A novel Eu³⁺-doped garnet-tpye tellurate red-emitting phosphor with high thermal stability and color purity

1. Introduction

The garnet-related family Li₃Ln₃Te₂O₁₂(Ln=Y, Pr, Nd, Sm-Lu) have been extensively studied as promising solid electrolytes for application in solid state rechargeable lithium-ion batteries for the last few decades [1-4]. In 2006, O’Callaghan et al. developed garnet-type Li₃Ln₃Te₂O₁₂ (Ln = Y, Pr, Nd, Sm-Lu) to investigate the relationship between Li site occupation and Li ion conductivity [1]. The lattice constant increases with increasing Ln ionic radius in Li₃Ln₃Te₂O₁₂. These Li₃Ln₃Te₂O₁₂ garnets have exhibited a fairly low ionic conductivity of ∼10⁻⁵ S cm⁻¹ at 600 °C with a high activation energy (>1 eV) [3]. In 2014, the crystal structures and conductivity data for the most of perspective Li-ion solid electrolytes based on garnet-type metal oxides have been recently reviewed by Thangadurai et al. [4].

Garnet host lattices are of considerable interest due to their wide applications as laser hosts and as phosphors for white light emitting diodes [5]. For example, trivalent rare earth doped Y₃Al₅O₁₂ (YAG) is one of the widely used systems of compounds for solid state lighting applications. Meanwhile, some new garnet-type compound can be constructed based on the garnet structural model, such as the green-emitting Ca₃Sc₂Si₃O₁₂:Ce³⁺, the orange-emitting Lu₂CaMg₂(Si, Ge)₃O₁₂:Ce³⁺, and the green-emitting Ca₂LaZr₂Ga₃O₁₂:Ce³⁺phosphors [6]. Therefore, the development of phosphors based on garnet-type materials is of great interest. As an important activator, the europium ion is one of the most studied lanthanide activators because of its singular luminescence properties, exhibiting pure red emission transitions with a series of sharp lines arising from the excited state ⁵D₀ to the lower energy state ⁷F_0-6. Eu³⁺ ions exhibit pure magnetic and electric dipole transitions which make it a very sensitive probe for the rare earth ion site structure/symmetry. ⁵D₀→⁷F₂ electric dipole (ED) transitions around 610 nm are highly hypersensitive, which is highly sensitive to the symmetry of the Eu³⁺ sites in the lattices; however, the magnetic dipole transitions (⁵D₀→⁷F₁) are not affected by the environment, and their emission intensities are often used as an internal standard [7].

However, luminescence properties of Eu³⁺-doped garnet-type Li₃Gd₃Te₂O₁₂ have not been studied yet. In this work, red emitting phosphors Li₃Gd₃(1-x)Eu_3xTe₂O₁₂(x = 0.01-0.30) were synthesized by the conventional solid-state reaction. The structure, composition and photoluminescence properties of Li₃Gd₃Te₂O₁₂:Eu³⁺ phosphors were investigated. In addition, the luminescence quenching of Eu³⁺ doping concentration and CIE on the photoluminescence spectra were demonstrated in detail.

2. Experimental Procedure

  The synthesis of Li₃Gd₃Te₂O₁₂ phosphors doped with Eu³⁺ ions was carried out via a high-temperature solid-state reaction method. Li₂CO₃ (99.99%), Gd₂O₃ (99.99%), TeO₂ (99.9%), and Eu₂O₃ (99.99%) as raw materials, they were purchased from Sigma-Aldrich without further purification and thoroughly mixed in an agate mortar. The mixtures were sintered in air at 900°C for 10 h. when the reaction was end at 900°C, the products were cooled down to room temperature without cooling devices. Finally, white powers were obtained by grinding. The relevant reaction formulas are as follows:

3Li₂CO₃+3(1-x)Gd₂O₃ + 4TeO₂ + 3xEu₂O₃ + 2O₂ = 2Li₃Gd₃(1-x)Eu_3xTe₂O₁₂+ 3CO₂

The crystal structure of phosphors were characterized for phase formation by using powder X-ray diffraction (XRD) analysis with a Philips X’Pert MPD (Philips, Netherlands) with Cu Kα radiation (λ = 1.5418 Å). The diffraction patterns were scanned within angular range of 10-70Ëš(2θ). The morphology and size of the phosphors were measured using a scanning electron microscope (SEM, JEOL JSM-6490). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were analyzed using a Hitachi F-4600 spectrophotometer at room temperature. The temperature-dependent PL spectra of the phosphor were recorded in air on an Edinburgh FLS 920 spectrometer equipped with a 450 W Xe lamp.

3. Results and discussion

Li₃Gd₃Te₂O₁₂ belongs to the cubic crystal system, space group of Iad (No.230), in the structure of Li₃Gd₃Te₂O₁₂, Gd³⁺ and Te⁶⁺ cations occupy the 8- and 6-fold sites, and Li⁺ ions are located exclusively in the tetrahedral (24d) sites, respectively. As shown in Fig. 1, this structure can be considered to be formed from two interpenetrating, body-centered lattices composed of edge-shared distorted [GdO₈] cubes [8, 9]. One of these frameworks composed of Gd (black sphere) and O (red sphere) is shown in Fig.1(b) along with selected polyhedra to illustrate the linkages between the [GdO₈] units. Tellurium in the [TeO₆] polyhedra is accommodated in an octahedral site that shares edges with an edge-linked [GdO₈] dimer.

 Fig. 2 shows the observed, calculated, and patterns of the Li₃Gd_2.55Te₂O₁₂:0.15Eu³⁺phosphors, confirmed from Rietveld analysis using GSAS software. The final refinement converged with weighted profile of χ² = 1.086, R_p = 24.4ï¼…, and R_wp = 33.9ï¼… for Li₃Gd_2.55Te₂O₁₂:0.15Eu³⁺. It is clear that all the diffraction peaks of these samples are in good agreement with the pure Li₃Gd₃Te₂O₁₂ (JCPDS 22-0683) and no second phase can be found, indicating that each sample is purity phase and that the substitution of Gd³⁺ by Eu³⁺ do not significantly influence the crystal structure. Li₃Gd₃Te₂O₁₂ belongs to the cubic system, and the lattice parameters are calculated to be a = b = c = 12.41 Å, V = 1911.24 ų, which are consistent with the literature [1]. As the similarity of valence and the ionic radii of Eu³⁺(r = 0.95 Å, CN = 8) is the closest to that of Gd³⁺(r = 0.94 Å, CN = 8), the doped Eu³⁺ is supposed to substitute for the Gd³⁺ sites [10].

 SEM analysis was carried out to investigate the surface morphology and particle sizes of the synthesized phosphor powder. Fig. 3 shows the representative SEM images of two different concentrations of Li₃Gd₃Te₂O₁₂:xEu³⁺(a, x = 0.05; b, x = 0.20). It seemed as if these small spherical particles combined together to form big crystallites. The size of particles is found to be in micrometer dimension. Meanwhile, the result indicated that doping content of Eu³⁺ content in Li₃Gd₃Te₂O₁₂:xEu³⁺from 0.05 to 0.20 mol did not alter the particle size and agglomeration. The grain size of phosphors is important for their applications in commercial WLEDs. In general, for practical bepowdering applications, the phosphors with micron particles can feed well the commercial demand for WLEDs. Therefore, a long ball-milling step is required to break up the agglomerations and improve the quality of the phosphor powder.

Figure 4 shows the excitation spectra of Li₃Gd_2.55Te₂O₁₂:0.15Eu³⁺ monitored at 613 nm emission (⁵D₀→⁷F₂) at room temperature. The broad band of 200-300 nm (No.1) centered at around 275 nm is called as charge transfer (CT) band which is ascribed to the charge-transfer state (CTS) transition of O²⁻→Eu³⁺ ions. The position of this band mightily relies on the host lattice. A sequence of sharp excitation bands(Nos.2-11)between 300 and 500 nm was attributable to the intra-configurational 4f-4f transitions of Eu³⁺ in the matrix, namely,⁷F₀ to ⁵F_J, ⁵H₆, ⁵H₃, ⁵D₄, ⁵L₈, ⁵G₃, ⁵G₂, ⁵L₆, ⁵D₃, and ⁵D₂at wavelengths300, 314, 321, 364, 368, 381, 386, 396, 419 and 466 nm respectively [11]. The strongest absorption band located at approximately 396 nm occurred from the ⁷F₀→⁵L₆ transition of Eu³⁺ ions. A suitable red-emitting ultraviolet light-emitting diode (UV-LED) phosphor should exhibit an absorption of around 400 nm (LED excitation wavelength). Obviously, the Li₃Gd₃Te₂O₁₂:Eu³⁺phosphor has a potential value for white lighting device.

Upon 396 nm excitation, the PL emission spectrum of the Li₃Gd₃Te₂O₁₂:Eu³⁺phosphors was measured as presented in Fig. 5. Clearly, the PL emission spectrum was dominated by a strong red emission with a center of about 613 nm due to the ⁵D₀ →⁷F₂ transition. Meanwhile, there also existed some relatively weak excitation peaks at 570, 596, 655 and 709 nm which are attributed to the 4f-4f transitions of Eu³⁺ ions from the excited state of ⁵D₀ to ⁷F₀, ⁷F₁, ⁷F₃ and ⁷F₄, respectively. Generally, the local symmetry of Eu³⁺ site in the crystal lattice can be mostly reflected by Eu³⁺ emission profile. When Eu³⁺ ion occupies a crystallographic site with inversion symmetry, its magnetic-dipole ⁵D₀→⁷F₁ orange emission is dominant, while the electric dipole ⁵D₀ →⁷F₂ red emission dominates when possessing the non-centrosymmetrical site [12]. Thus, the I_0-2/I_0-1 emission ratio can be used in lanthanide-based systems as a probe for the local surroundings of a cation. As shown in Fig. 5, in comparison with that of the 5D₀→7F₁transition, the emission intensity of the 5D₀→7F₂ transition was much stronger, and the I_0-2/I_0-1 ratio was about 4.84. They demonstrated that the Eu³⁺ ions occupied the low symmetry sites with non-inversion centers in Li₃Gd₃Te₂O₁₂ host lattice. This ratio value is larger in comparison with those of the other Eu³⁺-doped phosphors. This larger ratio is favorable to improve the red color purity.

The intensity of luminescence in phosphors is usually affected by the variation in concentration of activators. Dependence of PL emission intensity of Li₃Gd₃Te₂O₁₂:Eu³⁺ phosphors on dopant concentration can be seen in Fig. 6. None of wavelength shift or peak was observed for a new site at high Eu³⁺ concentrations. The emission intensity of the phosphor initially increases up to 15 mol%. The maximum intensity is observed at 15 mol% and after this it starts decreasing. The decrease in the emission intensity is due to concentration quenching effect.

The concentration quenching of luminescence is observed when the energy transfer from one activator to another. Blasse has pointed out that if the activator is introduced solely on Z ion sites, x_c is the critical concentration, N is the number of Z ions in the unit cell and V is the volume of the unit cell, then there is on the average one activator ion per V/x_cN [13]. The critical transfer distance (R_c) is approximately equal to twice the radius of a sphere with this volume:

The critical transfer distance of the centerEu³⁺ in Li₃Gd₃Te₂O₁₂:Eu³⁺ phosphor by taking the appropriate values of V, N, and x_c (1911.24 ų, 8, and 0.15, respectively) is 14 Å.

The intensity of multipolar interaction can be determined from the change in the emission intensity. The emission intensity is related to the emitting level which has the multipolar interaction. The emission intensity (I) per activator ion is given by the formula [14]:

where χ is the activator concentration; Q is a constant of multipolar interaction and equals 3, 6, 8, or 10 for the nearest-neighbor ions, dipole-dipole, dipole-quadrupole or quadrupole-quadrupole interaction, respectively; and K and β are constants under the same excitation condition for the given host crystal [14, 15]. Then we use this equation to fit the experimental results of the relationship between integrated emission intensity and Eu³⁺ concentration. The curve of lgI/x vs. lgx in Li₃Gd₃Te₂O₁₂: Eu³⁺ phosphor based on Fig. 6 is shown in Fig. 7. The figure clearly shows that the relation between lgI/x and lgx is approximately linear and the slope is about -1.0. The Q value calculated based on the linear fitting using Eq. (2) is 3.0. This finding indicates that the concentration quenching of the Eu³⁺-site emission centers is caused by the energy transfer around the nearest-neighbor ions in the Li₃Gd₃Te₂O₁₂:Eu³⁺ phosphor. The similar phenomenon has been reported in the Sr_1.7Zn_0.3CeO₄: Eu³⁺ phosphor [16].

Both the maintenance of the chromaticity and brightness of white light output are favored by a lower-temperature quenching in the solid-state lighting application. Figure 8 represents the temperature-dependent PL spectra of Li₃Gd₃Te₂O₁₂: Eu³⁺ excited at 396 nm from 300 K to 460 K. The PL intensity almost unchanged with increase of temperature from 300 K to 460 K. The temperature dependence of the integrated emission intensities normalized at the 300 K value. The sample remained at about 82% of the intensity measured at room temperature, even the sample was heated to 420 K (the temperature at which LEDs typically operate). The thermal quenching temperature T₅₀, the temperature at the 50% emission intensity, was above 500 K for Li₃Gd₃Te₂O₁₂:Eu³⁺. The Eu³⁺-activated Li₃Ba₂Gd₃(MoO₄)₈ red phosphor shows lower quenching temperature and only remain 60% of the room temperature emission intensity at 200 °C. The good thermal quenching performance is similar with K₂Ba₅Si₁₂O₃₀:Eu²⁺, BaTiF₆:Mn⁴⁺, Sr₃Lu_0.2(PO₄)₃:0.8Eu³⁺phosphor [18-20]. Furthermore, the emission wavelengths showed no shift with increasing temperature. The small decrease in the emission intensity and good color purity stability at higher temperature indicates that the phosphor Li₃Gd₃Te₂O₁₂:Eu³⁺ has good thermal stability and can serve a potential red emitting phosphor for white LEDs.

In order to clarify the thermal quenching behavior and to calculate the activation energy, the Arrhenius equation is fitted to the thermal quenching data of Li₃Gd₃Te₂O₁₂:Eu³⁺ [21]:

Where I₀ means the initial intensity at room temperature, I(T) means the intensity at  temperature T, c is a constant, k is Boltzmann’s constant (8.62 – 10⁻⁵eV/K), and E_ais the activation energy for the thermal quenching process fitted with the thermal quenching data. The inset in Figure 9 plots ln[(I₀/I)−1] versus 1/T for Li₃Gd₃Te₂O₁₂:Eu³⁺. Linear regression showed that the thermal activation energy E_a for quenching was calculated to be ~ 0.22 eV. The thermal quenching of the emission intensity of Eu³⁺-activated phosphors was due to the excited electrons easily jumping into the CTS band after absorbing thermal energy at high temperatures, which the probability of non-radiative transition may increase. Thus, the emission intensity of Eu³⁺-activated phosphors decreased with increased temperature [22, 23].

  The emission spectra of Li₃Gd₃Te₂O₁₂:0.15Eu³⁺ and commercial Y₂O₃:Eu³⁺ excited at 396 nm were then compared in Fig. 10. Remarkably, the integral emission intensity of Li₃Gd₃Te₂O₁₂:0.15Eu³⁺ was 3.03 times than that of Y₂O₃:Eu³⁺. The CIE chromaticity coordinates of the phosphors were calculated to be (0.642, 0.332) for Li₃Gd₃Te₂O₁₂:0.15Eu³⁺ according to its PL spectra, which are shown in the CIE 1931 chromaticity diagram in the insets of Fig. 10. It was found that the CIE coordinates of the present red phosphor are more close to those of the NTSC standard CIE chromaticity coordinate values for red (0.67, 0.33) standard value, which is better than those of the commercial red phosphors Y₂O₃:Eu³⁺ (0.49, 0.32) [24] and Y₂O₂S:Eu³⁺ (0.65, 0.36) [25]. Furthermore, to better understand the red emission of the Eu³⁺-activated Li₃Gd₃Te₂O₁₂ phosphors, the color purity was calculated according to the following expression described by Fred Schubert [26]:

where (x, y) denotes the CIE coordinate of the synthesized compounds, (x_i, y_i) presents the color coordinate of the white illumination and the (x_d, y_d) is the color coordinates of the dominant wavelength. The dominant wavelength point can be calculated from the intersection of the connecting line between the equal energy point and the sample point. The color purity of Li₃Gd₃Te₂O₁₂:0.15Eu³⁺ (0.642, 0.332) phosphors is determined to be around 92.6%. This indicates high color purity and excellent chromaticity coordinate characteristics. The inset image in Fig. 10 shows that strong red emission was observed with the naked eyes when Li₃Gd₃Te₂O₁₂:0.15Eu³⁺is under a 365 nm UV lamp.

4. Conclusion

A novel garnet-type red-emitting phosphor Li₃Gd₃Te₂O₁₂:Eu³⁺ was prepared by the convenient solid-state reaction. The excitation and emission spectra and the dependence of luminescence on temperature were studied. The excitation spectra indicate that this phosphor can be effectively excited by near-UV light, which matches the emission wavelength of near-UV-LED chips well. The phosphor shows intense red emission, which has a high quenching temperature and can keep a stable color purity with elevated temperature. The optimum dopant concentration of Eu³⁺ ions in Li₃Gd₃Te₂O₁₂:Eu³⁺ was around 15 mol%, and the critical transfer distance of Eu³⁺ was calculated to be 14 Å. The concentration quenching is probably caused by the energy transfer among the nearest-neighbor ions in the Li₃Gd₃Te₂O₁₂:Eu³⁺ phosphor. Because of its good excitation profile and stable luminescence properties at high temperature, Eu³⁺-doped Li₃Gd₃Te₂O₁₂ phosphors are a potential red phosphors for NUV chip-based WLEDs and display devices.