Novel Eu3+-doped Garnet-tpye Tellurate Red-emitting Phosphor

Novel Eu3+- do drugs Garnet-tpye Tellurate Red-emitting PhosphorA novel Eu3+- do drugs garnet-tpye tellurate going-emitting phosphor with high caloric perceptual constancy and colourize artlessnessIntroductionThe garnet-related family Li3Ln3Te2O12(Ln=Y, Pr, Nd, Sm-Lu) have been extensively studied as promising satisfying electrolytes for application in solid state rechargeable lithium-ion batteries for the last few decades 1-4. In 2006, OCallaghan et al. developed garnet-type Li3Ln3Te2O12 (Ln = Y, Pr, Nd, Sm-Lu) to investigate the relationship mingled with Li send occupation and Li ion conductivity 1. The wicket gate uninterrupted increases with increasing Ln ionic radius in Li3Ln3Te2O12. These Li3Ln3Te2O12 garnets have exhibited a middling low ionic conductivity of 105 S cm1 at 600 C with a high activation cogency (1 eV) 3. In 2014, the lechatelierite 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 swarm lattices be of considerable interest due to their wide applications as optical maser hosts and as phosphors for sinlessness light emitting diodes 5. For example, trivalent r be earth doped Y3Al5O12 (YAG) is one of the widely apply systems of compounds for solid state fervour applications. Meanwhile, few new garnet-type compound rear be constructed based on the garnet morphologic model, such as the green-emitting Ca3Sc2Si3O12Ce3+, the orange-emitting Lu2CaMg2(Si, Ge)3O12Ce3+, and the green-emitting Ca2LaZr2Ga3O12Ce3+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 mirthful luminescence properties, exhibiting virgin wild firing off regenerations with a series of sourish word of mouths arising from the frenzied state 5D0 to the lower push bu tton state 7F0-6. Eu3+ ions exhibit pure magnetic and electric dipole changes which make it a very subtile probe for the r atomic number 18 earth ion site structure/ balance wheel. 5D07F2 electric dipole (ED) transitions roughly 610 nm are highly hypersensitive, which is highly sensitive to the counterweight of the Eu3+ sites in the lattices however, the magnetic dipole transitions (5D07F1) are not affected by the environment, and their firing intensities are often used as an internal precedent 7.However, luminescence properties of Eu3+-doped garnet-type Li3Gd3Te2O12 have not been studied yet. In this work, red emitting phosphors Li3Gd3(1-x)Eu3xTe2O12(x = 0.01-0.30) were synthe size of itd by the conventional solid-state reaction. The structure, composition and photoluminescence properties of Li3Gd3Te2O12Eu3+ phosphors were investigated. In addition, the luminescence meet of Eu3+ doping tautness and CIE on the photoluminescence spectra were demonstrated in detail.2. Experim ental 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 still purification and thoroughly mixed in an agate mortar. The mixtures were sintered in aerate at 900C for 10 h. when the reaction was end at 900C, the products were cooled mass to means temperature without cooling devices. Finally, white powers were obtained by grinding. The relevant reaction constructions are as follows3Li2CO3+3(1-x)Gd2O3 + 4TeO2 + 3xEu2O3 + 2O2 = 2Li3Gd3(1-x)Eu3xTe2O12+ 3CO2The crystal structure of phosphors were characterized for phase formation by victimization powder X-ray diffraction (XRD) compendium with a Philips XPert MPD (Philips, Netherlands) with Cu K ray of light ( = 1.5418 ). The diffraction patterns were s advisened within angular range of 10-70(2). The morphology and si ze of the phosphors were measured apply a s female genital organning electron microscope (SEM, JEOL JSM-6490). The photoluminescence (PL) and photoluminescence temper (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 mass spectrometer equipped with a 450 W Xe lamp.Results and discussionLi3Gd3Te2O12 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, severally. As shown in Fig. 1, this structure apprize be considered to be formed from two interpenetrating, body-centered lattices composed of edge-shared distorted GdO8 cubes 8, 9. angiotensin-converting enzyme of these frameworks composed of Gd (black sports stadium) and O (red sphere) is shown in Fig.1(b) alon g with selected polyhedra to illustrate the linkages amidst 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 ob overhauld, mensural, and patterns of the Li3Gd2.55Te2O120.15Eu3+phosphors, confirmed from Rietveld analysis using GSAS software. The final refinement converged with weighted visibility of 2 = 1.086, Rp = 24.4, and Rwp = 33.9 for Li3Gd2.55Te2O120.15Eu3+. It is go off that all the diffraction full stops of these samples are in good agreement with the pure Li3Gd3Te2O12 (JCPDS 22-0683) and no second phase can be embed, indicating that each sample is rectitude phase and that the substitution of Gd3+ by Eu3+ do not significantly regularise the crystal structure. Li3Gd3Te2O12 belongs to the cubic system, and the lattice parameters are calculated to be a = b = c = 12.41 , V = 1911.24 3, which are consistent with the literature 1. As the similarity of valence and the ionic radii of Eu3+(r = 0.95 , CN = 8) is the scrawnyst to that of Gd3+(r = 0.94 , CN = 8), the doped Eu3+ is supposed to replacing 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 niggardnesss of Li3Gd3Te2O12xEu3+(a, x = 0.05 b, x = 0.20). It seemed as if these scummy 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 Li3Gd3Te2O12xEu3+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 bar is required to break up the agg lomerations and improve the quality of the phosphor powder.Figure 4 shows the excitation spectra of Li3Gd2.55Te2O120.15Eu3+ monitored at 613 nm emanation (5D07F2) at room temperature. The vast circle of 200-300 nm (No.1) centered at around 275 nm is called as charge off (CT) band which is ascribed to the charge- broadcast state (CTS) transition of O2Eu3+ 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 5FJ, 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 squiffyest absorption band located at approximately 396 nm occurred from the 7F05L6 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 waveleng th). Obviously, the Li3Gd3Te2O12Eu3+phosphor has a potence value for white lighting device.Upon 396 nm excitation, the PL emission spectrum of the Li3Gd3Te2O12Eu3+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 somewhat relatively weak excitation peaks at 570, 596, 655 and 709 nm which are attributed to the 4f-4f transitions of Eu3+ ions from the evoke 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 5D07F1 orange emission is superior, while the electric dipole 5D0 7F2 red emission dominates when possessing the non-centrosymmetrical site 12. Thus, the I0-2/I0-1 emission ratio can be used in lanthanide-based syst ems as a probe for the local surroundings of a cation. As shown in Fig. 5, in equivalence with that of the 5D07F1transition, the emission strength of the 5D07F2 transition was much stronger, and the I0-2/I0-1 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 people of colour purity.The lastingness of luminescence in phosphors is usually affected by the renewal in concentration of activators. Dependence of PL emission intensity level of Li3Gd3Te2O12Eu3+ 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 decreasi ng. The precipitate in the emission intensity is due to concentration fulfil effect.The concentration quenching of luminescence is observed when the nothing transfer from one activator to another. Blasse has pointed out that if the activator is introduced exclusively on Z ion sites, xc is the critical concentration, N is the number of Z ions in the unit cell and V is the volume of the unit cell, consequently there is on the average one activator ion per V/xcN 13. The critical transfer distance (Rc) is approximately equal to twice the radius of a sphere with this volumeThe critical transfer distance of the centerEu3+ in Li3Gd3Te2O12Eu3+ phosphor by taking the appropriate values of V, N, and xc (1911.24 3, 8, and 0.15, respectively) is 14 .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 14where is the activator concentration Q is a unending 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 are constants under the same excitation condition for the given host crystal 14, 15. Then we use this equation to go away the experimental results of the relationship between integrated emission intensity and Eu3+ concentration. The arc of lgI/x vs. lgx in Li3Gd3Te2O12 Eu3+ phosphor based on Fig. 6 is shown in Fig. 7. The jut clearly shows that the relation between lgI/x and lgx is approximately linear and the sky 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 Li3Gd3Te2O12Eu3+ phosphor. The similar phenomenon has been reporte d in the Sr1.7Zn0.3CeO4 Eu3+ phosphor 16.both the maintenance of the hue 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 dependance of the integrated emission intensities normalized at the 300 K value. The sample remained at about 82% of the intensity measured at room temperature, redden the sample was heated to 420 K (the temperature at which LEDs typically operate). The caloric quenching temperature T50, the temperature at the 50% emission intensity, was above 500 K for Li3Gd3Te2O12Eu3+. 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 operation is similar wi th K2Ba5Si12O30Eu2+, BaTiF6Mn4+, Sr3Lu0.2(PO4)30.8Eu3+phosphor 18-20. Furthermore, the emission wavelengths showed no shift with increasing temperature. The small decrease in the emission intensity and good color purity stableness at higher temperature indicates that the phosphor Li3Gd3Te2O12Eu3+ has good thermal stability and can serve a potential red emitting phosphor for white LEDs.In order to brighten the thermal quenching behavior and to calculate the activation energy, the Arrhenius equation is fitted to the thermal quenching data of Li3Gd3Te2O12Eu3+ 21Where I0 means the initial intensity at room temperature, I(T) means the intensity at temperature T, c is a constant, k is Boltzmanns constant (8.62 105eV/K), and Eais 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 Li3Gd3Te2O12Eu3+. Linear regression showed that the thermal activation energy Ea for quenching was calculated to b e 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 Li3Gd3Te2O120.15Eu3+ and commercial Y2O3Eu3+ excited at 396 nm were then compared in Fig. 10. Remarkably, the integral emission intensity of Li3Gd3Te2O120.15Eu3+ was 3.03 times than that of Y2O3Eu3+. The CIE chromaticity coordinates of the phosphors were calculated to be (0.642, 0.332) for Li3Gd3Te2O120.15Eu3+ according to its PL spectra, which are shown in the CIE 1931 chromaticity plot 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 develop than those of the commercial red phosphors Y2O3Eu3+ (0.49, 0.32) 24 and Y2O2SEu3+ (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 26where (x, y) denotes the CIE coordinate of the synthesized compounds, (xi, yi) presents the color coordinate of the white illumination and the (xd, yd) 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 Li3Gd3Te2O120.15Eu3+ (0.642, 0.332) phosphors is determined to be around 92.6%. This indicates high color purity and fantabulous chromaticity coordinate characteristics. The inset image in Fig. 10 shows that strong red emission was observed with the naked eyes when Li3Gd3Te2O120.15Eu3+is under a 365 nm UV lamp. ConclusionA novel garnet-type red-emitting phosphor Li3Gd3Te2O12Eu3+ 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 best dopant concentration of Eu3+ ions in Li3Gd3Te2O12Eu3+ was around 15 mol%, and the critical transfer distance of Eu3+ was calculated to be 14 . The concentration quenching is credibly caused by the energy transfer among the nearest-neighbor ions in the Li3Gd3Te2O12Eu3+ 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.