Emitting-tunable Eu^(2+/3+)-doped Ca_(8−x)La_(2+x) (PO₄)_6−x(SiO₄)_xO₂ apatite phosphor for n-UV WLEDs with high-color-rendering

Abstract

Currently, developing single-phased white light phosphors based on a single-doped activator is an efficient approach to blends of bicolor/tricolor phosphors for realizing phosphor-converted white light emitting diodes (pc-WLEDs) with a high color rendering index (CRI) and low correlated color temperature (CCT). Here, we present high CRI (R_a = 93–95) and low CCT (3500–6000 K) white lights by cosubstituting [Ca²⁺–P⁵⁺] for [La³⁺–Si⁴⁺] in the solid solution Ca_(8→2)La_(2→8)(PO₄)_6−x(SiO₄)_xO₂:Eu²⁺/Eu³⁺ (CLPSO_Eu). The results are attributed to the presence of multi Ca²⁺ sites due to possible mixing nanophases and the simultaneous occupancy of Ca²⁺ and La³⁺ sites by Eu, resulting in the mixing of blue (466 nm) and green emissions (540 nm) for Eu²⁺, and red emission (613 nm) for Eu³⁺, which were perfectly confirmed using X-ray Rietveld refinement, photoluminescence spectra and extended X-ray absorption fine structure. These findings not only imply that the as-prepared CLPSO_Eu are promising single-phased white light phosphor for near-UV based WLEDs but also offer a novel avenue to design high CRI white light phosphors based on a tunable Eu²⁺/Eu³⁺.

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1. Introduction

Near ultraviolet (n-UV) based white light-emitting diodes (WLEDs) that are composed of 380–420 nm LED chips and bicolor/tricolor phosphors are emerging as promising high CRI and low CCT lighting sources.^1–5 Unfortunately, this type of WLED easily suffers from a complicated fabrication process and color unbalance. An available alternative is to develop single-phased white light phosphors.⁶ The common single-phased white light infers the energy transfer among activator ion pairs such as Ce³⁺/Eu²⁺ → Mn²⁺, Ce³⁺/Eu²⁺ → Tb³⁺, Mn²⁺.^7–10 However, the Mn²⁺ red emission belongs to weak d–d transitions, resulting in the low luminescence efficiency and poor thermal stability.¹¹ To reduce the energy loss in the energy transfer process, single-activator-doped white light phosphors are highly desired, in which the emission spectra of the single activator must cover the entire visible light range.¹² Eu²⁺ is one of the most popular activators and its 4f–5d transition can be tuned from 400 to 700 nm depending on its coordination environment.^13–18 Recently, single Eu²⁺-activated materials were designed as single-phased white light phosphors by changing host component, such as Sr₅(PO₄)_3−x(BO₃)_xCl:Eu²⁺.¹² However, to obtain enough full width at half maximum (FWHM) of Eu²⁺ emission is still a challenge. Especially, more red component should be covered for presenting high CRI white light.¹⁹ Eu³⁺-activated materials are potential red-emitting phosphors in pc-WLEDs due to the typical 4f–4f transitions (⁵D_0,1–⁷F_J, J = 0–6) in the range of 570–700 nm from Eu³⁺.^20–22 Moreover, cation substitution has been confirmed as an effective strategy to adjust luminescent colors and properties of phosphor materials.^23–31 In the previous study, we have demonstrated the color-tunable emission from blue or green light to red light based on the valance transformation (3+ to 2+) through cation cosubstitution.³² However, there are few reports to present single-phased white light emission based on simultaneously blue, green, red-emitting Eu²⁺/Eu³⁺-activated phosphors. Herein, we realize high CRI white lights with low correlated color temperatures by cosubstituting [La³⁺–Si⁴⁺] for [Ca²⁺–P⁵⁺] in the solid solution Ca_(8→2)La_(2→8)(PO₄)_6−x(SiO₄)_xO₂:Eu²⁺/Eu³⁺.

2. Experimental section

Chemicals and materials

CaCO₃ (Aldrich, ≥99.95%), La₂O₃ (Aldrich, ≥99.999%), SiO₂ (Aldrich, ≥99.995%), (NH₄)₂HPO₄ (Aldrich, ≥99.9%), and Eu₂O₃ (Aldrich, ≥ 99.999%) were purchased from Sigma-Aldrich Corporation. All of the initial chemicals were used without further purification. Aluminum oxide crucibles were used to sinter the phosphor samples.

Preparation

A series of Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (x = 0–6) compounds were prepared by a conventional high-temperature solid state reaction. The doping concentration of Eu was fixed at 2 atom.% of Ca/La. All samples were synthesized in a reductive atmosphere and Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (0 ≤ x ≤ 6) samples were correspondingly denoted as CLSPO-x (x = 0–6). Stoichiometric amounts of CaCO₃, La₂O₃, SiO₂, (NH₄)₂HPO₄, and Eu₂O₃ were thoroughly mixed and pestled in an agate mortar for 1 h. Then, the powder mixtures were placed in aluminum oxides crucibles and sintered in a horizontal tube furnace at 1300 °C for 8 h with a reducing atmosphere of H₂ (8%) and N₂ (92%) atmosphere. After the furnace slowly cooled to room temperature, the sintered products were grinded again, generating the final phosphor powders. The as-prepared Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (x = 0, 1, 2, 3, 4, 5, 6) samples were denoted as CLSPO-0, 1, 2, 3, 4, 5, and 6, respectively. Phosphor-converted solid-state lighting devices were fabricated around discrete UV LEDs by using gold wires for electrical operation. The LEDs emit with λ_max = 395 nm. Homogeneous mixtures of the as-prepared phosphors and transparent silicone resin were cured on the LED chip. After the packaging was completed, the optical properties of the device were measured in an integrating sphere under forward DC bias conditions.

Characterization

Finely ground powders were used in all of the measurements. The phase purity of all samples were analyzed using X-ray diffraction (XRD) obtained in a D8 Focus diffractometer (Bruker, Karlsruhe, Germany) at a scanning rate of 1° min⁻¹ in the 2θ range from 5° to 120°, and the counting time was 5 s per step with Ni-filtered Cu Kα radiation (λ = 0.15406 nm). XRD Rietveld profile refinements of the structural models and texture analysis were performed with the use of General Structure Analysis System (GSAS). The photoluminescence measurements were recorded with a Fluoromax-4P spectrophotometer (Horiba Jobin Yvon, New Jersey, USA) equipped with a 450 W xenon lamp as the excitation source. Both excitation and emission spectra were set up to be 1.0 nm with the width of the monochromator slits adjusted as 0.50 nm. XANES of Eu L₃ edge was recorded with a wiggler beamline BL17C (the synchrotron X-ray diffraction wavelength λ = 0.774907 Å) at National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The thermal stability of luminescence of phosphor materials were measured by Fluoromax-4P spectrometer connected a heating equipment (TAP-02). The Commission Internationale de l'Eclairage chromaticity color coordinates, color rendering index (R_a), and CCT of WLED devices were measured by Starspec SSP6612.

3. Results and discussion

The crystal structure and phase purity of the studied samples were confirmed by powder X-ray diffraction (XRD). Fig. 1a shows the Rietveld XRD refinement of the representative Ca_5.88La_3.92Eu_0.2(PO₄)₄(SiO₄)₂O₂ (CLSPO-2) sample.³³ The refined structure parameters indicate that the studied sample crystallizes in hexagonal phase with space group P6_3/m (176), a = b = 9.47 Å, c = 6.92 Å, α = β = 90°, γ = 120°, V = 538.13 ų and Z = 1. The atom positions, fraction factors, and thermal vibration parameters were refined by convergence and satisfied well the reflection conditions, R_wp = 7.84%, R_p = 4.77%, and χ² = 6.86. The results imply the formation of single-phased solid solution (CLSPO-2). Fig. 1b shows the XRD patterns for Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (0 ≤ x ≤ 6) samples at 30–35°. The XRD patterns of x = 0 and x = 6 samples can be well assigned to the standard Ca₈La₂(PO₄)₆O₂ (CLPO, JCPDS no. 33-0287) and Ca₂La₈(SiO₄)₆O₂ (CLSO, JCPDS no. 29-0337) phases, respectively.^34,35 A linear shift of XRD diffraction peaks at 30–35° to small-angle direction with the substitution of [Ca²⁺–P⁵⁺] for [La³⁺–Si⁴⁺] (Fig. 1b) is attributed to the smaller ion radius of r[Ca²⁺+P⁵⁺] (1.23 Å, CN = 7; 1.35 Å, CN = 9) than that of r[La³⁺–Si⁴⁺] (1.36 Å, CN = 7; 1.476 Å, CN = 9). This phenomenon not only indicates the successful incorporation of [La³⁺–Si⁴⁺] into the lattice by substituting [Ca²⁺–P⁵⁺] but also confirms that the CLSPO-x (x = 0–6) can maintain single phase and form solid solutions in the whole series.³² Interestingly, two (112) crystal planes that respectively belong to CLPO and CLSO hosts simultaneously appear in CLPSO-2 and CLPSO-3 samples. This finding imply a possible mixing nanophases of two crystal structures, which could induce the coexistence of different coordination environments for activator ions.²³ A schematic spatial view of the Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu solid solution unit cell is shown in Fig. 1c. Ca/La/Eu occupy the nine-coordinated 4f sites with C₃ point symmetry and the seven-coordinated 6h sites with C_s point symmetry.^33–35 Along the c-axis, Ca/LaO₉ tetrakaidecahedron chains at 4f sites share faces, while Ca/LaO₇ decahedrons at 6h sites share vertexes and edges. The neighbouring tetrakaidecahedron and decahedron are connected through tetrahedral PO₄ groups and share edges. With the substitution of Si⁴⁺ for P⁵⁺, the relative Ca²⁺/La³⁺ ratio will change according to the charge compensation principle, accompanying the site change of the Eu activator at Ca²⁺ and La³⁺ sites.

Fig. 1 The Rietveld XRD refinement of (a) Ca_5.88La_3.92Eu_0.2(PO₄)₄(SiO₄)₂O₂ sample by the GSAS program, and (b) enlarged XRD patterns at 2θ = 30–35° of Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (CLPSO-x) (x = 0–6) samples. Observed and calculated XRD patterns, baseline, and their difference are depicted with black crosses, red solid line, green solid line, and blue solid line, respectively. The short vertical lines show the positions of Bragg reflections of the calculated pattern. (c) A schematic crystal structure and cation-cosubstitution of CLPSO-x along c-axis.

Fig. 2a and b depict the normalized photoluminescence excitation (PLE) and emission spectra (PL) of CLPSO-x samples prepared at 1300 °C. In previous report, it was found that the PL properties of Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂ phosphors were related to the synthesis temperature.^23,32 The Eu²⁺ ions in Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂ exhibit blue emission (400–600 nm) and green emission (420–650 nm) when the sintering temperatures are 1250 °C and 1350 °C, respectively.^32,33 The current CLSPO-0 sample show a broad absorption band from 250 nm to 500 nm centered at 395 nm due to the typical 5d–4f transitions of Eu²⁺. Interestingly, under 395 nm UV excitation, it gives a super wide emission band from 420 nm to 700 nm with two emission centers at 467 nm and 556 nm, respectively. Obviously, this super wide emission band also originates from the 5d–4f transitions of Eu²⁺ ions, and it simultaneously contains the Eu²⁺ emissions of Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂-1250 °C and Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂-1350 °C. The results reveal that two kinds of Eu²⁺ lattice environments appear in CLSPO-0 host at an intermediate sintering temperature (1300 °C). According to the previous conclusions in Eu²⁺-doped (Sr_1−xBa_x)Si₂O₂N₂ system, the super wide Eu²⁺ emission in Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂-1300 °C single-phased sample should be attributed to the nanophase segregation and mixing Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂-1250 °C and Ca_7.84La_1.96Eu_0.2(PO₄)₆O₂-1350 °C phases, which is consistent with the XRD results.²³ It is noted that some previous report also ascribed the same wide Eu²⁺ emission to replace at two different sites. It maybe the complexes of the two situation. Although the emission spectrum of CLSPO-0 sample covers a wide range from 420 nm to 700 nm, it's CIE color coordinates (0.316, 0.386) locates at yellow-green junction region due to the deficiency of red component. In addition, there is no obvious characteristic emission of Eu³⁺, revealing that Eu mainly occupy the lattice sites of Ca²⁺ ions and thus present +2 form. Interestingly, the Eu²⁺ emission gradually decreases and the characteristic red emissions of Eu³⁺ that are assigned to ⁵D₀–⁷F_0–3 transitions, i.e., 577 nm (0–0), 588 nm (0–1), 613 nm (0–2), 650 nm (0–3), 700 nm (0–4), appear and gradually rise with increasing [La³⁺–Si⁴⁺] concentration. Moreover, the enhanced relative emission intensity of Eu³⁺/Eu²⁺ with increasing x value, implying a gradual transformation from Eu²⁺ to Eu³⁺ with the substitution of [La³⁺–Si⁴⁺] for [Ca²⁺–P⁵⁺]. Beyond x ≥ 2, the emission intensity of Eu³⁺ is obviously higher than that of Eu²⁺. Finally, at x = 6 the Eu²⁺ emission vanishes, and only the characteristic emission of Eu³⁺ can be observed. The result indicates that the Eu mainly enter into the La³⁺ sites in Ca₂La₈(SiO₄)₆O₂ parent lattice and show +3 form. When monitoring with the maximum emission at 613 nm (⁵D₀–⁷F₂), the PLE spectra of x = 3–6 samples mainly consist of a O_2p → Eu_4f charge transfer band (CTB) in 250–335 nm and several 4f–4f transition lines from 350 nm to 500 nm.¹⁹Especially, the excitation intensities of ⁷F₀–⁵L₆ (395 nm) and ⁷F₀–⁵D₂ (463 nm) transitions are almost equivalent to or even surpass the O_2p → Eu_4f CTB absorption intensity, revealing that these samples could be effectively excited by 395 nm n-UV and 463 nm blue light. In general, the transformation of Eu²⁺ to Eu³⁺ happens with the substitution of [Ca²⁺–P⁵⁺] for [La³⁺–Si⁴⁺] in the CLSPO-x host, in which the Eu^2+/3+ can coexist and be controllably adjusted at appropriate x value. The finding leads to the emission colors of the studied samples from yellow-green region to red light across the white light region, as shown by the luminescence photos in Fig. 2c. The CIE color coordinates of CLSPO-x samples are listed in Table 1 and Fig. 3. Evidently, the color coordinates gradually shift from (0.316, 0.386) to (0.618, 0.383), further confirming the tunable emission colors. Especially, the x = 2–4 are well distributed at different white light region with the different ratio of Eu²⁺/Eu³⁺, which could be used as potential single-activator-doped single-phased white light phosphors. The cation-cosubstitution derived.

Fig. 2 (a) The normalized photoluminescence excitation (PLE, λ_em = 466, 556, 613 nm) and (b) emission (PL, λ_ex = 395 nm) spectra of CLPSO-x (x = 0–6) samples prepared at 1300 °C. (c) The luminescent photos of CLPSO-x (x = 0–6) samples under 395 nm UV are also shown. Eu²⁺(I) and Eu²⁺(II) means the crystal lattice environments of blue and green emission, respectively.

Table 1 CIE color coordinates and emission positions of Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (CLPSO-x) (0 ≤ x ≤ 6) prepared at 1300 °C, respectively (λ_ex = 395 nm)

Sample	X	Y	Peak (nm)
x = 0	0.316	0.386	468
x = 1	0.322	0.375	540
x = 2	0.337	0.363	615
x = 3	0.358	0.358	614
x = 4	0.423	0.356	614
x = 5	0.595	0.398	615
x = 6	0.618	0.383	615

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Fig. 3 CIE diagram of Ca_(8−x)La_(2+x)(PO₄)_6−x(SiO₄)_xO₂:2% Eu (CLPSO-x) (0 ≤ x ≤ 6) samples (λ_ex = 395 nm).

Eu²⁺/Eu³⁺-tunable strategy could be employed to design the desired single-phased white light in the apatite system.

To further verify the valence transformation of Eu in CLSPO-x, a powerful evidence, namely, the normalized Eu L₃-edge extended X-ray absorption fine structure (XANES) spectra of the studied samples were collected in Fig. 4a.^20,32 Clearly, two peaks at 6975 eV and 6985 eV are observed, which are attributed to the electron transitions of 2p_3/2 → 5d in Eu²⁺ and Eu³⁺, respecitvely.^20,32 The reason is the bigger shielding effect of nuclear potential by an additional 4f electron in Eu²⁺ than that in Eu³⁺. For the CLSPO-0, Eu almost exists in the form of 2+ except for a small amount of residual Eu³⁺, and this finding is consistent with the PL results. Moreover, the obviously higher emission of ⁵D₀ → ⁷F₂ at 613 nm with respect to the ⁵D₀ → ⁷F₁ emission at 588 nm indicates that the Eu³⁺ ions mainly enter the deviating inverse symmetry crystalline sites.^32,36 For the substitution of [Ca²⁺–P⁵⁺] for [La³⁺–Si⁴⁺], the absorption intensity of Eu²⁺ systematically increases, while that of Eu³⁺ correspondingly decreases, as shown in Fig. 4a. Finally, the Eu almost evolves to the 3+ format x = 6. The results convincingly demonstrate that Eu³⁺ and Eu²⁺ could coexist and mutually transform through the cation cosubstitution. A possible transformation mechanism between Eu³⁺ and Eu²⁺ is proposed, as shown by Fig. 4b. In the initial Ca₈La₂(PO₄)₆O₂ lattices, the Eu ions preferentially occupy the Ca²⁺ (6h) sites, presenting the Eu²⁺ broad band emission. A moderate sintering temperature induces the mixing nanophases of Ca₈La₂(PO₄)₆O₂ and Ca₂La₈(SiO₄)₆O₂ in the domain region, which further generates the multi-emitting sites of Eu²⁺ ions and produces a yellow-green emission. With the introduction of Si⁴⁺, the La³⁺ ions at 6h sites simultaneously enter the Ca²⁺/Eu²⁺ sites to balance charge, and thus Eu²⁺ ions gradually evolve to be Eu³⁺ ions. For the starting structure, the La³⁺ ions occupy all 6h sites and half of 4f sites in the ending Ca₂La₈(SiO₄)₆O₂ host. Therefore, the Eu²⁺ ions are finally completely transformed to Eu³⁺ ions. In general, the cation cosubstitution strategy could drive the coexistence and adjustment of Eu²⁺/Eu³⁺ in apatite structures, generating the single-phased white light phosphors for n-UV based WLEDs.

Fig. 4 (a) Normalized Eu L₃-edge X-ray absorption near edge structure (XANES) spectra of the representative CLPSO-x (x = 0, 1, 2, 4, 6) samples. (b) A schematic mechanism to explain the transformation between Eu²⁺ and Eu³⁺ with the cosubstitution of [Ca²⁺–P⁵⁺] for [La³⁺–Si⁴⁺].

As an important index to evaluate the efficiency and stability of pc-WLEDs devices, the thermal quenching behaviour of CLPSO-x samples from room temperature to 573 K was examined and shown in Fig. 5a.^37–42 Given a non-radiative transition under high working temperature, it is reasonable to observe the progressive emission attenuation of the studied samples.^43–47 It is noted that the thermal stability of x = 2–6 samples are obviously better than that of x = 0, 1 samples. Especially, a prominent improvement appears at x = 2, which can obtain 56% of the original emission intensity at room temperature. This is mainly because the outside electrons of Eu have a larger shielding effect for the 4f–4f transitions relative to its 5d–4f transitions.^32,36 Therefore, a weak electron-photon coupling interaction occurs and leads to a low thermal quenching under high temperature environment.^32,36 Finally, the x = 0 and x = 2 samples were fabricated to be single-phased white light-emitting devices combined with the 395 nm n-UV chips. Fig. 5b shows the corresponding electroluminescence (EL) spectra of the packaged WLEDs devices under a 200 mA forward bias current, which are basically in agreement with their PL results. Simultaneously, the packaged WLEDs present high R_a and low CCT, namely, R_a = 95.9, CCT = 5690 K for x = 0 sample and R_a = 93.2, CCT = 3680 K for x = 2 sample, respectively. The CIE color coordinates of x = 0 and 2 samples are calculated to be (0.33, 0.33) and (0.38, 0.34), respectively. Therefore, excellent warm white light with high R_a could be realized in the single Eu-doped apatite phosphor by the cation cosubstitution.

Fig. 5 Thermal quenching behaviour of photoluminescence for CLPSO-x (x = 0–6) samples (λ_ex = 395 nm). The insert shows the corresponding activation energy E_a. (b) Electroluminescence spectra of the CLPSO-0 (Top) and CLPSO-2 (Bottom) samples incorporated into an 395 nm-InGaN LED at 200 mA forward bias current. The CCT, R_a and corresponding luminescence photos (the insets) of the white lights are listed.

4. Conclusions

In conclusions, a novel Eu-activated single-phased white light phosphor was developed based on the presence of multi-emitting Eu²⁺ and the transformation between Eu²⁺ and Eu³⁺ by cation cosubstitution of [La³⁺–Si⁴⁺] for [Ca²⁺–P⁵⁺] in apatite analogue. The XRD patterns demonstrate that the formation of Ca_8−xLa_2+x(PO₄)_6−x(SiO₄)_xO₂:2% Eu solid solutions. Ultra broad Eu²⁺ emission from 420 nm to 600 nm imply the mixing nanophases of Ca₈La₂Eu(PO₄)_6−x(SiO₄)_xO₂ samples at different sintering temperatures. The PL and XANES spectroscopy of Eu L₃ edge convincingly confirm the valence transformation from Eu³⁺ to Eu²⁺ by the substitution of [La³⁺–Si⁴⁺] for [Ca²⁺–P⁵⁺]. Because of a whole cover across the blue, green emission from Eu²⁺ and red emission from Eu³⁺ in CLSPO phosphors, we obtained high R_a warm white lights (95.9 for x = 0; 93.2 for x = 2), with the corresponding CCT of 5690 K and 3680 K, respectively. The newly developed single Eu-activated white light phosphors exhibit potential applications in single-phase white emitting n-UV pumped LEDs devices.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (Grants No. NSFC 51672259, 21301162, 51672265, 91433110, U1301242) and the Ministry of Science and Technology of Taiwan (Contract no. MOST 104-2113-M-027-007-MY3).

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