High-temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors stabilized at room temperature by incorporating phosphorus ions

Abstract

In this work, high-temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors stabilized at room temperature are synthesized by incorporating a suitable amount of phosphorus ions. The crystalline structures and the influence of phosphorus ions on the crystalline phases as well as the photoluminescent properties for phosphorus stabilized α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors are investigated in detail. The results indicate that the α′_H- and α-Ca₂SiO₄ high-temperature crystalline phases can be quenched at room temperature with incorporation of 12% and 42% phosphorus ions (replacing Si ions), respectively, shielding the stability effect of Eu²⁺ ions on the β-form (low Eu²⁺ concentration) and α′_L-form (high Eu²⁺ concentration). Tunable emissions from green to orange-red light are observed with the increase of the Eu²⁺ doping concentration in both of α′_H-Ca₂Si_0.88P_0.12O₄:Eu²⁺ and α-Ca₂Si_0.58P_0.42O₄:Eu²⁺ phosphors, indicating their promising prospect in applications of LED lighting.

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Introduction

In recent years, white light emitting diodes (WLEDs) have been championed as the next-generation lighting source by researchers owing to its outstanding advantages of high efficiency, energy saving and long lifetime.^1,2 The performances of WLEDs, such as luminous efficiency, color rendering index (CRI) and correlated color temperature (CCT), are strongly dependent on the luminescent properties of the phosphor, which is one of the key materials in fabricating white LEDs. During the past decades, the discovery and development of phosphors applied in WLEDs have moved from a single family of phosphor compound, the Ce³⁺ doped Y₃Al₅O₁₂, to a plethora of rare earth ions doped aluminates, sulfides, nitrides and molybdates.^3–6 Nonetheless, these phosphor systems are still suffering from the drawbacks of themselves in the practical applications, such as deficiency of red component easily results in lower CRI and higher CCT of WLEDs for Y₃Al₅O₁₂:Ce³⁺ phosphor; poor thermal and chemical stability is announced for sulfides phosphors; higher cost raw materials and rigorous synthesis equipment are required for nitrides phosphors. Phosphor materials with characters of excellent luminescent properties, outstanding chemical stability and facile synthesis process are desired for the urgent demands in WLEDs. Alternatively, silicates phosphors application for WLEDs have achieved rapid development in recently years, exampling as M₃MgSi₂O₈:Eu²⁺,Mn²⁺, M₃SiO₅:Ce³⁺, MAl₂Si₂O₈:Eu²⁺,Mn²⁺, MSiO₃:Eu²⁺, MSi₂O₄:Eu²⁺ (M = Ca, Sr, Ba, Zn, Mg) etc.,^7–11 in view of their merits of strong absorption ability in the near-UV blue light region, high quantum efficiency and excellent chemical stability as well as low cost.

As a member of silicates phosphors, the exploitation of rare earth-doped dicalcium silicate phosphors could be dated back to in 1860s.¹² It was well recognized that dicalcium silicate is a polymorph, at least five of crystalline phases, γ-, β-, α′_L-, α′_H-, α-Ca₂SiO₄, have been known in ascending order with respect to the phase-stability temperature by previous high-temperature X-ray investigations.¹³ Basing on the kaleidoscope of crystal-phases (γ-, β-, α′_L-, α′_H-, α-Ca₂SiO₄) of dicalcium silicate, a plenty of rare earth doped phosphors with variable luminescence have been exploited for applications in WLEDs.^14–23 As for rare earth-doped dicalcium silicate phosphors, the doping rare-earth ions play the role of crystal-phase-stabilizer (CPS) on β-form at the same time. This is the reason why the early reported Ca₂SiO₄-based phosphors were confined to β-form.^12,14–16 Until to recent years, other crystalline-phase Ca₂SiO₄ phosphors with diversity of luminescent properties were exploited gradually by incorporating suitable CPS to shield the stability effect of rare earth ions on β-form. Such as, Jang and Kalaji et al. reported the production of γ-Ca₂SiO₄:Ce³⁺ yellow emitting phosphor by incorporating Li⁺ or Al³⁺ ions, which was employed as γ-form CPS.^17,18 Xia' and Lin' as well as our previous works showed that incorporation of Sr atoms into Ca₂SiO₄ might make α′_L-form quenchable to room temperature, resulting in the formation of α′_L-Ca_2−xSr_xSiO₄:Ce³⁺ and α′_L-Ca_2−xSr_xSiO₄:Eu²⁺ phosphors.^19–21 In addition, Sato reported the occurrence of phase transformation from β-form to α′_L-form in Ca_2−xEu_xSiO₄ system induced by the increasing the Eu²⁺ doping content and α′_L-Ca₂SiO₄:Eu²⁺ red-emitting phosphors was obtained when the Eu²⁺ doping concentration is larger than x = 0.2.²² Very recently, a systematic research about the entire five crystalline-phases of Eu²⁺ doped dicalcium silicate phosphors with tunable emission was demonstrated by our group.²³ To the best of our knowledge, however, there are only a very few papers pertaining to the investigation of luminescence of rare earth ions in α′_H- and α-Ca₂SiO₄, which are high-temperature crystalline-phases in thermodynamics. One of these is our recent publication about the five crystalline-phases of Eu²⁺ doped dicalcium silicate phosphors;²³ another paper described the luminescence of α-Ca₂Si_0.71P_0.29O₄:Eu²⁺ phosphor stabilized by incorporating phosphate ions.²⁴ Therefore, further efforts are needed to be paid on the exploitation of rare earth doped high-temperature α′_H- and α-Ca₂SiO₄ phosphors and the understanding of luminescence for these phosphor materials. Additionally, exploitation of high-temperature α′_H- and α-Ca₂SiO₄ phosphors will enrich greatly the systems of silicates phosphors.

Herein, we focused our attentions to synthesize high-temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors stabilized at room temperature by incorporating suitable content of phosphorus ions, which is employed to act as CPS on α′_H- and α-forms with various doping concentration. The influence of phosphorus ions on the crystal phase and photoluminescent properties of Eu²⁺ ions in α′_H- and α-Ca₂SiO₄ hosts are investigated in detail.

Experimental section

Traditional solid-state reaction method was employed to prepare high temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors, using CaCO₃ (99.999%), nano-SiO₂ (99.9%, 30 ± 5 nm), Eu₂O₃ (99.99%), and NH₄H₂PO₄ (99.9%) as the raw material powders. According to the designed stoichiometric compositions, Ca_2−xSi_1−mP_mO₄:xEu²⁺ (m = 0.12, 0.42; x = 0.02, 0.04, 0.08, 0.10, 0.20, 0.30, 0.40), incorporating with suitable content of phosphorus ions, raw material powders were weighted precisely and grinded together in an agate mortar with a small amount of ethanol. Then the dried mixture was transferred into alumina crucibles and sintered at 1100 °C for 5 h with one intermediary grinding, following which the powders were second sintered in the tube furnace at 1450 °C for 10 h. For all sintering processes, N₂–H₂ (N₂/H₂ = 92 : 8 in volume) reduction atmosphere was performed. Upon cooling down to room temperature and grinding to fine powders, final α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors samples were obtained.

The X-ray diffraction (XRD) patterns of the as-received phosphors were measured by a diffractometer (Rigaku, RINT Ultima-III, Japan) with a graphite monochromator using Cu-Kα radiation, operating at 40 kV and 40 mA. The scanning rate for phase identification was fixed at 4° min⁻¹ with a 2θ range from 20° to 50°, and the data for the Rietveld analysis were collected in a step-scanning mode with a step size of 0.01° and 2 s counting time per step over a 2θ range from 5° to 130°. Photo-luminescence (PL) and photoluminescence of excitation (PLE) spectra of samples were recorded by a fluorescence spectrometer (Hitachi F-4600, Japan). The diffuse-reflectance spectra (DRS) were recorded by UV-visible spectrometer (TU-1901, China) with integration sphere. CIE chromatic coordinates for the prepared phosphors were examined on a PMS-50 Plus UV-Vis-Near IR Spectro-photocolorimeter (EVERINE, China) system equipped with phosphor excitation equipment (PE-II, λ_ex = 365 nm). The quantum efficiency of samples was measured in QY-2000 integration sphere fluorescence spectrometer equipped with a 450 W Xe lamp as excitation source. Thermal stability of the phosphors was tested by an exciting spectra and thermal quenching analyzer for phosphor (Everfine Co. Ltd, EX-1000, China).

Results and discussion

Among the dicalcium silicate polymorphs, γ-Ca₂SiO₄ is the thermodynamically stable phase at room temperature, with the orthorhombic olivine types of structure (space group Pbnm); β-Ca₂SiO₄, a metastable monoclinic phase (space group P2₁/n) under room conditions; α′_L- and α′_H-Ca₂SiO₄ orthorhombic phases stable at higher temperature, and α′_L-Ca₂SiO₄ (space group Pna2₁) is generally considered to be a superstructure of α′_H-Ca₂SiO₄ (space group Pama); while the structure of highest-temperature α-Ca₂SiO₄ is still uncertain and is proposed to have trigonal or hexagonal symmetry. It was reported that high temperature crystalline-forms β-, α′_H- and α-Ca₂SiO₄ could be stabilized by incorporating certain amounts of phosphate ions.^25,26 The phase diagram of Ca₂SiO₄–Ca₃(PO₄)₂ system is depicted in Fig. 1(a). In this phase diagram, Ca₂SiO₄ and Ca₃(PO₄)₂ is indexed as C₂S and C₃P respectively; phase α′-C₂S indicates the higher temperature phases α′_L- or α′_H-C₂S; phase A is the highest-temperature phase α-C₂S; phase R is the solid solution of α′-C₂S and α-C₃P; phase S is the solid solution of 5Cao–P₂O₅–SiO₂. It shows that with increasing amounts of phosphate first the β-phase and then the α′-phase of Ca₂SiO₄ is stabilized. With about 50 wt% Ca₃(PO₄)₂ finally the highest temperature form α-Ca₂SiO₄ is quenchable at room temperature. This feature endows the opportunity in synthesizing high-temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors.

Fig. 1 (a) Phase diagram of Ca₂SiO₄(C₂S)–Ca₃(PO₄)₂(C₃P) system, phase α′-C₂S indicates the higher temperature phases α′_L- or α′_H-C₂S, phase A indicates the highest-temperature phase α-C₂S, phase R is the solid solution of α′-C₂S and α-C₃P, phase S is the solid solution of 5Cao–P₂O₅–SiO₂; (b) XRD patterns for as-prepared Ca_1.98Si_1−mP_mO₄:0.02Eu²⁺ (m = 0.00, 0.12, 0.42) phosphors incorporated variable amount of phosphate ions; Rietveld plots and the main parameters of the processing and refinement for (c) α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ sample; and (d) α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ sample.

The crystalline phase of our prepared Ca_1.98Si_1−mP_mO₄:0.02Eu²⁺ (m = 0.00, 0.12, 0.42) samples incorporated variable amount of phosphate ions was measured by the XRD, as shown in Fig. 1(b). These diffraction patterns were indexed to be β-Ca₂SiO₄ (ICSD no. 79550), α′_H-Ca₂SiO₄ (ICSD no. 82997) and α-Ca₂SiO₄ (ICSD no. 82999) respectively. β-Modification was identified for the phosphate ions free sample because of the appearance of Eu²⁺ ions, which acts the CPS on β-form. Higher temperature phase α′_H-Ca₂SiO₄ and highest-temperature phase α-Ca₂SiO₄ were obtained when the incorporation amount of phosphate ions is m = 0.12 and m = 0.42 respectively. These observations are well agreeable with the results reported in elsewhere.^25,26 The powder diffraction data of α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ and α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ samples were then refined to clear the their crystalline structure via the Rietveld analysis performed by using TOPAS 4.2. The Rietveld plots for these two samples and main parameters of the processing and refinement were shown in Fig. 1(c) and (d) respectively. We can find that the α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ sample could be indexed to the orthorhombic cell (Pnma) with parameters of a = 6.831 Å, b = 5.436 Å and c = 9.394 Å. While α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ sample could be indexed to the hexagonal cell (p3̄m1) with parameters of a = 5.394 Å, b = 5.394 Å, c = 7.171 Å. Obvious shrinkage of crystal cell were found for these phosphate ions incorporated samples owing to the replacement of larger Si⁴⁺ ions (0.40 Å) by smaller P⁵⁺ ions (0.31 Å), comparing with the corresponding original crystalline cells free of phosphate ions (α′_H-Ca₂SiO₄: a = 6.871 Å, b = 5.601 Å and c = 9.556 Å; α-Ca₂SiO₄: a = 5.532 Å, b = 5.532 Å, c = 7.327 Å). The α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors stabilized by the incorporation of phosphate ions might give important insights into the luminescence of Eu²⁺ ions in high temperature Ca₂SiO₄-modifications. It is worth to note that there would generate vacancies on the Ca-sublattice to balance the charge difference as Si⁴⁺ ions were preplaced by P⁵⁺ ions.

As above mentioned that phase transformation from β-form to α′_L-form in Ca_2−xEu_xSiO₄ system was observed by researchers if the Eu²⁺ doping content was larger x = 0.20 mol.^22,23 Fig. 2(a) showed the XRD patterns for the prepared Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ (x = 0.02, 0.04, 0.08, 0.10, 0.20, 0.30, 0.40) phosphor samples with variable Eu²⁺ doping concentration. It is obviously observed that the crystalline-phase for all these samples incorporated with 0.12 mol phosphate ions can be well indexed as α′_H-phase independent on the Eu²⁺ doping content. Apparently, the stable existence of α′_H-phase even with higher Eu²⁺ dopants (0.20–0.40 mol) obtained in this work is ascribed to the incorporation of phosphate ions, which acts as a strong CPS on α′_H-form in suppressing the phase-transformation induced by higher Eu²⁺ dopants. Fig. 2(b) depicted the diffuse reflectance spectra (DRS) of α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphor samples doped with various Eu²⁺ concentration and that of pure α′_H-Ca₂Si_0.88P_0.12O₄ host without doping Eu²⁺ ions. For the pure α′_H-Ca₂Si_0.88P_0.12O₄ host, a strong absorption band below 300 nm was observed, corresponding to the valence-to-conduction band transitions of α′_H-Ca₂Si_0.88P_0.12O₄ host lattice. In the Eu²⁺ doped α′_H-Ca₂Si_0.88P_0.12O₄ phosphor samples, strong absorption bands are in the wavelength range from 250 nm to 500 nm, which can be attributed to the transition absorption from ground state 4f⁷ to its excited states 4f⁶5d of Eu²⁺ doped in α′_H-Ca₂Si_0.88P_0.12O₄ hosts. With the increase of Eu²⁺ ions doped content, an obvious red-shift for the transition absorption band of Eu²⁺ ions is observed. In addition, the absorption intensity is enhanced with the increase of doped content of Eu²⁺ ions.

Fig. 2 (a) XRD patterns; (b) diffuse reflectance spectra (DRS); (c) normalized PL spectra (λ_ex = 350 nm); and (d) PLE spectra monitored at emission peaks for as-prepared α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphor samples doping with variable Eu²⁺ ions concentration. Diffuse reflectance spectrum of pure α′_H-Ca₂Si_0.88P_0.12O₄ host without doping Eu²⁺ ions was given a reference in (b).

The normalized PL spectra under the excitation of 350 nm for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphor samples were illustrated in Fig. 2(c). α′_H-Ca₂Si_0.88P_0.12O₄:Eu²⁺ phosphors showed broad band emission with variable wavelength peaks, depending on the Eu²⁺ doping concentration. These broad emission bands originate from the 4f⁶5d → 4f⁷ transition of Eu²⁺ ions doped in α′_H-Ca₂Si_0.88P_0.12O₄ lattice. It is obviously observed that the emission peak shifts from 490 nm to 640 nm with the increasing of Eu²⁺ doping concentration from 0.02 to 0.40 mol. The red-shifting phenomenon is widely reported in Eu²⁺ ions doped phosphors, resulting from higher doping concentrations. The most common reason for this is an increase of the crystal field splitting, which lowers the 5d → 4f transition energy. However, the increasing substitution of larger Eu²⁺ ions for smaller Ca²⁺ ions will result in the lattice expansion of host lattice. This lattice expansion should instead shift the 5d → 4f emission of Eu²⁺ to the higher-energy side by decreasing the crystal field splitting strength. Another possible reason for the red-shift resulting from higher concentrations of Eu²⁺ dopants is thought to be re-absorption. In this situation, the re-absorption reduces the higher-energy portion of the emission band, while the longer-wavelength portion gains intensity. Hence, a characteristic feature for re-absorption induced red shift is no change in the emission band shape. Re-absorption mechanism contributes to the red-shift for lower Eu²⁺ dopant samples (x = 0.02–0.10 mol) in Fig. 2(c), taking into account about the consistent band shape. However, an obvious change about the emission band shape is observed for Eu²⁺ dopant concentrations between x = 0.10 to x = 0.40. For the higher Eu²⁺ dopant samples (x = 0.20–0.40 mol), their emission band could be split into two independent Gaussian peaks, indicating a contribution from two types of Eu²⁺ luminescence centers. As stated in our previous work, two Ca²⁺ sites (Ca(1)O₁₅ polyhedra and Ca(2)O₁₁ polyhedra) exist in the α′_H-Ca₂SiO₄ host, which will lead to two types of Eu²⁺ luminescent centers.²³ For samples with lower doping concentrations (x = 0.02–0.10 mol), the Eu²⁺ ions prefer to occupy the larger Ca²⁺(1) sites rather than the smaller Ca²⁺(2) sites (the average Ca²⁺–O²⁻ bond length for Ca²⁺(1) sites (2.52 Å) is longer than that in Ca²⁺(2) sites (2.50 Å)). It should be expected that the nephelauxetic effect and crystal-field splitting for Eu²⁺ ions occupying Ca²⁺(2) sites will be stronger than that of Eu²⁺ in Ca²⁺(1) sites owing to the shorter Eu²⁺–O²⁻ bond. Thus, the emissions for lower dopant concentrations α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ (x = 0.02–0.10 mol) phosphor systems were assigned to the substitution of Eu²⁺ in Ca²⁺(1) sites. As the Eu²⁺ dopant concentration increases to 0.20 mol, the emission band was split into two bands, indicating the valid occupancies of Eu²⁺ ions in both of Ca²⁺(1) and Ca²⁺(2) sites. Further increases in the Eu²⁺ concentration leads to a more pronounced emission band above 600 nm (Eu²⁺ in Ca²⁺(2) sites), whereas the emission band below 600 nm (Eu²⁺ in Ca²⁺(1) sites) gets weaker and eventually disappears at a Eu²⁺ dopant concentration of 0.40 mol due to concentration quenching. Therefore, the red-shift observed in larger Eu²⁺ dopants samples could be ascribed to the variation of valid Eu²⁺ luminescent center from Ca²⁺(1) sites to Ca²⁺(2) sites.

The PLE spectra monitored at emission peaks for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphor samples were shown in Fig. 2(d). Broad excitation bands in the range of 250 nm to 500 nm were traced for these phosphors, indicating the promising prospect in fabricating white LEDs with near-UV or blue LED chips. With the increase of Eu²⁺ dopant concentration, the excitation band edge shifts to longer wavelength, which is agreeable with the observation in DRS (Fig. 2(b)).

As observed in Fig. 1(b), highest temperature α-phase could be quenchable at room temperature by incorporating 0.42 mol phosphate ions. The XRD patterns for 0.42 mol phosphate ions stabilized Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ (x = 0.02, 0.04, 0.08, 0.10, 0.20, 0.30, 0.40) phosphor samples doped with variable Eu²⁺ content are depicted in Fig. 3(a). Well crystallized α-phase is identified for these samples independent on the Eu²⁺ doping content, owing to the strong stabilized effect of 0.42 mol phosphate ions incorporation on α-form dicalcium silicate. Similar with the observation in α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphors, the transition absorption band of Eu²⁺ ions in α-Ca_2−xSi_0.58P_0.42O₄ phosphors becomes stronger and behaviors a red-shift with the increasing of Eu²⁺ doping concentration, as shown in Fig. 3(b).

Fig. 3 (a) XRD patterns; (b) diffuse reflectance spectra (DRS); (c) normalized PL spectra (λ_ex = 350 nm); and (d) PLE spectra monitored at emission peaks for as-prepared α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺: xEu²⁺ phosphor samples doping with variable Eu²⁺ ions content. Diffuse reflectance spectrum of pure α-Ca_2−xSi_0.58P_0.42O₄ host without doping Eu²⁺ ions was given a reference in (b).

The normalized PL under the excitation of 350 nm and PLE spectra monitored at emission peaks for α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors doped with variable Eu²⁺ content were illustrated in Fig. 3(c) and (d) respectively. Similar red shift from 508 nm to 640 nm is also observed on the normalized PL spectra for α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors with the increasing of Eu²⁺ dopant concentration. The slight red shift without changing the emission band shape for lower Eu²⁺ dopant concentrations (x = 0.02–0.10 mol) was due to re-absorption mechanism. The obvious change in the emission band shape was observed for concentrations between x = 0.10 to x = 0.40, implying the variation of valid luminescent centers in α-Ca_2−xSi_0.58P_0.42O₄ host. The obvious red-shift induced by larger Eu²⁺ ions dopant concentration could be ascribed to a change of the Eu²⁺ luminescent centers between Ca²⁺(1) to Ca²⁺(2) sites in α-Ca_2−xSi_0.58P_0.42O₄ lattice. Ca(1)O₁₄ polyhedra with longer Ca²⁺–O²⁻ bond length (2.68 Å) is responsible for the green emitting light while Ca(2)O₆ polyhedra with shorter Ca²⁺–O²⁻ bond length (1.95 Å) is assigned to the deep red emitting light. Broad excitation bands in the range of 250 nm to 500 nm were traced for α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors, showing potential application in white LEDs.

The relative PL spectra under the excitation of 350 nm for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphors and α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors were given in Fig. 4(a) and (b) respectively. The optimal Eu²⁺ doping concentration (highest luminous intensity) for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphors and α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors were detected to be 0.04 mol and 0.02 mol, respectively. As the doping concentration above the optimal values, the emission intensity of phosphors begins to decrease owing to the concentration quenching effect. In addition, the emission peaks show an obviously red-shift with the increasing of the Eu²⁺ dopant concentration. Fig. 4(c) and (d) showed the Commission International del’Eclairage (CIE) chromaticity coordinates and corresponding emitting photographs for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ and α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors under the excitation of 365 nm. It could be seen that the emitting color of the phosphors can be easily modulated from the green light to the yellow-red light by increasing the Eu²⁺ doping concentration for both α′_H-Ca₂Si_0.88P_0.12O₄:Eu²⁺ phosphors and α-Ca₂Si_0.58P_0.42O₄:Eu²⁺ phosphors. Accordingly, the corresponding CIE coordinates of Ca_1.79−xSi_0.58P_0.42O₄:xEu²⁺ change from green region to orange-red region, along with the increase of the Eu²⁺ concentration.

Fig. 4 Relative PL spectra (λ_ex = 350 nm) for as-prepared (a) α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphors samples; (b) α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors samples doped with variable Eu²⁺ content; CIE chromaticity coordinates and the corresponding emitting photographs (λ_ex = 365 nm) for as-prepared (c) α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphors samples; (d) α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors samples doped with variable Eu²⁺ concentration.

In the LEDs application, especial for those higher power LEDs, excellent thermal stability for phosphors is in favor of keeping the chromaticity and output brightness. Fig. 5 represented the relative emission intensity of as-prepared α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ and α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ phosphors samples as a function of temperature using commercial YAG:Ce³⁺ phosphor as a benchmark. As the temperature increases from room temperature to 210 °C, the emission intensity for α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ and α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ samples decreased by 65% and 70% of the initial value respectively. Obviously, the thermal stability of the high temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors was much inferior, comparing with the YAG:Ce³⁺ phosphor reference (intensity decrease about 17% of the initial value). The poor thermal stability for α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ and α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ phosphors might be ascribed to the thermodynamic instability of high-temperature crystalline α′_H- and α-Ca₂SiO₄ phases.

Fig. 5 Temperature dependent emission intensity for as-prepared α′_H-Ca_1.98Si_0.88P_0.12O₄:0.02Eu²⁺ (λ_ex = 400 nm) and α-Ca_1.98Si_0.58P_0.42O₄:0.02Eu²⁺ (λ_ex = 400 nm) phosphors samples as well as the commercial YAG:Ce³⁺ (λ_ex = 460 nm) reference.

Luminescence quantum efficiency (η_QE) is very important for phosphor materials in the practical application. The absolute η_QE values (listed in Table 1) for as-prepared α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ phosphors and α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors samples were calculated through the following equation:

η_QE = L_S/(E_R − E_S)

in which L_S is the emission spectrum of the studied phosphor sample, E_S and E_R is the excitation profile of the exciting light with the sample and without the sample in the sphere, respectively. The measured η_QE values for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ (x = 0.02, 0.04, 0.08, 0.20, 0.40) phosphor samples were determined as 34.73%, 36.01%, 21.27%, 8.70% and 10.12% respectively. As for the prepared α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ (x = 0.02, 0.04, 0.08, 0.20, 0.40) phosphor samples, η_QE values were measured to be 22.49%, 15.73%, 11.16%, 6.85% and 3.70% respectively. Because of the concentration quenching effect, the η_QE value for the prepared phosphors decrease gradually with the increase of Eu²⁺ doping content.

Table 1 The measured luminescence quantum efficiency η_QE values for as-prepared α′_H-Ca₂Si_0.88P_0.12O₄:Eu²⁺ phosphors and α-Ca₂Si_0.58P_0.42O₄:Eu²⁺ phosphors samples doped with variable Eu²⁺ content

Quantum efficiency	x = 0.02	x = 0.04	x = 0.08	x = 0.20	x = 0.40
α′H-Ca2−xSi0.88P0.12O4:xEu^2+	34.73%	36.01%	21.27%	8.70%	10.12%
α-Ca2−xSi0.58P0.42O4:xEu^2+	22.49%	15.73%	11.16%	6.85%	3.70%

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Conclusions

In summary, high-temperature crystalline α′_H- and α-Ca₂SiO₄:Eu²⁺ phosphors were quenched at room temperature by incorporating suitable content of phosphorus ions and their photoluminescent properties were investigated in this work. Eu²⁺ doped α′_H- and α-Ca₂SiO₄ high-temperature crystalline phase phosphors could be stabilized to room temperature with incorporation of 12% and 42% phosphorus ions, respectively, independent on the Eu²⁺ doping concentration. The variation of the Eu²⁺ luminescent centers between multi-Ca sites in both of α′_H- and α-Ca₂SiO₄ lattices resulted by the increasing Eu²⁺ incorporation concentration was identified to be the reason for the tunable emissions from green to orange-red light. The quantum efficiency for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ and α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ was found to be decrease with the increase of Eu²⁺ doping concentration owing to the concentration quenching effect. The excellent tuning characters for α′_H-Ca_2−xSi_0.88P_0.12O₄:xEu²⁺ and α-Ca_2−xSi_0.58P_0.42O₄:xEu²⁺ phosphors presented in this work exhibited their various applications prospect in solid-state lighting combing with blue chip or near-UV chip.

Acknowledgements

We gratefully acknowledge the financial support by the National Natural Science Foundation of China (no. 50872091 and 51102265) and Program of Discipline Leader of Colleges and Universities (Tianjin, China) and “Foreign Experts” Thousand Talents Program (Tianjin, China).

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Footnote

† These authors contributed equally to this work and should be considered as co-first authors.

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