Multi-color luminescence properties and energy transfer behaviour in host-sensitized CaWO₄:Tb³⁺,Eu³⁺ phosphors

Abstract

A series of host-sensitized and color-tunable CaWO₄:Tb³⁺,Eu³⁺ phosphors were prepared via a high-temperature solid-state reaction route, and the crystal structure and luminescence properties, especially the energy transfer behavior, were investigated in detail. Under UV radiation, the CaWO₄ host presents a broad emission band from about 320 to 600 nm centered around 415 nm, ascribed to the charge transfer in WO₄²⁻ groups; while Eu³⁺ and Tb³⁺ ion doped CaWO₄ samples show both host emission and respective emission lines derived from the characteristic f–f transitions of activators, which present abundant emission colors owing to an efficient energy transfer from the host to Eu³⁺/Tb³⁺ ions. The energy transfer from the host to Eu³⁺ and Tb³⁺ was evidenced by directly observing appreciable overlap between the excitation spectrum of the host and the emission spectrum of Eu³⁺/Tb³⁺, as well as by the decreased decay time of host emission with increasing Eu³⁺/Tb³⁺ concentration. The energy transfer mechanisms in CaWO₄:Eu³⁺/Tb³⁺ phosphors have been determined to be a resonant type via dipole–dipole interaction. In the Eu³⁺ and Tb³⁺ co-doped system, CaWO₄:Eu³⁺,Tb³⁺, the energy transfer phenomenon not only occurs from host to activators, but also from Tb³⁺ to Eu³⁺, resulting in color-tunable emission including white light by simply adjusting the doping concentration of Eu³⁺ and Tb³⁺ ions. The PL properties of the as-prepared materials indicate their promising application in solid-state lighting fields.

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1. Introduction

Recently, the development of inorganic luminescent materials has attracted fast growing interest due to their indispensable applications in lighting apparatus (e.g., fluorescent tubes and white light-emitting diodes), display devices (e.g., cathode ray tubes, field emission displays, and vacuum fluorescent displays), solid-state lasers, biological labeling, medical devices, and so on.^1–9 In particular, white light-emitting diodes (WLEDs) have received wide consideration due to their favorable properties of energy saving, high energy efficiency, long operation time, and environmental friendliness.^10–15 Therefore, more attention has been paid to searching for highly efficient phosphors with respect to the wide possible applications.^16–19

Rare earth ions have been playing an irreplaceable role in modern lighting and display fields due to the abundant emission colors based on their 4f–4f or 4f–5d transitions.^20–26 In particular, the Eu³⁺ ions, as one of the most common and widely used activators, can primarily emit red color originating from its characteristic transitional emissions of ⁵D_0,1,2–⁷F_J (J = 4, … , 0). Tb³⁺-doped phosphors have been broadly applied as green emitting materials based on ⁵D₄–⁷F₅ transition of Tb³⁺ ions.^27,28 However, the f–f transitions of rare earth ions generally have low excitation efficiencies because of their forbidden parity selection rules. Therefore, utilizing the host sensitization via energy transfer from the excited host to rare earth ions is an effective way to enhance the emission intensity of activators.

Metal tungstates have emerged as an important family of luminescent materials owing to their unique structural, excellent thermal stability and interesting luminescence properties.^29–33 Among them, calcium tungstate (CaWO₄) is a self-activated phosphor that can emit a high-efficiency blue emission when excited by ultraviolet light, which due to the tetrahedral WO₄²⁻ groups in the host lattices.^34,35 The luminescent properties of CaWO₄ and rare earth ions doped CaWO₄ have been described a few years ago.³⁶ Unfortunately, in those reports for Eu³⁺ or Tb³⁺ doped CaWO₄, there is not a clear description about their photoluminescence (PL) and relevant luminescent mechanism. Furthermore, detailed investigations on PL of CaWO₄:A (A = Eu³⁺ and/or Tb³⁺) have not been performed. Accordingly, in this paper, we report the synthesis of Eu³⁺, Tb³⁺ single-doped and Eu³⁺, Tb³⁺ codoped CaWO₄ phosphors using a solid-state reaction process, and investigated their PL properties and energy transfer mechanism in more detail. Under UV excitation, energy transfer from host to activators exists in the singly-doped samples, which leads to tunable emission color from blue to red for CaWO₄:Eu³⁺ and green for CaWO₄:Tb³⁺. While, in Eu³⁺ and Tb³⁺ co-doped system, CaWO₄:Eu³⁺,Tb³⁺, energy transfer phenomenon not only occurs from host to activators, but also from Tb³⁺ to Eu³⁺. Furthermore, a white emission in a single-composition was achieved by blending simultaneously the blue, green, and red emissions of host, Tb³⁺, and Eu³⁺ in the CaWO₄:8%Eu³⁺,4%Tb³⁺ phosphor. It suggests that the as-prepared products would have good potential application in the solid-state lighting fields.

2. Experimental

2.1 Materials and synthesis

The CaWO₄:Eu³⁺, CaWO₄:Tb³⁺, and CaWO₄:Eu³⁺,Tb³⁺ samples were all prepared through a high-temperature solid-state method with CaCO₃ (A.R.), WO₃ (A.R.), Tb₄O₇ (99.99%), and Eu₂O₃ (99.99%). In a typical process, the stoichiometric amounts of the raw materials were first thoroughly mixed by grinding them in an agate mortar with an appropriate amount of ethanol and then dried at 80 °C for 0.5 h. After reground for 20 min, the powder mixtures were loaded into the alumina crucible and transferred to the box furnace to calcine at 1100 °C for 6 h in air condition with a heating rate of 3 °C min⁻¹. Finally, the prepared samples were furnace-cooled to room temperature and then ground once more for the subsequent measurements.

2.2 Characterization

The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2400 X-ray diffractometer at a scanning rate of 10° min⁻¹ in the 2θ range from 5° to 80° with graphite-monochromatized CuKα radiation (λ = 0.15405 nm). The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra as well as quantum efficiency were measured using a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescent decay curves were measured using an FLS920 spectrofluorometer (Edinburgh Instruments). All the measurements were performed at room temperature (RT).

3. Result and discussion

3.1 Phase identification and structure

The composition and phase purity of as-prepared representative samples CaWO₄ host, CaWO₄:4%Eu³⁺, CaWO₄:4%Tb³⁺, CaWO₄:4%Tb³⁺,2%Eu³⁺ and CaWO₄:6%Tb³⁺,1%Eu³⁺ were firstly identified by XRD patterns, as shown in Fig. 1. It is obvious that all diffraction peaks of the synthesized phosphors are well assigned to the tetragonal phase of CaWO₄ according to the JCPDS card no. 41-1431, and no traces of impurity phases are detected, implying that the Eu³⁺ and Tb³⁺ ions can be completely incorporated into the host lattice and their introductions do not cause any significant changes to the crystal structure. The CaWO₄ crystallizes belong to a scheelite-type tetragonal structure with the space group of I4₁/a (88) and cell parameters of a = b = 5.243 Å, c = 11.373 Å, V = 312.63 ų and Z = 4.^37,38 Fig. 2 shows the unit cell structure of CaWO₄ and the coordination environments of the Ca²⁺ and W⁶⁺ sites. It can be seen that the tungsten (W) atoms are surrounded by four oxygen atoms, forming the tetrahedral [WO₄] clusters, where the tetrahedral angles are slightly distorted. The calcium (Ca) atoms are coordinated by eight oxygen atoms ([CaO₈] clusters) and have a S₄ point symmetry with no inversion center.³⁹ There is only one type of Ca cationic site with a Wyckoff position 4b for activators to accommodate attributed to the similar ionic radius of Ca²⁺ [r = 1.00 Å for coordination number (CN) = 8] and Eu³⁺ (r = 0.95 Å for CN = 8) and Tb³⁺ (r = 0.92 Å for CN = 8).

Fig. 1 XRD patterns of representative CaWO₄ host (a), CaWO₄:4%Eu³⁺ (b), CaWO₄:4%Tb³⁺ (c), CaWO₄:4%Tb³⁺,2%Eu³⁺ (d), CaWO₄:6%Tb³⁺,1%Eu³⁺ (e) samples. The standard data of CaWO₄ (JCPDS no. 41-1431) is shown as a reference.

Fig. 2 The unit cell structure of CaWO₄ and the coordination environment of cations in the host lattice.

3.2 Photoluminescence properties analysis

Fig. 3 illustrates the PLE and PL spectra of as-prepared CaWO₄, CaWO₄:Eu³⁺, and CaWO₄:Tb³⁺ samples, respectively. The excitation spectrum of CaWO₄ (Fig. 3a) contains a broad band from 200 to 300 nm with a maximum at 248 nm, which is ascribed to the charge-transfer absorption from the oxygen ligands to the central tungsten atoms inside the WO₄²⁻ complex.⁴⁰ Upon excitation at 248 nm, the emission spectrum of pure CaWO₄ phosphors displays a broad band ranging from 320 to 600 nm with a maximum at 415 nm. The excitation and emission spectra obtained are mainly owing to the charge transfer absorption within the WO₄²⁻ groups. From the viewpoint of molecular orbital theory, these bands in the excitation and emission spectra can be attributed to the transitions from the ¹A₁ ground state to the high vibration level of ¹B (¹T₂) and from the low vibration level of ¹B (¹T₂) to the ¹A₁ ground state within the WO₄²⁻ ion.⁴¹ Fig. 3b presents the PL excitation and emission spectra of CaWO₄:2%Eu³⁺ phosphors. When monitoring by the red emission of Eu³⁺ (617 nm, ⁵D₀ → ⁷F₂), the PLE spectrum of CaWO₄:2%Eu³⁺ reveals a broad band ranging from 200 to 300 nm ascribed to the combined absorptions of host and Eu³⁺–O²⁻ charge transfer transition.⁴² In addition, the narrow lines beyond 300 nm are the characteristic f–f transitions (320 nm, ⁷F₀ → ⁵H₆; 363 nm, ⁷F₀ → ⁵D₄; 383 nm, ⁷F₀ → ⁵G₂; 395 nm, ⁷F₀ → ⁵L₆; 417 nm, ⁷F₀ → ⁵D₃) of Eu³⁺ within its 4f⁶ configuration.⁴³ Upon excitation at 276 nm, both the wide host emission band and Eu³⁺ 4f–4f characteristic transition emission lines appear, namely, ⁵D_0,1 excited states to the ⁷F_J ground states including ⁵D₀ → ⁷F₁ (594 nm), ⁵D₀ → ⁷F₂ (617 nm), ⁵D₀ → ⁷F₃ (657 nm) and ⁵D₀ → ⁷F₄ (706 nm), respectively.⁴⁴ It is well-known that Eu³⁺ is an outstanding structural probe for exploring the local environment in a host lattice. The electric dipole transition ⁵D₀ → ⁷F₂ is a hypersensitive one, which is strongly influenced by the chemical environment around the Eu³⁺ ions. When Eu³⁺ ions are located at a low symmetry site, the ⁵D₀ → ⁷F₂ transition often dominates in the emission spectrum. Nevertheless, the magnetic dipole transition (⁵D₀ → ⁷F₁) is independent from the local crystal field environment.⁴⁵ From the results in Fig. 3b, the emission intensity of the electric dipole transition is considerably higher than that of the magnetic dipole transition, implying that the Eu³⁺ ions occupy the sites without inversion symmetry, which is consistent with the crystal structure of CaWO₄.

Fig. 3 PL excitation and emission spectra of CaWO₄ (a), CaWO₄:2%Eu³⁺ (b) and CaWO₄:2%Tb³⁺ (c) samples.

As for CaWO₄:2%Tb³⁺ phosphors, the excitation spectra monitoring the emission at 546 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ is shown in Fig. 3c. It can be seen that the excitation spectrum of CaWO₄:2%Tb³⁺ consists of a strong broad band from 200 to 300 nm with a maximum at 264 nm due to the WO₄²⁻ groups as described above, and the other weak lines in longer wavelength region are attributed to the f–f transition within the Tb³⁺ 4f⁸ configuration.⁴⁶ Upon excitation into the WO₄²⁻ group at 264 nm, not only the broad host emission but also the Tb³⁺ 4f–4f transition emission lines [383 nm (⁵D₃ → ⁷F₆), 415 nm (⁵D₃ → ⁷F₅), 437 nm (⁵D₃ → ⁷F₄), 490 nm (⁵D₄ → ⁷F₆), 546 nm (⁵D₄ → ⁷F₅), 590 nm (⁵D₄ → ⁷F₄), and 624 nm (⁵D₄ → ⁷F₃) for Tb³⁺ ion] can be clearly observed.⁴⁷ Moreover, it is worth noting that the presence of the excitation peak of the WO₄²⁻ groups in the excitation spectrum of Eu³⁺/Tb³⁺ indicates that there is an energy transfer from the WO₄²⁻ groups of host to Eu³⁺/Tb³⁺ ions.⁴⁸

In addition, we can deduce that the energy transfer primarily takes place from the WO₄²⁻ groups of host to Eu³⁺/Tb³⁺ by the spectral overlaps between the emission bands of host and the characteristic excitation peaks of Eu³⁺/Tb³⁺ ions in phosphors, as shown in Fig. 4. Fig. 5 illustrates the decay curves of activators characteristic emissions in CaWO₄:4%Eu³⁺ (λ_ex = 276 nm, λ_em = 617 nm) and CaWO₄:4%Tb³⁺ (λ_ex = 264 nm, λ_em = 546 nm), which can be well fitted with a single exponential formula as follows:^49–51

I = I₀ exp(−t/τ) (1)

where I₀ and I are the luminescence intensities at time 0 and t, respectively, and τ is the decay lifetime. The value of lifetimes were determined to be 552.7 and 621.8 μs for Eu³⁺ and Tb³⁺, respectively, which are consistent with the previous results reported in the literature^6,52 and much longer than that of the host emission below in Fig. 7. These results illustrate that the Eu³⁺ and Tb³⁺ ions not only can be accommodated into this type of host but also they can present their characteristic emission spectra.

Fig. 4 PL emission spectrum of CaWO₄ (a) and PL characteristic excitation spectra of activators in CaWO₄:2%Eu³⁺/2%Tb³⁺ (b and c).

Fig. 5 The decay curves for the luminescence of Eu³⁺ ions in CaWO₄:4%Eu³⁺ samples (a) and (b) Tb³⁺ ions in CaWO₄:4%Tb³⁺ samples.

The variations in the emission spectra as a function of Eu³⁺/Tb³⁺ concentration in CaWO₄:x%Eu³⁺/y%Tb³⁺ (x = 0–20, y = 0–10) are shown in Fig. 6. In the CaWO₄:x%Eu³⁺ system, the emission intensity of the host decreases sharply with increasing Eu³⁺ concentration under the excitation of 276 nm UV. However, the emission intensity of Eu³⁺ initially increases until the Eu³⁺ content becomes 8%, and subsequently descends with further increasing the Eu³⁺ concentration due to the concentration quenching effect as shown in Fig. 6a. This phenomenon can confirm the existence of energy transfer from the host to the activators in CaWO₄:Eu³⁺ samples. In order to provide further evidence to validate the energy transfer from the host to Eu³⁺ ions in CaWO₄:x%Eu³⁺, we investigated the decay curves and lifetimes of host (λ_ex = 276 nm, λ_em = 415 nm) emission with the increasing Eu³⁺ (x = 0, 1, 4, 6, 8) concentration in CaWO₄:xEu³⁺ (Fig. 7a). The decay curves can be well fitted using a second-order exponential function as given by the following equation:^53–56

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I represents the luminescent intensity; A₁ and A₂ are constants; t is the time; τ₁ and τ₂ are short- and long-decay components, respectively. According to these parameters, the average decay times (τ*) can be determined by the following equation:

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (3)

Fig. 6 PL spectra of CaWO₄:x%Eu³⁺ and CaWO₄:y%Tb³⁺ samples as a function of (a) Eu³⁺ and (b) Tb³⁺ doping concentrations, respectively.

Fig. 7 Decay curves of host emission in (a) CaWO₄:xEu³⁺ samples with different Eu³⁺ doping concentrations (b) CaWO₄:yTb³⁺ samples with different Tb³⁺ doping concentrations.

On the basis of eqn (3), the average lifetimes of host are determined to be 9.39, 8.95, 6.82, 4.60 and 3.64 μs for Eu³⁺ concentrations of 0%, 1%, 4%, 6%, and 8%, respectively. The declined decay times of host with increasing Eu³⁺ concentration strongly certifies the occurrence of energy transfer from host to Eu³⁺ in CaWO₄:Eu³⁺ phosphors. Also, such energy transfer phenomenon demonstrates that the as-synthesized CaWO₄:xEu³⁺ is a solid solution, in which Eu³⁺ has successfully incorporated into CaWO₄ lattice,⁵⁷ agreeing well with the XRD results. The efficiency (η_T) of energy transfer from the host to Eu³⁺ in CaWO₄:x%Eu³⁺ system can be calculated using the following equation:⁵⁸

η_T = 1 − I_S/I_S0 (4)

where η_T is the energy transfer efficiency, and I_S0 and I_S are the luminescence intensity of the host in the absence and presence of an activator (Eu³⁺), respectively. As shown in Fig. 8a, the η_T value from host to activators in CaWO₄:x%Eu³⁺ system is plotted as a function of concentrations of Eu³⁺ ions, which show that the η_T value monotonously increase to reach maximums at 98% under 276 nm UV radiation, indicating the energy transfer from the host to Eu³⁺ becomes more and more efficient with the increasing concentration of Eu³⁺ ions.

Fig. 8 Energy transfer efficiency from (a) host to Eu³⁺ in CaWO₄:x%Eu³⁺ samples and (b) host to Tb³⁺ in CaWO₄:y%Tb³⁺ samples.

In order to determine the energy transfer mechanism from host to activators in CaWO₄:x%Eu³⁺ phosphors, it is necessary to know the critical distance (R_c) between the neighbouring Eu³⁺ ions. According to Blasse,^59,60 the critical distance can be expressed as

where V represents the volume of the unit cell, N refers to the number of the host cations in the unit cell, and X_c is the critical concentration of the activator. For CaWO₄ host, N = 4 and V = 312.63 ų, X_c is 0.08 for Eu³⁺; therefore, the critical distance (R_c) was calculated to be about 12.32 Å. With the increase of Eu³⁺ content, the energy transfer from the host to Eu³⁺ becomes more efficient, and the probability of energy migration between activators increases simultaneously. When the distance becomes small enough, the concentration quenching phenomenon takes place and the energy migration is hindered. In general, there are two main mechanisms for a non-radiative energy transfer process: one is exchange interaction, and the other is electric multipolar interactions.⁶¹ The value of R_c acquired above indicates the little possibility of exchange interaction since it can be predominant only for about 5 Å.^62–64 Therefore, we can infer that the energy transfer mechanism from the host to Eu³⁺ is dominated by the electric multipolar interactions. On the basis of Dexter's energy transfer formula of multipolar interactions and Reisfeld's approximation, the following relationship can be attained:^65,66where η_S0 and η_S represent the luminescence quantum efficiency of the host in the absence and presence of Eu³⁺ ions, respectively. C is the concentration of the Eu³⁺ ions. The value for α = 6, 8, and 10 corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The values of η_S0/η_S also can be approximately calculated by the ratio of related luminescence intensities as^67–70where I_S0 and I_S stand for the luminescence intensity of the host without and with the Eu³⁺ ions, respectively. The relationship between I_S0/I_S and C^α/3 according to the above equation are shown in Fig. 9. It is obvious that a linear relationship can be observed only when α = 6, implying that the energy transfer from the host to Eu³⁺ ions takes place through the dipole–dipole interaction mechanism.

Fig. 9 Dependence of I_S0/I_S of host emission on (a) C^6/3_Eu, (b) C^8/3_Eu, and (c) C^10/3_Eu.

Similarly, Tb³⁺ ions could also be sensitized by host transition through energy transfer process as confirmed by the PL spectra of CaWO₄:y%Tb³⁺ shown in Fig. 6b and the decay lifetimes presented in Fig. 7b. The maximal energy transfer efficiency from host to Tb³⁺ ions is 97%. The critical distance (R_c) was calculated to be about 15.51 Å, and the energy transfer mechanism from the host to Tb³⁺ ions is also dipole–dipole interaction (Fig. S1, ESI†).

In order to enrich the emission colors, the Eu³⁺ and Tb³⁺ codoped CaWO₄ phosphors were synthesized. Fig. 10 shows the PL emission spectra for CaWO₄:4%Tb³⁺,x%Eu³⁺ (x = 0, 1, 2, 4, 6, 8, 10) and CaWO₄:1%Eu³⁺,y%Tb³⁺ (y = 0, 1, 2, 4, 6) upon 264 nm UV excitation. We can observe that the intensities of host and Tb³⁺ emissions in CaWO₄:4%Tb³⁺,x%Eu³⁺ both decreases monotonously with an increase in the Eu³⁺ content, whereas the emission intensity of Eu³⁺ first increases and saturates at x = 8, as depicted in Fig. 10a, which illustrates the energy transfers from the host and Tb³⁺ to Eu³⁺ ions can take place. Energy transfer between Tb³⁺ and Eu³⁺ is a well-known phenomenon since the ⁵D₄ → ⁷F_6,5,4,3 emissions of Tb³⁺ are effectively overlapped with the ⁷F_0,1 → ⁵D_0,1,2 absorptions of Eu³⁺.^16,52 In addition, the decrease in the lifetimes of Tb³⁺ with increasing Eu³⁺ concentration in CaWO₄:4%Tb³⁺,x%Eu³⁺ system strongly demonstrates the existence of an energy transfer process from Tb³⁺ to Eu³⁺ ions (Fig. S2, ESI†). In CaWO₄:1%Eu³⁺,y%Tb³⁺ system, the host emission intensity decreases monotonously, while the emission intensity of Tb³⁺ increases gradually with an increase in Tb³⁺ concentration (Fig. 10b). However, the intensity of Eu³⁺ emission varies little. The result suggests that the energy transfer from the host to Tb³⁺ is more apt to that from Tb³⁺ to Eu³⁺ ions in these phosphors.

Fig. 10 PL emission spectra of (a) CaWO₄:4%Tb³⁺,x%Eu³⁺ and (b) CaWO₄:1%Eu³⁺,y%Tb³⁺ samples as a function of Eu³⁺ and Tb³⁺ concentration, respectively.

Table 1 summarizes the CIE chromaticity coordinates of CaWO₄:Eu³⁺/Tb³⁺ and CaWO₄:Eu³⁺,Tb³⁺ samples. All the values were calculated on the basis of their corresponding PL spectrum. The CIE chromaticity coordinates of CaWO₄:Eu³⁺/Tb³⁺ phosphors can vary from (0.149, 0.100) for CaWO₄ host to (0.625, 0.307) for CaWO₄:20%Eu³⁺ and (0.255, 0.560) for CaWO₄:10%Tb³⁺, which correspond to the color changes from blue to red and green, respectively. In CaWO₄:4%Tb³⁺,x%Eu³⁺ system, the CIE chromaticity coordinate varies from (0.212, 0.302) to (0.434, 0.383) corresponding to x = 0 and 10, respectively. In particular, the CIE chromaticity coordinates for CaWO₄:4%Tb³⁺,8%Eu³⁺ is (0.320, 0.324), which is very close to the standard value of white emission (0.323, 0.333). In CaWO₄:1%Eu³⁺,y%Tb³⁺ system, the CIE coordinate shifts from (0.197, 0.109) to (0.277, 0.462) corresponding to y = 0 and 6, respectively. For the practical applications, the quantum yield (QY) is an important parameter for phosphors. Therefore, the QY of these representative phosphors, CaWO₄ (blue emission), CaWO₄:10%Tb³⁺ (green emission), CaWO₄:20%Eu³⁺ (red emission) and CaWO₄:4%Tb³⁺,8%Eu³⁺ (white emission), are measured. The value of QY is 31.9% for CaWO₄, 24.6% for CaWO₄:10%Tb³⁺, 20.3% for CaWO₄:20%Eu³⁺ and 17.2% for CaWO₄:4%Tb³⁺,8%Eu³⁺. It is believed that the QY of these phosphors could be further improved by optimizing the synthesis conditions and chemical composition. The variety of CIE chromaticity coordinates for the as-prepared samples and digital luminescence photographs of representative samples upon UV lamp excitation at 254 nm are illustrated in Fig. 11, which indicate that the tunable color can be realized by doping different activators and controlling the doping concentration of the activators. These results indicate that the as-obtained products emerge the advantage of multicolor emissions in the visible region and have potential applications in solid-state lighting fields.

Table 1 CIE chromaticity coordinates for CaWO₄:x%Tb³⁺,y%Eu³⁺ samples

Sample no.	CaWO4:x%Eu^3+/y%Tb^3+	Excitation	CIE (x, y)
1	x = 0	248	(0.149, 0.100)
2	x = 1	276	(0.197, 0.109)
3	x = 2	276	(0.222, 0.117)
4	x = 4	276	(0.265, 0.135)
5	x = 6	276	(0.350, 0.179)
6	x = 8	276	(0.486, 0.247)
7	x = 12	276	(0.613, 0.301)
8	x = 20	276	(0.625, 0.307)
9	y = 1	264	(0.175, 0.149)
10	y = 2	264	(0.187, 0.199)
11	y = 4	264	(0.212, 0.302)
12	y = 6	264	(0.236, 0.407)
13	y = 8	264	(0.251, 0.542)
14	y = 10	264	(0.255, 0.560)

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Sample no.	CaWO4:4%Tb^3+/x%Eu^3+	Excitation	CIE (x, y)
11	x = 0	264	(0.212, 0.302)
15	x = 1	264	(0.229, 0.337)
16	x = 2	264	(0.245, 0.367)
17	x = 4	264	(0.257, 0.346)
18	x = 6	264	(0.294, 0.329)
19	x = 8	264	(0.320, 0.324)
20	x = 10	264	(0.434, 0.383)

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Sample no.	CaWO4:1%Eu^3+/y%Tb^3+	Excitation	CIE (x, y)
2	y = 0	264	(0.197, 0.109)
21	y = 1	264	(0.230, 0.232)
22	y = 2	264	(0.251, 0.303)
23	y = 4	264	(0.269, 0.401)
24	y = 6	264	(0.277, 0.462)

---

Fig. 11 The variety of the CIE chromaticity coordinates of the as-prepared phosphors. The insets are digital photographs of representative samples upon 254 nm UV lamp box excitation.

4. Conclusions

In summary, a series of Eu³⁺ and Tb³⁺ singly-doped and co-doped CaWO₄:Eu³⁺/Tb³⁺ phosphors have been prepared by high-temperature solid-state reaction method. Under UV excitation, the CaWO₄ host can emit an intense blue light. When Eu³⁺/Tb³⁺ were singly doped into the host, the tunable emission colors occur attributed to the energy transfer from the host to the activators. The monotonous decreases in the lifetimes of host with increasing activator concentrations strongly certify the existence of energy transfer from the host to activators in CaWO₄:Eu³⁺/Tb³⁺. The energy transfer mechanism from the host to Eu³⁺ and Tb³⁺ in CaWO₄:Eu³⁺/Tb³⁺ phosphors has been determined to be a resonant type via dipole–dipole interaction. Moreover, by adjusting the concentration ratio of Eu³⁺ and Tb³⁺ ions, a white emission in a single-composition was realized by blending simultaneously the blue, green, and red emissions of host, Tb³⁺, and Eu³⁺ in the CaWO₄:8%Eu³⁺,4%Tb³⁺ phosphor. These results indicate that the as-prepared CaWO₄:Eu³⁺, CaWO₄:Tb³⁺, and CaWO₄:Eu³⁺,Tb³⁺ phosphors have potential application in solid state-lighting technology.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21476045) and the Excellent talents plan project for universities of Liaoning province (LJQ2014055).

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Footnote

† Electronic supplementary information (ESI) available: The decay curves of Tb³⁺ in CaWO₄:4%Tb³⁺,x%Eu³⁺ with changing Eu³⁺ concentrations, and dependence of I_S0/I_S of host emission on (a) C^6/3_Tb, (b) C^8/3_Tb, and (c) C^10/3_Tb respectively. See DOI: 10.1039/c6ra01862a

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