Tunable emission color and mixed valence state via the modified activator site in the AlN-doped Sr₃SiO₅:Eu phosphor

Abstract

Commercially white LEDs have cool white light and a low color rendering index (CRI) due to the absence of red light emission. Considering the respective characteristic luminescence properties of Eu²⁺ and Eu³⁺ ions, the combination of the two activators into a single host compound shows promisingly technical feasibility to realize warm w-LEDs with a high CRI but it is still a challenge in practice. Here, we confirmed the mixed europium valence states in Sr₃Si_1−xAl_xO_5−2xN_x:Eu phosphors, synthesized by a solid-state reaction method under a H₂/N₂ atmosphere. The crystal structure was characterized by X-ray diffraction and found to crystallize in the Sr₃SiO₅ phase. The samples show broad excitation bands (250–400 nm) and tunable emission colors from yellow to red, and the CIE chromaticity coordinates of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0, 0.03, 0.2) were (0.518, 0.474), (0.607, 0.385) and (0.630, 0.367), respectively. The modification of activator sites caused by Al³⁺–N³⁻ replacing Si⁴⁺–O²⁻ explained the coexistence of Eu²⁺/Eu³⁺ ions, which was demonstrated by Rietveld refinement, photoluminescence analysis and fluorescence lifetime studies. Furthermore, the obtained phosphors also possessed good thermal stabilities. These results suggest that the Sr₃Si_1−xAl_xO_5−2xN_x:Eu phosphors show potential applications in w-LEDs.

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1 Introduction

Due to fossil fuel depletion and global climate change, all countries call for concerted actions to realize energy conservation and carbon reduction. As one of the representatives in the technical field, solid-state lighting (SSL) offers a promising alternative to conventional lighting sources such as incandescent bulbs and fluorescent tubes. White light-emitting diodes (w-LEDs) have gained general consensus for being the next-generation lighting source owing to its high energy efficiency, long lifetime, short response time, energy savings, environmental friendliness, and material hardness.^1–3 Currently, white LEDs typically comprise a blue InGaN chip and yellow (Y,Gd)₃(Al,Ga)₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphors. But with long-wavelength spectral emission lacked, this phosphor-converted LEDs (pc-LEDs) solution suffers from high color temperature and low color rendering index.⁴ Nitride phosphors, such as CaAlSiN₃:Eu²⁺ and M₂Si₅N₈:Eu²⁺ (M = Ca, Sr, Ba), are commonly used as red-lighting compensation for w-LEDs. But the rather broad emission bands of these materials greatly limits the maximum achievable luminous efficacies of high-quality warm-white pc-LEDs because a significant part of the light is produced outside the sensitivity range of the human eye.^5,6 On these accounts, novel phosphor materials with optimized luminescence properties are urgently required.

Among all rare earth ions, europium is the most well-known activator, and both the divalent or trivalent state ions have found enormous applications in lighting and display fields because of their abundant emission colors based on the 5d–4f or 4f–4f transitions.⁷ Since the f–d transitions of Eu²⁺ seriously depend on the crystal field effects imposed by the host lattice, its emissions in inorganic phosphors usually occur as broad emission bands and the color can range from ultraviolet to deep red. And Eu³⁺ ions, frequently used as red-emitting activator, present a series of characteristic sharp emission bands assigned to the electronic transitions of ⁵D₀ → ⁷F_J (J = 0, 1, 1, 2, 3 and 4). Yet Eu³⁺-activated phosphors often show low absorption efficiency because the 4f–4f transitions are parity-forbidden.^8,9 Earlier research pointed that combining Eu²⁺ and Eu³⁺ ions in a single host lattice could be a possibility to generate white light with superior color coordinates, which avoid the limit of spectra adjustment and energy loss in multiple rare-earth co-doped phosphors.¹⁰ But until now, relative few researches have been conducted on the coexistence of Eu²⁺ and Eu³⁺ ions in a single host lattice. Initial studies about concomitance of the two activators found that Eu²⁺ in some special compounds, which have stiff three-dimensional enclosed crystal structures such as BO₄, SO₄, AlO₄, SiO₄ or PO₄ tetrahedron groups, can be obtained by synthesis in air at high temperature.^11–14 Later, similar phenomenon of mixed valance states were found in LaAlO₃:Eu and La_1−xSr_xAlO₃:Eu, meanwhile the Eu²⁺ ions appear to substitute the trivalent rare earth sites, hitherto, which is still hard to achieve even now.^15,16 An abnormal reduction mechanism for above Eu³⁺ to Eu²⁺ was proposed on the basis of charge compensation, in which these tetrahedron groups play the electron transferring roles. Recently, a new strategy based on crystal-site engineering, such as designing the host composition with two types of sites for various valence' cations to occupy or choosing appropriate dopants to tune the valence state via controlling the activator site, was reported to realize the coexistence of Eu²⁺/Eu³⁺ in Ba₂(Ln_1−zTb_z)(BO₃)Cl:Eu (Ln = Y, Gd and Lu), Ca₁₂Al_14−zSi_zO₃₂F_2−z:Eu and Ca_1+xY_1−xAl_1−xSi_xO₄:Eu.^17–20 Actually, this crystal-site engineering approach has been often used to design inorganic functional materials because it can customize the photoluminescence properties of a given inorganic substance through changing the coordination environment at the site occupied by the luminescent center ion in the crystal.^21–23

Due to its high luminous efficiency and thermal stability, Sr₃SiO₅:Eu²⁺ can also function as an excellent yellow-orange phosphor for w-LEDs application.²⁴ However, as with available YAG:Ce³⁺ phosphor, the photoluminescence of Sr₃SiO₅:Eu²⁺ phosphor lacks necessary red-emission parts. Hence, efforts have been devoted to improving the intensity in red spectral region through partial substitution like (Sr,Ba,Mg)₃SiO₅:Eu²⁺ and Sr₃SiO₅:Ce³⁺,Eu²⁺,Al³⁺.^25–28 In this work, we synthesized a series of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.5) phosphors under a reducing atmosphere. The obtained samples presented not only broad excitation bands (ranging from 250 to 400 nm) but also tunable emission colors, originated from the characteristic emissions of Eu²⁺ and Eu³⁺. The coexistence of mixed valence states was confirmed through photoluminescence (PL) and fluorescence lifetime analysis. We investigated the effects of Al³⁺–N³⁻ substitution on Sr₃SiO₅:Eu²⁺ structure by Rietveld refinement analysis. The valence change results from structural modification-induced and the valence stability of europium was explained based on crystal-site engineering. Especially, the Eu²⁺/Eu³⁺ activated Sr_2.97Si_1−xAl_xO_5−2xN_x phosphors possessed additional red emissions and excellent thermal stability, which hold great prospect for w-LEDs applications.

2 Experimental section

2.1 Materials and synthesis

Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.5) phosphors were prepared from appropriate stoichiometric mixture of SrCO₃ (A. R.), SiO₂ (A. R.), AlN (A. R.), and Eu₂O₃ (A. R.) by traditional high temperature solid-state reaction method. All raw materials were purchased commercially without further treatment. In a typical process, 0.005 mol of starting materials were weighted and mixed thoroughly in an agate mortar according to different values of x. Then the powder mixtures were annealed at 1500 °C for 6 h in a corundum crucible under the reducing atmosphere (N₂/H₂ = 95 : 5%). The as-prepared samples were cooled to room temperature naturally, and after discarding the top layer of the nongrinded products, the interior of uniform samples was ground into powder for next characterization. For comparison, samples of Sr_2.97SiO₅:0.03Eu³⁺, Sr_2.97SiO₅:0.03Eu³⁺,0.03Li⁺, Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu³⁺ (x = 0.1) synthesized in air and Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu³⁺ (x = 0.1) synthesized in N₂ were also obtained under same sintered procedure.

2.2 Characterization

Crystal structure of the samples was analyzed by X-ray powder diffractometer (SHIMADZU, XRD-6000, Cu Kα radiation, λ = 0.15406 Å). The data were collected in a 2θ range from 20° to 70° with a scanning step of 0.02° and a scanning rate of 4° min⁻¹. The powder diffraction pattern for Rietveld analysis was collected with the same diffractometer. The step size of 2θ was 0.016°, and the counting time was 1 s per step. The crystal structure parameters were obtained using UnitCell software based on the Rietveld refinement data. Room-temperature photoluminescence and photoluminescence excitation (PLE) spectra were measured with a Hitachi F-7000 (Tokyo, Japan) spectrophotometer equipped with a 150 W xenon arc lamp. Temperature-dependent (300–460 K) photoluminescence spectra were conducted using a heating apparatus (OXFORD instrument, Oxon, UK) in combination with the above Hitachi 7000 spectrophotometer. Decay curves were measured using a FSP920 Time Resolved and Steady State Fluorescence Spectrometers (Edinburgh Instruments) equipped with a 450 W Xe lamp, a 150 W nF900 flash lamp, TM300 excitation monochromator and double TM300 emission monochromators and thermo-electric cooled red-sensitive PMT.

3 Results and discussion

Fig. 1a gives the XRD patterns of the as-prepared Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.2) phosphors synthesized by traditional solid-state reaction methods. The standard data for Sr₃SiO₅ phase (JCPDF card no. 26-0984) are also presented in the figure for comparison. As seen, all diffraction peaks of the samples agree well with the pure Sr₃SiO₅ phase, which indicate the formation of solid solutions, and suggest that the introduction of europium and AlN cannot affect the main crystal structure through partial replacement. As the XRD patterns show in ESI Fig. S1,† unknown impurities (two extra diffraction peaks at 31.0° and 32.5°) began to appear when x value exceeds 0.3. Sr₃SiO₅ phase has a tetragonal crystal structure with space group P4/ncc and lattice constants of a = 6.948 Å, c = 10.75 Å, Z = 4. The typical crystal structure of Sr₃SiO₅ was exhibited in Fig. 1b: Si⁴⁺ forms one kind of tetrahedral and builds up to a SiO₄ ring, and Sr²⁺ have two different crystal sites surrounded by SiO₄ and coordinated by six O²⁻; the three-dimensional tetragonal crystal structure of Sr₃SiO₅ is further formed by sharing the O²⁻ between SiO₄ rings. It is worth noting that the octahedron of Sr2 sites has larger volume than that of the Sr1 sites.²⁷

Fig. 1 (a) XRD patterns of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.2), together with the standard data for Sr₃SiO₅ (JCPDF card no. 26-0984) as reference and (b) crystal structure of Sr₃SiO₅ and coordination environment of Sr atom.

Herein, we actually obtained Sr_2.97SiO₅:0.03Eu²⁺ (x = 0) under a reducing atmosphere of H₂/N₂, as previously reported.^25,27,29,30 Fig. 2 shows the room-temperature excitation and emission spectra of sample Sr_2.97SiO₅:0.03Eu²⁺ (x = 0). The PLE spectrum (λ_em = 580 nm) spans from near-UV to visible area, which is useful for effective excitation in w-LEDs applications. And the PL spectrum (λ_ex = 335 nm) exhibits a broad emission band peaked at 580 nm with full width at half maximum (FWHM) of 70.6 nm, attributed to typical 4f⁶5d → 4f⁷ electron transitions of Eu²⁺ ions. After enlargement of the emission spectrum between 400 nm and 500 nm, a small emission peak at 467 nm was observed (see insert of Fig. 2). As mentioned above, there are two different crystallographic Sr²⁺ sites and the octahedron of Sr2 sites has larger volume than that of the Sr1 sites. Therefore, it is believed that two Eu²⁺ emission centers exist in Sr₃SiO₅:Eu²⁺, and most Eu²⁺ ions incorporate into Sr2 sites corresponding to the main emission at 580 nm.

Fig. 2 Room-temperature PLE (λ_em = 580 nm) and PL (λ_ex = 335 nm) spectra of Sr_2.97SiO₅:0.03Eu²⁺ (x = 0).

Through the crystal-site engineering approach, we introduced Al³⁺–N³⁻ into Sr₃SiO₅ structure to replace Si⁴⁺–O²⁻ resulting in Sr₃Si_1−xAl_xO_5−2xN_x:Eu phosphors under a reducing atmosphere. We noted that a slight excess of negative charge is formed due to the substitution and the formation of Eu³⁺. The structure offers many mechanisms to compensate this excess charge, the most possible being slight off-stoichiometry between N³⁻ and O²⁻. Although original idea of this substitution operation was only supposed to further improve the photoluminescence performances of Sr₃SiO₅:Eu²⁺ phosphor, additional narrow red emission was observed, which is unexpected but quite valuable. Because it is possible to obtain both Eu²⁺ and Eu³⁺ emissions in a single host lattice and then compensate the red emission in current w-LEDs. Fig. 3a displays the emission spectra of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.5) phosphors under the excitation of 340 nm. Obviously, several sharp emission bands between 570–715 nm appear with increasing x, accompanied by the broad emission bands at 580 nm of Eu²⁺ within 0 < x ≤ 0.03 ranges. Given the well-known characteristic 4f–4f electron transitions of Eu³⁺, we concluded that these narrow emissions come from Eu³⁺ ions, meaning the coexistence of two activators Eu³⁺ and Eu²⁺. And with more amount of AlN incorporating, the photoluminescence intensity of Eu³⁺ have a maximum value at x = 0.3, and the luminescence intensity of Eu²⁺ decrease to completely disappearing. Fig. 3b further lists the photographs and Commission International de I'Eclairage (CIE) coordinates of the selected samples (x = 0, 0.02, 0.025, 0.03, 0.1, 0.2). As shown, the luminescence colors tune from yellow to red, corresponding CIE coordinates of these samples are regularly shifted from (0.518, 0.474) to (0.630, 0.367). Apparently, the substitution of Si⁴⁺–O²⁻ by Al³⁺–N³⁻ in Sr₃SiO₅:Eu host compound gets the ability to fix the valence state of part europium ions as trivalent under the reducing atmosphere, and improve the photoluminescence intensity of Eu³⁺. In general, the utilization of crystal-site engineering approach commonly brings with the changes of coordination environment and crystal site size, which can indeed determine the valence state of activator ions and influence the photoluminescence properties of phosphors in some cases.^18,31 Detailed mechanisms for the valence stability in this study will be discussed below.

Fig. 3 (a) Emission spectra (λ_ex = 335 nm) of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.5), (b) dependence of CIE chromaticity coordinates on various x values upon 340 nm excitation, and (c) PLE (λ_em = 619 nm) and PL (λ_ex = 310 nm) spectra of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0 and 0.1) and Sr_2.94SiO₅:0.03Eu³⁺,0.03Li⁺ prepared in various conditions.

First, we synthesized a series of samples with Li⁺ co-doping or under different atmosphere, namely, Sr_2.97SiO₅:0.03Eu³⁺, Sr_2.97SiO₅:0.03Eu³⁺,0.03Li⁺, Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu³⁺ (x = 0.1) prepared in air and Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu³⁺ (x = 0.1) prepared in N₂, to investigate the effect of synthesis conditions and local charge unbalance on photoluminescence properties of phosphors. Corresponding PLE (λ_em = 619 nm) and PL (λ_ex = 310 nm) spectra of these phosphors and sample Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.1) prepared in N₂/H₂ are given in Fig. 3c. It is not hard to find that all emission spectra exhibit the same characteristic ⁵D₀ → ⁷F_J (J = 1, 2, 3 and 4) line-emissions of Eu³⁺ ions, in which the strongest emission is located at 619 nm (⁵D₀ → ⁷F₂). This further confirms that the sharp red emissions in Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu are attributed to Eu³⁺ ions, and also means that Eu³⁺ originating from raw materials Eu₂O₃ fail to be reduced even under the reducing atmosphere after AlN co-doping. The Li⁺ co-doping sample shows slightly enhanced emission compared to Sr_2.97SiO₅:0.03Eu³⁺ due to charge compensation. While the addition of AlN especially under the reducing atmosphere is able to dramatically increase the emission intensities, probably because the incorporation result in crystalline optimization and H₂ can restrain the generation of defects like oxygen vacancy.³² The PLE spectra consist of three excitation peaks: the broadband (250–380 nm) with a maximum value at 310 nm corresponds to the charge transfer band (CTB) of O²⁻ → Eu³⁺, and the other two peaks at 391 and 461 nm are caused by the ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ transition absorption of Eu³⁺ ions, respectively.³³

Second, efforts have also been taken to figure out how the substitution influences the coordination environment around luminescence centers and crystal structure. As we know, the intensity ratio (R) of the electric (⁵D₀ → ⁷F₂) and magnetic (⁵D₀ → ⁷F₁) dipole allowed transitions of Eu³⁺ provides an easy way to assess ligand symmetry of the Eu³⁺-site.³⁴ Because ⁵D₀ → ⁷F₂ is dependent on ligand symmetry and ⁵D₀ → ⁷F₁ is not, high R values typically indicate low ligand symmetry and high bond covalency. As seen in Fig. S2,† a continuous increase of R (from 0.54 to 3.97) is observed with the increasing x. This observation is taken as evidence for the incorporation of Eu³⁺ species into a less symmetric environment with more covalent character. In Sr₃SiO₅:Eu²⁺ structure, the EuO₆ octahedral neighbors with the SrO₆ octahedral by linking to the SiO₄ tetrahedral, which means that the (Sr,Eu)O₆ octahedral is closely surrounded by SiO₄ tetrahedral to form a highly condensed framework. The doping AlN into Sr₃SiO₅ structure was proposed to replace Si⁴⁺–O²⁻ to form host composite Sr_2.97Si_1−xAl_xO_5−2xN_x.

Detailed crystal structure change information is plotted in Fig. 4a. Based on the ions radius: r⁴(Si⁴⁺) = 0.26 Å, r⁴(Al³⁺) = 0.39 Å, r⁴(N³⁻) = 1.46 Å and r⁴(O²⁻) = 1.38 Å,³⁵ the replacement of Si⁴⁺–O²⁻ by Al³⁺–N³⁻ contributes to the expansion of (Si/Al)(O/N)₄ tetrahedral, which will shrink the (Sr/Eu)O₆ octahedral. In the cases of Ca₁₂Al_14−zSi_zO₃₂F_2−z:Eu and Ca_1+xY_1−xAl_1−xSi_xO₄:Eu, the occupation sites of Eu³⁺ in CaO₆F and CaO₉ get to expand after corresponding substitutions, which could favor the reduction Eu³⁺ to Eu²⁺.^17,18 Instead, here the shrinkage of (Sr/Eu)O₆ octahedral gives rise to the possibility of the activators maintaining as Eu³⁺. The variation of unit cell parameters (a, b, c, V) of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0–0.05) dependent on x values is given in Fig. 4b, which clearly shows that they gradually decrease with minimum values at x = 0.02 and then increase with increasing x values. The variation trend indicated the successful incorporation of AlN and formation of solid solutions of Sr_2.97Si_1−xAl_xO_5−2xN_x. Since the ion radius of Eu³⁺ [r⁶(Eu³⁺) = 0.947 Å] is much smaller than that of Sr²⁺ [r⁶(Sr²⁺) = 1.18 Å] and Eu²⁺ [r⁶(Eu²⁺) = 1.17 Å],³⁵ the unit cell contraction caused by the emerge of Eu³⁺ are predominant at the beginning, then the unit cell expansion caused by more incorporation of AlN accounted for the main, resulting in “V” type change trend of the cell parameters. Detailed XRD patterns from 30° to 31.5° also confirm this, as illustrated in Fig. S3,† the diffraction peaks shift to the larger 2θ angles until x = 0.02 and then to the opposite side. From the point view of applied physics, multiple factors, such as coordination environment, crystal site size and band gap of the compound, are the reasons for valance stability of lanthanide. In theory, Dorenbos pointed that the location of the lanthanide ground-state energy relative to the Fermi energy in the un-doped inorganic compound (referred to energy difference E_Ff) determines the valance stability of lanthanide, assuming that the latter energy is located midway between the top of the valence band and the bottom of the conduction band.^36,37 Accordingly, the crystal-site engineering approach actually results in the change of E_Ff and then determines the preferred valence of Eu in compound.

Fig. 4 (a) Detailed crystal structure change information of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu: (Sr/Eu)O₆ octahedral surrounded by (Si/Al)O₄ tetrahedral, (b) dependence of unit cell parameters a, b, c, V of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu on x values.

In order to further understand the effect of AlN incorporation on the coordination environment of the activators, Fig. 5 depicts the lifetime decay curves for the samples Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0, 0.01 and 0.02), monitored at λ_ex = 340 nm and λ_em = 619 nm. It is found that all decay curves can be well fitted by a second-order exponential equation³⁸

I(t) = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (1)

where I(t) is the luminescence intensity at time t and I₀ is the luminescence intensity initially, A₁ and A₂ are constants, and τ₁ and τ₂ are the lifetimes of the exponential components. The average lifetime τ* can be obtained by the formula as follows³⁸

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (2)

Fig. 5 Decay curves (λ_ex = 340 nm, λ_em = 619 nm) of Eu in Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0, 0.01 and 0.02).

The decay times of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0, 0.01 and 0.02) are determined to be 1.452, 1.366 and 1.388 μs, respectively, suggesting that the coordination environment around europium is indeed affected by the substitution and influences the valence stability of Eu. Fig. S4† also gives the decay curves of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.03–0.2), well fitted by eqn (1). The samples (x = 0.03–0.2) have almost same calculated lifetimes (about 1.33 μs), inducing that the coordination environment of the activators seems to no longer change, that is to say, the reduction process of Eu³⁺ to Eu²⁺ was completed at around x = 0.03.

Fig. 6 gives the PLE and PL spectra of sample Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.02). PLE spectra monitored at 619 and 703 nm both show strong excitation bands around 250–350 nm, originating from the CTB of O²⁺–Eu³⁺. Yet the PLE spectrum monitored at 580 nm reveals a broad band from 250 to 500 nm with the maximum at 349 nm, attributed to the 4f⁷ → 4f⁶5d¹ electron transition of the Eu²⁺ ions. Interestingly, strong emissions of Eu²⁺ (4f⁷ → 4f⁶5d¹, broadband around 580 nm) and Eu³⁺ (⁵D₀–⁷F_{1, 2, 3, 4}, 570–590, 619, 659 and 703 nm, respectively) ions can be obtained simultaneously under the excitation of 290–350 nm ranges. This is because the excitation bands of Eu²⁺ and Eu³⁺ ions have big overlap, which is quite useful for the effective excitation for phosphors with the coexistence of Eu²⁺ and Eu³⁺ activators.

Fig. 6 Room-temperature PLE (λ_em = 580, 619 and 703 nm) and PL (λ_ex = 290–350 nm) spectra of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.02).

Due to the inevitable problems of thermal stability in high-power LEDs applications, thermal quenching behaviours of phosphors must be taken into consideration. Thus, the temperature-dependent (300–460 K) PL spectra of selected samples (x = 0, 0.02 and 0.2) were measured and given in Fig. 7. As expected, the emission intensities of all samples decline with increasing temperatures (as shown in Fig. 7a–c), resulting from the stronger nonradiative transition probability at high temperature.³⁹ Fig. 7d gives the integrated PL intensities of these samples at different temperatures. When the temperature rises to 460 K, the integrated areas of samples x = 0, 0.02 and 0.2 (λ_ex = 340 nm) are decreased to 71.4%, 61.8% and 34.6% of their respectively initial intensities at room temperature, suggesting the emissions of Eu²⁺ ions possess better thermal stability than that of Eu³⁺ ions. As a proof of the concept, the integrated areas of sample x = 0.02 (λ_ex = 310 nm) at 460 K are about 55.6% of its intensity at room temperature, corresponding temperature-dependent PL spectra of sample x = 0.02 excited at 310 nm were shown in Fig. S5.† To further understand the temperature dependence behaviours, the activation energy of nonradiative transition was fitted (see Fig. 7d) and calculated by Arrhenius equation⁴⁰

I(T) = I₀/[1 + C exp(−E_A/kT)] (3)

where I₀ and I(T) are the intensity at temperature T = 0 and T(K), respectively. k is the Boltzmann' constant while C is a rate constant for the thermally activated escape. E_A is the activation energy for the thermal quenching process. Thus, the E_A for samples of x = 0, 0.02 and 0.2 excited at 340 nm and sample of x = 0.02 excited at 310 nm are calculated to be 0.504, 0.395, 0.311 and 0.348 eV, respectively, which indicate relatively good thermal stability. Note that, the emission band of Eu²⁺ has a light blueshift from 580 nm to 575 nm with increasing temperature. This phenomenon can be ascribed to the thermally active phonon-assisted tunnelling from the excited states of the lower-energy side to those of the high-energy side in the configuration coordinate diagram.^41,42

Fig. 7 PL (λ_ex = 310 nm and 340 nm) spectra of (a) Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0), (b) Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.02), and (c) Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.2) under various temperatures (300–460 K). (d) Dependence of integrated PL intensities on measured temperature for samples x = 0, 0.02 and 0.2.

4 Conclusions

In summary, we have successfully synthesized series of yellow-to-red emitting Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (0–0.5) phosphors under the H₂/N₂ atmosphere. In the ranges of x = 0–0.2, the samples perform as pure Sr₃SiO₅ phase and the CIE chromaticity coordinates can be tuned from (0.518, 0.474) to (0.630, 0.367). Within 0 < x < 0.03, two activators Eu²⁺ and Eu³⁺ coexist in compound, obviously confirmed by the photoluminescence spectra. When the x value exceeds 0.03, the phosphors exhibit extremely strong characteristic line emissions (⁵D₀ → ⁷F_J, J = 1, 2, 3 and 4) of Eu³⁺ ions, and the emission at 619 nm of sample with x = 0.1 is nine times than that of sample Sr_2.97SiO₅:Eu³⁺ prepared in air. Meanwhile, the excitation spectra of all samples show broad excitation band varying from 250 to 400 nm, which match well with the near-UV LEDs chip. The valence state of europium dopants can be fixed as trivalent state once the replacement of Si⁴⁺–O²⁻ by Al³⁺–N³⁻ occurs, because the substitution changes the coordination environment and crystal site size of the activators based on the analysis of Rietveld refinement, photoluminescence and fluorescence lifetime. The selected phosphors of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0.02, 0.2) also show a good thermal stability with activation energy of 0.395 and 0.311 eV, respectively, which quite benefits for the application in w-LEDs.

Acknowledgements

The present work was supported by the National Natural Science Foundations of China (Grant No. 51203053, 51372091), the Foundation for High-level Talents in Higher Education of Guangdong Province, the Project for Construction of High-level University in Guangdong Province, and the Teamwork Projects funded by the Guangdong Natural Science Foundation (Grant No. S2013030012842).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04387a

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