞)hν]ⁿ = A(hν − E_g) (1)

where hν and A refer to the photon energy and a proportional constant, respectively; R_∞ = R_sample/R_standard; n = 1/2 for an indirect transition or 2 for a direct transition; and F(R_∞) is the remission (Kubelka–Munk) function, defined as³⁵

F(R∞) = K/S = (1 − R_∞)²/2R_∞ (2)

where K, and S represent the absorption and scattering coefficient, respectively. Through the linear extrapolation of [F(R_∞)hν]ⁿ = 0 (as shown in the inset of Fig. 4), the E_g value was determined to be about 4.83 eV. When 8 mol% of Ce³⁺ was doped into the Lu₂Mg₂Al₂Si₂O₁₂ host lattice, the DR spectrum exhibits enhanced absorption and two broad valleys (300–350 nm and 400–500 nm), which is attributable to the 4f–5d transitions of Ce³⁺ and consistent with the excitation spectrum.

Fig. 4 Diffuse reflectance spectra of Lu₂Mg₂Al₂Si₂O₁₂ host and Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor. Inset: the band gap calculation of Lu₂Mg₂Al₂Si₂O₁₂ host.

Fig. 5 shows the room-temperature photoluminescence excitation (PLE) and emission (PL) spectra of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺. The PLE spectrum (monitored at 575 nm), ranging from 300–540 nm, possesses a strong excitation band (about 370–540 nm) peaking at about 436 nm and a weak excitation band (about 300–360 nm) peaking at about 325 nm, corresponding to transitions of Ce³⁺ from the ground state (4f) to the two lowest excited levels (5d₁ and 5d₂, respectively). As also shown in Fig. 5, the PL spectrum of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ under 436 nm excitation exhibits an asymmetric broad band (about 480–750 nm) peaking at 575 nm assigned to the 5d–4f transitions of Ce³⁺.^36,37 The full width at half maximum (FWHM) of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ is calculated to be about 144 nm. Moreover, the asymmetric emission band can be resolved into two Gaussian profiles with peaks centered at 554 nm and 614 nm, respectively, which are attributed to the transitions from the lowest 5d excited state to the ²F_5/2 and ²F_7/2 ground states.^38,39 And the energy difference Δk between these two bands is about 1764 cm⁻¹, which is close to the theoretical energy difference between the ²F_5/2 and ²F_7/2 levels (∼2000 cm⁻¹).^40,41

Fig. 5 PLE (λ_em = 575 nm) spectrum and PL (λ_ex = 436 nm) spectrum of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺.

In order to optimize the doping concentration of Ce³⁺ ion for this new garnet phosphor, the PL (λ_ex = 436 nm) spectra of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ samples with different concentrations of Ce³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.1, 0.12) were measured as shown in Fig. 6. The emission intensity increases with Ce³⁺ concentration increasing at first, reaches a maximum when x = 0.08, and then declines when Ce³⁺ concentration exceeds this critical concentration (also see the inset of Fig. 6) on account of concentration quenching effect.⁴² The concentration quenching mainly results from non-radiative energy transfer among Ce³⁺ ions, whose occurring possibility increases as the content of Ce³⁺ increases. Three main mechanisms responsible for non-radiative energy transfer are exchange interaction, radiation reabsorption and multipolar interaction, respectively.^43,44 To make clear which mechanism dominates, it is requisite to obtain the critical distance (R_c) of energy transfer. The critical distance (R_c) among Ce³⁺ ions can be estimated based on the following formula:^45,46

where V, x_c, and N refer to the unit cell volume, the critical concentration of activator, and the number of crystallographic sites in the unit cell that can be occupied by activator ions, respectively. In this case, the values V = 1677.991(28) ų, x_c = 0.08, and N = 24, and the obtained R_c value is 11.86. It's less likely that the exchange interaction mechanism plays the dominant role for the non-radiative energy transfer, because this mechanism generally requires a forbidden transition (the R_c value is ∼5 Å).⁴⁷ Besides, the radiation reabsorption mechanism has an obvious effect only when there is a considerable overlap between the excitation and emission spectra. Thus, it is deduced that the electric multipolar interaction should take the main responsibility for non-radiative energy transfer between activator ions for Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor.

Fig. 6 PL spectra of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ (0.02 ≤ x ≤ 0.12). Insert: the dependence of emission intensity and peak wavelength on Ce³⁺ concentration.

Furthermore, the inset of Fig. 6 also depicts the dependence of peak wavelength on Ce³⁺ dopant concentration. It can be found that the peak wavelength of emission spectra gradually red-shifts from 561 nm for x = 0.02 to 580 nm for x = 0.12. The redshift could be ascribed to the following two factors. First, the possibility of energy transfer from the Ce³⁺ ions at higher-energy sublevels of 5d to those at lower-energy sublevels increases with the increasing Ce³⁺ ion content, reducing the probability of the higher-energy emission. Second, as the substitution for smaller Lu³⁺ ions by larger Ce³⁺ ions increases, the crystal field strength surrounding Ce³⁺ ions enlarges, which causes a larger splitting of 5d levels and thus decreases the emission energy from the lowest excited level (5d₁) to the ground state (4f) of Ce³⁺.²³ For these two factors, it is observed that the emission band of Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ gradually red-shifts as the Ce³⁺ ion content increases.

Fig. 7 depicts the normalized PL spectra of the orange-yellow-emitting Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ and the commercial yellow-emitting YAG: Ce³⁺ under 450 nm excitation. It is observed that the orange-yellow-emitting Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor prepared by us contains more red-component emission than the commercial YAG: Ce³⁺ phosphor. In addition, the bandwidth of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ (FWHM = 144 nm) is wider than that of YAG: Ce³⁺ (FWHM = 124 nm). The results indicate that higher-quality white light obtained by coupling 455 nm blue InGaN chips with the orange-yellow-emitting Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor is expected. In addition, the particularly broad emission band of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor may be due to the diverse local distortions of Ce³⁺. Since the dodecahedral, octahedral and tetrahedral sites near Ce³⁺ ions are occupied by Lu³⁺/Mg²⁺, Mg²⁺/Al³⁺ and Al³⁺/Si⁴⁺, respectively, the different distribution ratio of cation ions around Ce³⁺ results in the site-to-site variation of local crystalline environment and accordingly inhomogeneous broadening of the Ce³⁺ emission spectra.⁴⁸

Fig. 7 Normalized PL spectra of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ and YAG: Ce³⁺.

Fig. 8 shows the fluorescence decay curves of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ phosphors with varying Ce³⁺ contents (x = 0.02, 0.04, 0.06, 0.08, 0.1, 0.12) excited at 436 nm and monitored at 570 nm. It was found that these decay curves had parallel decay behaviours and all the curves can be fitted well by the mono-exponential function:⁴⁹

I(t) = A₁ exp(−t/τ) (4)

where I(t) and A₁ represent the photoluminescence intensity at time t and the initial intensity immediately after the exciting pulse, respectively, and t refers to fluorescence lifetime. And the decay times were determined by the fitting to be 66.8, 65.6, 64.4, 63.9, 62.9 and 62.2 ns for Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.1, 0.12) phosphors, respectively. As Ce³⁺ concentration increases, both the possibility of energy transfer from Ce³⁺ ions to quenching sites and the energy transfer rate between Ce³⁺ ions increase, resulting in the decrease of the lifetime.⁵⁰ In addition, the well-fitting results by mono-exponential decay illustrate that Ce³⁺ ions only occupy one crystallographic site (24c) in the Lu₂Mg₂Al₂Si₂O₁₂ host in accordance with the Rietveld refinement result of Lu_1.94Mg₂Al₂Si₂O₁₂: 0.06Ce³⁺.

Fig. 8 Decay curves and calculated lifetime values of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ phosphors with different Ce³⁺ concentrations (0.02 ≤ x ≤ 0.12).

The temperature-dependent performance of phosphor is one of the significant technological parameters, which has a great effect on the luminous efficacy and color rendering performance of the w-LEDs, especially for phosphor-converted w-LEDs based on blue chips. The temperature-dependent emission spectra of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor under 436 nm excitation are presented in Fig. 8. As temperature rises from 25 to 200 °C, the emission intensity gradually declines due to the thermal quenching. All the spectral shape keeps similar with a weak blue shift (575 nm at 25 °C vs. 571 nm at 200 °C). The blue shift can be ascribed to the thermally activated phonon-assisted excitation from a lower-energy sublevel to a higher-energy sublevel of Ce³⁺ in the excited state.⁵¹ The variation of emission intensity with temperature is also shown in the inset of Fig. 9. When the temperature reaches 150 °C, the emission intensity of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor drops to 74.9% of its initial intensity, indicating that this phosphor could exhibit excellent performance in the operation temperature range of w-LEDs. In case of the thermal quenching process only generated by the nonradiative relaxation process, the activation energy (barrier energy for thermal quenching) can be calculated by the Arrhenius equation:^52,53

where I₀ is the initial emission intensity; I_T represents the emission intensity at different testing temperatures; c is a constant; ΔE is the activation energy of thermal quenching; and k is the Boltzmann constant (8.629 × 10⁻⁵ eV K⁻¹). By linear fitting the relationship of ln[(I₀/I_T) − 1] vs. 1000/T with a slope of −4.075 got, the activation energy was calculated to be 0.351 eV (as shown in Fig. 10). The relatively high activation energy of this phosphor also illustrates its excellent thermal properties.

Fig. 9 Temperature-dependent PL (λ_ex = 436 nm) spectra of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor. Insert: the relative emission intensity of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor varying with temperature.

Fig. 10 Arrhenius fitting of the PL intensity of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor as a function of temperature.

3.3. CIE chromaticity coordinates, quantum efficiency, and w-LED device performance

The CIE chromaticity coordinates of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ with different Ce³⁺ doped concentrations (0.02 ≤ x ≤ 0.12) under 436 nm excitation are depicted in Fig. 11. The color coordinates are shown as ranging from (0.458, 0.516) for x = 0.02 to (0.493, 0.493) for x = 0.12, which means that these phosphors may serve as orange-yellow-emitting phosphors for w-LEDs applications. Besides, the image of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor under blue light radiating is also presented in the inset of Fig. 11, and intense orange-yellow light can be clearly caught by the naked eye. Furthermore, the internal and external QE of the optimal Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor under 436 nm excitation are measured to be about 72.6% and 53.3%, respectively. Generally, the QE value could be further improved by optimizing the preparation conditions.

Fig. 11 The CIE coordinates of Lu_2−xMg₂Al₂Si₂O₁₂: xCe³⁺ (0.02 ≤ x ≤ 0.12) and the image of Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor under blue light lamp.

The Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor is great appropriate as a color converter for blue chip w-LEDs due to its strong blue light absorption and orange-yellow light emission. To further demonstrate the potential of Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor for pc-wLEDs application, the Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor and a blue InGaN chip (λ_ex = 455 nm) were used to fabricate a w-LED device. Fig. 12 presents the electroluminescence (EL) spectrum of the as-fabricated w-LED device under 50 mA current operating. The device emits warm bright light with the CIE chromaticity coordinates of (0.3485, 0.3216) and relatively low correlated colour temperature (CCT) of 4718 K. And the colour rendering index (R_a) of this w-LED lamp was checked out to be 84.5, which is higher than that of the w-LEDs using blue chips pumped by YAG: Ce³⁺ phosphor (R_a = 75).⁵⁴ Consequently, Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ displays significant potential as a w-LED phosphor, achieving high R_a and low CCT without blending additional red phosphor.

Fig. 12 Electroluminescence spectrum of a w-LED consisting of a blue InGaN chip and the Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor. Insert: as-prepared w-LED and w-LED in operation.

4. Conclusions

In summary, a new phosphor Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ has been successfully synthesized via the high-temperature solid–state reaction method. Rietveld structural refinement results make sure the formation of a single garnet phase. Under excitation with n-UV to blue light, the Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor exhibited intense broad-band orange-yellow emission (peaking at about 575 nm, FWHM = 144 nm) containing more red component than that of YAG: Ce³⁺. The optimal dopant concentration of Ce³⁺ was x = 0.08, of which the internal quantum efficiency was about 72.6%. The corresponding concentration quenching mechanism is considered to be the electric multipolar interaction. When heated up to 150 °C, the Lu_1.92Mg₂Al₂Si₂O₁₂: 0.08Ce³⁺ phosphor still remained at about 75% of the emission intensity at room temperature. The w-LED device fabricated with a blue chip and the Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ phosphor displayed the CIE color coordinate of (0.3485, 0.3216), CCT of 4718 K, and R_a value of 84.5. Those results indicate that Lu₂Mg₂Al₂Si₂O₁₂: Ce³⁺ may be a promising candidate for applications in n-UV and blue light excited warm white LEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (2014CB643801).

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