Tunable Optical Properties and Enhanced Thermal Quenching of Non

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Tunable Optical Properties and Enhanced Thermal Quenching of Non-Rare-Earth Double-Perovskite (Ba1−xSrx)2YSbO6:Mn4+ Red Phosphors Based on Composition Modulation Jiasong Zhong,† Daqin Chen,*,†,‡ Shuo Yuan,† Meijiao Liu,§ Yongjun Yuan,† Yiwen Zhu,† Xinyue Li,† and Zhenguo Ji† †

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China College of Physics and Energy, Fujian Normal University, Fuzhou, 350117, P. R. China § Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China

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S Supporting Information *

ABSTRACT: Non-rare-earth Mn4+-doped double-perovskite (Ba1−xSrx)2YSbO6:Mn4+ red-emitting phosphors with adjustable photoluminescence are fabricated via traditional high-temperature sintering reaction. The structural evolution, variation of Mn4+ local environment, luminescent properties, and thermal quenching are studied systematically. With elevation of Sr2+ substituting content, the major diffraction peak moves up to a higher angle gradually. Impressively, with increasing the substitution of Ba2+ with Sr2+ cation from 0 to 100%, the emission band shifts to short-wavelength in a systematic way resulting from the higher transition energy from excited states to ground states. Besides, this blue-shift appearance can be illuminated adequately using the crystal field strength. The thermal quenching of the obtained solid solution is dramatically affected by the composition, with the PL intensity increasing 16% at 423 K going from x = 0 to 1.0. The w-LEDs component constructed by coupling the UV-LED chip with red/green/blue phosphors demonstrate an excellent correlated color temperature (CCT) of 3404 K, as well as color rendering index (CRI) of 86.8.

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INTRODUCTION In comparison to incandescent and fluorescent lamps using tungsten filament and mercury vapor sources, semiconductorbased LEDs are extensively used in various illumination fields because of the many excellent characteristics, such as higher energy efficiency, smaller size, longer lifetime, and more environmentally friendly.1−5 Currently, the commercial white LEDs (w-LEDs) are phosphor-converted LEDs, which structured by coupling a Y3Al5O12:Ce3+(YAG:Ce3+) yellow phosphor with a blue LED chip.6 Although it has been commercialized and is widely used, it suffers from major shortcomings, for instance a high CCT and poor CRI on account of the deficiency of red components.7 As a consequence, its broader applications is restricted, such as indoor lighting and medical lighting.8 To achieve high CRI and low CCT, a phosphor with preferable red-emitting performance is demand to be added into the system.9 Therefore, considerable efforts are devoted to discover red-emitting phosphors suitable for the application of w-LEDs; Usually, they are rare earth ions, such as Eu3+ and Eu2+, activated (oxy)nitride compounds. However, several challenging issues still exist in the commercial Eu2+-doped nitride red phosphors, such as unsatisfactory luminescence involve photon reabsorption © XXXX American Chemical Society

and broad band emission, as well as harsh fabrication process, which restrict their widespread applications.10−12 Therefore, development and investigation of non-rare-earth doped phosphors for illumination and other optoelectronic device is significant. Lately, Mn4+ ion as an effective non-rare-earth activator for red-emitting phosphor has attracted great interest because of its special spectroscopic properties.9,13 It is well-known that Mn4+doped phosphors exhibit broad absorption from UV to blue region ascribed to the 4A2g → (4T1g, 2T2g, and 4T2g) transitions and show a red emitting within the range of 620−780 nm originated from the Mn4+:2E → 4A2g transition.13−16 Furthermore, the Mn4+ activators usual occupy in octahedral site and the red-emitting is affected by coordination surroundings.9,17 As a result, various Mn4+-doped materials, such as oxides and fluoride, are commonly used for the host structure of phosphors.18 Fluoride compounds, exhibiting a red emission peak situated at ∼630 nm, are deemed to be suitable matrix for Mn4+ because of their outstanding thermal stability and optical Received: April 9, 2018

A

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. XRD Rietveld refinement results of Ba2YSbO6: Mn4+ and Sr2YSbO6: Mn4+. The experimental data are black crosses. The Rietveld fit and difference curve are given in red and blue lines, respectively. Bragg reflections are indicated with green short vertical lines.

CN = 6), the bond length between Sb5+ and O2− ions decreases from 2.233 to 2.002 Å. The enhanced interaction between ions generates a stronger crystal field (CF) environment in Sr2YSbO6:Mn4+ phosphor than that in Ba2YSbO6:Mn4+ one, leading to blue-shift of Mn4+ luminescence.

properties. Regretfully, it has poor tolerance to humidity. Although such shortcoming can be partially addressed by covering the hydrophobic materials on the surface of phosphors, the procedure probably leads to a loss in luminescence and increase the cost.12,19 In addition, the requirement of toxic hydrofluoric acid also limits its practical usage and quantity production.11,12,20 As an alternative, Mn4+-activated oxide compounds have been developed for its ecofriendly preparation procedure and high chemical stability. Generally, PL peak of Mn4+-doped oxide phosphors vary with host structures and locate in the red waveband range of 650− 730 nm (e.g., Sr4Al14O25:Mn4+ (652 nm),21 SrMgAl10O17: Mn4+ ,Li+ (663 nm),22 Y3Al5O 12:Mn4+ (673 nm),23 LiAl 5 O 8 :Mn 4+ (680 nm), 24 Gd 2 ZnTiO 6 :Mn 4+ (705 nm), 25 MgAl2Si2O8:Mn4+(710 nm),26 and SrTiO3:Mn4+(723 nm)27). Thus, to blue-shift their photoluminescence peak toward 650 nm, the host structures can be changed due to the covalence of “Mn4+-ligand” bonding can strongly influence the emission.9 Recently, one promising strategy for modifying the composition of the host lattice is substitution, including neighboring cation/ anion (e.g., Al3+−Si4+, O2−−F−), same family (e.g., Li+−Na+− K+, Ca2+−Sr2+−Ba2+), and anionic group (e.g., (PO4)3−− (BO3)3−, (PO4)3−−(SiO4)4−) substitution.28 By appropriate substitution, the spectral range, chemical/thermal stability, as well as quantum efficiency, can be optimized because of the modification of activator environment. Using these strategies, a large number of significant phosphor materials have been discovered, and new products with adjustable optical performances are endlessly developed by scientists.29 To date, many researchers devoted their interesting to modify the luminescent properties of Eu2+/Ce3+ by varying the structural composition and crystal structure of solid solution host.30−32 However, there is little report on studying the structural property of the Mn4+ site through color-tuning technique. Cao et al. reported the redshift of emission from 668 to 672 nm through substitution on the Lu3+ site with the large Y3+ in Lu3−xYxAl5O12: Mn4+, resulting from the stronger crystal field strength of LuAG than that of YAG.33 Regrettably, there is no systematical study on the relationship of structural composition, thermal stability and luminescent property. Herein, Mn4+-activated double-perovskite type antimonate Ba2YSbO6 red phosphor have been prepared. The structural evolution, PL performance, stability against temperature, decay times, and CRI values are thoroughly investigated. Specifically, with the substitution of Ba2+(1.36 Å, CN = 6) by Sr2+(1.13 Å,

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EXPERIMENTAL SECTION

Materials Synthesis. A series of (Ba1−xSrx)2YSbO6:Mn4+ samples with various Sr2+ contents ranging from 0 to 100% were prepared successfully by a conventional high-temperature sintering reaction. All the products were prepared under the same condition. The starting materials, including BaCO3 (99.9%), SrCO3 (99.9%), Y2O3(99.99%), Sb2O5(99.99%), and MnCO3 (99.9%), were used without any purification. The starting materials were weighted with stoichiometric and mixed thoroughly in an agate mortar, and then put it into a corundum crucible. Subsequently, the mixture sintered at 1500 °C for 8 h. Finally, these samples were cooled to room temperature naturally and grounded for further measurements. Fabrication of LED Devices. The w-LEDs devices were fabricated by combing UV-LED chip (∼365 nm) with the mixture of blue phosphor (BaMgAl10O17:Eu2+), green phosphor [Ba3La6(SiO4)6:Eu2+], and red phosphor (Sr2YSbO6:Mn4+), as well as epoxy resin. First, the mixed phosphors were blended with epoxy resin, and then deposited on the surface of the LED chip. Lastly, the w-LEDs were obtained by solidifying at 80 °C for 2 h. Structure and Optical Characterization. The phase compositions of the as-synthesized products were studied by X-ray diffraction (XRD) on a Bruker D8 advance diffractometer (40 kV, 30 mA). The XRD patterns were recorded from 10° to 80° at a scanning rate of 0.05 deg/s. The X-ray Rietveld refinement data were collected over 2θ in the range of 10°−120° at intervals of 0.01° and analyzed using the General Structure Analysis System (GSAS) program.34 The band structure for Ba2YSbO6 was calculated based on the density functional theory (DFT) and carried out using the Cambridge Serial Total Energy Package (CASTEP) code.35 Meanwhile, the exchange and correlation potentials were treated by the local-density approximation (LDA).36,37 The diffuse reflectance (DR) spectra were recorded on an UV−vis-NIR spectrometer (Shimadzu UV-3600), using BaSO4 white powder as a standard reference. The PLE and PL spectra, as well as decay curves at room temperature, were determined using a fluorescence spectrometer (Edinburgh Instrument FS5) with a 150 W continuous and pulsed xenon lamp. The temperature-dependent properties of the as-obtained products were examined by a computer-controlled electric furnace measured from 303 to 513 K, which set up a FS5 spectrophotometer. The photoelectric parameters of the fabricated LEDs devices, including CCT, CRI, luminous efficacy (LE), and Commission International de l’Eclairage (CIE) chromaticity coordinates were recorded from an integrating sphere (HAAS-2000; Hangzhou, China) at an operating current of 60 mA. B

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Structure Parameters of the Obtained Ba2YSbO6:0.005Mn4+ and Sr2YSbO6:0.005Mn4+ Phosphors crystal system space group units, Z a (Å) V (Å3) Rp (%) Rwp (%) Re (%) x2

Ba2YSbO6

Ba2YSb0.995Mn0.005O6

Sr2YSbO6

Sr2YSb0.995Mn0.005O6

cubic Fm3̅m 4 8.4117 595.18

cubic Fm3̅m 4 8.4084 594.48 4.81 7.17 4.53 2.499

cubic Fm3̅m 4 8.405 593.76

cubic Fm3̅m 4 8.4003 592.77 5.74 8.19 5.11 2.568

Figure 2. (a) Representative XRD patterns of (Ba1−xSrx)2YSbO6:0.005Mn4+ (0 ≤ x ≤ 1.0). (b) The unit cell parameters show a shrinkage in the lattice constants a and V with the increases of Sr2+ concentration in the (Ba1−xSrx)2YSbO6 (0 ≤ x ≤ 1.0) solid solution.

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as-prepared (Ba1−xSrx)2YSbO6:0.005Mn4+ (0 ≤ x ≤ 1.0) phosphors are plotted in Figure 2b, in which both a and V linearly decrease with increase of Sr2+ content. As the smaller Sr2+ is substituted for Ba2+, the a and V decreasing from 8.4084 to 8.4003 Å and 594.48 to 592.77 Å3, respectively, indicating that Sr2+ ions have entered into Ba2YSbO6 lattice via substituting Ba2+ ones. Crystal structures of the studied phosphors have great influence on their emissive performance. Both Ba2YSbO6 and Sr2YSbO6 belong to A2BB′O6 double perovskite structure (Figures 3a and 3b and Figure S1). They have a cubic Fm3̅ (225) space group with the corresponding cell parameters are a = b = c = 8.417 Å and a = b = c = 8.405 Å, respectively. The crystal structure of Sr2YSbO6 is isostructural with Ba2YSbO6 as well as the reported Ba2GdNbO6 and Ba2YNbO6, in which [YO6] and [SbO6] octahedrons are cross-linked each other through sharing O2− ion in the corner and Ba2+/Sr2+ ions are present in the cubic vacancies between octahedrons, as displayed in Figure S2.38,39 In general, Mn4+ ions prefer to occupying octahedral site, in which the splitting of Mn4+ 3d state is confirmed by CF strength.25 Herein, there are two types of cation ions, [YO6] and [SbO6], coordinated by six oxygen, which are suitable for Mn4+ substitution. Based on similar ionic radius, Mn4+(0.54 Å, CN = 6) will replace Sb5+ (0.61 Å, CN = 6) site, rather than Y3+ one (0.90 Å, CN = 6). The bond lengths between Sb5+ and O2− in Ba2YSbO6 and Sr2YSbO6 matrix are 2.233 and 2.002 Å, respectively, as presented in Figure 3c and 3d.

RESULTS AND DISCUSSION Phase Analysis and Structure Characteristics. XRD Rietveld refinements for Ba2YSbO6:0.005Mn4+ and Sr2YSbO6: 0.005Mn4+ are conducted using high intensity, high resolution XRD data, as presented in Figure 1. Crystal structure data of Ba2YSbO6 (ICSD no. 155253) and Sr2YSbO6 (ISCD no. 157887) are applied as the original structure pattern. Rietveld refinements illustrate that the experimental data are well consistent with the structural model. Meanwhile, the final refinement details as well as the standard cell parameters of Ba2YSbO6 (ICSD no. 155253) and Sr2YSbO6 (ISCD no. 157887) are summarized in Table 1. It is confirmed that Ba2YSbO6:Mn4+ and Sr2YSbO6:Mn4+ crystallizes in a cubic system with the unit cell volume slightly smaller than those of pure Ba2YSbO6 (V = 595.18 Å3) and Sr2YSbO6 (V = 593.76 Å3) crystal, respectively. The shrinkage of cell volume is attributed to the substitution of Sb5+ (0.61 Å, CN = 6) by Mn4+ (0.54 Å, CN = 6) with smaller ionic radius. The XRD patterns of (Ba1−xSrx)2YSbO6:0.005Mn4+ (0 ≤ x ≤ 1.0) phosphors are depicted in Figure 2a. The diffraction peaks of the synthesized products are in good agreement with Ba2YSbO6 (ICSD no. 155253) and Sr2YSbO6 (ISCD no. 157887), suggesting that the prepared samples are single phase without other impurities. Interestingly, as the amount of Sr2+ increases, the diffraction peaks gradually move up to a higher angle since the small Sr2+(1.13 Å, CN = 6) ion replaces large Ba2+(1.36 Å, CN = 6) one. To gain further information, the lattice parameter (a), as well as crystal cell volume (V), for the C

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a, b) Unit cell structure of Ba2YSbO6 and Sr2YSbO6 viewed along a-axis. The coordination environment of Sb sites and the [SbO6] octahedron in Ba2YSbO6 (c) and Sr2YSbO6 (d) is also depicted.

Figure 4. (a) Calculated energy band structure of Ba2YSbO6. (b) PDOS and TDOS for Ba2YSbO6.

x = 0 to 1.0, the band gap changes from 4.50 to 4.72 eV, indicating the variation of crystal field environments. With the help of the refinement data, the electronic structure of the Ba2YSbO6 host can be calculated based on DFT method using the CASTEP module.29−31 Moreover, the density function is used the LDA as the theoretical basis, and the distribution curves are displayed in Figure 4a. Obviously, the maximum value of valence band (VB) on the top point and the minimum one of conduction band (CB) on the bottom point are found almost at Γ spot, indicating that the matrix is a direct band gap semiconductor.40 Compared to the band gap of experimental value (4.50−4.72 eV), the value of Ba2YSbO6 matrix (∼3.57 eV) is small. This is reasonable since the LDA generally underestimates the scale of band gap.40,41 To further resolve the components for the evaluated energy band, the total density of states (TDOS) and partial density of states (PDOS) of Ba2YSbO6 are depicted in Figure 4b. The region below the Fermi level is divided to four zones. The VB region on the bottom-most from −25 to −18 eV is consisted of Y-4p states.

The shorting of distance between atoms is result from the decrease of lattice parameter as the number of atomic in each cell remains constant.33 Therefore, since the Mn4+ interaction between atoms is strengthened, the Mn4+ experience a stronger CF when Sr2+ doping content increases. Electronic Band Structure. DR spectra of the obtained (Ba1−xSrx)2YSbO6:0.005Mn4+ with different Sr2+ concentrations are depicted in Figure S3. Obviously, two typical discernible peaks located at ∼350 (28571 cm−1) and ∼510 nm (19608 cm−1) assigned to the spin-allowed Mn4+:4A2g → (4T1g and 4T2g) transitions, and an inconspicuous peak situated at ∼390 nm (25641 cm−1) derived from spin-forbidden Mn4+:4A2g →4T2g transition can be observed. Furthermore, the broad band between 230 and 320 nm is originated from Mn4+−O2− charge transfer (CT) band and host absorption. Meanwhile, based on those DR spectra, the band gaps of the prepared (Ba1−xSrx)2YSbO6: 0.005Mn4+ products can be derived by the Tauc relation, as depicted in Figure S3b. When Sr2+ contents increase from D

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) PLE and PL spectra of Ba2YSbO6:Mn4+ and Sr2YSbO6: Mn4+ phosphors (λex = 350 nm; λem = 685 nm). (b) Tanabe−Sugano energy level diagram.

The second band region from −18 to −13 eV is mostly composed of O-2s and Sb-5s5p states. The third band at −11 eV is primarily originated from the states of Ba-5p. The fourth region from −10 to 0 eV is mainly originated from Sb-5s5p and O-2p states. Obviously, the VB predominantly originates from the states of O-2s, O-2p, Y-4p, Sb-5p, and Ba-5p, while the CB is mainly constituted of Sb-5p and Ba-4d states. Herein, we can predict that the energy levels of Mn4+ ions in Ba2YSbO6 share insignificantly interference with the conduction and valence bands owing to the large band gap, demonstrating that Ba2YSbO6 should provide an appropriate band gap for Mn4+ and be a suitable emissive host for Mn4+.42,43 Luminescent Properties. PL and PLE spectra of Mn4+doped Ba2YSbO6 and Sr2YSbO6 products are measured at room temperature and shown in Figure 5a. When monitored at 685 nm, several excitation bands attributing to the Mn4+ dipole− dipole (d−d) transitions and Mn4+−O2− CT band in the range of 250 to 570 nm are observed.44 As displayed in Figure 5a, four Gaussian peaks situated at 323 (30960 cm−1, CT), 355 (28169 cm−1, 4A2 → 4T1), 398 (25126 cm−1, spin-forbidden 4 A2 → 2T2), and 528 nm (18939 cm−1, spin-allowed 4A2 → 4T2) for Ba2YSbO6:Mn4+ and 303 (33003 cm−1, CT), 346 (28902 cm−1, 4 A2 → 4T1), 387 (25840 cm−1, spin-forbidden 4A2 → 2T2), and 510 nm (19608 cm−1, spin-allowed 4A2 → 4T2) for Sr2YSbO6:Mn4+ are decomposed.45 Upon 350 nm excitation, the strongest PL peak at 681 nm for Sr2YSbO6:Mn4+ and 691 nm for Ba2YSbO6:Mn4+ in the range of 620 to 750 nm, originating from Mn4+:2E → 4 A2g transition in a [SbO6] octahedral environment, can be found.46 Compared to Ba2YSbO6:Mn4+(691 nm), a blue shift occurs in Sr2YSbO6:Mn4+(681 nm), implying the different CF environment on Mn4+. As is well-known, Mn4+ ion is sensitive to CF environments due to its special 3d3 electron configuration. Therefore, to comprehend the influence of CF strength on the luminescence properties of Ba2YSbO6:Mn4+ and Sr2YSbO6: Mn4+, the Racah parameters (B, C) and CF strength (Dq) are calculated. The value of Dq is evaluated with the assistance of the energy of 4A2g → 4T2g transition47,48 Dq =

where the mean peaks energy gap of Ba2YSbO6:Mn4+ and Sr2YSbO6:Mn4+ are 18939 and 19608 cm−1, respectively. Besides, the value of B is evaluated according to the difference of peak energy between 4A2g → 4T1g and 4A2g → 4T2g transitions (9230 cm−1 for Ba2YSbO6:Mn4+ and 9294 cm−1 for Sr2YSbO6: Mn4+), as the following equation:49 Dq 15(δ − 8) = 2 B δ − 10δ

δ can be obtained as follows: δ=

E(4 A 2g → 4 T1g ) − E(4 A 2g → 4 T 2g ) Dq

(3)

Finally, based on the energy of 2Eg → 4A2g (14684 and 14472 cm−1 for Ba2YSbO6:Mn4+ and Sr2YSbO6:Mn4+, respectively), the value of C is estimated as follows: E(2 E g → 4 A 2g) B

=

3.05C 1.8B − + 7.9 B Dq

(4)

On the basis of the eqs 1−4, the final calculated Dq, B and C in Ba2YSbO6:Mn4+ and Sr2YSbO6:Mn4+ products are 1894, 1007, 3480 cm−1 and 1961, 1000, 3354 cm−1, respectively. Thus, Dq versus B can be calculated to be 1.88 and 1.96 for Ba2YSbO6 and Sr2YSbO6, respectively. Therefore, owing to the decreased lattice parameter with increasing of Sr2+ concentrations, the interactions of Mn4+−Mn4+ and Mn4+−O2− are enhanced, resulting in a stronger crystal field of Mn4+. From the calculated Dq/B values, we can find that Sr2YSbO6 has stronger CF strength than that of Ba2YSbO6. This result is well agreement with the above conclusions. Generally, Mn4+ ion only situated in an octahedral, which can offer complex spectral characteristic in various CF environments because of its special 3d3 electron configuration.50 Therefore, owing to the different site symmetry and CF strength of the doped host, the luminescence of Mn4+ is diverse from each other. The electron transitions of Mn4+ in different energy levels can be described using the well-known Tanabe−Sugano energy diagram, as depicted in Figure 5b. Notably, except for 2Eg and

E(4 A 2g → 4 T 2g ) 10

(2)

(1) E

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) Integrated emission intensity of the Mn4+ at various Sr2+ doping contents. Inset is the digital images of the obtained phosphors under sunlight and 365 nm UV light. (b) Normalized PL spectra of the as-prepared (Ba1−xSrx)2YSbO6:0.005Mn4+ (0 ≤ x ≤ 1.0) phosphors. Inset is the corresponding amplification PL spectra ranging from 670 to 700 nm. (c) Decay curves of the Mn4+: 2E → 4A2 in (Ba1−xSrx)2YSbO6:0.005Mn4+ (0 ≤ x ≤ 1.0) samples under 350 nm excitation.

T1g levels, most multiplets are highly rely on the CF strength.51 The 4T1g and 4T2g levels are originated from the t22 electronic orbital, while others come from t23e orbital, including 4A2g, 2T1g, 2 T2g, and 2Eg levels. A large spectral bandwidth for the transition can be observed due to the large lateral displacement between ground state 4A2g and 4T1g(or 4T2g).52 Furthermore, on the basis of the spin selection rule, 4A2g → (4T1g and 4T2g) transitions are spin-allowed; hence, the relatively large bandwidths of the strong excitation or absorption between these transitions can be expected. Meanwhile, the sharp emission lines derived from the spin-forbidden 2Eg → 4A2 transition can be found. As presented in Figure 5b, the Dq/B values of Sr2YSbO6 than Ba2YSbO6 are 2

indicated with red and green short vertical lines, respectively. Compared to Ba2YSbO6 one, the stronger CF strength observed in Sr2YSbO6 is ascribed to the shorter band length of Mn4+−O2− ligand in Sr2YSbO6 (2.002 Å) than that of Ba2YSbO6 (2.233 Å). This result is consistent with the above conclusions. To determine the optimal Mn4+ concentration, PL spectra of Ba2YSbO6:xMn4+ (0.001≤ x ≤ 0.02) are measured under 350 nm excitation, as shown in Figure S4. It can be found that the position of emission peak keeps almost unchanged with increment of Mn4+ concentrations from 0.1 mol % to 2 mol %. PL intensity decline gradually as the concentrations of Mn4+ exceed 0.5 mol % owing to concentration quenching. Hence, we F

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Temperature-dependent emission spectra of as-prepared (Ba1−xSrx)2YSbO6:Mn4+(0 ≤ x ≤ 1.0) with varying Sr2+ concentrations (x): (a) x = 0, (b) x = 0.5, and (c) x = 1.0 from 303 to 513 K. (d) The integrated PL intensities versus temperature for the (Ba1−xSrx)2YSbO6:Mn4+ phosphors. Inset shows the fitted lines based on the Arrhenius equation to determine activation energy.

keep Mn4+ content at 0.5 mol % and tuned regularly the molar ratio of Ba:Sr between 2:0 and 0:2 to discuss our design of shifting and enhancing the emission of Mn4+ systematically. The relative intensities of the Mn4+ emission bands gradually enhance as the enlargement of x. When x = 1.0, and the emission intensity of Sr2YSbO6: Mn4+ displays 3.53 times than that of Ba2YSbO6: Mn4+, as revealed in Figure 6a. Additionally, the normalized PL spectra of (Ba1−xSrx)2YSbO6:0.005Mn4+(0 ≤ x ≤ 1.0) are depicted in Figure 6b. Impressively, the PL spectra exhibit a blue shift from 691 to 681 nm as the Ba2+ ions are substituted by Sr2+ ones in Ba2YSbO6. After cation substitution, the energy gap between 4 A2g level and 2E one of Mn4+can be neglected since these two levels are insensitive to the modification of crystal field.53 Compared with the reported Lu3−xYxAl5O12:Mn4+ red phosphor with a red shift of 4 nm,33 the obtained (Ba1−xSrx)2YSbO6: Mn4+ products exhibit a great progress with a blue shift of 10 nm. The major blue shift is mainly attributed to the higher CF strength of Mn4+ in Sr2YSbO6 (1.96) than that in Ba2YSbO6 (1.88). Higher-energy splitting results in higher transition energy from excited states to ground ones, which leads to blue-shift of emission.54 Additionally, the luminescence decay curves of (Ba1−xSrx)2 YSbO6:0.005Mn4+ (0 ≤ x ≤ 1.0) samples measured at excitation wavelength of 350 nm with λem= 680 nm are displayed in Figure 6c. The lifetime is increased from 0.1592 to 0.3458 ms with increasing of Sr2+ contents. As above-mentioned, the variation trend of fluorescence lifetime is similar to that of PL intensity, confirming that difference CF strength produces diverse effects on Mn4+ decay times for the (Ba1−xSrx)2YSbO6:0.005Mn4+(0 ≤ x ≤ 1.0) samples. Furthermore, the quantum efficiency of the as-prepared samples is determined to 32.8%−46.2% under 350 nm excitation according to two measurement approaches.55 To realize their possible application in w-LEDs, the thermal stability is one of important factors needing consideration.

Temperature-dependent PL spectra of (Ba1−xSrx)2YSbO6: 0.005Mn4+ with various Sr2+ concentrations (x) are presented in Figures 7a−7c. Evidently, as the temperature elevates from 303 to 513 K, the emission intensity descends gradually without any shifting or broadening for three typical Ba2YSbO6: 0.005Mn4+, BaSrYSbO6:0.005Mn4+, and Sr2YSbO6:0.005Mn4+ phosphors. Interestingly, the body color of the obtained Sr2YSbO6:Mn4+ sample shows the better temperature stability than the other members under the 350 nm UV excitation. The tendency is depicted more apparently in Figure 7d, in which the emission intensity reduced gradually as the temperature increasing above 303 K. When the temperature raised up to 423 K, the PL intensity of Ba2YSbO6:0.005Mn4+, BaSrYSbO6: 0.005Mn4+ and Sr2YSbO6:0.005Mn4+ drops to 69%, 74%, and 85% of the initial intensity at 303 K, respectively. Compared with other reported Mn4+-activated oxide phosphor, the as-prepared sample exhibits a relatively high thermal quenching temperature. Additionally, this phenomenon can be elaborated by the activation energy (ΔE) using the Arrhenius equation56 IT =

I0 1 + exp( −ΔE /kT )

(5)

where I0 and IT stand for the emission intensity at initial and given temperature (303−513 K), respectively. On basis of above equation, a plot of ln(I0/IT − 1) to 1/kT can be gained, as depicted in inset of Figure 7d. The ΔE values are 0.2104, 0.2640, and 0.3237 eV for Ba2YSbO6:Mn4+, BaSrYSbO6:Mn4+, and Sr2YSbO6:Mn4+, respectively, which comparable to that of CaAlSiN3:Eu2+ (0.20−0.24). Therefore, the retained intensities at 150 °C for these two phosphors are found to be similar (about 80−90%).57,58 Both high thermal quenching temperature and high activation energy of Mn4+ luminescence are originated from the decrease of ionic distance between O2− and Sb5+ in the G

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Inorganic Chemistry

Figure 8. (a) CIE chromaticity diagram of five constructed w-LEDs devices via combining UV LED chip with the mixture of blue and green phosphors without (b) and with (c−f) red phosphor. (b−f) EL spectra of the constructed w-LEDs with various weight ratios of red/green/blue (0, 5:47.5:47.5, 10:45:45, 15:42.5:42.5, 20:40:40). Inset shows the photograph of their corresponding LED images under 60 mA forward bias currents.

white light turns from cool to warm. Fortunately, the values of CCT (4021−3404 K), CRI (83.7−86.8), and LE (18.61−15.51 lm/W) in LED devices including 15% or 20% Sr2YSbO6:Mn4+ red phosphors are still accredited for indoor illumination.

[SbO6] octahedron, resulting in the increase of activation barrier for thermal quenching.59 Performance of LED Devices. On the basis of the above results, we can find that the as-prepared (Ba1−xSrx)2YSbO6:Mn4+ phosphors are of superior luminescence performances and excellently thermal quenching resistances. Therefore, investigating their potential application on the LED devices is very significance. To validate the available application, we chose Sr2YSbO6:Mn4+ phosphor for the example to fabricate the w-LEDs. Herein, the w-LEDs are constructed by mixing blue phosphor (BaMgAl10 O 17 :Eu 2+ ), green phosphor [Ba3 La 6 (SiO4)6:Eu2+]60 and red phosphor (Sr2YSbO6:Mn4+) with a UV-LED chip. Figure 8 shows the CIE chromaticity coordinates and typical electroluminescence (EL) spectra of the fabricated w-LEDs under 60 mA forward bias current. Herein, when the content of Sr2YSbO6:Mn4+ phosphor is chosen, the other phosphors are equally included. Apparently, significant difference can be found for the w-LEDs constructed with various ratios of red/ green/blue phosphors, as presented in Figure 8b−8f. Their corresponding important photoelectric parameters with various ratios of R/(R+G+B) are tabulated in Table S1. When the amount of Sr2YSbO6:Mn4+ phosphor increases, the red emission peak at ∼685 nm originated from Mn4+:2Eg → 4A2g transition becomes obvious. Meanwhile, the performance of the fabricated w-LEDs experience decreasing CCT from 7618 to 3404 K and increasing CRI from 68.7 to 86.8 by enhancing the weight ratio of Sr2YSbO6:Mn4+ from 0 wt % to 20 wt %. Unfortunately, the LE declines from 31.08 to 15.51 lm/W, probably owing to the reduction in light extraction after the addition of red phosphors. Specially, the chromaticity coordinates of these five LED are (0.2918, 0.3307), (0.32378, 0.3476), (0.3587, 0.3644), (0.3774, 0.3720), and (0.4082, 0.3831), respectively, which all locate in or near the blackbody locus (Figure 8a). Correspondingly, the yielded

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CONCLUSIONS Mn -doped double perovskite (Ba1−xSrx)2YSbO6:Mn4+ solidsolutions with adjustable PL emissions were prepared through a conventional high-temperature sintering reaction. Rietveld refinements confirmed that these solid-solutions belonged to a cubic Fm3̅m space group. The substitution of Ba2+ by Sr2+ inducedual shift of diffraction peaks toward a higher angle, monotonous decrease of lattice parameter from 8.4084 to 8.4003 Å and increase of band gap from 4.50 to 4.72 eV. Correspondingly, Mn4+ emission peaks blue-shifted from 691 to 681 nm upon 350 nm light excitation, owing to the promotion of Mn4+2 Eg energy state position. This result is mainly ascribed to the alteration of Mn4+ CF strength by cationic substitution in the (Ba1−xSrx)2YSbO6: Mn4+ hosts. Impressively, the thermal stability is significantly improved after complete replacement of Ba2+ by Sr2+, where 69% to 85% of the initial PL intensities were retained at 423 K, respectively. Finally, w-LEDs were fabricated by coupling the mixed red/green/blue phosphors with UV-LED chip, and the corresponding CCT and CRI can be easily tuned from 7618 to 3404 K and from 68.7 to 86.8 by simply modifying the contents of Sr2YSbO6:Mn4+. All the results manifest that the prepared composition series may find promising application in warm LED.

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4+

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00947. H

DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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Crystal structure of Ba2YSbO6, Ba2GdNbO6, and Ba2YNbO6; diffuse reflectance spectra; PL spectra of Ba2YSbO6:xMn4+; and photoelectric parameters (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daqin Chen: 0000-0003-0088-2480 Yongjun Yuan: 0000-0002-1823-3174 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Zhejiang Province Natural Science Foundation of China (LY18E020006), the Natural Science Foundation of Zhejiang for Distinguished Young Scholars (LR15E020001), and National Nature Science Foundation of China (51502066 and 51572065).

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DOI: 10.1021/acs.inorgchem.8b00947 Inorg. Chem. XXXX, XXX, XXX−XXX