Color tuning and energy transfer in Eu²⁺/Mn²⁺-doped Ba₃Y(PO₄)₃ eulytite-type orthophosphate phosphors

Abstract

Eulytite-type orthophosphate phosphors Ba₃Y(PO₄)₃:Eu²⁺, Mn²⁺ and Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) were synthesized by high-temperature solid state reactions under reductive atmospheres. Their photoluminescence showed a surprising red-shift in the emission spectrum with the increase in the ionic radius of Ln in the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphors system, which arises from the splitting of the 5d energy level. The phase formation, luminescence properties, and energy-transfer mechanism from the Eu²⁺ to the Mn²⁺ ions, and the CIE coordinates in the Ba₃Y(PO₄)₃:Eu²⁺, Mn²⁺ phosphors were investigated. From powder X-ray diffraction (XRD) analysis, the formation of the single-phased Ba₃Y(PO₄)₃ in a cubic crystal system with the space group I4̄3d (no. 220) was confirmed. With the doping of Mn²⁺, the spectral overlap between the emission spectrum of Eu²⁺ and the excitation spectrum of Mn²⁺ allows resonance-type energy transfer to occur from Eu²⁺ to Mn²⁺ with the mechanism carefully studied by luminescence spectra, energy transfer efficiency and decay times. By increasing the Mn²⁺ doping concentration in the Ba₃Y(PO₄)₃:Eu²⁺, Mn²⁺ phosphors, the emission colors can be tuned from yellowish-green through yellow and ultimately to orange. Such color tuning emissions originate from the change in intensity between the 4f–5d transitions of the Eu²⁺ ions and the ⁴T₁–⁶A₁ transitions of the Mn²⁺ ions through the energy transfer from the Eu²⁺ to the Mn²⁺ ions. In particular, compared with the commercial YAG:Ce phosphor, our developed phosphor contains a larger amount of red-emitting component; thus, it possesses favorable properties for application in warm white LEDs.

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1. Introduction

White light-emitting diodes (LEDs) have attracted tremendous attention due to their broad application prospects in full-color flat-panel displays, back-lighting sources for liquid-crystal displays and general solid-state lighting sources.^1–6 Currently, the most common approach to realizing white LEDs is to combine an InGaN-based blue diode with yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor. However, white LEDs based on YAG:Ce phosphor exhibit an unsatisfactory high correlated color temperature (T_c > 4500 K) and low color rendering indexes (R_a < 80) for general illumination due to the weak emission in the red spectral region.^7–9 In this regard, it is essential to develop orange-yellow-emitting phosphors, which have more emission saturation in the red than YAG:Ce. Although many attempts have been made to develop orange-yellow-emitting phosphors based on nitride and oxynitride, and despite their superior properties, almost all the nitride- and oxynitride-type phosphors require critical preparation conditions, such as high temperature, high pressure, and carbothermal reactions,^10–12 which limit their application. Therefore, it is important to develop oxide-type orange-yellow-emitting phosphors using comparatively mild synthesis routes for white LEDs.

Recently, eulytite-type orthophosphate compounds with the general composition M₃Ln(PO₄)₃ (M = alkaline earth; Ln = trivalent rare earth) have been extensively reported as a suitable phosphor host due to their excellent thermal stability, simple synthesis conditions, and good optical properties when doped with rare-earth activators.^13–15 In the compound M₃Ln(PO₄)₃, the Ln³⁺/M²⁺ pairs of cations are disordered on a single crystallographic site (C₃ point group symmetry), while the oxygen atoms of the phosphate groups are distributed over several partially occupied sites. This type of compound was found to show not only disorder in the cation but also an oxygen sub-lattice disorder.^16–18 Thus, this disordered structure provides a family of sites for the doping ions to occupy. Therefore, it is worth studying new efficient oxide-type orange-yellow-emitting luminescent materials with structures derived from eulytite-type compounds. These facts prompted us to study the luminescence properties of Ba₃Y(PO₄)₃ activated by Eu²⁺ and Mn²⁺ ions. In addition, to the best of our knowledge, the effect of Eu²⁺- and Mn²⁺-doping in the Ba₃Y(PO₄)₃ host has not been reported to date.

In this study, a series of orange-yellow-emitting eulytite-type orthophosphate phosphors, Ba₃Y(PO₄)₃:Eu²⁺, Mn²⁺ (BYP:Eu²⁺, Mn²⁺), were prepared by a conventional solid-state reaction. The photoluminescence excitation and emission spectra, energy transfer mechanism, and color tuning properties were investigated in detail. In particular, the luminescent properties of the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphor system are discussed in terms of crystal field strength and the Eu–O bond.

2. Experimental section

2.1. Materials and synthesis

The powder samples Ba_3(1−x−y)Y(PO₄)₃:xEu²⁺, yMn²⁺ (BYP:xEu²⁺, yMn²⁺) and Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) were synthesized by a solid-state reaction method. The reactants BaCO₃ (A.R. (Analytical Reagent)), Ln₂O₃ (Ln = Lu, Y and Gd; 99.99%), NH₄H₂PO₄ (A.R.), Eu₂O₃ (99.99%) and MnCO₃ (A.R.) were weighed according to stoichiometric ratio. After mixing and grinding, the mixtures were heated at 1300 °C for 4 h under a 10% H₂–90% N₂ gas mixture. Finally, the as-synthesized samples were slowly cooled to room temperature.

2.2. Measurements and characterization

The phase purity of the obtained phosphors were carefully checked using powder X-ray diffraction (XRD) analysis (Bruker AXS D8) in the 2θ range from 10° to 80° with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm) operating at 40 kV and 40 mA. A step size of 0.02° (2θ) was used with a scanning speed of 10° min⁻¹ in the 2θ range. The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The quantum efficiency (QE) was analyzed with a PL quantum efficiency measurement system (C9920-02, Hamamatsu Photonics, Shizuoka) containing a 150 W xenon lamp. All the measurements mentioned above were performed at room temperature.

3. Results and discussion

3.1. Phase identification

Fig. 1 shows the powder X-ray diffraction (XRD) patterns of the as-synthesized BYP:0.005Eu²⁺, yMn²⁺ (y = 0–0.10) samples in contrast to the standard card of Ba₃Y(PO₄)₃ (JCPDS 44-0318) for comparison. It was found that all the diffraction peaks of the obtained samples can be indexed to the standard card of Ba₃Y(PO₄)₃ and no detectable impurity phase appeared. The results indicate that the introduction of the dopants Eu²⁺/Mn²⁺ did not cause any significant change in the host structure in our experimental doping-content ranges and the doping ions Eu²⁺/Mn²⁺ completely entered the host lattice. In addition, one can see that the marked diffraction peak at the 2-theta value of 47.5° was right-shifted to a higher angle with an increase in the Mn²⁺ concentration, which can be ascribed to the successful substitution of the larger Ba²⁺ by the smaller Mn²⁺ ions. The unit cell constants a and V of the BYP:0.005Eu²⁺, yMn²⁺ (y = 0–0.10) samples were calculated by refining the powder XRD data according to the Rietveld method. From Fig. 2, one can see that the unit cell constants a and V for the BYP:0.005Eu²⁺, yMn²⁺ samples decreases linearly with an increase in Mn²⁺ concentration, which is consistent with the Vegard's law. The XRD analysis and change in the unit cell constants confirm that the dopants Eu²⁺ and Mn²⁺ have been homogeneously incorporated into the BYP host lattice. In consideration of the effective ionic radii of the cations and the electric charge balances, it was predicted that Eu²⁺ and Mn²⁺ prefer to randomly occupy the Ba²⁺ sites in the BYP host structure.

Fig. 1 XRD patterns of the BYP:0.005Eu²⁺, yMn²⁺ samples. The standard data for Ba₃Y(PO₄)₃ (JCPDS card no. 44-0318) is shown as a reference.

Fig. 2 The relationship between the unit cell constants (a and V) and Mn²⁺ content (y).

3.2. Photoluminescence properties

Fig. 3a displays the PLE and PL spectra of the BYP:0.005Eu²⁺ phosphor. The PLE spectra showed a broad absorption between 250 and 420 nm, which was assigned to the 4f⁷ → 4f⁶5d¹ transition of the Eu²⁺ ions. The broad excitation band matches well with the UV LED chips for application in white LEDs. Upon excitation at 350 nm, the PL spectrum comprises several broad bands from 400 to 700 nm, which are attributed to the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions. The shape of the PL spectrum appears to be asymmetric and broad, indicating that the present host lattice has a family of sites for the Eu²⁺ ions to occupy. The crystal structure of Ba₃Y(PO₄)₃ and the coordination environment of the Ba/Y and P are depicted in the ESI.† The Ba₃Y(PO₄)₃ host contains two possible different orientations of the [PO₄] tetrahedron, corresponding to the two sets of partially occupied oxygen positions. The Ba²⁺/Y³⁺ pairs of cations are disordered on a single crystallographic site (C₃ point group symmetry), while the oxygen atoms of the phosphate groups are distributed over two partially occupied sites. To distinguish the different luminescence sites of the Eu²⁺ ions, the emission curve has been decomposed into four well-separated Gaussian components with peaks at 2.95 eV, 2.40 eV, 2.26 eV, and 2.05 eV (corresponding to 420 nm, 516 nm, 548 nm, and 606 nm, respectively) on an energy scale, as depicted in Fig. 4. These results indicate that there are four luminescence Eu²⁺ centers in the BYP host lattice with different local coordination environments, which is consistent with the fact that the composition of Ba₃Y(PO₄)₃ was found to show not only a disorder in the cation but also an oxygen sublattice disorder.^17,18 As shown in Fig. 4, the Eu(B) centered at 2.40 eV (516 nm) may act as the main luminescence center due to its relatively high intensity.

Fig. 3 PLE and PL spectra of (a) the BYP:0.005Eu²⁺ phosphor, (b) the BYP:0.06Mn²⁺ phosphor, and (c) the BYP:0.005Eu²⁺, 0.06Mn²⁺ phosphor.

Fig. 4 The emission spectrum of BYP:0.005Eu²⁺ and its corresponding Gaussian components on an energy scale.

According to the increase in unit cell volume from Ba₃Lu(PO₄)₃:Eu²⁺ to Ba₃Y(PO₄)₃:Eu²⁺ to Ba₃Gd(PO₄)₃:Eu²⁺, we would expect the crystal field splitting to decrease in the same order, resulting in a blue-shift of the emission wavelength. Instead, as shown in Fig. 5, the Ba₃Gd(PO₄)₃:Eu²⁺ compound has the most red-shifted optical properties. The increase in lattice parameters and unit cell volume do not facilitate the prediction of the observed optical properties. Fig. 5 depicts the normalized PL spectra of the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphors. The position of the emission peaks is observed to be slightly shifted to a longer wavelength with the successive increase in the radius of the rare-earth metal ions Lu³⁺, Y³⁺, and Gd³⁺. The crystal field strength increases with a decrease in the bond length; the correlation between the crystal field strength and the bond length is D_q ∝ 1/R⁵, where D_q is the crystal field strength and R is the bond length between the central ion and its ligands.^19–21 Accordingly, the increase in the crystal field could lower the 5d excited state energy level and red-shift the emission peaks for the 5d → 4f transition of Eu²⁺.²² That is to say, the shorter the Eu–O bond is, the longer the emission wavelength can be observed. As mentioned above, the bond length of Eu–O could be greatly influenced by the surrounding ions due to the disorder of the Eu²⁺/Ba²⁺/Ln³⁺ ions at a single C₃ site.²³ In the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphor system, the large ionic radii of the Gd³⁺ ions leads to a stressful compression of the neighboring Eu–O bonds when Ln = Gd, while in the Ba₃Lu(PO₄)₃:Eu²⁺ phosphor, some stress is released owing to the smaller ionic radius of Lu³⁺ than that of Gd³⁺, and thus the bond length of Eu–O becomes longer. This result indicates the highest degree of distortion in Ba₃Gd(PO₄)₃:Eu²⁺, confirming that this compound has the most red-shifted optical properties. The increased degree of distortion causes a larger crystal field splitting. As shown in the inset of Fig. 5, as the ionic radii of Lu, Y and Gd are successively increased, the crystal field splitting becomes larger with a decrease in the Eu–O bond length, leading to a different decrease between the 4f and 5d energy levels of the Eu²⁺ ions. As a result, a red shift of the emission peak is caused with an increase in the ionic radius of Ln in the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphor system. Furthermore, the Eu²⁺ ions that occupy different sites may also contribute to this red shift. Table S1 (See ESI†) displays the integral area of the four Gaussian components (A, B, C, and D) for Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphors. One can see that the integral area of the PL spectrum for the A and B sites decrease, while the integral area for the D site increases from Lu to Y and eventually to Gd. The change in the integral area of the four Gaussian components (A, B, C, and D) is consistent with this red shift.

Fig. 5 The normalized PL spectra of the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphors. The inset shows the schematic energy level diagram of Eu²⁺ ions in the Ba₃Ln(PO₄)₃ host lattice.

Fig. 3b depicts the PLE and PL spectra of BYP:0.06Mn²⁺. The PLE spectrum shows several distinct peaks centered at 365, 405, 500, and 535 nm, which were assigned to the d–d transition of the Mn²⁺ ion from the ground level ⁶A₁(⁶S) to the ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], ⁴T₂(⁴G), and ⁴T₁(⁴G) excited levels, respectively.^24,25 Upon excitation at 365 nm, the PL spectrum exhibits a broad emission band centered at 600 nm, which can be ascribed to the spin-forbidden d–d transition from ⁴T₁(⁴G) to the ground state ⁶A₁(⁶S) of Mn²⁺ ions. Mn²⁺-doped phosphors typically exhibit a broad emission band, and the emission wavelength depends strongly on the crystal-field strength of the host lattice. The stronger the crystal field is, the longer emission wavelength is expected to be observed. At the same time, the coordination number also has a great effect on the emission color.²⁴ Thus, the appearance of the reddish-orange emission band of the Mn²⁺ ions indicates that the Mn²⁺ ions substitute the Ba²⁺ sites. Because the d–d transition in Mn²⁺ centers is at parity and spin-forbidden, its excitation intensity is not sufficiently high in the UV wavelength regions.²⁵ Therefore, co-dopants such as Eu²⁺ or Ce³⁺ are often added to increase the sensitizing action of the crystal lattice.

To enhance the absorption of the samples in the UV region, the dopant Eu²⁺ is expected to act as a sensitizer to transfer energy to the Mn²⁺ ions. As shown in Fig. 3a and b, by comparing the PL spectrum of the BYP:0.005Eu²⁺ and the PLE spectrum of BYP:0.06Mn²⁺, an apparent spectral overlap can be observed. On the basis of the Dexter's theory,²⁶ an effective resonance type energy transfer is expected to take place from the Eu²⁺ to the Mn²⁺ ions. This can be further confirmed by the PL and PLE spectra of BYP:0.005Eu²⁺, 0.06Mn²⁺, as shown in Fig. 3c. For the Eu²⁺ and Mn²⁺ codoped sample, upon excitation into the PLE band of the Eu²⁺ ions, the PL spectrum consists of a yellowish-green band, assigned to the f–d transitions of the Eu²⁺ ions, and a reddish-orange band, attributed to the ⁴T₁–⁶A₁ transitions of the Mn²⁺ ions. In addition, one can see that the PLE spectrum monitoring the emission of the Mn²⁺ is similar to that monitoring the emission of the Eu²⁺ ions, demonstrating the existence of energy transfer from the Eu²⁺ to the Mn²⁺ ions.^27,28 Based on the discussion above, it can be concluded that Eu²⁺ ions can be excited by UV irradiation and that they strongly absorb the energy and then transfer the energy to Mn²⁺. Therefore, the emission from both Eu²⁺ and Mn²⁺ can be observed in a single host.

Fig. 6 depicts the PL spectra of the BYP:0.005Eu²⁺, yMn²⁺ samples with a fixed Eu²⁺ content of 0.005 and a varying Mn²⁺ content y in the range of 0–0.10. Upon excitation at 350 nm, the PL spectra exhibit several broad emission bands, which can be ascribed to the emission of Eu²⁺ and Mn²⁺. Thus, we can adjust the relative intensity of the emission bands by tuning the amounts of the activator, and a multicolor emission can be obtained in a single host through the principle of energy transfer. From Fig. 6, one can see that the PL intensity for Eu²⁺ decreases monotonically with an increase in the Mn²⁺ doping content. Moreover, the reddish-orange emission of the Mn²⁺ ions increases initially until it reaches a maximum at y = 0.06, beyond which it decreases, which is ascribed to the Mn²⁺–Mn²⁺ internal concentration quenching. The observed variations in the emission intensity of the Eu²⁺ and Mn²⁺ ions strongly indicate the energy transfer from Eu²⁺ to Mn²⁺. Moreover, as shown in the inset of Fig. 6, one can see that the emission peak of the Mn²⁺ ions shifts toward long-wavelength from 576 to 611 nm with an increase of the Mn²⁺ content y from 0.005 to 0.10, which could be assigned to the change of the crystal field strength. Because the radius of the Mn²⁺ ions is smaller than that of Ba²⁺ ions, the crystal lattice of BYP shrinks after doping Mn²⁺ ions into the BYP host lattice to replace Ba²⁺ ions, as confirmed by the shift of XRD pattern to higher angles, and then the unit cell constants for the BYP:0.005Eu²⁺, yMn²⁺ samples decrease linearly with the increase in Mn²⁺ content (Fig. 1 and 2). This leads to the enhancement of the crystal field strength surrounding the Mn²⁺ ions and further results in a larger crystal field splitting of the Mn²⁺ 3d energy levels, which brings the lowest 3d state of Mn²⁺ closer to its ground state, and finally gives a red shift to the PL emission peak of the Mn²⁺ ion.^24,25

Fig. 6 PL spectra for the BYP:0.005Eu²⁺, yMn²⁺ phosphors with varying Mn²⁺ doping content (y). The inset shows the dependence of the emission peak of the Mn²⁺ on Mn²⁺ content (y).

To further explore the energy transfer processes and energy transfer efficiency from the Eu²⁺ to the Mn²⁺ ion, the fluorescence decay curves of Eu²⁺ in the BYP:0.005Eu²⁺, yMn²⁺ samples, monitoring the emission of Eu²⁺ at 516 nm, were measured and investigated, as depicted in Fig. 7. It should be noted that the decay curves in the single Eu²⁺-doped BYP:0.005Eu²⁺ sample deviate from a single exponential, indicating that there is more than one luminescent Eu²⁺ center in the host lattice, which is also consistent with the crystal structure of the host lattice. From Fig. 7, one can see that all the decay curves of Eu²⁺ in the BYP:0.005Eu²⁺, yMn²⁺ (y = 0–0.10) samples deviate from a single exponential rule, and this deviation is more evident with an increase in the Mn²⁺ doping concentration. Because of the non-exponential decay of Eu²⁺ in all the samples, the average fluorescence lifetime of Eu²⁺ has been defined as follows:^29,30

where I(t) is the fluorescence intensity at time t. Based on eqn (1), the average fluorescence lifetimes of Eu²⁺ were calculated and are depicted on the left axis in Fig. 8. It can be observed that the lifetime of the Eu²⁺ in the BYP:0.005Eu²⁺, yMn²⁺ (y = 0–0.10) samples decrease monotonically with an increase in the Mn²⁺ doping concentration, further confirming the energy transfer from Eu²⁺ to Mn²⁺ in the BYP:0.005Eu²⁺, yMn²⁺ samples. Assuming that all the excited Mn²⁺ ions decay radioactively, the energy transfer efficiency (η_T) from Eu²⁺ to Mn²⁺ can be obtained according to the data of the average lifetimes by the following equation:³¹

η_T = 1 − τ/τ₀ (2)

where τ and τ₀ are the decay times of the sensitizer Eu²⁺ in the presence and absence of the activator Mn²⁺, respectively. Accordingly, the energy transfer efficiencies (η_T) were calculated as a function of the Mn²⁺ content, and are also illustrated in Fig. 8. As shown on the right axis in Fig. 8, one can see that the energy transfer efficiency from Eu²⁺ to Mn²⁺ increases gradually as the content of Mn²⁺ increases from 0 to 0.10. At y = 0.10 in the BYP:0.005Eu²⁺, yMn²⁺ samples, the energy transfer efficiency can reach 55.8%. This suggests that the energy transfer efficiency from Eu²⁺ to Mn²⁺ is partial and depends strongly on the Mn²⁺ doping content. Compared with Ce³⁺–Tb³⁺ and Ce³⁺–Mn²⁺ codoped eulytite-type phosphors,^32,33 the energy transfer efficiency from Eu²⁺ to Mn²⁺ in the BYP:0.005Eu²⁺, yMn²⁺ samples is relatively low. The main reason for this may be attributed to the small spectral overlap between the emission band of Eu²⁺ and the excitation band of Mn²⁺ in the BYP:Eu²⁺, Mn²⁺ phosphors.

Fig. 7 Photoluminescence decay curves of Eu²⁺ in the BYP:0.005Eu²⁺, yMn²⁺ phosphors displayed on a logarithmic intensity scale (excited at 355 nm, monitored at 516 nm).

Fig. 8 Dependence of the energy transfer efficiency η_T and the fluorescence lifetime of Eu²⁺ on the Mn²⁺ content (y).

In general, the energy transfer between the Eu²⁺ and Mn²⁺ ions mainly takes place via an exchange interaction and electric multipolar interaction. According to the Dexter's energy transfer formula for exchange and multipolar interactions, the following relation can be given:^34–36

In(I₀/I) ∝ C (3)

where I and I₀ are the integrated emission intensities of the PL spectrum for the sensitizer Eu²⁺ in the presence and absence of the activator Mn²⁺, respectively, and C is the sum doping concentration of Eu²⁺ and Mn²⁺. The linear relationship ln(I₀/I) − C corresponds to the exchange interaction, and (I₀/I) − C^n/3 with n = 6, 8, and 10 corresponds to the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The relationships of ln(I₀/I) − C and (I₀/I) − C^n/3 are illustrated in Fig. 9. A liner relation is observed when n = 8. This clearly indicates that the dominant interaction mechanism for the Eu–Mn energy transfer in the BYP host is based on the dipole–quadrupole interaction, which is similar to that previously investigated and observed for Sr₃Lu(PO₄)₃:Eu²⁺, Mn²⁺,¹⁴ Sr₃La(PO₄)₃:Eu²⁺, Mn²⁺ (ref. 37) and Ba₃Lu(PO₄)₃:Eu²⁺, Mn²⁺.³⁸

Fig. 9 (a) Dependence of ln(I₀/I) of Eu²⁺ on C; and I₀/I of Eu²⁺ on (b) C^6/3, (c) C^8/3, and (d) C^10/3.

The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the BYP:0.005Eu²⁺, yMn²⁺ phosphors with varying Mn²⁺ concentration were determined based on their relevant PL spectrum, and are depicted in Fig. 10 and Table 1. The Eu²⁺ doping content was fixed at 0.005 as the concentration of Mn²⁺ was increased from 0 to 0.10; through the energy transfer of Eu²⁺ → Mn²⁺, the corresponding color tone of the BYP:0.005Eu²⁺, yMn²⁺ phosphor shifts from yellowish-green through yellow and ultimately to orange. Accordingly, UV-pumped white LEDs with improved chromaticity quality can be fabricated using a UV-LED chip with our developed BYP:0.005Eu²⁺, yMn²⁺ phosphor system and the blue-emitting BAM:Eu²⁺ (BaMgAl₁₀O₁₇:Eu²⁺) phosphor. Consequently, it is clear that white LEDs with tunable correlated color temperature (CCT) in the triangle ABK region (Fig. 10) can be produced to meet the needs of different illumination applications by simply varying the Mn²⁺ doping concentration (y) in the BYP:0.005Eu²⁺, yMn²⁺ phosphor system. In particular, compared with the commercial YAG-based white LED system, these UV-pumped white LEDs can produce warm white light with low CCT, which is more suitable for room lighting. This is mainly due to the as-developed phosphor, which contains more red-emitting component than YAG:Ce. In addition, it should be noted that the experimental conditions and reactants composition should be optimized to improve the quantum efficiency.

Fig. 10 CIE chromaticity diagram for the BYP:0.005Eu²⁺, yMn²⁺ phosphors excited at 350 nm.

Table 1 Comparison of the CIE chromaticity coordinates (x, y), emissive colors, and quantum efficiency (QE) for the BYP:0.005Eu²⁺, yMn²⁺ phosphors excited at 350 nm

Sample no.	Sample composition (y)	CIE coordinates (x, y)	Emissive colors	Quantum efficiency (QE)
A	0	(0.321, 0.456)	Yellowish-green	17%
C	0.005	(0.342, 0.461)	Yellowish-green	19%
D	0.01	(0.356, 0.459)	Yellow	15%
E	0.02	(0.381, 0.456)	Yellow	20%
F	0.04	(0.413, 0.449)	Yellow	15%
G	0.06	(0.439, 0.441)	Orange	23%
H	0.08	(0.450, 0.437)	Orange	14%
K	0.10	(0.457, 0.436)	Orange	12%

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4. Conclusion

In summary, a series of eulytite-type orthophosphate phosphors Ba₃Y(PO₄)₃:Eu²⁺, Mn²⁺ and Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) were synthesized via a high-temperature solid-state reaction process. In the order of Lu, Y, and Gd for the Ba₃Ln(PO₄)₃:Eu²⁺ (Ln = Lu, Y and Gd) phosphor system, a red shift of the emission peak is caused with increases in the ionic radius owing to the increase in the crystal field strength. The energy transfer from the Eu²⁺ to the Mn²⁺ ions in the BYP:Eu²⁺, Mn²⁺ phosphors was confirmed to occur via a dipole–quadrupole mechanism by the Dexter theoretical model. Upon the excitation of UV light, the emissive colors of the obtained phosphors could be tuned from yellowish-green through yellow and ultimately to orange through energy transfer by adjusting the doping content ratio between Eu²⁺ and Mn²⁺. In particular, an orange emission with CIE coordinates of (0.439, 0.441) and a QE of 23% could be achieved in the BYP:0.005Eu²⁺, 0.06Mn²⁺ phosphor. A red-shift emission of the Mn²⁺ ions toward long-wavelength from 576 to 611 nm was observed in the BYP:0.005Eu²⁺, yMn²⁺ phosphor system with the increase of Mn²⁺ content y from 0.005 to 0.10, which could be assigned to the change of the crystal field strength. The energy transfer efficiency increases gradually as the content of Mn²⁺ increases; at y = 0.10, the energy transfer efficiency can reach 55.8%. This suggests that the energy transfer efficiency from Eu²⁺ to Mn²⁺ is partial and depends strongly on the Mn²⁺ doping content.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grant no. 21401130), the Opening Research Fund of the State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (RERU2014005).

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Footnote

† Electronic supplementary information (ESI) available: The crystal structure of Ba₃Y(PO₄)₃ viewed along [100] direction (Fig. S1). The coordination environment of Ba/Y viewed along [111] direction (Fig. S2). The coordination environment of P viewed along [100] direction (Fig. S3). Comparison of the integral area of the four Gaussian components for Ba₃Ln(PO₄)₃:Eu²⁺ (Table S1). See DOI: 10.1039/c5ra06347g

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