A multi-centre activated single-phase white light phosphor with high efficiency for near-UV based WLEDs

Abstract

Single-phase white light phosphors for phosphor-converted white light-emitting diodes (pc-WLEDs) have become a hot spot and attracted considerable attention. However, the unsatisfactory luminous efficiency and colour rendering index of this kind of device hinder their large-scale applications in the field of lighting. Herein, a novel multi-centre activated single-phase white light phosphor, Gd₂Sr₃B₄O₁₂:Ce³⁺,Tb³⁺,Sm³⁺, has been designed and prepared. The crystal structures of the host and the doped samples are characterized with XRD and Rietveld refinements. The band gap of the host calculated with the density functional theory is consistent with the value obtained from the UV-Vis reflection spectrum. Benefiting from the energy transfer paths of Ce³⁺–Tb³⁺, Ce³⁺–Sm³⁺ and Tb³⁺–Sm³⁺, which are quantitatively evaluated with photoluminescence spectra and decay curves, the intensities of the characteristic emission peaks of the three dopants are found to be closely interlocked and the emission colour of the phosphors can be precisely tuned by adjusting the relative ratio of the three ions. Under the excitation of near-UV light, the two phosphors of Gd₂Sr₃B₄O₁₂:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and Gd₂Sr₃B₄O₁₂:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ emit white light with CIE coordinates of (0.32, 0.35) and (0.34, 0.34), and the quantum efficiencies are 46.8% and 34.9%, respectively. The temperature-dependent photoluminescence spectra of the phosphors show that the quenching temperature (T_50%) is as high as 450 K. Finally, WLED devices with maximal colour rendering index of 87.5 are obtained by fabricating the two single-phase white light phosphors with 365 nm LED chips. The balanced and preferable comprehensive performances of Gd₂Sr₃B₄O₁₂:Ce³⁺,Tb³⁺,Sm³⁺ demonstrate that the as-prepared single-phase white light phosphor can be a promising candidate for fabricating near-UV chip-based WLEDs.

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1. Introduction

Recently, solid-state lighting has become rapidly growing research field owing to extensive worry about environmental and energy issues.^1,2 Phosphor-converted white light-emitting diodes (pc-WLEDs) are considered to be a promising solid-state lighting source for their emerging advantages of high efficiency, energy saving, environmental friendliness, etc.^3–5 Two approaches are employed to develop commercialized WLEDs. The first method is fabricating blue InGaN chips with yellow phosphors, typically Y₃Al₅O₁₂:Ce³⁺. However, the colour rendering index (R_a) of this kind of WLEDs is strongly restricted by the absence of cyan and red regions in emission spectra.^6–8 The second strategy is coating the mixture of multi-colour phosphors on near-UV LED chips.⁹ The R_a of this kind of WLEDs is much higher than that of the former, but the reduction of luminous efficiency caused by the strong reabsorption in blue light region limits its application.^10,11 Therefore, producing white light in a single matrix has gained much attention.¹¹

Generally, there are three routes to generate white light in single-phase phosphors activated with rare-earth ions.^10,12 The first is doping single rare-earth ions into certain hosts, such as narrow-band emitting BiF₃:Tb³⁺, CaIn₂O₄:Eu³⁺ and Ca₉NaZn(PO₄)₇:Dy³⁺, or broadband emitting Ba₃Lu₄O₉:Ce³⁺ and (Rb,K)₂CaPO₄F:Eu²⁺.^13–17 Nevertheless, the f–f forbidden transitions of Tb³⁺, Eu³⁺ and Dy³⁺ make the quantum efficiency of the former too low to meet the requirements of WLED applications, for example, the internal quantum efficiencies of CaIn₂O₄:Eu³⁺, Ca₉NaZn(PO₄)₇:Dy³⁺ and BiF₃:Tb³⁺ are 10%, 19.2% and 3.44%, respectively. Although the quantum efficiency of the phosphors adopting f–d allowed transitions is significantly increased, the application of such phosphors in high-quality WLED devices is still restricted by the imbalance of emission spectra.¹⁸ White light can also be generated by the co-excitation of two kinds of rare earth ions, as in Sr₂LaGaO₅:Dy³⁺,Sm³⁺ and LaOCl:Tb³⁺,Eu³⁺.^19,20 However, since such single-phase white light phosphors have strict requirements on the co-excitation wavelength, the quality of white light will be significantly reduced when the excitation wavelength is changed.^10,18 The third route is co-doping two or three kinds of ions with energy transfer processes into certain hosts. The processes make the luminescence of the activators be controlled by the sensitizers, but the slight variation of excitation wavelength does not significantly change the quality of white light. So, this route for generating white light has been widely concerned.^10,18

Nowadays, many single-phase white light phosphors via energy transfer have been reported. Zheng et al. prepared the white light-emitting phosphor, Ba₉La₂Si₆O₂₄:Bi³⁺,Eu³⁺, with satisfying thermal stability (T_50% = 498 K), and fabricated a high colour rendering index (R_a = 87) WLED device with the phosphor and 365 nm chip.²¹ SrBPO₅:Ce³⁺,Dy³⁺ phosphor prepared by Vinodkumar et al. exhibits relatively high thermal stability and colour stability.⁴³ Ca₃YAl₃B₄O₁₅:Ce³⁺,Tb³⁺,Sm³⁺ prepared by Khan et al. shows both high R_a and thermal stability.¹⁸ Though single-phase white light phosphors via energy transfer have made great progress in improving luminescent properties, few of them can achieve satisfying balanced overall performances, including but not limited to quantum efficiency, R_a and thermal stability. Therefore, it still needs to be addressed to meet the demands of high-quality WLEDs.

Ce³⁺ ion usually emit broad blue light originating from the 4f–5d transition, which makes it a suitable sensitizer for single-phase white light phosphors. Tb³⁺ and Sm³⁺ ions are essential green and red activators in traditional phosphors. However, because of the f–f parity-forbidden transitions, the luminescence efficiencies of Tb³⁺ and Sm³⁺ are relatively low, which strongly restricts their applications. Taking advantages of the broadband emission of Ce³⁺ and the energy transfer from Ce³⁺ to Tb³⁺ or Sm³⁺, the emission efficiencies of Tb³⁺ or Sm³⁺ may be greatly improved.^18,22 The choice of host materials is also important for researchers to obtain phosphors with satisfying luminescent properties. Borates are considered to be favourite hosts for their excellent stability, low synthetic temperature, high ultraviolet transparency and wide bandgap.^22,23 As a new type of rare-earth and alkali-halide-based double borates, RE₂M₃B₄O₁₂ have been investigated for their excellent optical properties.^23–25 According to the structure models of Y₂Sr₃B₄O₁₂ and La₂Sr₃B₄O₁₂, the structure of Gd₂Sr₃B₄O₁₂ belongs to Pnma. There are two distinct cationic sites in Gd₂Sr₃B₄O₁₂, among which the [BO₃] planar triangles fill in and form a network structure.²³

Herein, the novel phosphor, Gd₂Sr₃B₄O₁₂:Ce³⁺,Tb³⁺,Sm³⁺, were designed and prepared to obtain white light emission by controlling the energy transfer processes of Ce³⁺–Tb³⁺, Ce³⁺–Sm³⁺ and Tb³⁺–Sm³⁺. The photoluminescent properties of singly-, doubly- and triply-doped phosphors were studied in detail to evaluate the three paths of energy transfer. By finely adjusting the relative ratio of the three ions to control the energy transfer processes, white light with CIE chromaticity coordinates of (0.32, 0.35) and (0.34, 0.34) can be obtained in Gd₂Sr₃B₄O₁₂:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and Gd₂Sr₃B₄O₁₂:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺, respectively. The quantum efficiency and the quenching temperature (T_50%) are as high as 46.8% and 450 K. A WLED device with maximal R_a of 87.5 is obtained by fabricating the as-prepared single-phase white light phosphor with 365 nm LED chip. Based on the balanced and preferable comprehensive performances, Gd₂Sr₃B₄O₁₂:Ce³⁺,Tb³⁺,Sm³⁺ can be a candidate for single-phase white light phosphors being potential application in WLEDs.

2. Experimental section

2.1. Sample preparation

All samples were prepared by conventional high temperature solid-state reaction. Raw materials of Sr₂CO₃ (A.R.), Gd₂O₃ (99.99%), CeO₂ (99.99%), Sm₂O₃ (99.9%), and Tb₄O₇ (99.99%) were purchased from Aladdin, and H₃BO₃ (A.R.) was purchased from Guangzhou Chemical Reagent Factory. All the raw materials were stoichiometrically weighed, and 5% excess H₃BO₃ were mixed with other reactants. After properly ground, the mixture was placed into an alumina crucible and preheated at 1073 K for 4 h. After cooling to room temperature, the mixture was ground again and heated up to 1373 K for 4 h under weak reducing atmosphere. Finally, the as-synthesized Gd₂Sr₃B₄O₁₂ (abbreviated to GSBO) and Gd_{2(1−x−y−z)}Sr₃B₄O₁₂:xCe³⁺,yTb³⁺,zSm³⁺ samples were crushed into powders and collected for further characterization. Two WLED devices were fabricated by coating GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ phosphors with 365 nm LED chips, respectively.

2.2. Measurements and characterization

The powder X-ray diffraction measurements were examined on a Rigaku D-Max 2200 X-ray diffraction system with Cu K_α radiation working at 40 kV and 26 mA. The refinement of XRD data was conducted by using JANA 2006 software. The microstructure and the element mapping were investigated by an FEI Quanta scanning with an energy-dispersive spectrometer. The photoluminescence emission (PL), excitation (PLE) spectra were measured with an FLS-980-combined time resolved and steady state fluorescence spectrometer (Edinburgh) with Xe/μs lamps. And fluorescence decay curves were obtained with the same fluorescence spectrometer equipped with a μs lamp (μF2, Edinburgh) and an EPLED-340 pulsed laser (central wavelength 345.4 nm, bandwidth 13.4 nm, pulse width 806.9 ps, Edinburgh). Temperature-dependent PL spectra with the temperature range from 300 K to 500 K were collected on the same instrument with an Oxford temperature controller. And the quantum efficiencies were obtained by a Hamamatsu C9920-03G absolute photoluminescence quantum efficiencies measurement system. The WLED devices were driven by LED300E programmable test power for LEDs under different forward bias current, and the electroluminescence (EL) spectra of WLED devices were measured with a HAAS-2000 high accuracy array spectroradiometer equipped with an integrating sphere (Hangzhou Everfine Photo-E-Info Co., Ltd).

2.3. Computational methods

The band structure and the density of state (DOS) of GSBO were carried out by first-principle calculations based on the density functional theory (DFT) with the Vienna Ab initio Software Package (VASP).²⁶ The interaction between ions and electrons was defined by the projector augments wave (PAW) pseudopotentials.²⁷ The Perdew–Burke–Ernzerhof (PBE) functional was used to perform the geometry optimizations.²⁸ The plant wave cutoff energy was set to 520 eV. The k-mesh of 3 × 2 × 3 Monkhorst–Pack (M–P) grid was used for geometry optimizations and 4 × 3 × 4 M–P grid was used during self-consistent relaxation. The convergence criterion for iteration was set to 10⁻⁵ eV per atom and the maximal force was set to 0.001 eV Å⁻¹.

3. Results and discussion

3.1. Crystal structure of GSBO and GSBO:Ce³⁺,Tb³⁺,Sm³⁺

The crystal structure of Gd₂Sr₃B₄O₁₂ (GSBO) is isostructural with Y₂Sr₃B₄O₁₂, which belongs to orthorhombic structure with space group of Pnma (No. 62).²³ The powder XRD patterns of the host and the co-doped samples depicted in Fig. 1(a) show that all the patterns can match well with the simulated one. According to the Bragg equation, the patterns will shift to smaller angles, when the smaller ions are replaced by the larger ions. As shown in the enlarged XRD patterns between 30° and 32°, the patterns of GSBO:0.03Ce³⁺,0.05Sm³⁺, GSBO:0.03Ce³⁺,0.10Tb³⁺ and GSBO:0.10Tb³⁺,0.05Sm³⁺ are shift to smaller angles. However, the pattern of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ is almost in the same position with the host, which should be caused by the substitution of Gd³⁺ by a large amount of Tb³⁺ in this triply-doped sample. In order to further analyse the phase purity, the Rietveld refinement was performed by using Y₂Sr₃B₄O₁₂ (ICSD#421574) as the starting model, and the fitting results are displayed in Tables S1 and S2, Fig. S1 (ESI†) and Fig. 1(b).²⁹ The reliability factors (R_wp and R_p) of both GSBO and GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ are less than 5%, suggesting that there is no distinct impurity phase and the doping of RE³⁺ will not cause any significant phase change. Cell parameters of the doped sample are a = 7.4244 Å, b = 16.0184 Å, c = 8.7620 Å, and V = 1042.04 ų, respectively. According to the fitting results, the unit cell volume of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ is smaller than that of GSBO matrix, which confirms the result of XRD of each samples. The crystal structure of GSBO along a-axis and the coordination environments of cations in the host are exhibited in Fig. 1(c) and (d). There are three different cation sites in the unit cell, named Sr1, Sr2 and Gd. Sr1 and Sr2 are eight-fold coordinated with O atoms and occupy 8d and 4c Wyckoff site, respectively. And Gd also locates at 8d Wyckoff site with 7-fold coordination. The ionic radii of Gd³⁺, Ce³⁺, Sm³⁺ and Tb³⁺ with coordination number (CN) of 7 are 1.00 Å, 1.07 Å, 1.02 Å and 0.98 Å, respectively.

Fig. 1 (a) The XRD patterns of GSBO, GSBO:0.03Ce³⁺,0.05Sm³⁺, GSBO:0.03Ce³⁺,0.10Tb³⁺, GSBO:0.10Tb³⁺,0.05Sm³⁺, and GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺, (b) the Rietveld refinement profile of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺, (c) the crystal structure of GSBO, and (d) the coordination environments of cations.

The similarity of ionic radii and valence states of RE³⁺ ions show that the doping ions are more likely to replace Gd³⁺ in the matrix. The possibility of cation substitution can be estimated by the empirical equation of Hume–Rothery, and the difference (D_r) between dopants and possible replaced cations should be less than 30%. The values of D_r can be estimated by the following equation:³⁰

in which, r_D(CN) is the ionic radius of dopants and r_R(CN) refers to the ionic radius of replaced ions. The values of D_r between the dopants (Ce³⁺, Sm³⁺ and Tb³⁺) and the possible replaced ions (Gd³⁺ or Sr²⁺) are shown in Table 1. As shown in the table, the radius difference percentages between the dopants and Gd³⁺ are significantly smaller than those between the dopants and Sr²⁺, which further indicates that Ce³⁺, Tb³⁺, Sm³⁺ will mainly replace Gd³⁺ rather than Sr²⁺ in GSBO.

Table 1 The radius difference percentage between replaced ions and dopants

Elements	Ionic radii	CN	D r
			Ce^3+	Sm^3+	Tb^3+
Sr^2+	1.26 Å	8	10.24%	14.37%	17.46%
Gd^3+	1.00 Å	7	7.00%	2.00%	2.00%

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Fig. 2 shows the scan electron microscope (SEM) image and the element mappings of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ sample. The smooth surface of the as-prepared sample with an average particle size of about 60 μm suggests good crystallization. The element mappings show the uniform distribution of all the metal elements, which confirms that all rare earth ions successfully enter the host lattice. The atomic percent of the metal elements in the as-prepared sample, the calculated ratio and the stoichiometric ratio of RE³⁺ in GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ are listed in Table S3.† And the mappings indicate that all the metal elements are homogeneously distributed. Besides, the ratio of RE³⁺ calculated with EDS results is consistent with the stoichiometric ratio of RE³⁺ in the sample, also proofing the homogeneous distribution of rare earth ions.

Fig. 2 SEM image of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and the element mappings of all the metal elements.

Fig. 3 shows the UV-Vis reflection spectra of the GSBO host, GSBO:0.05Ce³⁺, and GSBO:0.03Ce³⁺,0.14Tb³⁺,0.07Sm³⁺. There is a sharp peak at 275 nm in the reflection spectrum of the host, which should come from the absorption of Gd³⁺ in the host lattice. An obvious broad absorption ranging from 225 nm to 400 nm and some sharp peaks at 402 nm, 416 nm and 474 nm can be observed in the reflection spectra of Ce³⁺-doped and triply-doped samples, which are attributed to the f–d transition of Ce³⁺ and the f–f transitions of Sm³⁺, respectively. Because of the overlapping of the absorption of Ce³⁺ (ranging from 300 nm to 400 nm) and Tb³⁺ (ranging from 310 nm to 390 nm), the f–f transitions of Tb³⁺ are not obvious in the reflection spectrum of GSBO:0.03Ce³⁺,0.14Tb³⁺,0.07Sm³⁺. The optical band gap (E_g) of the host can be calculated by using the Tauc and Kubelka−Munk equations:^31,32

where α is the absorption coefficient, R stands for the diffuse reflectance of the sample, hν represents the photon energy, and A is the proportionality constant. The exponent n refers to the nature of optical band gap, e.g., n = 1/2 represents indirect bandgap semiconductor. As shown in the inset of Fig. 3a, E_g of GSBO is estimated to be 5.78 eV by extrapolating the tail of the curve. From the calculated band structure of GSBO as displayed in Fig. 3b, the band gap value is estimated to be 4.59 eV. Since the band gap calculation adopting generalized gradient approximation (GGA) function always underestimates bandgap, the calculated value will be smaller than optical band gap.^33,34 The total and projected electron density of states (TDOS/PDOS) of GSBO are presented in Fig. S2.† According to the calculation, the conduction band of GSBO is mainly contributed to Gd-s and Sr-d orbitals and the valence band is composed of O-p orbitals.

Fig. 3 (a) The UV-Vis reflection spectra of GSBO, GSBO:0.05Ce³⁺ and GSBO:0.03Ce³⁺,0.14Tb³⁺,0.07Sm³⁺ (inset with the optical band gap of GSBO) and (b) the band structure of the host.

3.2. Photoluminescent properties of singly-doped GSBO phosphors

Fig. 4 depicts the photoluminescence emission (PL) and excitation (PLE) spectra of GSBO:Ce³⁺ samples. As presented in Fig. 4a, GSBO:0.03Ce³⁺ shows a broad excitation band ranging from 300 nm to 400 nm, which is derived from the transition of 4f ground state to 5d excited state of Ce³⁺. Under 350 nm excitation, the sample shows a characteristic blue emission ranging from 360 nm to 600 nm with a peak at 425 nm and the full width at half maximum (FWHM) of 103 nm. The Gaussian fitting result is shown in Fig. S3,† the PL spectrum of GSBO:0.03Ce³⁺ can be well fitted with two sub-bands, which belongs to the emission from 5d excited state to the ground states of Ce³⁺ (²F_5/2 and ²F_7/2). The concentration-dependent PL spectra in Fig. 4b show that the emission intensity reaches the maximal value when x = 0.03 and decreases with increasing the doping concentration of Ce³⁺ because of the concentration quenching of luminescence. The decay curves of Ce³⁺-doped samples in Fig. 4c can be fitted by the following equation:^19,34where I_t is the luminescent intensity at time t, and τ is the lifetime. Since the change of the doping concentration is small, the lifetimes show little variation from 30.94 ns to 29.01 ns (details in Table S5, ESI†).

Fig. 4 (a) The PL and PLE spectra of GSBO:0.03Ce³⁺, (b) the concentration-dependent PL spectra of GSBO:xCe³⁺ (inset with the dependence of the emission intensity with the doping concentration of Ce³⁺), (c) the decay curves of GSBO:xCe³⁺ (x = 0.01–0.05), and (d) the I–H fitting of GSBO:0.05Ce³⁺.

In order to further illustrate the concentration quenching mechanism, the Inokuti–Hirayama (I–H) model with the following equation was employed:^34,35

where I₀ represents the luminescent intensity at t = 0, and τ₀ is the lifetime of donors without acceptors. Since the energy migration should be a unique energy transfer where donors and acceptors are the same ions, we chose the lifetime of the least quenched samples (GSBO:0.01Ce³⁺) as τ₀ to fit the decay curve of GSBO:0.05Ce³⁺. Q stands for the macroscopic parameter, and S = 6, 8, 10 represents the dominant of dipole–dipole (D–D), dipole–quadrupole (D–Q) and quadrupole–quadrupole (Q–Q) interactions, respectively. Here, the fitting results of each value of S are presented in Fig. 4d. Because of the value of R² for S = 6, 8, 10 is too close to each other, I–H fitting for quenched samples were conducted (Fig. S4†). According to the fitting results, D–D interaction should be the dominant energy migration mechanism in Ce³⁺ singly-doped samples.

The PLE, PL and concentration-dependent PL spectra of GSBO:Tb³⁺ are displayed in Fig. 5a and b. When monitoring at 542 nm, several sharp peaks located within 300 nm to 380 nm can be observed, which are derived from the f–f transitions of Tb³⁺. Under 377 nm excitation, four sharp peaks located at 488 nm, 542 nm, 583 nm, and 623 nm are ascribed to ⁵D₄ to ⁷F_J (J = 6, 5, 4, 3) transitions, respectively. The luminescence of GSBO:Tb³⁺ shows concentration quenching when the doping concentration of Tb³⁺ is about 0.50. Fig. 5c and d exhibit the luminescent spectra of Sm³⁺-doped samples. Similar to Tb³⁺ singly-doped samples, the PLE spectrum of Sm³⁺ doped sample contains a series of narrow peaks ranging from 270 nm to 500 nm. Upon 402 nm excitation, Sm³⁺-doped sample shows orange-red emission which is attributed to the transitions of ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, 11/2). The lifetimes of Tb³⁺ and Sm³⁺ singly-doped samples are listed in Tables S6 and S7 (ESI†), both of which show a monotonic decreasing trend with the increasing of the concentration of Tb³⁺ and Sm³⁺. And by fitting the decay curve of the most quenched Tb³⁺ and Sm³⁺ singly-doped samples, their dominant energy migration mechanism should be D–D interaction (Fig. S5†).

Fig. 5 (a) The PL and PLE spectra of GSBO:0.50Tb³⁺, (b) the concentration-dependent PL spectra of GSBO:yTb³⁺ (inset is about the dependence of the emission intensity with the doping concentration of Tb³⁺), (c) the PL and PLE spectra of GSBO:0.03Sm³⁺, and (d) the concentration-dependent PL spectra of GSBO:zSm³⁺ (inset is about the dependence of the emission intensity with the doping concentration of Sm³⁺).

3.3. Energy transfer between dopants in doubly-doped GSBO phosphors

Because the emission intensities of each doping ion in single-phase white light phosphors are usually interrelated by energy transfer, a minor variation of doping concentration of RE³⁺ will cause a significant change of the colour of emission. It is important to identify the energy transfer between doped ions for achieving white light in a single-phase phosphor. Hence, the doubly-doped phosphors of GSBO:Ce³⁺,Tb³⁺, GSBO:Ce³⁺,Sm³⁺ and GSBO:Tb³⁺,Sm³⁺ were prepared and their PL spectra were measured and are presented in Fig. 6. In order to fully exclude the influence of co-activation of dopants, the excitation wavelengths of each doubly-doped phosphor should be carefully chosen (Fig. S6†). As depicted in Fig. 6a, when excited with 330 nm, GSBO:0.03Ce³⁺,yTb³⁺ phosphors show the characteristic emissions both from Ce³⁺ (broadband ranged at 400 nm–525 nm) and from Tb³⁺ (sharp peaks within 450 nm–650 nm). And with increasing y value, the emission from Ce³⁺ decreases constantly, while the emission from Tb³⁺ shows an increasing trend, which indicates the energy transfer from Ce³⁺ to Tb³⁺ takes place in GSBO:Ce³⁺,Tb³⁺. The decrease in the blue/green (B/G) ratio with increasing the doping concentration of Tb³⁺ as shown in Fig. 6b suggests the emission colour of GSBO:0.03Ce³⁺,yTb³⁺ phosphors can be tuned from blue to green. The decay curves of the phosphors are presented in the inset of Fig. 6c. The calculated results of with eqn (4) show that τ of Ce³⁺ at 425 nm decreases constantly with increasing the doping concentration of Tb³⁺ (Table S8†). The critical distance (R_C) between each dopants are calculated in ESI and the results are listed in Table S4.† Because the R_C values for both Ce³⁺–Tb³⁺, Ce³⁺–Sm³⁺ and Tb³⁺–Sm³⁺ are larger than 5 Å, it indicates that the dominant interaction mechanism should be electric multipolar interaction. By fitting the decay curve of GSBO:0.03Ce³⁺,0.20Tb³⁺ with eqn (5), the energy transfer mechanism can be easily determined as D–D interaction.

Fig. 6 (a) The PL spectra of GSBO:0.03Ce³⁺,yTb³⁺, (b) the B/G ratio of GSBO:0.03Ce³⁺,yTb³⁺, (c) the I–H fitting of GSBO:0.03Ce³⁺,0.20Tb³⁺, (d) the PL spectra of GSBO:0.03Ce³⁺,zSm³⁺, (e) the B/R ratio of GSBO:0.03Ce³⁺,zSm³⁺, (f) the I–H fitting of GSBO:0.03Ce³⁺,0.09Sm³⁺, (g) the PL spectra of GSBO:0.50Tb³⁺,zSm³⁺, (h) the G/R ratio of GSBO:0.50Tb³⁺,zSm³⁺, and (i) the I–H fitting of GSBO:0.50Tb³⁺,0.09Sm³⁺.

The changing trends observed in PL spectra (Fig. 6d and g), the blue/red (B/R) ratio (Fig. 6e), the green/red (G/R) ratio (Fig. 6h), τ of Ce³⁺ at 425 nm (Fig. 6f and Table S9†), and τ of Tb³⁺ at 542 nm (Fig. 6i and Table S10†) for GSBO:0.03Ce³⁺,zSm³⁺ and GSBO:0.50Tb³⁺,zSm³⁺ are similar to those for GSBO:0.03Ce³⁺,yTb³⁺. To exhibit the colour variation directly, the CIE chromaticity diagram for doubly-doped samples is presented in Fig. S7.† With increasing the concentration of acceptors, the CIE coordinates of the doubly-doped samples gradually deviate from those of the singly-doped samples. The mechanisms of energy transfer from Ce³⁺ to Sm³⁺ and from Tb³⁺ to Sm³⁺ are also dominated by D–D interaction.

3.4. Photoluminescent and thermal quenching properties of GSBO:Ce³⁺,Tb³⁺,Sm³⁺ phosphors

Based on understanding the mechanisms of energy transfer in singly- and doubly-doped phosphors, it is easy to speculate the energy transfer processes in GSBO:Ce³⁺,Tb³⁺,Sm³⁺ phosphors (as shown in Fig. 7a). Due to the existence of energy transfer pathways between any two RE³⁺ ions, it could be hard to infer the colour variation when the doping concentrations of Tb³⁺ and Sm³⁺ change at the same time. In order to obtain white light emission in the GSBO matrix, Tb³⁺ ions could serve as the energy transfer channel to tune the colour. Hence, triply-doped samples were synthesized by fixing the doping concentration of Ce³⁺ and Sm³⁺. The PL spectra of GSBO:0.03Ce³⁺,yTb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,yTb³⁺,0.07Sm³⁺ are depicted in Fig. 7b and c. Under 350 nm excitation, these triply-doped samples exhibit the characteristic emission of Ce³⁺, Tb³⁺ and Sm³⁺. Fig. 7d illustrates the variation of their emission intensities. The emission from Tb³⁺ shows an upward trend and reaches the maximum at y = 0.14, while the emission from Ce³⁺ decreases constantly and the emission from Sm³⁺ increases monotonically. The decay curves of triply-doped phosphors at 425 nm were measured and are shown in Fig. S8 (ESI†). The lifetime of Ce³⁺ is largely reduced by the co-doping of Tb³⁺ and Sm³⁺, dropping from 30.41 ns to 22.51 ns or 20.81 ns (Tables S11 and S12, ESI†).

Fig. 7 (a) The schematic energy-level diagram of Ce³⁺, Tb³⁺ and Sm³⁺ and the energy transfer processes in GSBO:Ce³⁺,Tb³⁺,Sm³⁺, (b) and (c) the concentration-dependent PL spectra of GSBO:0.03Ce³⁺,yTb³⁺,zSm³⁺ (y = 0.02–0.22 and z = 0.05, 0.07), and (d) the emission peaks variation of GSBO:0.03Ce³⁺,yTb³⁺,zSm³⁺ (y = 0.02–0.22 and z = 0.05, 0.07) in intensities.

As shown in Fig. 8a and c, the emission intensity from each dopant is closely interlocked in the triply-coped phosphors. Hence, by adjusting the doping concentrations of Tb³⁺ and Sm³⁺, the CIE coordinates can be precisely tuned. In order to directly present the emission colour of as-synthesized phosphors under near-UV excitation, the correlated CIE coordinates of GSBO:0.03Ce³⁺,yTb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,yTb³⁺,0.07Sm³⁺ phosphors are exhibited in Fig. 8b and d. White light can be achieved in GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ samples with CIE coordinates of (0.32, 0.35) and (0.34, 0.34), respectively, which is quite close to standard white light (0.33, 0.33). Apart from the PL properties mentioned above, the internal quantum efficiency (IQE) is also a crucial factor to evaluate the performances of phosphors. The IQE is defined by the equation below:^36,37

where L_s is the emission spectra of phosphors, E_r and E_s are the reflections of excitation with and without samples. As shown in Fig. S9,† the measured η_int values of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ are 46.8% and 34.9%, respectively.

Fig. 8 (a) The R/G/B ratio variation of GSBO:0.03Ce³⁺,yTb³⁺,0.05Sm³⁺ (y = 0.02–0.22), (b) the CIE chromaticity diagram of the corresponding samples, (c) the R/G/B ratio variation of GSBO:0.03Ce³⁺,yTb³⁺,0.07Sm³⁺ (y = 0.02–0.22) and (d) the CIE chromaticity diagram of the corresponding samples.

To further analyse the thermal quenching behaviours of the as-synthesized phosphors, the temperature-dependent PL spectra with the temperature range of 300 K–500 K were measured. As shown in Fig. 9a, the emission intensity of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ shows a downward tendency owing to the thermal quenching. The normalized integrated intensity of the temperature-dependent PL spectra is depicted in Fig. 9b. The integrated intensity at 423 K can be maintained at more than 60% of that at room temperature and the quenching temperature (T_50%) is as high as 450 K. The emission intensity variations of Ce³⁺ (at 425 nm), Tb³⁺ (542 nm) and Sm³⁺ (at 599 nm) are presented in Fig. 9c. The emissions from Ce³⁺, Tb³⁺ and Sm³⁺ constantly decrease with increasing temperature. Since the f–f transition remains insensitive to the variation of temperature compared to f–d transition, the thermal quenching rate of Ce³⁺ is faster than Tb³⁺ and Sm³⁺. Fig. 9d exhibits the thermal activation energy calculation of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ sample with the Arrhenius equation:^38,39

Fig. 9 (a) The temperature-dependent PL spectra of GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺, (b) the trend of integrated intensity of the whole spectra, (c) the trend of Ce³⁺ (at 425 nm), Tb³⁺ (542 nm) and Sm³⁺ (at 599 nm) and (d) the Arrhenius fitting of the integral intensities of the temperature-dependent PL spectra.

By fitting with vs. , the thermal activation energy can be obtained to be 0.2226 eV.

3.5. The electroluminescence properties of WLED devices

Finally, WLED devices were fabricated by coating the phosphors, GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ and GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺, with 365 nm LED chips. Fig. 10 and Fig. S8† display the EL spectra of the as-fabricated devices with an inset of the images of corresponding packed WLED devices. As shown in Fig. 10a and Fig. S10a,† both GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ and GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ can be effectively excited by the n-UV LED chip and emit bright white light, the R_a of the two WLED devices is 87.5 and 77.3, respectively. Fig. 10b and Fig. S10b† display the EL spectra of the as-fabricated WLED devices under various currents. It can be seen that the shape and peak position of the EL spectra have little variation. However, the EL intensity slightly decreases after the current exceeds 150 mA, which should be caused by the temperature rise under high current.¹¹ Because the application prospect of phosphors cannot be evaluated with any single property, IQE, R_a and T_50% were chosen to compare our work with some other recent studies. The comprehensive comparison of the as-synthesized phosphors with other single-phase white light phosphors is listed in Table 2. It can be seen that the performances of as-prepared phosphors are balanced and the comprehensive performance is preferable. Hence, the comparison result shows that the as-synthesized phosphors could have a potential application in WLEDs.

Fig. 10 (a) The EL spectrum of the WLED fabricated by GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ with 365 nm LED chip and (b) the EL spectra of the as-fabricated WLED under various forward bias currents.

Table 2 The comparison of IQE, maximal R_a and T_50% of some single-phase white light phosphors with GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺

Phosphors	λ ex (nm)	IQE (%)	Max. Ra	T 50% (K)	Ref.
Gd2Sr3B4O12:0.03Ce^3+,0.18Tb^3+,0.05Sm^3+	350	46.8	77.3	450	This work
Gd2Sr3B4O12:0.03Ce^3+,0.22Tb^3+,0.07Sm^3+	350	34.9	87.5	—	This work
Rb2CaP2O7:0.005Ce^3+,0.01Eu^2+	310	74.36	88.4	∼430	40
NaSrBO3:0.01Ce^3+,0.08Tb^3+,0.06Sm^3+	360	48.2	80.1	480	22
Sr2LaGaO5:0.02Bi^3+,0.05Eu^3+	370	∼47.3	78.5	350	41
Sr3Ce0.55La0.45(PO4)3:0.05Eu^3+	365	37	90	402	42
Ca3Y0.65Al3B4O15:0.03Ce^3+,0.20Tb^3+,0.12Sm^3+	345	34.84	84.4	>500	18
SrBPO5:0.01Ce^3+,0.012Dy^3+	296	28.26	—	>423	43
SrBaLaGaO5:0.02Bi^3+,0.05Eu^3+	370	∼24.9	83.1	∼375	41
GdBO3:0.02Ce^3+,0.12Tb^3+,0.015Eu^3+	360	20.2	—	—	10
Ba2LaGaO5:0.02Bi^3+,0.05Eu^3+	370	∼14.7	92.2	∼325	41
Ca0.99In2O4:0.01Eu^3+	250	10	—	—	14
Mg2Gd8(SiO4)6O2:0.03Ce^3+,0.01Mn^2+	365	7.93	87.4	373	44
Li3Gd2.34Te2O12:0.15Bi^3+,0.07Pr^3+	297	5	—	<300	45
NaSr0.9425VO4:0.05Dy^3+,0.0075La^3+	347	3.26	—	—	46
Bi4BPO10:0.07Dy^3+	395	—	40	—	47
Sr2LaGaO5:0.03Dy^3+,0.07Sm^3+	320	—	91.4	400	19

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4. Conclusions

In conclusion, Ce³⁺-, Tb³⁺- and Sm³⁺-doped Gd₂Sr₃B₄O₁₂ phosphors have been successfully synthesized by the conventional high temperature solid-state reaction. The XRD and Rietveld refinements of GSBO host and the doped samples have shown that the substitution of Ce³⁺, Tb³⁺ and Sm³⁺ ions for Gd³⁺ in the host has no any significant effect on the phase purity. The band structure of the host has been calculated with the first-principle calculations based on the density functional theory, and the band gap is calculated to be 4.59 eV, which is close to the value obtained from the UV-Vis reflection spectrum. The Inokuti–Hirayama model has been used to fit the photoluminescence spectra of the three singly-doped phosphors and the fitted results have indicated that the dipole–dipole interaction is the dominant mechanism of energy transfer in the three phosphors.

The photoluminescence spectra and the decay curves of GSBO:Ce³⁺,Tb³⁺, GSBO:Ce³⁺,Sm³⁺ and GSBO:Tb³⁺,Sm³⁺ phosphors have shown that there are energy transfer processes of Ce³⁺–Tb³⁺, Ce³⁺–Sm³⁺ and Tb³⁺–Sm³⁺, and the mechanisms of all the energy transfer processes are dominated by dipole–dipole interaction. The intensities of the characteristic emission peaks of the three dopants in GSBO:Ce³⁺,Tb³⁺,Sm³⁺ have been found to be closely interlocked and the emission colour of the phosphors can be precisely tuned by adjusting the relative ratio of the three ions. Bright white light with CIE coordinates of (0.32, 0.35) and (0.34, 0.34) have been obtained in GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺ and GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺, respectively. Under 350 nm excitation, the internal quantum efficiency up to 46.8% has been achieved in GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺, which is relatively high in RE³⁺ doped single-phase white light phosphors. The temperature-dependent photoluminescence spectra of the phosphors have shown that the quenching temperature (T_50%) is as high as 450 K. Two WLED devices with high R_a, 87.5 for GSBO:0.03Ce³⁺,0.22Tb³⁺,0.07Sm³⁺ and 77.3 for GSBO:0.03Ce³⁺,0.18Tb³⁺,0.05Sm³⁺, have been fabricated by coating the prepared single-phase white light phosphors with near-UV chips. The balanced and preferable comprehensive luminescent properties manifest that Gd₂Sr₃B₄O₁₂:Ce³⁺,Tb³⁺,Sm³⁺ is an outstanding single-phase white light phosphor activated with multi-centre via energy transfer, which has a potential application in near-UV pumped WLEDs for lighting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by grants from the National Natural Science Foundation of China (NSFC No. 51972347 and 21771195) and the joint projects of NSFC with Guangdong Province (No. U22A20135 and U1301242).

References

1. L. Wang, R.-J. Xie, T. Suehiro, T. Takeda and N. Hirosaki, Down-conversion nitride materials for solid state lighting: recent advances and perspectives, Chem. Rev., 2018, 118, 1951–2009.
2. M.-H. Fang, Z. Bao, W.-T. Huang and R.-S. Liu, Evolutionary generation of phosphor materials and their progress in future applications for light-emitting diodes, Chem. Rev., 2022, 122, 11474–11513.
3. Y.-C. Lin, M. Karlsson and M. Bettinelli, Inorganic phosphor materials for lighting, Top. Curr. Chem., 2016, 374, 21.
4. S. Gu, M. Xia, C. Zhou, Z. Kong, M. S. Molokeev, L. Liu, W.-Y. Wong and Z. Zhou, Red shift properties, crystal field theory and nephelauxetic effect on Mn⁴⁺-doped SrMgAl_10−yGa_yO₁₇ red phosphor for plant growth LED light, Chem. Eng. J., 2020, 396, 125208.
5. X. Zhao, Y. Wang, Z. Pan, Y. Lu, J. Li and M. Wu, Synthesis of Sm³⁺-doped YGa_1.5Al_1.5(BO₃)₄ phosphor via mechanical activation-assisted solid-state reaction, Dalton Trans., 2023, 52, 4808–4818.
6. T. Hu, Y. Gao, M. S. Molokeev, Z. Xia and Q. Zhang, Eu²⁺ stabilized at octahedrally coordinated Ln³⁺ site enabling red emission in Sr₃LnAl₂O_7.5 (Ln = Y or Lu) phosphors, Adv. Opt. Mater., 2021, 9, 2100077.
7. J. Qiao, L. Ning, M. S. Molokeev, Y.-C. Chuang, Q. Zhang, K. R. Poeppelmeier and Z. Xia, Site-selective occupancy of Eu²⁺ toward blue-light-excited red emission in a Rb₃YSi₂O₇:Eu phosphor, Angew. Chem., Int. Ed., 2019, 58, 11521–11526.
8. J. Ding, M. Kuang, S. Liu, Z. Zhang, K. Huang, J. Huo, H. Ni, Q. Zhang and J. Li, Sensitization of Mn²⁺ luminescence via efficient energy transfer to suit the application of high color rendering WLEDs, Dalton Trans., 2022, 51, 9501–9510.
9. Y. Zhuo, S. Hariyani, J. Zhong and J. Brgoch, Creating a green-emitting phosphor through selective rare-earth site preference in NaBaB₉O₁₅:Eu²⁺, Chem. Mater., 2021, 33, 3304–3311.
10. J. Li, Q. Liang, J.-Y. Hong, J. Yan, L. Dolgov, Y. Meng, Y. Xu, J. Shi and M. Wu, White light emission and enhanced color stability in a single-component host, ACS Appl. Mater. Interfaces, 2018, 10, 18066–18072.
11. D. Zhang, X. Zhang, B. Zheng, Q. Sun, Z. Zheng, Z. Shi, Y. Song and H. Zou, Li⁺ ion induced full visible emission in single Eu²⁺-doped white emitting phosphor: Eu²⁺ site preference analysis, luminescence properties, and WLED applications, Adv. Opt. Mater., 2021, 9, 2100337.
12. M. Shang, C. Li and J. Lin, How to produce white light in a single-phase host?, Chem. Soc. Rev., 2014, 43, 1372–1386.
13. D. Lin and P. Du, Room temperature synthesis of Tb³⁺-activated BiF₃ green-emitting nanoparticles with high thermal stability for near-ultraviolet white light-emitting didoes, Appl. Phys. A: Mater. Sci. Process., 2021, 127, 65.
14. X. Liu, C. Lin and J. Lin, White light emission from Eu³⁺ in CaIn₂O₄ host lattices, Appl. Phys. Lett., 2001, 90, 081904.
15. D. Zhu, M. Liao, Z. Mu and F. Wu, Preparation and luminescence properties of Ca₉NaZn(PO₄)₇:Dy³⁺ single-phase white light-emitting phosphor, J. Electron. Mater., 2018, 47, 4840–4844.
16. S. Ma, J. Zhang, J. Chen, Z. Yang, S. Wang and Z. Ye, Synthesis, crystal structure and photoluminescence properties of novel Ba₃Lu₄O₉:Ce³⁺ orange-red phosphors for white light emitting diodes, J. Alloys Compd., 2020, 819, 153047.
17. D. Wu, C. Shi, J. Zhou, Y. Gao, Y. Huang, J. Ding and Q. Wu, Full-visible-spectrum lighting enabled by site-selective occupation in the high efficient and thermal stable (Rb, K)₂CaPO₄F: Eu²⁺ solid-solution phosphors, Chem. Eng. J., 2022, 430, 133062.
18. W. U. Khan, L. Zhou, X. Li, W. Zhou, D. Khan, S.-I. Niaz and M. Wu, Single phase white LED phosphor Ca₃YAl₃B₄O₁₅:Ce³⁺,Tb³⁺,Sm³⁺ with superior performance: color-tunable and energy transfer study, Chem. Eng. J., 2021, 410, 128455.
19. Z. Zhang, J. Li, N. Yang, Q. Liang, Y. Xu, S. Fu, J. Yan, J. Zhou, J. Shi and M. Wu, A novel multi-center activated single-component white light-emitting phosphor for deep UV chip-based high color-rendering WLEDs, Chem. Eng. J., 2020, 390, 124601.
20. G. Li, Z. Hou, C. Peng, W. Wang, Z. Cheng, C. Li, H. Lian and J. Lin, Electrospinning derived one-dimensional LaOCl: Ln³⁺ (Ln = Eu/Sm, Tb, Tm) nanofibers, nanotubes and microbelts with multicolor-tunable emission properties, Adv. Funct. Mater., 2010, 20, 3446–3456.
21. Z. Zheng, P. Xie, P. Yi, J. Zhao, L. Liu and F. Yi, Achieving white-light/tunable emissions via the controllable energy transfer in the single Ba₉La₂Si₆O₂₄:Eu³⁺,Bi³⁺ phosphor for UV converted white LEDs, J. Lumin., 2020, 221, 117052.
22. M. Xin, D. Tu, H. Zhu, W. Luo, Z. Liu, P. Huang, R. Li, Y. Cao and X. Chen, Single-composition white-emitting NaSrBO₃:Ce³⁺,Sm³⁺,Tb³⁺ phosphors for NUV light-emitting diodes, J. Mater. Chem. C, 2015, 3, 7286–7293.
23. Y. Zhang, Z. Lin and G. Wang, Synthesis, growth, structure and characterization of the new laser host crystal Sr₃Y₂(BO₃)₄, Laser Phys. Lett., 2013, 10, 075806.
24. B. V. Mill, A. M. Tkachuk, E. L. Belokoneva, G. I. Ershova, D. I. Mironov and I. K. Razumov, Spectroscopic studies of Ln₂Ca₃B₄O₁₂:Nd³⁺ (Ln = Y, La, Gd) crystals, J. Alloys Compd., 1998, 275–277, 291–294.
25. B. Wei, Z. Hu, Z. Lin, L. Zhang and G. Wang, Growth and spectral properties of Er³⁺/Yb³⁺-codoped Ca₃Y₂(BO₃)₄ crystal, J. Cryst. Growth, 2004, 273, 190–194.
26. G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186.
27. P. E. Blöchl, Projector augmented-wave method, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979.
28. J. P. Perdew, K. Burke and M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett., 1996, 77, 3865–3868.
29. V. Petříček, M. Dušek and L. Palatinus, Crystallographic computing system JANA2006: General features, Z. Kristallogr. - Cryst. Mater., 2014, 229, 345–352.
30. N. Yang, Z. Zhang, L. Zou, J. Chen, H. Ni, P. Chen, J. Shi and Y. Tong, A novel red-emitting phosphor with an unusual concentration quenching effect for near-UV-based WLEDs, Inorg. Chem. Front., 2022, 9, 6358–6368.
31. Y. Wei, G. Xing, K. Liu, G. Li, P. Dang, S. Liang, M. Liu, Z. Cheng, D. Jin and J. Lin, New strategy for designing orangish-red-emitting phosphor via oxygen-vacancy-induced electronic localization, Light: Sci. Appl., 2019, 8, 15.
32. X. Li, B. Milićević, M. D. Dramićanin, X. Jing, Q. Tang, J. Shi and M. Wu, Eu³⁺-activated Sr₃ZnTa₂O₉ single-component white light phosphors: emission intensity enhancement and color rendering improvement, J. Mater. Chem. C, 2019, 7, 2596–2603.
33. M. D. Dramicanin, L. Marciniak, S. Kuzman, W. Piotrowski, Z. Ristic, J. Perisa, I. Evans, J. Mitric, V. Dordevic, N. Romcevic, M. G. Brik and C. G. Ma, Mn⁵⁺-activated Ca₆Ba(PO₄)₄O near-infrared phosphor and its application in luminescence thermometry, Light: Sci. Appl., 2022, 11, 279.
34. R. Shi, B. Li, C. Liu and H. Liang, On doping Eu³⁺ in Sr_0.99La_1.01Zn_0.99O_3.495: The photoluminescence, population pathway, de-excitation mechanism, and decay dynamics, J. Phys. Chem. C, 2016, 120, 19365–19374.
35. R. Shi, J. Xu, G. Liu, X. Zhang, W. Zhou, F. Pan, Y. Huang, Y. Tao and H. Liang, Spectroscopy and luminescence dynamics of Ce³⁺ and Sm³⁺ in LiYSiO₄, J. Phys. Chem. C, 2016, 120, 4529–4537.
36. L. Cao, W. Li, B. Devakumar, N. Ma, X. Huang and A. F. Lee, Full-spectrum white light-emitting diodes enabled by an efficient broadband green-emitting CaY₂ZrScAl₃O₁₂:Ce³⁺ garnet phosphor, ACS Appl. Mater. Interfaces, 2022, 14, 5643–5652.
37. S. Zhao, S. Liao, R. Shi, J. Zhang, Y. Han and S. Lian, Tuning emission color of Eu²⁺-activated phosphor through phase segregation, Chem. Eng. J., 2023, 452, 139640.
38. Z. Zhang, J. Yan, Q. Zhang, G. Tian, W. Jiang, J. Huo, H. Ni, L. Li and J. Li, Enlarging sensitivity of fluorescence intensity ratio-type thermometers by the interruption of the energy transfer from a sensitizer to an activator, Inorg. Chem., 2022, 61, 16484–16492.
39. Y. Chen, F. Liu, Z. Zhang, J. Hong, M. S. Molokeev, I. A. Bobrikov, J. Shi, J. Zhou and M. Wu, A novel Mn⁴⁺-activated fluoride red phosphor Cs₃₀(Nb₂O₂F₉)₉(OH)₃·H₂O: Mn⁴⁺ with good waterproof stability for WLEDs, J. Mater. Chem. C, 2022, 10, 7049–7057.
40. Y. Yin, M. Zhu, Y. Zhang, C. Hu, Z. Wang, Y. Che, J. Lu, R. Sun, J. Zhao, B. Teng, J. Li, S. Sun and D. Zhong, Achievement of high-efficiency sunlight-like emission in Rb₂CaP₂O₇: Ce³⁺, Eu²⁺ phosphors, J. Lumin., 2022, 242, 118547.
41. D. Liu, P. Dang, X. Yun, G. Li, H. Lian and J. Lin, Luminescence color tuning and energy transfer properties in (Sr,Ba)₂LaGaO₅:Bi³⁺,Eu³⁺ solid solution phosphors: Realization of single-phased white emission for WLEDs, J. Mater. Chem. C, 2019, 7, 13536–13547.
42. P. Dai, Q. Wang, M. Xiang, T.-M. Chen, X. Zhang, Y.-W. Chiang, T.-S. Chan and X. Wang, Composition-driven anionic disorder-order transformations triggered single-Eu²⁺-converted high-color-rendering white-light phosphors, Chem. Eng. J., 2020, 380, 122508.
43. P. Vinodkumar, S. Panda, G. Jaiganesh, R. K. Padhi, U. Madhusoodanan and B. S. Panigrahi, SrBPO₅: Ce³⁺, Dy³⁺ - A cold white-light emitting phosphor, Spectrochim. Acta, Part A, 2021, 253, 119560.
44. B. Zheng, X. Zhang, D. Zhang, F. Wang, Z. Zheng, X. Yang, Q. Yang, Y. Song, B. Zou and H. Zou, Ultra-wideband phosphor Mg₂Gd₈(SiO₄)₆O₂:Ce³⁺,Mn²⁺: Energy transfer and pressure-driven color tuning for potential applications in LEDs and pressure sensors, Chem. Eng. J., 2022, 427, 131897.
45. A. Bindhu, A. S. Priya, J. I. Naseemabeevi and S. Ganesanpotti, Deciphering crystal structure and photophysical response of Bi³⁺ and Pr³⁺ co-doped Li₃Gd₃Te₂O₁₂ for lighting and ratiometric temperature sensing, J. Alloys Compd., 2022, 893, 162246.
46. M. Chen, K. Qiu, P. Zhang, W. Zhang and Q. Yin, Enhancement of NaSrVO₄:Dy³⁺-white-phosphor photoluminescence via La³⁺ co-doping, Ceram. Int., 2019, 45, 22547–22552.
47. Y. Zheng, H. Li, T. Yang, J. Li, S. Yang and J. Zhu, Bi₄BPO₁₀:Dy³⁺: A single-phase white-emitting phosphor for light-emitting diodes, Mater. Today Chem., 2022, 26, 101199.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00685a

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