Highly efficient red-emitting Ca2YSbO6:Eu3+ double perovskite phosphors for warm WLEDs

Highly efficient red-emitting Eu3+-doped double perovskite Ca2YSbO6 phosphors have been successfully prepared by the traditional high-temperature solid state method. The phase purity, photoluminescence and decay properties as a function of the Eu3+ concentration have been investigated in detail. The XRD results demonstrate that all of the obtained phosphors can be assigned to a pure monoclinic structure. Upon 464 nm excitation, a strong red emission situated at 614 nm (5D0–7F2) indicates that Eu3+ ions occupy a site with low symmetry. The quenching concentration of Eu3+ ions reaches as high as 70 mol% and the quenching mechanism is discussed. Especially, the prepared phosphor exhibits a high quantum efficiency of 92.1% and superior thermal stability, with PL intensity at 423 K up to 81.6% of that at room temperature. Moreover, a warm white light with a correlated color temperature of 4720 K and a color rendering index of 82.6 is achieved by fabricating a Ca2YSbO6:Eu3+ phosphor in a 460 nm blue-InGaN chip together with the commercial Y3Al5O12:Ce3+ yellow phosphor.


Introduction
In the past couple of decades, white light-emitting diodes (WLEDs), as the fourth generation of illumination appliances, have received signicant interest due to their superior characteristics such as low electricity consumption, long operating lifetime, high brightness, good reliability, fast response, environmental friendliness, etc. [1][2][3][4] Currently, the mainstream commercial phosphor-converted WLEDs (pc-WLEDs) are realized by combining a blue InGaN LED chip with a yellow YAG:Ce 3+ phosphor. 5,6 Although this combination has some advantages, this kind of white light demonstrates a low color rending index (CRI) and high correlated color temperature (CTT) owing to the deciency of red light in the emission spectra. 7 Therefore, in order to improve the CRI and CCT of pc-WLEDs, much effort has been devoted to develop a novel red phosphor. 8,9 However, the most widely used rare earth ion activated red phosphors such as Y 2 O 3 :Eu 3+ , Y 2 O 2 S:Eu 3+ and CaAlSiN 3 :Eu 2+ also exhibit several limitations including low efficiency, chemical instability and complicated preparation processes, which prevent their broad application for lighting. [10][11][12] Thus, it is urgent to search for a novel red phosphor with excellent stability, high conversion efficiency and low concentration quenching, which can be synthesized easily.
As the most important factor to acquire efficient redemitting phosphors, different host lattices will demonstrate diverse optical properties due to the surrounding of the given optical center. 13 It is well known that Eu 3+ has generally been used as a luminescent center, however, the high efficiency red emission is mainly to be blamed for the low quenching concentration and improper coordination environment. 14 Therefore, it is essential to nd a suitable host matrix, in which the crystal structure benets an increase in distance among the Eu 3+ ions. 15 Recently, ordered double perovskite type oxides with the general formula A 2 BB 0 O 6 (A ¼ Ca, Sr, Ba, Y, La, Gd; B ¼ Y, La, Gd, Mg, Zn, Li; B 0 ¼ Sb, Ta, Nb, Ti) or AA 0 BB 0 O 6 (A ¼ Ca, Sr, Ba, Na, Li; A 0 ¼ Y, La, Gd; B ¼ Mg, Zn; B 0 ¼ W, Mo, Sb, Ta, Nb) have been intensively investigated owing to their good thermal stability and chemical properties. 10,16 In typical crystal structures of double perovskites, both the B and B 0 sites are coordinated with six oxygen atoms by corner-sharing alternately, while the A (or A 0 ) site is coordinated with eight or twelve oxygen atoms based on the distortion of the crystal structure. Besides, the BO 6 and B 0 O 6 sites can reduce the symmetry of the A (or A 0 ) ones and produce various coordination environments for doping with rare earth ions. 17,18 Therefore, the double perovskite oxide is a suitable candidate matrix for phosphors by structurally modulating.
Hence, we chose Ca 2 YSbO 6 as the host for a red phosphor, in which the B and B 0 sites are occupied by Y 3+ and Sb 5+ ions, respectively. In addition, Ca 2 YSbO 6 belongs to an ordered perovskite structure and has a monoclinic crystal system with the space group of P2 1 /m, which is very suitable for luminescent ions. 19 A Y 3+ ion and six O 2À ions are coordinated to form [YO 6 ] octahedrons, providing the opportunity for an Eu 3+ ion to occupy the Y 3+ lattice according to its similar ionic radius and the same valence. Recently, Bi 3+ and Eu 3+ ion co-doped Ca 2 -YSbO 6 phosphor materials have been synthesized, and the luminescence enhancement mechanism was also investigated. 10 However, the inuences of structure symmetry on high concentration quenching, thermal stability and quantum efficiency of Ca 2 YSbO 6 :Eu 3+ red phosphors have not been studied in detail. Besides, the potential application for WLEDs is also unconsidered. Therefore, in this study, a series of Ca 2 Y 1Àx Eu x -SbO 6 (0.1 # x # 1.0) red-emitting phosphors have been prepared by the conventional high temperature solid-state reaction. In addition, the relationship between crystal structure and optical performance is also investigated and discussed in detail.

Characterization
The crystal structure and phase purity of the nal products were recorded on a MiniFlex 600 X-ray diffractometer with CuKa radiation (l ¼ 1.5405Å) at a scanning rate of 1 min À1 in the 2q range from 10 to 80 . The crystal structure parameters were rened by the Rietveld method using the General Structure Analysis System (GSAS). The morphology of the as-obtained product was studied using an FEI Apreo HiVac scanning electron microscope (SEM). The photoluminescence excitation (PLE) and emission (PL) spectra as well as luminescence decay lifetimes of the prepared phosphors were performed on an Edinburgh FS5 spectrometer using a 150 W continuous and pulsed xenon lamp as the excitation source, respectively. The internal quantum efficiency (IQE) of the synthesized phosphors was recorded on the Edinburgh FS5 spectrometer using an integrating sphere coated with BaSO 4 . The temperature dependent emission spectra were measured using a homemade temperature control system in the temperature range of 303-523 K.

Fabrication of WLEDs
The WLED devices were fabricated with the commercial YAG:Ce 3+ yellow phosphor (purchased from XinLi Illuminant Co. LTD) and blue GaN chips ($460 nm) with/without the asprepared Ca 2 YSbO 6 :Eu 3+ red phosphor. The mass ratio of yellow phosphor versus red one was 1 : 3. These phosphors were mixed with epoxy resin thoroughly and coated on the surface of the LED chips. The photoelectric parameters of the assembled WLEDs, including electroluminescence (EL) spectra, color rendering index (CRI), correlated color temperature (CCT), and luminescent efficiency (LE), were collected using an integrating sphere equipped with a CCD detector (HAAS-2000).

Phase characterization and morphology analysis
The representative Ca 2 YSbO 6 :0.1Eu 3+ sample was measured using powder X-ray diffraction (XRD), and the related data were analyzed using the Rietveld renement method, as illustrated in Fig. 1a. Owing to the absence of structural information on Ca 2 YSbO 6 , we used the crystallographic data of Ca 3 TeO 6 (ICSD #230130) as the initial structural model. 10 The nal renement results of Ca 2 YSbO 6 :Eu 3+ are listed in Table S1, † which index to a monoclinic structure with the lattice parameters a ¼ 5. 3 . The enlargement in cell volume certies that the smaller Y 3+ (r ¼ 1.019Å, CN ¼ 8) lattice has been successful replaced by the larger Eu 3+ (r ¼ 1.066 A, CN ¼ 8) ions in the Ca 2 YSbO 6 . The schematic illustration of the Ca 2 YSbO 6 structure is displayed in Fig. 1b 6 ] provides a better opportunity for Eu 3+ ions to occupy the Y 3+ lattice.
The phase structures of the as-obtained Ca 2 YSbO 6 :Eu 3+ phosphors with various Eu 3+ concentrations were veried by XRD, as demonstrated in Fig. 2a. All the diffraction peak positions and relative intensities of the products appear at the same angles. No impurity and other phases can be found even with a doping concentration of 100 mol%, indicating that the nal products have been successfully prepared and the doping with Eu 3+ ions does not affect the crystal structure of the Ca 2 YSbO 6 lattice. Magnication of the dominant diffraction peaks ranging from 30-33 is presented in Fig. 2b. It can be clearly seen that these peaks shi slightly to a lower angle value with the increase in Eu 3+ concentration. This phenomenon can be explained according to Bragg's equation, 2d sin q ¼ nl, in which q represents diffraction angle, d corresponds to interplanar distance, and n and l are constants. Based on the effective ionic radii and the same valence state, Eu Thus, the peak shiing is attributed to the replacement of the smaller Y 3+ ions by larger Eu 3+ ones. This result further indicates that Eu 3+ ions have successfully entered into the Ca 2 YSbO 6 lattice.
The surface morphology of the obtained Ca 2 Y 0.3 SbO 6 :0.7Eu 3+ sample was measured using SEM analysis, as illustrated in Fig. S1. † Regular phosphor particles with the size range from 0.5 to 2.5 mm can be observed.

Luminescence properties
The PLE and PL spectra of the as-synthesized Ca 2 Y 0.3 SbO 6 :0.7-Eu 3+ powders are given in Fig. 3. When monitored at 612 nm, the PLE spectrum consists of several sharp peaks in the range of 350-500 nm and a broad band ranging from 200-350 nm, which are ascribed to the Eu 3+ ions' characteristic inner-4f transitions and the charge transfer band (CTB) of the O 2À -Eu 3+ interaction, respectively. The sharp peaks located at 316 nm, 361 nm, 380 nm, 393 nm, 413 nm and 464 nm are mainly attributed to the 7 F 0 / 5 H 5 , 7 F 0 / 5 D 4 , 7 F 0 / 5 L 7 , 7 F 0 / 5 L 6 , 7 F 0 / 5 D 3 and 7 F 0 / 5 D 2 transitions of the Eu 3+ ion, respectively. Among them, two of the strongest sharp peaks at 393 nm and 464 nm can be found, which are in good agreement with the emission wavelength of NUV and blue LED chips. 20 Upon 464 nm excitation, the characteristic emission peaks of Eu 3+ ions corresponding to the 5 D 0 / 7 F J (J ¼ 0, 1, 2, 3, 4) transitions are clearly observed. Especially, the most intense emission peak situated at 612 nm results from the electric dipole transition 5 D 0 / 7 F 2 , which demonstrates that Eu 3+ occupies a site with low symmetry in the double perovskite Ca 2 YSbO 6 lattice. 21,22 This non-inversion center environment of Eu 3+ ions is benecial to acquire a red phosphor with high color purity. 23 In order to study the effect of Eu 3+ concentration on the luminescence, the PL spectra of Ca 2 Y 1Àx SbO 6 :xEu 3+ (0.1 # x # 1.0) samples under the excitation of 464 nm are collected and presented in Fig. 4a. All of the samples display similar spectral proles with the dominant red emission band located at 612 nm. As expected, the emission intensity was enhanced with the increase in Eu 3+ doping concentration and reaches maximum at x ¼ 70 mol%, as depicted in Fig. 4b. Then, the strength gradually decreases with further increasing Eu 3+ concentration due to the well-known concentration quenching effect, which is ascribed to the probability of the energy migration between Eu 3+ -Eu 3+ increasing. Herein, the high quenching concentration (70 mol%) in the Eu 3+ doped Ca 2 YSbO 6 double perovskite mainly originates from the different energy migrations between intralayer and interlayer luminescence centers. 21,24 In the structure of Ca 2 YSbO 6 , [YO 6 ] and [SbO 6 ] are connected by corner-sharing alternately with a layered structure, which is benecial to damp the concentration quenching among Eu 3+ -Eu 3+ ions. Thus, higher emission intensity and luminescence efficiency can be realized via doping with a higher Eu 3+ concentration. 15 Besides, the critical distance (R c ) among Eu 3+ ions for energy transfer is an important parameter, which can be evaluated based on the Blasse theory. 25 In this study, the R c is estimated to be $7.1Å, which is larger than 5Å. Therefore, the concentration quenching is mainly governed by multipolar interactions. Herein, the type of interaction mechanism of Eu 3+ ions in Ca 2 YSbO 6 is determined by the relationship between log(I/x) and log(x) using the following expression: 26   where I represents the emission intensity; x stands for the dopant concentration; K and b are constants for the given host; q ¼ 6, 8, and 10 correspond to dipole-dipole (d-d), dipolequadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. As illustrated in the inset of Fig. 4b, the relationship between log(x) and log(I/x) is found to be linear, and the slope is tted to be about À1.51. Therefore, q is calculated to be 4.53, which is close to 6, suggesting that the concentration quenching mechanism among Eu 3+ ions in the Ca 2 YSbO 6 is d-d interaction.
In order to obtain additional information on the concentration quenching behavior of the Eu 3+ ions, the luminescence decay curves of Ca 2 YSbO 6 :Eu 3+ with various Eu 3+ concentrations under the excitation of 464 nm are investigated, as illustrated in Fig. 5. All the decay curves of Eu 3+ emission can be well tted with a rst order exponential as follows: 27 where I(t) and I 0 correspond to the luminescence intensities at times t and 0, A is a constant, and s is the decay time for the exponential components. The obtained average lifetimes monotonically decrease from 1.513 ms to 0.4733 ms with increasing Eu 3+ concentrations from x ¼ 0.1 to x ¼ 1.0, which is mainly attributed to the enhanced non-radiative energy transfer among Eu 3+ ions with shortening the distance between Eu 3+ -Eu 3+ . Besides, in order to further certify the emitting center in the Ca 2 YSbO 6 lattice, the luminescence decay curves of Ca 2 -Y 0.8 SbO 6 :0.2Eu 3+ excited from 250-420 nm and monitored at 612 nm were measured, as depicted in Fig. 6. It can be clearly seen that the prole of all these curves is similar and the calculated s values are almost the same. This result further conrms that the Eu 3+ environment in this system is unique.
To evaluate the luminescence performance of the asobtained phosphor materials for practical application, the internal quantum efficiency (IQE) was recorded. The measured spectrum of the Ca 2 Y 0.5 SbO 6 :0.5Eu 3+ phosphor as an example and reference sample is given in Fig. 7, and the value can be estimated using the formula 28 where L S is the luminescence spectrum; E R and E S represent the spectra with and without Ca 2 Y 0.5 SbO 6 :0.5Eu 3+ phosphor,    1.0, respectively. The variation tendency of IQE is in agreement with the concentrationdependent PL spectra discussed above, which is further evidence for the transformation of energy transfer caused by non-radiation. Especially, it is worth emphasizing that the IQE of the as-obtained phosphors is much higher than that of commercial phosphor Y 2 O 2 S:Eu 3+ (IQE: 35%), 29 illustrating that it has promising application prospects in solid-state lighting.
As an indispensable performance objective in evaluating practical applications in solid-state lighting, the temperature quenching property has been investigated. The temperaturedependent PL emission spectra of the Ca 2 Y 0.3 Eu 0.7 SbO 6 sample recorded from 303 to 523 K are illustrated in Fig. 8a. Unsurprisingly, as the temperature rises from 303 to 523 K, the emission peak remained almost unchanged except the PL emission intensity decreased gradually. Especially, the asprepared Ca 2 YSbO 6 :Eu 3+ demonstrates a very low luminescence quenching behavior with its PL intensity still maintaining 81.6% at 423 K in comparison with that of the initial intensity at 303 K, as depicted in Fig. 8b. The excellent thermal stability is mainly attributed to its rigid network structure of double perovskite, which could adequately minimize the emission loss with enhancing temperature. 30,31 Besides, in order to investigate the temperature dependence of luminescence in depth, the activated energy DE can be obtained by using the modied Arrhenius equation as follows 32-34 where I T and I 0 refer to the emission intensity of Ca 2 Y 0.3 Eu 0.7 -SbO 6 recorded at a different given temperature and the initial intensity at 303 K, respectively; k and A represent constants. The graph of ln(I 0 /I T À 1) versus 1/kT is plotted and illustrated in Fig. 8c. It can be clearly seen that the experimental data is consistent with the tting curve with the slope of À0.1838. Thus, DE is determined to be approximately 0.1838 eV, higher than some previous reports for red phosphors, such as Na 3 -La 2 (PO 4 ) 3 :0.02Eu 3+ (0.0718 eV), 35 or BaZrGe 3 O 9 :Eu 3+ (0.175 eV). 23 This relatively high DE demonstrates that the as-prepared phosphors have good thermal stability, further conrming that the red-emitting phosphor has potential application in LED devices. Besides, the energy difference between 5 D 0 and 7 F J of Eu 3+ is illustrated in Fig. S2. † In this case, a very low luminescence quenching behavior mainly originates from the variations in temperature insufficient to match the energy level difference ($1.414 eV). 36 Therefore, the obtained Ca 2 YSbO 6 :Eu 3+ phosphors exhibit excellent thermal stability. Additionally, the temperature-dependent luminescence decay curves of Ca 2 Y 0.3 Eu 0.7 SbO 6 monitored at 612 nm were also measured, as illustrated in Fig. 9d. All the curves are well tted to a single exponential and the decay behavior of Ca 2 Y 0.3 Eu 0.7 -SbO 6 weakly depends on the temperature. With the temperature increased to 523 K, the lifetime has slight variation, which further veries that the as-prepared phosphor has excellent thermal stability.

Electroluminescent performance of the fabricated WLEDs
To further evaluate the potential applications of the Ca 2 -YSbO 6 :Eu 3+ phosphor, a prototype WLED device was fabricated by coupling a blend of the as-prepared red phosphor, the commercial YAG:Ce 3+ yellow phosphor and transparent silicon  resin on a 460 nm blue LED chip. The electro-luminescence (EL) spectra of the InGaN blue chip, the fabricated YAG:Ce 3+ -based and Ca 2 YSbO 6 :Eu 3+ -YAG:Ce 3+ LEDs under 60 mA current excitation are depicted in Fig. 9. The corresponding EL spectrum in Fig. 9b contains two emission bands: the one at 460 nm belongs to the blue chip and the other at 550 nm is ascribed to the emission of YAG:Ce 3+ . Except the above emission bands, the peak located at 612 nm attributed to the emissions of Ca 2 YSbO 6 :Eu 3+ can be observed in Fig. 9c. Bright cool white light is found (Fig. 9b, inset), and the CIE chromaticity coordinates, luminescent efficiency (LE), color rendering index (R a ) as well as correlated color temperature (CCT) are measured to be (0.308, 0.341), 118.6 lm W À1 , 75.4 and 6666 K, respectively. Thus, in order to improve the R a and CCT of the LED, the asprepared Ca 2 YSbO 6 :Eu 3+ phosphor was added and the high performance warm WLEDs with a low CCT of 4720 K and high CRI of 82.6 can be achieved. Besides, the CIE diagram shis to (0.353, 0.377) and the LE is 101.9 lm W À1 . These results illustrate that the added Ca 2 YSbO 6 :Eu 3+ red-emitting phosphor can effectively improve the photoelectric parameter of WLED devices.

Conclusions
In summary, highly efficient red-emitting Ca 2 Y 1Àx SbO 6 :xEu 3+ (0.1 # x # 1.0) phosphors with high concentration quenching and quantum efficiency have been successfully prepared by the traditional high-temperature solid state route. All the samples can be indexed to a pure monoclinic structure. The PLE and PL spectra demonstrate that the Ca 2 YSbO 6 :Eu 3+ phosphors can be efficiently excited by NUV (393 nm) and blue light (464 nm), which match well with the UV and blue LED. Upon 464 nm excitation, a bright red emission located at 612 nm corresponding to the electric dipole transition 5 D 0 -7 F 2 of Eu 3+ can be observed. The optimal Eu 3+ doping concentration reaches as high as 70 mol%, and the concentration quenching mechanism is attributed to the dipole-dipole interaction. Besides, the high quenching concentration mainly originates from different  energy migrations between intralayer and interlayer luminescence centers in the Ca 2 YSbO 6 lattice. Importantly, the IQE of Ca 2 Y 0.3 SbO 6 :0.7Eu 3+ reaches up to 92.1%. Besides, the obtained phosphor demonstrates excellent thermal stability, where the emission intensity still maintains 81.6% at 423 K compared to that of the initial intensity at 303 K. This result is ascribed to its rigid network structure of double perovskite. Finally, a warm white light with CIE chromaticity coordinates of (0.353, 0.377), CCT of 4720 K and CRI of 82.6 can be achieved by efficiently fabricating Ca 2 YSbO 6 :Eu 3+ phosphor in a 460 nm blue-InGaN chip together with the commercial Y 3 Al 5 O 12 :Ce 3+ yellow phosphor. All of the results illustrate the promising prospect of Ca 2 YSbO 6 :Eu 3+ as a potential red-emitting phosphor for application in blue/UV converted WLEDs.

Conflicts of interest
There are no conicts to declare.