Luminescence and energy transfer of a color tunable phosphor: Tb³⁺ and Eu³⁺ co-doped ScPO₄

Abstract

A series of novel emission-tunable ScPO₄:xTb³⁺, yEu³⁺ phosphors were prepared by a high temperature solid-state reaction. The phase purity was examined using X-ray diffraction refinement. X-ray photoelectron spectroscopy (XPS) and the crystal information, luminescence properties and energy transfer between Tb³⁺ and Eu³⁺ are analyzed systematically. The cross relaxation from ⁵D₃ to ⁵D₄ of single doped Tb³⁺ in the host is investigated. The energy transfer between Tb³⁺ and Eu³⁺ has been demonstrated by the decay times, which are ascribed to the dipole–dipole (d–d) mechanism, and the η_T reaches 54.4%. Additionally, the energy transfer critical distance between Tb³⁺ and Eu³⁺ was calculated to be about 12.95 Å. The emission color can be adjusted from green to yellow to orange-red by tuning the ratio of Tb³⁺/Eu³⁺. The ScPO₄:0.03Tb³⁺, 0.025Eu³⁺ exhibits good thermal stability, indicating its great potential in w-LED applications.

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Introduction

Recently, solid-state lighting based on white light-emitting diodes (w-LEDs) has become a prevailing trend for illumination technology due to their significant advantages such as high luminous efficiency, low energy consumption, long lifetime, and being environmentally friendly. Therefore there is great potential for them to be considered as the next generation of light sources.^1–3 At present, in order to acquire the high quality white light, the w-LEDs could be fabricated with phosphors of three individual primary colours: red, blue and green, which can offer suitable correlated colour temperature and high colour rendering index.⁴

The rare earth (RE) have played a dominating role in the development of modern lighting and display fields because of their broad emission spectrums based on the 4f–4f or 5d–4f transitions. As the increasing demand of the white lighting, the researchers have devoted great efforts to obtain the color tuning of single phased phosphors with different doping via the energy transfer between RE ions.^5–7 It is found that Tb³⁺ can be used as an activator in different hosts, and its emissions mainly attribute to the ⁵D₃–⁷F_J (blue) and ⁵D₄–⁷F_J (green). With the increasing concentration of Tb³⁺, the cross relaxation between the ⁵D₃ and ⁵D₄ energy level takes place owing to the interaction of the Tb³⁺ ions, which results in the enhanced green emission of ⁵D₄–⁷F_J.^8,9 The Eu³⁺ ion is also be one of the most frequently used red or orange-red emitting activators doping in the luminescent materials. It has better lumen equivalency and colour rendering for the reason that the emission line of Eu³⁺ ions is slight host-dependence.^10,11 Moreover, researchers have paid more attention to the energy transfer processes involving the Tb³⁺ and Eu³⁺ ions in co-doping compounds in order to realize the colour tuning at present.^7,12

To our best knowledge, the phosphates are considered as the excellent hosts for luminescent materials because of their good chemical stability and low cost. Especially the compounds YPO₄ and ScPO₄ represent orthophosphate that are structurally related to the lanthanide orthophosphate and with the properties of LnPO₄.^13,14 Up to now, there is not any related reports about the photoluminescence (PL) properties and energy transfer from Tb³⁺ to Eu³⁺ occurring in this host under UV excitation. Furthermore, the potential application of the ScPO₄ phosphors for w-LEDs hasn't been involved. In our work, a series of ScPO₄:xTb³⁺, yEu³⁺ phosphors have been observed via the high temperature solid-state reaction method. A systematic study on their crystal structure, luminescence properties, thermal stability, quantum efficiency, energy transfer between Tb³⁺ and Eu³⁺ and applications in w-LEDs were carried out in detail respectively. White LEDs was fabricated by combing an UV LED chip (λ_max = 385 nm) with the ScPO₄:0.03Tb³⁺, 0.01Eu³⁺ phosphors along with the commercial phosphors BAM:Eu²⁺. All corresponding LED optical parameters were demonstrated.

Experimental

ScPO₄:xTb³⁺, yEu³⁺ phosphors investigated in this work were synthesized by a conventional solid-state reaction method. The raw materials include initial rare-earth oxides, Sc₂O₃ (99.99%), Tb₄O₇ (99.99%) and Eu₂O₃ (99.99%), and (NH₄)₂HPO₄ (A.R.). All chemicals were used directly without any further purification and then were intimately mixed and milled with stoichiometric ratios in an agate mortar, and then sintered at 1200 °C for 4 h in the air to obtain the samples. Finally, the samples were furnace-cooled to room temperature, and were crushed and pulverized for various characterizations.

The crystalline phases of the as-prepared phosphors were identified by X-ray diffraction (XRD; D8 Advance diffractometer, Germany), using Cu-Kα1 radiation (λ = 0.15406 nm) at a tube-voltage of 40 kV and a tube-current of 30 mA with a step size of 0.02 and a scanning rate of 2° min⁻¹. The step scanning rate (2θ ranging from 10° to 100°) used for Rietveld analysis was 2 s per step with a step size of 0.01. Powder diffraction data were obtained by the Rietveld method using the computer software Jana2006.¹⁵ The X-ray photoelectron spectroscopy (XPS) was obtained on an ESCALAB 250xi (ThermoFisher, England) electron spectrometer. Diffuse reflection spectra on as-synthesized phosphors powder samples were detected on a UV-vis-NIR spectrophotometer (SHIMADZU UV-3600) attached to an integrating sphere. BaSO₄ was used as a reference standard. Room temperature photoluminescence (PL) measurements of all samples were recorded on a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer equipped with a photomultiplier tube operating at 400 V, and using a 150 W Xe lamp as the excitation source. The photoluminescence decay curves and quantum efficiency were determined by a spectrofluorometer (HORIBA, JOBIN YVON FL3-21) with an integral sphere, and the 370 nm pulse laser radiation (nano-LED) was used as the excitation source. The EL spectrum was detected by the FLS 980 Spectrometer.

Results and discussion

The phase purity of all samples is examined by XRD. As illustrates in Fig. 1, the similar XRD patterns of samples ScPO₄ (a), ScPO₄:0.03Tb³⁺ (b) and ScPO₄:0.03Tb³⁺, 0.02Eu³⁺ (c) were all corresponded to the standard pattern of tetragonal ScPO₄ (JCPDS 84-336) and no other impurity phase exit. Therefore, the crystal structure of the samples were not significantly influenced by the dopant Tb³⁺, Eu³⁺ ions. For further demonstrating the phase purity and the occupancy of Tb³⁺ and Eu³⁺ ions in the host ScPO₄, the Rietveld refinement of ScPO₄ and ScPO₄:0.03Tb³⁺, 0.02Eu³⁺ phosphors were analyzed via the Jana2006 program as shown in Fig. 2 and ScPO₄ was served as an initial structural model. As the results revealed, neither the host nor the phosphors doped with Tb³⁺ and Eu³⁺ ions generated any impurity or secondary phases in ScPO₄. ScPO₄ crystallizes in the tetragonal system with space group I4₁/amd (141). The lattice parameters of the host were fitted to be a = b = 6.578 (7) Å, c = 5.796(3) Å, V = 250.5(5) ų, the weighted profile R-factor (R_wp) and the expected R-factor (R_p) are 5.52% and 3.75%, respectively. As doped with Tb³⁺ and Eu³⁺, the parameters were a = b = 6.595(7) Å, c = 5.807(4) Å, V = 252.6(2) ų. The refinement data became R_wp = 5.36% and R_p = 3.59%. Moreover the fractional atomic coordinates, occupancies and isotropic thermal parameters of this sample are summarized in Table 1. The volumetric expansion of the sample with Tb³⁺ and Eu³⁺ doping indicates that the Tb³⁺ and Eu³⁺ occupied the Sc³⁺ sites, which can be ascribed to the relationship of the ionic radius of the host cation Sc³⁺, and doping ions Tb³⁺, Eu³⁺ (R_Eu³⁺ > R_Tb³⁺ > R_Sc³⁺). Fig. 3 shows the schematic illustration of the crystal structure of ScPO₄ viewed from the c axis and the Sc³⁺ ions locate at the eight coordinated sites while the P⁵⁺ ions at the four coordinated sites.¹⁴

Fig. 1 XRD patterns of as-prepared ScPO₄ (a), ScPO₄:0.03Tb³⁺ (b), ScPO₄:0.03Tb³⁺, 0.02Eu³⁺ (c) phosphors and the standard pattern of JCPDS 84-336.

Fig. 2 Powder XRD patterns (×) of (a) ScPO₄ host and (b) ScPO₄:0.03Tb³⁺, 0.02Eu³⁺ samples with their corresponding Rietveld refinement (solid line) and residuals (bottom).

Table 1 Fractional atomic coordinates and isotropic displacement parameters (Ų) of ScPO₄ obtained from the Jana2006 software using X-ray powder diffraction data at room temperature^ab

Atom	x	y	z	Uiso	Occ.
a Space group: I41/amd (141); a = b = 6.578(7) Å, c = 5.796(3) Å, V = 250.5(5) Å^3; 2θ-interval = 10–100°; Rwp (%) = 5.52, Rp (%) = 3.75, χ^2 = 2.58.b Space group: I41/amd (141); a = b = 6.595(7) Å, c = 5.807(4) Å, V = 252.6(2) Å^3; 2θ-interval = 10–100°; Rwp (%) = 5.36, Rp (%) = 3.59, χ^2 = 2.22.
ScPO4
Sc	0	0.75	0.625	0.0135(2)	1
P	0	0.75	0.125	0.0324(3)	1
O	0	0.9308(27)	0.2929(32)	0.0064(2)	1
ScPO4:0.03Tb^3+, 0.02Eu^3+
Sc	0	0.75	0.625	0.0075(5)	0.943
Tb	0	0.75	0.625	0.0075(5)	0.033
Eu	0	0.75	0.625	0.0075(5)	0.024
P	0	0.75	0.125	0.0109(1)	1
O	0	0.9310(94)	0.2938(8)	0.0138(9)	1

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Fig. 3 Schematic illustration of the crystal structure of ScPO₄ viewed from the c axis, and the coordination environment around Sc³⁺.

To further determine the chemical states of europium and terbium, the X-ray photoelectron spectroscopy XPS technique was carried out on the sample Sc_0.96PO₄:0.03Tb³⁺, 0.01Eu³⁺ as shown in Fig. 4 The typical survey XPS spectrum reveals a series of photoelectron peaks corresponding to Sc2p₁, P2p₁, O1s, Tb3d₅, Eu3d₃ and C1s, respectively. The occurring of C peak is due to the adventitious hydrocarbon of the XPS instrument. The binding energies around 403.06, 133.46, 531.24, 1241.78 and 1136.46 eV correspond well to the Sc³⁺, P⁵⁺, O²⁻, Tb³⁺ and Eu³⁺ ions respectively.^16,17 Moreover, there is no other ions including the divalent europium have been detected. We can deeply confirm the successful preparation of Tb³⁺ and Eu³⁺ co-doped ScPO₄ phosphors combined with the XRD analysis.

Fig. 4 High-resolution of XPS survey spectrum of ScPO₄:0.03Tb³⁺, 0.01Eu³⁺.

The excitation (PLE) and emission (PL) spectra of the sample ScPO₄:0.03Tb³⁺ are shown in the Fig. 5(a). The PLE spectrum monitored at 550 nm presents strong bands from 200 to 500 nm. The spectrum centered at 230 nm and 275 nm assign to a spin-allowed 4f⁸–4f⁷5d¹ transition with higher energy (230 nm, ΔS = 0) and a spin-forbidden 4f⁸–4f⁷5d¹ transition with lower energy (275 nm, ΔS = 1) of Tb³⁺ ion, respectively.^18,19 The others from 300 to 500 nm correspond to the intra-configurational 4f⁸–4f⁸ transitions from ⁷F₆ levels to ⁵H₆ (306 nm), ⁵D₀ (323 nm), ⁵G₂ (358 nm), ⁵D₃ (376 nm) levels.²¹ It can be expected that the excitation wavelength at 370 nm can be found from the typical commercial chips in the near future. The PL spectra of the as prepared ScPO₄:xTb³⁺ (x ≤ 0.09) consist of five main peaks within the scope of 400 to 700 nm which are due to the transitions of ⁵D_3,4–⁷F_J (J = 3, 4, 5, 6) as labelled in the Fig. 5(b). It can be seen clearly that ⁵D₄ emissions are stronger than ⁵D₃ emissions. For the Tb³⁺ ion, there is an efficient cross-relaxation via the resonant energy transfer processes due to the similarity of energy gap between ⁵D₃–⁵D₄ (5915 cm⁻¹) and ⁷F₆–⁷F₀ (6000 cm⁻¹). It can be expressed as follows:^20,21

⁵D₃(Tb³⁺) + ⁷F₆(Tb³⁺) − ⁵D₄(Tb³⁺) + ⁷F₀(Tb³⁺) (1)

Fig. 5 The spectral patterns of ScPO₄:0.03Tb³⁺ (a); the PL spectra of ScPO₄:xTb³⁺ (x ≤ 0.09) (inset: the ⁵D₃ and ⁵D₄ emission intensity with different concentrations of Tb³⁺) (b).

The Tb³⁺ levels are involved in several transitions owing to the high degeneracy and have a low energy ground state ⁷F_J (J = 6, …, 0) and excited states ⁵D₃ and ⁵D₄. Generally speaking, with different doping concentrations of Tb³⁺ in host matrix, the blue and green emissions can be emitted due to the transitions of ⁵D₃ or ⁵D₄ to ⁷F_J.²² In our work, when the doping concentration of the Tb³⁺ is increasing, the positions of the emission transitions have no shifts but the change of their intensities. As revealed in the inset of Fig. 5(b), the emission intensity of ⁵D₃ was strengthened with the increasing of Tb³⁺ doping concentrations initially and then reduced after reaching its maximum intensity, while the concentration quenching does not take place on the ⁵D₄ emissions even at the high Tb³⁺ concentrations (x = 0.09). At present the concentration quenching is under further investigation. A diagram of the energy levels of Tb³⁺ for the cross-relaxation processes is illustrated in the Fig. 6, revealing the energy transfer from the high level of ⁵D₃ to the low energy level of ⁵D₄. A similar phenomenon of Tb³⁺ have been reported in the host CaSc₂O₄,⁷ CaYAlO₄,⁹ Y₂O₃,²³ KSrPO₄.²⁴

Fig. 6 The energy level structure of Tb³⁺, Eu³⁺ and the energy-transfer mechanism among them.

As the Eu³⁺ singly doped ScPO₄ host, the PL and PLE spectra are shown in the Fig. 7. It consists of two excitation bands from 200 to 300 nm, which is due to the host absorption centered at 215 nm and the charge-transfer band (CTB) between Eu³⁺ and O²⁻ at the peak of 265 nm.^25–27 The CTB is formed from the transition of 2p electrons of O²⁻ to the empty 4f orbitals of Eu ions. The long wavelength side (300–500 nm) results from the features of f–f transitions of Eu³⁺ within its 4f⁶ configuration, corresponded to ⁷F₀–⁵F₃ (300 nm), ⁷F₀–⁵H₆ (325 nm), ⁷F₀–⁷D₄ (362 nm), ⁷F₀–⁵L₇ (384 nm), the strong sharp excitation ⁷F₀–⁵L₆ (397 nm) and the weak one ⁷F₀–⁵D₂ (480 nm) respectively.^11,28 Under the excitation of 397 nm, we can find the representative emissions peaks (596 nm, 623 nm, 660 nm, 705 nm) associating with the transitions ⁵D₀–⁷F_J (J = 1, 2, 3, 4).

Fig. 7 The spectral patterns of ScPO₄:0.03Eu³⁺.

In order to further study the energy absorption of the ScPO₄ phosphors, Fig. 8 show the diffuse reflectance spectra of ScPO₄ host, ScPO₄:0.03Tb³⁺ and ScPO₄:0.03Tb³⁺, 0.025Eu³⁺. It can be seen obviously a strong energy absorption band in the 200–350 nm region from the host material. When the Tb³⁺ doped the host, it exhibit an obvious absorption from 230 to 275 nm assigned to the f–d transition of the Tb³⁺ ions. Moreover, with the introduction of Eu³⁺ ions, there is another narrow absorption lines at 397 nm ascribed to the f–f transition of Eu³⁺ ions. Above results are matching well with the PLE spectra (Fig. 5 and 7) and demonstrates the potential of the ScPO₄:Tb³⁺, Eu³⁺ phosphors for the solid-state lighting.

Fig. 8 Diffuse reflectance spectra of ScPO₄ host (a), ScPO₄:0.03Tb³⁺ (b), and ScPO₄:0.03Tb³⁺, 0.025Eu³⁺ (c).

As revealed in Fig. 9(a) series of samples with varied Eu³⁺ concentration and the fixed concentration of Tb³⁺ are synthesized and their PL spectra are observed schematically upon excitation of 370 nm to investigate their energy transfer (ET). One can see that the intensity of the Tb³⁺ emission is found to decrease monotonically with increasing of Eu³⁺ content due to the markedly energy transfer between Tb and Eu. These results provide an evidence for the ET between Tb³⁺ and Eu³⁺. Meanwhile, the intensity of the Eu³⁺ reaches a maximum at y = 0.025 and then occurs concentration quenching resulting in the weakening of emission intensity (Fig. 9(b)). The series of phosphors were eventually realized to tune from green, yellow to orange-red via co-doped Tb³⁺ and Eu³⁺. To observe the emission hue after doping Eu³⁺ concentration more directly, the Commision International de'LEclarirage (CIE) chromaticity coordinates, correlated colour temperature (CCT), quantum efficiency and CIE chromaticity diagram for the ScPO₄:0.03Tb³⁺, yEu³⁺ and ScPO₄:0.03Eu³⁺ phosphors are measured and are shown in Table 2 and Fig. 10, respectively. As the Table 2 depicts the variation of chromaticity coordinates (x, y) of the ScPO₄:0.03Tb³⁺, yEu³⁺ which are calculated based on the corresponding emission spectrum. The phosphors can be modulated from green (0.2936, 0.4558) to orange-red (0.4553, 0.3547) as the inset of Fig. 8 shown. Meanwhile, it is found from Table 2 that the colour temperature of the samples was modulated from 6562 K to 2279 K with a decreasing trend and QE values for ScPO₄:0.03Tb³⁺, xEu³⁺ declined with elevating the x value.

Fig. 9 The PL spectra of ScPO₄:0.03Tb³⁺, yEu³⁺ (y = 0–0.05) phosphors under N-UV excitation (λ_ex = 370 nm) (a); dependence of the relative emission intensity of Tb³⁺ (550 nm) and Eu³⁺ (598 nm) in the ScPO₄ phosphors as a function of Eu³⁺ mol concentration (b).

Table 2 The CIE chromaticity coordinates (x, y), correlated colour temperature (CCT) and QE values of ScPO₄:0.03Tb³⁺, yEu³⁺ and ScPO₄:0.03Eu³⁺ phosphors

Sample no.	Sample composition (y)	CIE coordinates (x, y)	CCT (K)	QE values (%)
1	0	(0.2936, 0.4558)	6562	15.46
2	0.005	(0.3570, 0.4217)	4833	11.3
3	0.01	(0.4027, 0.3888)	3556	7.02
4	0.015	(0.4239, 0.3870)	3106	—
5	0.025	(0.4444, 0.3821)	2704	5.8
6	0.03	(0.4433, 0.3735)	2722	—
7	0.05	(0.4553, 0.3547)	2279	3.09
8	—	(0.6285, 0.3687)	2045	—

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Fig. 10 CIE chromaticity coordinates of ScPO₄:0.03Tb³⁺, yEu³⁺, ScPO₄:0.03Eu³⁺ and the commercial phosphor BAM:Eu²⁺ under 370 nm excitation and the digital images of 1, 7 and 8 at 365 nm in UV box.

To furtherly verify the existence of energy transfer between Tb³⁺ and Eu³⁺ in the samples of ScPO₄:0.03Tb³⁺, yEu³⁺ (y = 0–0.050), we measured the luminescence decay curves of Tb³⁺ 550 nm emission and calculated the corresponding lifetimes of Tb³⁺ with the increasing concentration of Eu³⁺. Moreover, the energy transfer efficiency is also presented in the inset of Fig. 11(a). In general, the curves all can be well fitted based on the following non-exponential equation:^29,30

where I(t) stand for the luminescence intensities at time t. The effective decay lifetimes at 550 nm of Tb³⁺ are determined to be 1.34, 1.18, 1.09, 0.839, 0.797, 0.795 and 0.611 ms for y = 0.005, 0.01, 0.02, 0.025, 0.04, 0.05, respectively. The lifetime of Tb³⁺ emission are monotonously decreasing as the increasing concentration of Eu³⁺, which demonstrates that the efficient energy transfer from Tb³⁺ ions to neighboring Eu³⁺ ions occur in the ScPO₄ host. Then the eqn (3) can be used to estimate the Tb³⁺–Eu³⁺ ET probability (P_Tb–Eu):^31,32where τ_x and τ₀ mean the corresponding lifetimes of the sensitizer (Tb³⁺) with and without the activator (Eu³⁺) doping for the same sensitizer concentration. The values of the calculated P_Tb–Eu are depicted in Fig. 11(b) obviously. The continuous increasing of the ET probabilities with the different Eu³⁺ content demonstrates an enhanced ET process between Tb³⁺ and Eu³⁺. Furthermore, the energy transfer efficiency η_T from a donor Tb³⁺ to acceptor Eu³⁺ would be estimated according to the expression as follows:³³where η_T means the energy transfer efficiency; τ_s and τ_so stand for the lifetimes of Tb³⁺ in the absence and the presence of Eu³⁺, respectively. As shown from the inset of Fig. 11(a), the efficiency of energy transfer ascends continually from 11.9% to 54.4% with the increasing of the concentration of Eu³⁺. The phenomenon is ascribed to be the decreased distance between the sensitizer ions (Tb³⁺) and activator ions (Eu³⁺).²¹ As is well-known to us that the energy transfer between a sensitizer and an activator take place via two interaction mechanism: exchange interaction or electric multipolar interaction. If the critical distance between the donor ions and acceptor ions is shorter than 5 Å, the exchange interaction occurs in the host.³⁴ Then in our work, the critical distance R_c for energy transfer was well estimated by the following formula suggested by Blasse:³⁵where V is the volume of a unit cell, N is the number of available sites for the dopant in the unit cell, and x_c is the critical concentration (the total concentration of Tb³⁺ and Eu³⁺ ions) at the point that the quenching occurs. Accordingly, the crystallographic parameter for ScPO₄ are V = 250.12 Å, N = 4 and x_c is about 0.055, which was derived from the quenching point (Tb³⁺ content is 0.03, Eu³⁺ content is 0.025) in the Fig. 11. Therefore, basing on the eqn (5), R_c was calculated to be about 12.95 Å and it is much greater than 5 Å which illustrates the electric multipolar interaction plays a great role in the process of energy transfer between Tb³⁺ and Eu³⁺.

Fig. 11 Decay curves, lifetime and η_T of ScPO₄:0.03Tb³⁺, yEu³⁺ under the excitation of 370 nm (a). Energy transfer probability (P_Tb–Eu) as a function of x value of Eu³⁺ concentration (b).

In the light of Dexter's energy transfer theory of electric multipolar interaction and Reisfeld's approximation, the energy transfer present in the ScPO₄ can be furtherly deduced by the following equation:^36,37

where η₀ and η mean the luminescence quantum efficiencies of Tb³⁺ in the absence and presence of Eu³⁺ doping, respectively; C is the concentration of Eu³⁺, and n = 6, 8 and 10 assigning to the dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively.³⁸ Then the value of η₀/η can be approximately computed by the ratio of related emission intensities as:

In which the I_so is the intensity while only doping Tb³⁺ and I_s is the intensities with different Eu³⁺ contents at fixed the concentration of Tb³⁺. Then in our work, the I_so/I_s versus C^n/3 (n = 6, 8, 10) plots are depicted in Fig. 12. From the figure we can clearly see the linear fitting behaviour and compare the values of the fitting accuracy R², so the results reveal that the energy transfer from Tb³⁺ to Eu³⁺ follows the dipole–dipole (d–d) interaction only when n = 6.

Fig. 12 Dependence of I_so/I_s of Tb³⁺ on C_Eu^6/3 (a), C_Eu^6/3 (b), C_Eu^6/3 (c).

Fig. 13 gives the temperature-dependent emission spectra of ScPO₄:0.03Tb³⁺, 0.025Eu³⁺. As the inset shows the trend of the relative emission intensity versus temperature, it can be concluded that the as-prepared samples exhibits an obvious temperature quenching which is due to the temperature-dependence of the electron–phonon interactions in both the ground state and excited state of the luminescence center and thermally activated photo-ionization of lanthanide.^39,40 Moreover, as the increasing temperature up to 150 °C, the PL intensities of the characteristic peaks of Tb³⁺ and Eu³⁺ decreased to 79% and 79.5% of the initial value at room temperature, respectively. As we know, the high power LEDs lamp often works at 150 °C, which means the present phosphor possesses good thermal stability potentially.

Fig. 13 Temperature-dependent PL spectra of ScPO₄:0.03Tb³⁺, 0.025Eu³⁺. The inset shows the emission intensity versus temperature.

In order to further demonstrate the potential application of ScPO₄:0.03Tb³⁺, xEu³⁺, a phosphor-converted LED device was fabricated via using N-UV LED chips (λ_max = 385 nm) combining a blue-emitting BAM:Eu²⁺ with yellow-emitting ScPO₄:0.03Tb³⁺, 0.01Eu³⁺ phosphor as the inset given in Fig. 14. The EL spectrum of the fabricated LED device was shown in Fig. 14 under forward-basis current of 2 mA. The emission bands centered at 456 nm and 516 nm are attributed to the BAM:Eu²⁺ phosphor, while the other bands are the emission peaks of ScPO₄:0.03Tb³⁺, 0.01Eu³⁺. The CIE coordinates, correlated color temperature (CCT) and color rendering index (R_a) of this fabricated w-LED lamp are determined to be (0.31, 0.28) as shown in Fig. 10, 7314 K and 72.5, respectively. The CRI value (R_a) is determined from the full set of the first eight CRIs shown in Table 3.

Fig. 14 EL spectrum of a white-emitting N-UV-LED chip (385 nm) comprising of BAM:Eu²⁺ (blue) and ScPO₄:0.03Tb³⁺, 0.01Eu³⁺ (yellow) phosphors driven by a 2 mA current.

Table 3 Full set of 14 CRIs and the R_a of the fabricated w-LED

Ra	R1	R2	R3	R4	R5	R6	R7	R8	R9	R10	R11	R12	R13	R14
72.5	72.5	84.2	81.8	69.6	71.4	73.5	82.5	44.8	51.9	57.3	54.5	75.6	89.2	66.1

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Conclusions

In summary, a single-phased, colour tunable emitting phosphors ScPO₄ single doped with Tb³⁺ and co-doped with Tb³⁺ and Eu³⁺ ions have been synthesized by the conventional high temperature solid-state reaction. As for the samples with solely doped Tb³⁺, the energy transfer from ⁵D₃ to ⁵D₄ and the cross relaxation phenomenon occurs which strengthen the energy transition from the high energy level ⁵D₃ to the low energy level ⁵D₄. The ET process is verified via the decay curves with the energy transfer efficiency (the maximum is 54.4%) and the energy level structure, which is well matched with the dipole–dipole (d–d) mechanism. The ET probability (P_Tb–Eu) was calculated to give another proof for the existing of ET between Tb³⁺ and Eu³⁺. The emission colours of the obtained phosphors could be tuned from green (0.2936, 0.4558) to orange-red (0.4553, 0.3547) by controlling the doping of Eu³⁺. The white LEDs packaged by an UV LED chip with the ScPO₄:0.03Tb³⁺, 0.01Eu³⁺ phosphors along with the commercial phosphors BAM:Eu²⁺ generate white light (0.31, 0.28) with color rendering index (R_a = 72.5) and a correlated color temperature (7314 K).

Acknowledgements

We gratefully thank the financial support from the National Natural Science Foundation of China (Grant No. 51472223), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. CET-12-0951) and the Fundamental Research Funds for the Central Universities (Grant No. 2652015008).

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