1. Introduction

The progress in phosphor-converted white LEDs was primarily enabled by the advances in the InGaN-based LEDs as excitation sources [1, 2]. Recently, significant advances in the development of deep-UV and mid-UV III-Nitride based emitters [3–7] are important for enabling the development of UV-based phosphor converted white LEDs. Lanthanide-ion-doped phosphors are efficient luminescent materials and irreplaceable components of phosphor-converted white LED (pc-WLED). Recently, much attention has been paid to obtain white light emission [8–10]. The formation of efficient, durable, and single-phase white-light-emitting phosphors is one of the important strategies because they can avoid the reabsorption for blue or UV light by the red-/green-emitting phosphors and the mixing of RGB phosphors. Consequently, it can enhance the luminescence efficiency and color reproducibility of the white light source, and reduce manufacturing costs [8, 11].

It is well known that the components of single-phase white light are normally contributed by co-doping activators in a single pure phase [12]. Ba₃MgSi₂O₈:Eu²⁺, Mn²⁺ is a typical example of a single-phased white-emitting phosphor, which shows three emission bands at around 442, 505, and 620nm [13]. There are many other phosphors co-doped with Eu²⁺, Mn²⁺ that could be used to realize white light emission, including SrZn₂(PO₄)₂ [14], Ba₂Ca(B₃O₆)₂ [15], M₃MgSi₂O₈(M = Ca, Sr, Ba) [16], Ca₂MgSi₂O₇ [17], CaAl₂Si₂O₈ [12], La_0.827Al₁₁.₉O_19.09 [18]and Ba₂SiO₃Cl₂ [19]. Recently, Ce³⁺–Eu²⁺ codoped white light phosphors are obtained such as Sr₃B₂O₆:Ce³⁺, Eu²⁺ [20]. There are some other co-doping ion pairs which could generate white light, such as Ce³⁺–Tb³⁺ in Ca₂Al₂SiO₇ [21], Bi³⁺–Eu³⁺ in Y₄MgSi₃O₁₃ [22], and Dy³⁺–Ce³⁺ in GdAl₃(BO₃)₄ [23, 24], as well as Ag-Eu³⁺ codoped oxygluoride glasses [25]. It has been known that Tm³⁺ ion is always used as efficient blue light emissive activator and Eu³⁺ is always used as efficient red emissive activator. The blue and red light mixed in suitable proportions will show a white light, however, to the best of our knowledge, up to now, Tm³⁺ and Eu³⁺ codoped single-phase white-light phosphors have not been reported.

Recently, vanadate-based phosphors drew much more attention due to the self-activated emitting properties of [VO₄]^3- group, the sensitization from [VO₄]^3- to rare earth ions as well as their long wavelength excitation and the excellent chemical stabilities [26–33]. The vanadate group, namely, [VO₄]^3-, where the central metal ion V is coordinated by four oxygen ligands in a tetragonal symmetry, has broad and intense charge transfer (CT) absorption bands in the UV region and some of them can produce intense broadband CT emission spectra from 400 to more than 700 nm related to the local structure [34–36]. When excited by UV light, these vanadates or rare earth ions-doped these materials have the capability to convert the ultraviolet emission into white light [29, 37, 38].

In this work, Tm³⁺ and/or Eu³⁺ codoped Ca₉Gd(VO₄)₇ (CGV) phosphors with whitlockite-like structure were synthesized by the conventional high temperature solid state reaction. The photoluminescence excitation (PLE) and photoluminescence (PL) of Tm³⁺ or/ and Eu³⁺ ions in CGV host were discussed. The energy transfer properties of Tm³⁺ and/or Eu³⁺ doped CGV phosphors were investigated. The color-tunable luminescence including white light in the CGV:Tm³⁺,Eu³⁺ phosphor was obtained by varying the relative doping concentrations of Tm³⁺ and Eu³⁺.

2. Experimental section

2.1 Preparation

The phosphors with nominal composition Ca₉Gd (VO₄)₇: xTm³⁺, yEu³⁺ (CGV: xTm³⁺, yEu^3+, in which x and y stand for the doping concentration of Tm³⁺ and Eu³⁺, respectively) were prepared using a high-temperature solid-state reaction method. Raw powders were prepared by stoichiometric mixtures of CaCO₃ (99.9%), V₂O₅ (99.9%), Gd₂O₃ (99.99%), Eu₂O₃ (99.99%) and Tm₂O₃ (99.99%). The concentrations of Tm³⁺ and Eu³⁺ dopants were varied in the range of 0-9mol% with respect to Gd³⁺ ions. The mixtures were ground in an agate mortar for 1h and homogeneously mixed. The powder mixtures were firstly preheated at 1023K for 24h, then calcined at 1123-1273K for 12h in air and at last cooled down to room temperature to obtain the final white samples.

2.2 Characterizations

Powder X-ray diffraction (XRD) measurements were taken on a D/MAX 2500 instrument (Rigaku) with a Rint 2000 wide angle goniometer and Cu Kα1 radiation (λ = 1.54056 A) at 40 kV and 100 mA. The diffraction patterns were scanned over an angular (2θ) range of 20−80° at intervals of 0.02° with a counting time of 0.6s per step. Photoluminescence (PL) studies were conducted on a fluorescence spectrophotometer (Photon Technology International) equipped with a 60 W Xe-arc lamp as the excitation light source. All the measurements were taken at room temperature.

3. Results and discussion

3.1 Phase identification and structural characteristics

Figure 1 presents the typical XRD patterns of Ca₉Gd (VO₄)₇: xTm³⁺, yEu³⁺ fired at various temperatures. All of the main diffraction peaks of the samples can be basically indexed to the standard data of Ca₉Gd (VO₄)₇ (JCPDs No. 48-1782) when the sintering temperature at 750°C. With increasing of fired temperature to 1000°C, the XRD peaks sharpen and intensify, indicative of the enhanced crystallinity of the Ca₉Gd(VO₄)₇ phase, as can be shown in Figs. 1(a), 1(b), and 1(f). From Figs. 1(a) and 1(b), we can see no other phase is detected, indicating that the obtained samples are single phase and Eu³⁺ ions have been successfully incorporated in the Ca₉Gd(VO₄)₇: yEu³⁺ host lattice by replacing Gd³⁺ ions due to their similar ionic radii and charge.

 

Fig. 1 Typical XRD patterns of Ca₉Gd(VO₄)₇: 5% Eu³⁺ as a function of the final calcinations temperature (a) 1000°C (b) 950°C (e) 850°C (f) 750°C and the standard data for (d) Ca₉Gd(VO₄)₇(JCPDS file No. 48-1782) and (e) Ca₃(VO₄)₂ (JCPDS file No. 46-0756)

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The crystal structures of Ca₉Gd (VO₄)₇ is isotypic with Ca₃ (VO₄)₂ (JCPDS no. 46-0756). Their diffraction peaks are similar, which is can be shown in Fig. 1(c). In Ca₃ (VO₄)₂, Ca²⁺ ions occupy five independent sites: Ca1-Ca5. The Ca1-Ca3 (18b) and Ca5 (6a) sites are fully occupied while the Ca4 sites (6a) is half-occupied [39]. There is one vacant site, Ca6 (6a), in the structure. Gd³⁺ ions are located at the Ca1, Ca2, and Ca5 sites together with the Ca sites in the crystal Ca₉Gd(VO₄)₇.

3.2 Photoluminescence properties

3.2.1 Photoluminescence properties of CGV: xTm³⁺ and CGV: yEu³⁺ phosphors

Figures 2(a) 2(b) 2(c), and 2(d) show the photoluminescence excitation (PLE) and emission (PL) spectra of pure CGV: 3%Tm³⁺ sample and CGV:5%Eu³⁺ sample. Under the excitation of 315 nm UV-light irradiation, CGV: 3%Tm³⁺ shows a strong blue emission (Fig. 2(b)) due to the typical ¹G₄→³H₆ transition of Tm³⁺ ions with a maximum at 477 nm. The (x, y) chromaticity coordinate of CGV is (0.197, 0.226) in the blue region. Upon the monitoring wavelength at 477nm (Fig. 2(a)), the excitation spectra of pure CGV: 3%Tm³⁺ sample consists of a broad band with peak at 315 nm covering from 200 to 350nm, which can be matched well with the UV-LED chips, and a sharp weak peak at 371 nm due to the characteristic f-f transition of Tm³⁺. The broad excitation spectrum band originates from O^2-→V⁵⁺ CT transition, which can be separated into two peaks at 294 (4.22eV) and 316 nm (3.92eV) corresponding to the ¹A₁ →¹T₂ and ¹A₁ →¹T₁ transition of VO₄^3- group, respectively [26, 35]. In Fig. 2(a), red dotted lines indicates excitation spectra fitted with two Gaussian curves (green dotted lines) corresponding to the two excitation bands (usually named as Ex₁, Ex₂).The energy difference between the peaks Ex₁and Ex₂ is 0.30eV which is ascribed to the energy difference between ¹T₂ and ¹T₁, as reported in [31, 40, 41].

 

Fig. 2 Typical photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the CGV: 3% Tm³⁺ sample and CGV: 5% Eu³⁺ sample: (a) PLE of CGV: 3% Tm³⁺ (λ_em = 477 nm) and its Gaussian components at 294 nm and 316 nm; (b) PL of CGV: 3% Tm³⁺ (λ_ex = 315 nm); (c) PLE of CGV: 5% Eu³⁺ (λ_em = 620 nm); (d) PL of CGV: 5% Eu³⁺ (λ_ex = 315 nm)

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Figure 2(d) shows the emission spectra of CGV: 5% Eu excited by 315 nm. The phosphor shows bright red color. The (x, y) chromaticity coordinate of CGV: 5% Eu is (0.632, 0.364) in the red region (Fig. 3).From Fig. 2(d), it can be seen that the dominant red emission bands of 617 and 620 nm are attributed to the electric dipole transition ⁵D₀ →⁷F₂, indicating that Eu³⁺ ions locate at the sites of non-inversion symmetry. The emission peaks at about 575, 596, 652, 700-706 nm are derived from the transition of ⁵D₀→⁷F₀, ⁵D₀ →⁷F₁, ⁵D₀→⁷F₃, and ⁵D₀ →⁷F₄, respectively, which are much weaker comparing with the intensity of ⁵D₀ →⁷F₂ . Consequently, ⁵D₀ →⁷F₂ red emission (617 and 621 nm) presents the most prominent intensity in the emission spectrum.

 

Fig. 3 The CIE chromaticity diagram for the CGV: 3% Tm³⁺ (point 1) and CGV: 5% Eu³⁺ (point 2).

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Figure 4 shows the PL and PLE spectra of CGV: 5% Eu and CGV: 3% Tm samples obtained at different fired temperatures. In Figs. 4(a) and 4(c), we can see that with increasing of the annealing temperature, the broad O-V CT bands were found red-shift due to the decrease of oxygen vacancy [42], and the intensity of PL spectra became weaker and weaker with increasing of calcining temperature, but the relative intensity coming from the f-f transitions of Eu³⁺ ions or Tm³⁺ almost keep unchanged, indicating that the region of chromaticity coordinate will not change under different temperature, while more intensity PL can be obtained at low temperature. This is due to the existence of very small amounts of intermediate during the reaction, which is maybe CaO, or vanadates. A certain amount of these intermediates can increase intensity of PL of CGV: 5% Eu and CGV: 3% Tm samples. With increasing of the fired temperature, the intermediates are gradually reacted, decreased and at last completely disappeared, which induces the decreasing of PL intensity of CGV: 5% Eu and CGV: 3% Tm samples.

 

Fig. 4 The PLE and PL spectra of CGV:5%Eu and CGV:3%Tm obtained after calcined at different temperatures.

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Figures 5(a), 5(b) 5(c), and 5(d) show the PLE and PL spectra of pure CGV: xTm³⁺ (x, mol%) sample and CGV: yEu³⁺ (y, mol%) sample as a function of their doping concentration of Tm³⁺ and Eu³⁺, respectively. Their excitation spectra have no shift with the peaks at 315 nm, as shown in Figs. 5(a) and 5(c). However, under the excitation of 315 nm, from the inset figures of Figs. 5(b) and 5(d), the PL emission intensity of Tm³⁺ and Eu³⁺ increases with the increase of its concentration, reaching a maximum value at 3% for CGV: xTm³⁺ and at 0.5% for CGV: yEu³⁺ (it is in the range of low concentration of Eu³⁺, however, its emission intensity become much stronger in the higher concentration of Eu³⁺, which is not discussed here.) . In one aspect, it may be due to the concentration quenching. In general, the concentration quenching of luminescence is due to the energy migration among the activator ions at the high concentrations. In the energy migration process the excitation energy will be lost at a killer or quenching site, resulting in the decrease of luminescence intensity [43]. In another aspect, in the crystal structures of CGV_, Gd³⁺ ions are located at the Ca1, Ca2, and Ca5 sites together with the Ca sites in the crystal Ca₉Gd(VO₄)₇. When Eu³⁺ ions were doped into the CGV crystals, they would occupy the sites of Gd³⁺, which indicates that Eu³⁺ would occupy the sites of Ca1, Ca2, and Ca5 sites. At low doping of Eu³⁺, Eu³⁺ ions would randomly occupy the different sites of Ca1, Ca2 or Ca5 firstly, instead of occupying only one stationary site. PL intensity at different site is different. When the concentration of Eu³⁺ ions is 0.5%, the Eu³⁺ ions occupying sites with low PL intensity dominates and their PL intensity would be low, which induces the decrease when concentration increased to 0.5%.

 

Fig. 5 (a) and (b) are PLE and PL spectra of CGV:yEu³⁺with different concentration of Eu³⁺, respectively. Inset in (b) is the dependence curve of PL intensity at 620nm on the concentration of Eu³⁺. (c), (d) are PLE and PL spectra of CGV:xTm³⁺ with different concentration of Tm³⁺, respectively. Inset in (d) is the dependence curve of PL intensity at 477nm on the concentration of Tm³⁺.

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3.2.1 Photoluminescence properties of CGV: xTm³⁺, yEu³⁺ phosphors

From above analysis of PL properties of CGV: xTm³⁺and CGV: yEu³⁺ phosphors, it is can be shown that under 315 nm excitation, CGV: xTm³⁺ can emit blue light and CGV: yEu³⁺ phosphor exhibits red light. It is well-known that blue and red light mixed in suitable proportions will show a white light. So in our current study, we tried to obtain white emission by codoping Tm³⁺ and Eu³⁺ as well as the elaborate choice of doping levels in the single CGV host lattice.

Figure 6 shows the representative PLE and PL spectra of CGV:xTm³⁺,yEu³⁺. Tm³⁺ and Eu³⁺ co-doped Ca₉Gd(VO₄)₇ phosphors showed a blue and red dual emission bands under the 315 nm UV excitation (Fig. 6(b)). Similar to Tm³⁺ and Eu³⁺-single-doped CGV, the peak at 477 nm comes from ¹G₄→³H₆ transition of Tm³⁺ ions and the stronger peak at 620 nm originates from electric dipole transition ⁵D₀ →⁷F₂ of Eu³⁺. Their excitation spectra are broad O-V CT band with the peak at 315 nm (Fig. 6(a)), which indicates that efficient energy transfer from O-V CT state to Tm³⁺ and Eu³⁺ ions occurs under the UV excitation. The schematic diagram of the excitation, emission and energy transfer in CGV:xTm³⁺,yEu³⁺ are shown in Fig. 6(e).

 

Fig. 6 (A) and (B) are representative photoluminescence excitation and emission spectra of CGV:xTm³⁺,yEu³⁺, respectively; (C) Spectra overlap between the photoluminescence emission spectrum of CGV:Tm³⁺ (black line) and the photoluminescence excitation spectrum of CGV:Eu³⁺ (red line); (D) Decay curves for the luminescence of Tm³⁺ ions in CGV:3%Tm³⁺, yEu³⁺ samples (excitation at 315nm, monitored at 477nm); (E) The schematic diagram of the excitation, emission and energy transfer in CGV:xTm³⁺,yEu³⁺.

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It has been known that the luminescence intensities of various rare earth ions can be enhanced or quenched by the energy transfer from other codoped rare earth ions. The comparison of the PL spectra for CGV:3%Tm and the PLE spectrum for CGV:1%Eu in Fig. 6(C) reveals a great spectral overlap between the emission band of Tm³⁺ centered at 477 nm and the excitation transition of ⁷F₀→⁵D₂ centered at 469 nm. The energy difference between ¹G₄ and ³H₆ of Tm³⁺ matches well with that between ⁵D₂ and ⁷F₀, which makes the energy transfer from Tm³⁺ to Eu³⁺ possible, as can be shown in Fig. 6(E). To further study the energy transfer process, the fluorescence decay curves of the Tm³⁺ ions in CGV:3%Tm³⁺,yEu³⁺ phosphors were measured by monitoring at 477 nm with irradiation of 315 nm. From Fig. 6(D), one can see that all the decay curves deviate from the single exponential rule. The lifetimes of Tm³⁺ ions are determined to be 84.6, 75.7, 74.3 and 65.7 μs with y = 0.1%, 0.5%, 1.5% and 2%, respectively, which can confirm the the presence of nonradiative energy transfer from Tm³⁺and Eu³⁺ since the transfer process shortens the life time of the excitated state Tm³⁺. Accordingly, an efficient resonance-type energy transfer from Tm³⁺ to Eu³⁺ is expected in CGV. Here, we investigated the energy transfer properties from Tm³⁺ to Eu³⁺ when they are codoped in the CGV host.

Figure 7(a) shows the PL spectra of CGV:3%Tm³⁺,yEu³⁺(y = 0.1%,0.5%,1%,1.5%,2%) samples with different Eu³⁺ concentration. Both the blue emission band at 477 nm and a red emission band at 620 nm are obtained. From the comparison figures of the two strong emission intensities at 477 and 620nm (Fig. 7(b)), it can be shown that the blue emission of Tm³⁺ decreases with the increase of Eu³⁺ doping concentration, while the red emission of Eu³⁺ increases, which indicates that the energy transfer occurs from Tm³⁺ to Eu³⁺. The same change tendency can be found in a series of phosphors CGV:5%Tm³⁺,yEu³⁺(y = 0.1%, 0.5%, 1%, 2%, 2.5% and 3%) and CGV: 7% Tm³⁺, yEu³⁺ (y = 0.1%, 0.5%, 1%, 1.5%, 2%), as can be shown in Fig. 7(b), 7(e) and 7(g), 7(h), respectively. The possible energy transfer mechanism is shown in the transfer mode (3) of Fig. 6(E).

 

Fig. 7 PL spectra, comparision of the two strong emission intensities at 477nm and 620 nm, and the corresponding CIE chromaticity diagram for CGV:xTm³⁺,yEu³⁺. (a),(b)and (c): CGV:3% Tm³⁺, yEu³⁺ (y = 0.1%, 0.5%, 1%, 1.5%, 2%); (d), (e) and (f): CGV: 5% Tm³⁺, yEu³⁺(y = 0.1%, 0.5%, 1%, 2%, 2.5% and 3%); (g), (h) and (i): CGV: 7% Tm³⁺, yEu³⁺ (y = 0.1%, 0.5%, 1%, 1.5%, 2%)

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The energy transfer efficiency from Tm³⁺ to Eu³⁺ of CGV: 3% Tm³⁺, yEu³⁺ was investigated. Generally, the energy transfer efficiency from a sensitizer to activator can be expressed as the following equation [44]

Where ${\eta }_T$ is the energy transfer efficiency and $I_{SO}$ and $I_S$ are the luminescence intensity of a sensitizer in the absence and presence of an activator, respectively. In the CGV:3%Tm³⁺,yEu³⁺ systems, Tm³⁺ can be regarded as the sensitizer and Eu³⁺ as the activator. Figure 8 shows the results of energy transfer efficiency from Tm³⁺ to Eu³⁺ based on the Eq. (1). ${\eta }_T$ increases with the increasing Eu³⁺ concentration. The maximum energy transfer efficiency can reach 67.5%. All of above results indicate that the energy transfer efficiency is very efficient.

 

Fig. 8 Energy-transfer efficiency from Tm³⁺ to Eu³⁺ in CGV: 3% Tm³⁺, yEu³⁺(y = 0.1%,0.5%,1%,1.5%,2%) samples.

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There are two main aspects to be responsible for the resonant energy-transfer mechanism: one is exchange interaction and the other is multipolar interaction. If energy transfer results from the exchange interaction, the critical distance between sensitizer and activator should be shorter than 4Ǻ. The critical distance R_Tm-Eu of energy transfer from the Tm³⁺ to Eu³⁺ can be calculated using the concentration quenching method. According to Blass [45], the critical distance R_Tm-Eu can be expressed by

Where $N$is the number of available sites for the dopant in the unit cell, $x_C$ is the total concentration of Tm³⁺ and Eu³⁺, and $V$ is the volume of the unit cell. In CGV crystal structure, three sites Gd³⁺ ions in 18b, due to the similar ion radius and identical valence to Gd³⁺, the Tm³⁺/Eu³⁺ are expected to enter into the sites of Gd³⁺, so $N=54$. V = 3890.85Ǻ³. The critical concentration $x_C$, at which the luminescence intensity of Tm³⁺ is half of that in the sample in the absence of Eu³⁺, is 0.04. Therefore, the critical distance of energy transfer was calculated to be about 15.10 Ǻ. The value is much longer than 4 Ǻ, indicative of little possibility of energy transfer from Tm³⁺ to Eu³⁺ via the exchange interaction mechanism. Thus, the electric multipolar interaction can take place for energy transfer. According to the Dexter’s energy transfer expressions of moltipolar interaction and Reisfeld’s approximation, the following relation can be given [14]

Where ${\eta }_0$ and $\eta$ are the luminescence quantum efficiency of Tm³⁺ in the absence and presence of Eu³⁺; C is the sum of the content of Tm³⁺ and Eu³⁺; n = 6,8,10 corresponding to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The value ${\eta }_0/\eta$ can be approximately calculated by the ratio of related luminescence intensities as

The I_SO/I_S-C^n/3 plots are represented in Fig. 9, and the relationships are observed when n = 6,8,10. The plot for n = 10 has the best linear fitting result (its fitting R value is the largest), which indicates that the energy transfer mechanism from Tm³⁺ to Eu³⁺ is a quadrupole-quadrupole interaction type.

 

Fig. 9 Dependence of I_SO/I_S of Tm³⁺ on (a)C^6/3, (b) C^8/3 and (c)C^10/3

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The Commission International d I’Eclairage (CIE) chromaticity coordinates for CGV: 3% Tm³⁺, yEu³⁺(y = 0.1%,0.5%,1%,1.5%,2%) phosphors were measured and summarized in Table 1, and the CIE coordinates of CGV: 3% Tm³⁺, yEu³⁺ are also represented in Fig. 7(c). With increasing of the Eu³⁺ concentration, the CIE coordinates vary systemically from A(0.240, 0.238), B(0.305, 0.257), C(0.411, 0.288), D(0.474, 0.305), to E(0.503, 0.313) corresponding to hues of blue, white, yellow and eventually to red, and thus the white light emission was realized for CGV: 3% Tm³⁺, 0.5%Eu³⁺. The similar variation tendency of CIE chromaticity coordinates for other two series CGV: 5% Tm³⁺, yEu³⁺(y = 0.1%, 0.5%, 1%, 2%, 2.5% and 3%) phosphors and CGV: 7% Tm³⁺, yEu³⁺ (y = 0.1%, 0.5%, 1%, 1.5%, 2%) phosphors were exhibited in Fig. 7(f) and 7(i). Adjusting suitable concentrations of Tm³⁺ and Eu³⁺ for CGV: 5%Tm³⁺, yEu³⁺ and CGV: 7%Tm³⁺, yEu³⁺ can realize white light emission, such as CGV: 5%Tm³⁺, 0.1%Eu³⁺, CGV: 5%Tm³⁺, 0.5%Eu³⁺ and CGV: 7%Tm³⁺, 0.5%Eu³⁺ . From the comparision figures of the two strong emission intensities at 477 nm and 620 nm (Fig. 7(b), 7(e), and 7(h)), we can make a conclusion that when the emission of Tm³⁺ at 477 nm has close intensity to that of Eu³⁺ at 620 nm, the white light can be realized.

Table 1. Chromaticity Coordinates (x,y) of the Tm³⁺and/ or Eu³⁺ Doped CGV Samples under 315 nm Excitation

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4. Conclusions

By a solid state reaction method, a series of Tm³⁺ and/or Eu³⁺ doped Ca₉Gd(VO₄)₇ single composition phosphors were synthesized, and their photoluminescence properties were investigated. The blue emission or red emission originating from the f-f transitions of Tm³⁺ or Eu³⁺ ions is observed in the Tm³⁺ or Eu³⁺ singly-doped Ca₉Gd(VO₄)₇ sample under UV excitation, respectively. Tm³⁺ and Eu³⁺ co-doped Ca₉Gd(VO₄)₇ phosphors showed a blue with the peak at 477 nm and red with the stronger peak at 620 nm dual emission bands under the UV excitation. The energy transfer from O^2--V⁵⁺ CT (charge transfer) energy to Tm³⁺ and Eu³⁺ ions as well as the energy transfer from Tm³⁺ to Eu³⁺ ions has been demonstrated. The energy transfer from Tm³⁺ to Eu³⁺ has been demonstrated to be a resonant type via a quadrupole-quadrupole mechanism and the critical distance of Tm³⁺ and Eu³⁺ ions by quenching concentration method has also been calculated to be 15.10 Ǻ. The photoluminescence intensity ratio of blue and red emission could be tuned by adjusting the concentration of Tm³⁺ and Eu³⁺ ions and as a result the emission color varies from blue to white to red. The white-light emission is realized in single phased phosphor of Ca₉Gd(VO₄)₇:Tm³⁺, Eu³⁺ by combining the Tm³⁺-emission and the Eu³⁺-emission.