Structure, energy transfer, and luminescence properties of NaLaMgWO₆: Tb³⁺,Eu³⁺ phosphors for solid-state lighting

Abstract

Novel NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphors with tunable color emissions were prepared by a solid-state reaction. The crystal structure, composition, and luminescent properties of the obtained phosphors were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and fluorescence spectrophotometer. The results reveal that NaLaMgWO₆ crystallizes in a monoclinic double perovskite structure with C2/m space group. Tb³⁺ and Eu³⁺ single-doped phosphors show the intense green and red emissions at 545 and 617 nm, corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ and the ⁵D₀ → ⁷F₂ transition of Eu³⁺, respectively, while for Tb³⁺ and Eu³⁺ co-doped phosphors, a tunable emission color from green to red is obtained by variation of the ratio of Eu³⁺ to Tb³⁺. The emission spectra and luminescence decay lifetimes verify the occurrence of energy transfer from Tb³⁺ to Eu³⁺ in NaLaMgWO₆: Tb³⁺, Eu³⁺ phosphor, and the energy transfer mechanism is dominated by the dipole–quadrupole interaction. Furthermore, the thermal stability of NaLaMgWO₆:Tb³⁺, Eu³⁺ phosphor was discussed from 305 to 515 K. The results exhibit the phosphors possess excellent thermal stability, and the activation energy is calculated to be 0.302 eV.

1 Introduction

Phosphor-converted white light emitting diodes (pc-LEDs) have been used in numerous fields, such as indoor and outdoor lighting, screen display, and safety warning, because of their environmental friendliness, long lifetime, high brightness, and reliability. Nowadays, the most common way to achieve white LEDs is to combine a blue LED chip (InGaN) with yellow-emitting phosphors (Y₃Al₅O₁₂:Ce³⁺) [1, 2]. However, the deficiency of red-emitting component leads to poor color-rendering index and low stability of color temperature that restrict its applications. To circumvent these drawbacks, ultraviolet (UV) LED chips can be combined with tricolor (blue, green, and red) phosphors to obtain high-quality white LEDs. Nevertheless, they still suffer from some problems, such as low luminous efficiency and poor color stability because of the strong reabsorption of blue light by the red and green phosphors. Using single-phase phosphors with tunable emission can effectively avoid the above problems [3, 4]. Therefore, it is important and urgent to explore single-phase tunable color phosphors with high luminescent efficiency under UV.

On the basis of energy transfer from the sensitizer to the activator, the combination of multiple activator ions in single-phase host is assumed to be a feasible method to realize tunable color emission [5]. In the rare earth family, the Eu³⁺ and Tb³⁺ ions are well considered as a promising red and green emitting activator for their characteristic ⁵D₀ → ⁷F₂ transition and ⁵D₄ → ⁷F₅ transitions, respectively. Meanwhile, when Tb³⁺ and Eu³⁺ ions are co-doped in a single-phase host, Tb³⁺ ion as a good sensitizer can transfer its energy to activator Eu³⁺ with the phonon spectrum. By adjusting the Eu³⁺/Tb³⁺ ratio, color-tunable emission can be realized in Tb³⁺ and Eu³⁺ co-doped phosphors [6, 7]. Consequently, plenty of researches on Tb³⁺ and Eu³⁺ ion co-doped phosphors have been proposed to obtain abundant emitting colors, such as CaGd₂(WO₄)₄:Tb³⁺, Eu³⁺ [6], KBaGd(WO₄)₃:Tb³⁺, Eu³⁺ [7], Sr₃La(PO₄)₃:Tb³⁺, Eu³⁺ [8], and Na₂MgSiO₄:Tb³⁺, Eu³⁺ [9]. In general, the luminescence properties of rare earth ions (Tb³⁺ and Eu³⁺) depend on the host crystal structure, so it is important to select an appropriate host to obtain efficient phosphors. Recently, double perovskite structure compounds with the general formula AA’BB’O₆ (where A = K, Na, Li; A′ = Gd, La; B = Ca, Mg; B′ = W, Mo) being host doped with a variety of activators have been studied extensively due to their excellent luminescence characteristics and chemical and physical properties [10,11,12,13]. AA’BB’O₆ compound exhibits the diversity of crystal structure with the different ion types of A/A’ and B/B’. Among them, the representative compound NaLaMgWO₆ possesses a monoclinic structure with the space group of C2/m. It has double ordering structural characteristics: the layered A site cation ordering (Na⁺/La³⁺) and rock salt B site cation ordering (Mg²⁺/W⁶⁺) [14]. The structure provides lots of crystal field to co-dope numerous rare earth ions, which is beneficial to tune the luminescent characteristics. Furthermore, NaLaMgWO₆ host shows broad and strong charge transfer band in the UV, and the energy could effectively transfer to rare earth activators, resulting in the enhancement of luminous efficiency. As a result, rare earth ion-doping NaLaMgWO₆ phosphors have been widely studied, especially Eu³⁺ single-doped NaLaMgWO₆:Eu³⁺ red-emitting phosphors, as the suitable candidate for high-performance white LEDs [13,14,15,16]. However, no report is available on the luminescence properties and the energy transfer between Tb³⁺ and Eu³⁺ in NaLaMgWO₆:Tb³⁺, Eu³⁺ phosphors.

In this paper, a series of Tb³⁺ and Eu³⁺ co-doped NaLaMgWO₆ phosphors were prepared by the solid-state reaction technique and their crystal structure, photoluminescence properties, and thermal stability were systematically discussed. Under excitation at 273 nm, NaLaMgWO₆:Tb³⁺, Eu³⁺ phosphors generate tunable color emission from green to red by tuning the ratio of Eu³⁺ to Tb³⁺. Evidence of energy transfer from Tb³⁺ to Eu³⁺ was illustrated via the emission spectrum and decay curve measurements.

2 Experimental section

The phosphor samples NaLa_1-x-yMgWO₆:xTb³⁺,yEu³⁺ (0 ≤ x ≤ 0.15; 0 ≤ y ≤ 0.8) were prepared via the high-temperature solid-state technique. Firstly, stoichiometric amounts of raw materials Na₂CO₃ (A.R), MgCO₃ (A.R), La₂O₃ (99.95%), Eu₂O₃ (99.9%), Tb₄O₇ (99.9%) and WO₃ (99.9%) were weighed and mixed thoroughly in a planetary ball mill. The ball-milled mixtures were put into a Petri dish and dried in a drying oven at 80 °C. Then, the homogeneous powders were calcined at 1100 °C for 4 h and cooled to room temperature with the furnace. Finally, the obtained samples were ground again for further characterizations.

The X-ray diffraction (XRD) patterns of the phosphors were collected using an X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu K_α (λ = 0.15418 nm) radiation at 40 kV and 40 mA. The morphology and composition analysis of the samples were obtained via a field emission scanning electron microscopy (Apreo + HiVac, FEI, America) equipped with energy-dispersive spectrometer (EDS). The photoluminescence spectra were characterized using a fluorescent spectrometer (F-7000, Hitachi, Japan), and TAP-02 high-temperature fluorescent controller was equipped to the spectrometer to measure the temperature-dependent emission spectra. The fluorescence decay curves were collected with a FluoroLog-3 fluorescence spectrophotometer (Jobin–Yvon, Horiba, France).

3 Results and discussion

3.1 Phase analysis

Figure 1 shows the XRD patterns of NaLa_0.9-yMgWO₆:0.1Tb³⁺, yEu³⁺ (0 ≤ x ≤ 0.8) phosphors. It is clearly seen that all the diffraction peaks are consistent with (NaLa)(MgW)O₆ standard diffraction card (JCPDS 37-0243). This result indicates that all studied phosphors have the same monoclinic double perovskite structure without any other impurity phase. In other words, Tb³⁺ and Eu³⁺ have been successfully incorporated into the host lattice without any structural changes. Furthermore, as shown in Fig. 1b, as the Eu³⁺ doping concentration increases, the diffraction peaks of NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors show a regularly shift toward higher angle which may be due to the replacement of smaller Eu³⁺ ion (r = 1.120 Å, CN = 9) for La³⁺ ion (r = 1.216 Å, CN = 9) [17].

Fig. 1

a XRD patterns of NaLa_0.9-yMgWO₆:0.1Tb³⁺, yEu³⁺ (0.1 ≤ y ≤ 0.8) phosphors; b the enlarged XRD patterns in the range of 30°–36°

To further explore the crystal structure and confirm that the Tb³⁺ and Eu³⁺ ions are successfully introduced into the NaLaMgWO₆ host, the Rietveld refinement of three representative samples NaLaMgWO₆, NaLa_0.9MgWO₆:0.1Tb³⁺, and NaLa_0.4MgWO₆:0.1Tb³⁺,0.5Eu³⁺ were performed by the Rietica software. According to the report on the structure evolution of NaLaMgWO₆ by Liu et al. [18], the un-doped NaLaMgWO₆ crystallizes in monoclinic structure with two possible space groups named P21 and C2/m, and the space group of C2/m has been proved to be more appropriate for the crystal structure of NaLaMgWO₆. Therefore, the space group C2/m was adopted as starting model for Rietveld refinement of above three samples, and the refined results are exhibited in Fig. 2. The red solid lines, crosses, blue bars, and green lines stand for calculated patterns, experimental data, Bragg positions, and differences between experimental data and calculated patterns, respectively. The refined R_p and R_wp for NaLaMgWO₆, NaLa_0.9MgWO₆:0.1Tb³⁺ and NaLa_0.4MgWO₆:0.1Tb³⁺,0.5Eu³⁺ are 5.308/5.683/6.138 (R_p) and 7.109/8.929/9.677 (R_wp), respectively, which indicates that un-doped and doped NaLaMgWO6 samples are good agreement with the space group C2/m. Moreover, the refined unite cell parameters are displayed in Table 1. It is obvious that the lattice parameters (a, b, c) and unite cell volume (V) of NaLa_0.9MgWO₆:0.1Tb³⁺ and NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ are smaller than those of un-doped NaLaMgWO6 host. The lattice shrinkage confirms further the successful substitution of smaller Tb³⁺/Eu³⁺ ions for La³⁺ ions, and the substitution has no obvious effect on the structure of NaLaMgWO₆.

Fig. 2

Refined XRD patterns of a NaLaMgWO₆, b NaLa_0.9MgWO₆:0.1Tb³⁺ and c NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ phosphors

Table 1 Crystallographic data for NaLaMgWO₆, NaLa_0.9MgWO₆:0.1Tb³⁺ and NaLa_0.4MgWO₆:0.1Tb³⁺,0.5Eu³⁺ based on Rietveld refinement

3.2 SEM and EDX study

In order to study the composition and element distribution of NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ phosphor, the elemental mapping and EDX spectrum are shown in Fig. 3. The sample contains spherical-like particles with good dispersion and uniform particle size. The particle size is in the range of 0.5–2 μm. In addition, EDX spectrum of the sample suggests that the sample is composed of Eu, Tb, Na, La, Mg, W, and O elements, and the elements of Tb, Eu, Na, La, Mg, and W are uniformly distributed throughout the whole particle. The results further manifest that Eu³⁺ and Tb³⁺ ions are incorporated into NaLaMgWO₆ host, and the NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphor can be achieved.

Fig. 3

SEM image, elemental mapping, and EDX spectrum of NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ phosphor

3.3 Photoluminescence properties

Figure 4a displays the excitation and emission spectra of Tb³⁺ single-doped NaLa_0.9MgWO₆ phosphors by monitoring excitation and emission wavelength at 273 nm and 545 nm, respectively. The excitation spectrum contains a strong broad band ranging from 250 to 350 nm and a series of weak narrow peaks in the range of 350–450 nm. The broad band located at 273 nm is ascribed to the spin-forbidden and spin-allowed 4 f ⁸ → 4 f ⁷5d transitions of Tb³⁺ ions. The weak peaks are located at 357 nm, 368 nm, and 378 nm, corresponding to ⁷F₆ → ⁵D₂, ⁷F₆ → ⁵L₁₀ , and ⁷F₆ → ⁵D₃ transitions of Tb³⁺ ions, respectively [19, 20]. Under the excitation of 273 nm, The emission spectrum of NaLa_0.9MgWO₆:0.1Tb³⁺ comprises of four emission peaks at 486 nm, 545 nm, 585 nm and 623 nm, which correspond to ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions of Tb³⁺ ions, respectively [21]. Moreover, the emission intensity of NaLa_1-xMgWO₆:xTb³⁺ phosphors is related to Tb³⁺ doping concentration. As shown in Fig. 4b, with the increase of Tb³⁺ doping content, the emission intensity of NaLa_1-xMgWO₆:xTb³⁺ phosphor increases initially and reaches maximum at x = 0.1, then exhibit a decrement tendency due to the effect of concentration quenching.

Fig. 4

a Excitation and emission spectra of NaLa_0.9MgWO₆:0.1Tb³⁺ phosphors (λ_Em = 545 nm and λ_Ex = 273 nm). b Emission spectra of NaLa_1-xMgWO₆:xTb³⁺ (x = 0.05, 0.075, 0.1, 0.125, 0.15). The inset in (a) shows amplification of excitation spectrum in the range of 350–400 nm

The Fig. 5 demonstrates the excitation and emission spectra of Eu³⁺ single-doped NaLaMgWO₆ phosphors. The excitation spectrum of NaLaMgWO₆:Eu³⁺ (Fig. 5a) consists of a broad band located at 313 nm with several line peaks covering from 350 to 500 nm. The former broad band is attributed to the charge transfer band (CTB) of O²⁻ → W⁶⁺ in WO₄²⁻ and the CTB of O²⁻ → Eu³⁺ transition. The latter narrow peaks are assigned to the typical 4-4f transitions of Eu³⁺ions. Figure 5b presents the emission spectra of NaLaMgWO₆:Eu³⁺ phosphors under excitation at 313 nm and 397 nm. The emission spectra have similar emission profile, which show the Eu³⁺ 4–4f transition peaks centered at 536 nm (⁵D₁ → ⁷F₁), 580 nm (⁵D₀ → ⁷F₀), 592 nm (⁵D₀ → ⁷F₁), 617 nm (⁵D₀ → ⁷F₂), 653 nm (⁵D₀ → ⁷F₃), and 695 nm (⁵D₀ → ⁷F₄) [15, 16]. Furthermore, the intensity of electric dipole transition (⁵D₀ → ⁷F₂, 617 nm) is much higher than that of magnetic dipole transition (⁵D₀ → ⁷F₁, 592 nm) for both emission spectra, indicating that the Eu³⁺ ions are placed to the non-centrosymmetric lattice sites in this double perovskite NaLaMgWO₆.

Fig. 5

a Excitation spectrum of NaLaMgWO₆: Eu³⁺ phosphors (λ_em = 617 nm). b Emission spectra of NaLaMgWO₆: Eu³⁺ phosphors (λ_ex = 313 nm and 397 nm)

Figure 6a presents the excitation of Eu³⁺ and the emission spectra of Tb³⁺. It can be seen that only small spectral overlap in the range of 475–510 nm could be found. Energy transfer between Tb³⁺ to Eu³⁺ generally increase with the spectral overlap. However, some literatures [7, 8] have proved that the energy transfer from the Tb³⁺ to Eu³⁺ can still occur through assisted the phonons spectrum even if little spectral overlap. Figure 6b shows the excitation spectra of NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors by monitoring with 617 nm. The excitation spectra are almost identical to that of NaLaMgWO₆: Eu³⁺ phosphors without any excitation peaks of Tb³⁺. This is due to that the excitation peaks of Tb³⁺ overlap with those of Eu³⁺ and the spectrum intensity of Tb³⁺ is weaker than that of Eu³⁺.

Fig. 6

a Excitation of Eu³⁺ and the emission spectra of Tb³⁺; b excitation spectra of NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors; c emission spectra of NaLa_0.9−yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors; d the relative emission intensity of Eu³⁺ (⁵D₀ → ⁷F₂) and Tb³⁺ (⁵D₄ → ⁷F₅) (λ_ex = 273 nm)

Figure 6c depicts the emission spectra of NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors by exciting with 273 nm. At y = 0.1, the emission spectrum of NaLa_0.8MgWO₆:0.1Tb³⁺,0.1Eu³⁺ phosphor contains both the typical ⁵D₄ → ⁷F_6,5,4,3 transitions of Tb³⁺ ions and the ⁵D_0,1 → ⁷F_0,1,2,3,4 transitions of Eu³⁺ ions. In addition, with the increase of Eu³⁺ doping content, the emission intensity of Tb³⁺ decreases monotonically, while the intensity of Eu³⁺ increases initially then reduces, and its maximum is observed when y = 0.5 (Fig. 6d). It demonstrates the presence of energy transfer from Tb³⁺ to Eu³⁺ and indicates that the color tuning of the emission from green to red could be realized by precisely changing the ratio of Eu³⁺/Tb³⁺ ions concentration. The corresponding CIE chromaticity coordinates and CIE chromaticity diagram of NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors are estimated and exhibited in Fig. 7. The emission color of NaLa_0.9MgWO₆:0.1Tb³⁺ phosphor is green with CIE coordinates of (0.293, 0.586) (Fig. 7a). With raising Eu³⁺ concentration, the emission hue changes from green (0.293, 0.586) to red (0.648, 0.351).

Fig. 7

CIE chromaticity coordinates for NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors under 273 nm excitation

3.4 Decay curves and energy transfer mechanism

To explore the energy transfer process from Tb³⁺ to Eu³⁺, the luminescence decay curves of NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors excited at 273 nm and monitored at 545 nm are measured and presented in Fig. 8.

Fig. 8

a Luminescence decay curves of Tb³⁺ for NaLa_0.9−yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors; b the corresponding lifetime and energy transfer efficiency as a function of Eu³⁺ concentration

It can be seen that the decay curves can be fitted with the following biexponential function:

$$I\left( t \right) = I_{o} + A_{1} \exp \left( {{{ - t} \mathord{\left/ {\vphantom {{ - t} {\tau_{1} }}} \right. \kern-\nulldelimiterspace} {\tau_{1} }}} \right) + A_{2} \exp \left( {{{ - t} \mathord{\left/ {\vphantom {{ - t} {\tau_{2} }}} \right. \kern-\nulldelimiterspace} {\tau_{2} }}} \right)$$

(1)

where I₀ and I(t) represent the luminescence intensities at time t=0 and t, respectively. τ₁ and τ₂ refer to the decay lifetimes for the exponential components. A₁ and A₂ are the fitting coefficient. The average lifetime can be derived from the following equation [22]:

$$\tau = {{\left( {A_{1} \tau_{1}^{2} + A_{2} \tau_{2}^{2} } \right)} \mathord{\left/ {\vphantom {{\left( {A_{1} \tau_{1}^{2} + A_{2} \tau_{2}^{2} } \right)} {\left( {A_{1} \tau_{1} + A_{2} \tau_{2} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {A_{1} \tau_{1} + A_{2} \tau_{2} } \right)}}$$

(2)

From the Eq. (2), the average decay lifetimes for NaLa_0.9−yMgWO_6:0.1Tb³⁺,yEu³⁺ phosphors are determined to be 0.435, 0.261, 0.184, 0.134, 0.0727, and 0.0521 ms for y = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, respectively. In general, the decay lifetime of the sensitizer will be shorten due to the influence of energy transfer from the sensitizer to the activator. As shown in Fig. 8, the decay lifetime of Tb³⁺ decreases monotonically with the increase of Eu³⁺ doping concentration, which strongly indicates that Tb³⁺ ion can act as a good sensitizer, transferring its energy to activator Eu³⁺ in the NaLa_0.9-yMgWO₆:0.1Tb³⁺,yEu³⁺ phosphors. On the basis of the obtained decay lifetimes, the energy transfer efficiency (η_ET) from Tb³⁺ to Eu³⁺ can be determined by the following formula [6, 7]:

$$\eta_{{{\text{ET}}}} = 1 - \frac{{\tau_{{\text{s}}} }}{{\tau_{{{\text{s}}0}} }}$$

(3)

where τ_s and τ_so refer to the lifetimes of the sensitizer Tb³⁺ with and without the activator Eu³⁺. According to the Eq. (3), the η_ET values of NaLa_0.9−yMgWO₆:0.1Tb³⁺,yEu³⁺ (y = 0, 0.1, 0.2, 0.3, 0.4, 0.5) phosphors are estimated and exhibited in Fig. 8b. The energy transfer efficiency η_ET increases with the increase of the Eu³⁺ content and reaches the maximum of 88.11% at y = 0.5, suggesting that the energy of Tb³⁺ can be efficiently transfer to Eu³⁺ in NaLaMgWO₆:Tb³⁺,Eu³⁺phosphors.

General speaking, the energy transfer from sensitizers to activators in host is associated with exchange and multipolar interaction [23]. The prerequisite of exchange interaction mechanism is that the critical distance (R_c) between the sensitizer and activator is less than about 0.5 nm. As suggested by Blasse, the critical distance (R_c) between Tb³⁺ and Eu³⁺ in NaLaMgWO₆ host can be estimated using the following formula [24]:

$$R_{C} = 2\left( {\frac{3V}{{4\pi \chi_{C} N}}} \right)^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}}}}$$

(4)

where V is the volume of the unit cell, N is the number of the La³⁺ lattice sites in the unit cell, and χ_c is the total concentration of Tb³⁺ and Eu³⁺. For NaLa_0.4MgWO₆:0.1Tb³⁺,0.5Eu³⁺ phosphors, V = 473.949 Å, N = 4, and χ_c = 0.6. Therefore, the critical distance (R_c) is estimated to be 7.23 Å. This value is longer than 5 Å for the exchange interaction, indicating the multipolar interaction play a leading role in the mechanism of energy transfer between Tb³⁺ and Eu³⁺.

According to Dexter’s energy transfer formula of multipolar interaction, the ratio of the luminescence quantum efficiency of the sensitizer in the absence of activation to the efficiency of the sensitizer in the presence of activation can be defined as [23, 25]:

$$\frac{{\eta_{0} }}{\eta } = C^{{{n \mathord{\left/ {\vphantom {n 3}} \right. \kern-\nulldelimiterspace} 3}}}$$

(5)

where η₀ and η are the quantum efficiencies of Tb³⁺ without and with Eu³⁺, respectively. C is the sum content of Tb³⁺ and Eu³⁺, and n = 6, 8, 10, which correspond to dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interaction, respectively. Experimentally, the value of η₀/η can be estimated approximately by the ratio of luminescence intensities (I₀/I) based on Reisfeld’s approximation [3, 26]. Figure 9 shows the relationships of I₀/I versus C ^n/3 for NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphors. It is obvious that the optimal linear relationship is observed when n = 8, indicating that the energy transfer from Tb³⁺ to Eu³⁺ is realized via dipole–quadrupole interaction.

Fig. 9

Dependence of I₀/I of Tb³⁺ on C_Tb+Eu^n/3

3.5 Thermal quenching properties

The thermal quenching temperature of the phosphors is a vital parameter for their practical application in high power white LEDs. The temperature-dependent emission spectra for NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ phosphors excited by 273 nm was measured and shown in Fig. 10a. The emission intensity declined gradually with the increase of the temperature, and the emission intensity at 455 K drops to 64.2% of the initial intensity at 305 K. It implies that NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ possesses excellent thermal stability. The thermal quenching is associated with the temperature of the electron–phonon interaction, affecting the ground state as well as the excited states [27]. The thermal quenching mechanism of NaLa_0.4MgWO₆:0.1Tb³⁺,0.5Eu³⁺ phosphors can be analyzed by calculating the activation energy according to the Arrhenius equation [5]:

$$I\left( T \right) = \frac{{I_{0} }}{{1 + c \exp \left( { - \Delta E/kT} \right)}}$$

(6)

Fig. 10

a Emission spectra of NaLa_0.4MgWO₆:0.1Tb³⁺,0.5Eu³⁺ phosphors at different temperature (λ_ex = 273 nm); b The plot of ln[(I₀/I_T) − 1] versus 1/kT

where I(T) and I₀ refer to the emission intensity at test temperatures and initial emission intensity, respectively. k is the Boltzmann constant (8.629 × 10^–5 eV/K), c is the constant, and ΔE corresponds to the activation energy. The equation can be converted to the natural logarithm [28]:

$$\ln \left( {\frac{{I_{0} }}{{I_{T} }} - 1} \right) = - \frac{\Delta E}{{kT}} + c^{\prime}$$

(7)

Figure 10b presents the plot of ln[(I₀/I_T) − 1] versus 1/kT. The experimental data can be linear fitted with a slope of − 0.302. Hence, based on the Arrhenius Eq. (7), the activation energy ΔE is approximately 0.302 eV for NaLa_0.4MgWO₆:0.1Tb³⁺, 0.5Eu³⁺ phosphors.

4 Conclusions

In conclusion, Novel NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphors with tunable emission color were successfully prepared by a solid-state reaction. X-ray diffraction study shows that NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphors have the monoclinic crystal structure with the C2/m space group, and Rietveld refinement demonstrates the effective replacement of La³⁺ by Tb³⁺ and Eu³⁺ in the host has no influence on the crystallinity of NaLaMgWO₆. Photoluminescence investigation reveals that with the increase of the ratio of Eu³⁺ to Tb³⁺, the emission color of NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphors can be tuned from green to red upon the excitation of ultraviolet light, and the corresponding CIE coordinates vary from (0.293, 0.586) to (0.648, 0.351). The energy transfer from Tb³⁺ to Eu³⁺ in NaLaMgWO₆ host is responsible for their tunable color emission, and the mechanism of energy transfer is confirmed to be the dipole–quadrupole interaction. Moreover, the thermal stability of NaLaMgWO₆:Tb³⁺,Eu³⁺ phosphors was also investigated and ΔE value is estimated to be 0.302 eV.