Monoclinic Lu2–xSmxWO6-Based White Light-Emitting Phosphors

Dec 18, 2017 - †Key Laboratory of Optoelectronic Information Materials of Hebei Province, College of Electron and Information Engineering, ‡Resear...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Monoclinic Lu2−xSmxWO6‑Based White Light-Emitting Phosphors: From Ground−Excited-States Calculation Prediction to Experiment Realization Yongyang Zhang,† Bangfu Ding,*,†,‡ Luqiao Yin,∥ Jiandi Xin,§ Rui Zhao,† Shukai Zheng,†,‡ and Xiaobing Yan† †

Key Laboratory of Optoelectronic Information Materials of Hebei Province, College of Electron and Information Engineering, Research Center for Computational Materials & Device Simulations, College of Electron and Information Engineering, and §College of Civil Engineering and Architecture, Hebei University, Baoding 071002, China ∥ Key Laboratory of Advanced Display and System Applications, Shanghai University, Ministry of Education, Shanghai, 200444, China ‡

S Supporting Information *

ABSTRACT: Through ground state and constrained density function calculations, Sm3+ ions luminescence in self-activated monoclinic Lu2WO6 was originated from intra 4f → 4f transitions, not inter 5d → 4f transitions. Theoretically the white luminescence was obtained by combining red and bluegreen emissions of 4f energy levels and W−O charge transfer transitions. Experimentally, pure and Sm3+ doping Lu2WO6 powders were synthesized using solid phase reaction calcined in air atmosphere. By the analysis of X-ray photoelectron spectroscopy and Rietveld refinement, element Sm charge state was trivalent, and Sm3+ doping was concentrationdependent selectively doping in three Lu sites. With the increase of Sm3+ concentrations, the color coordinates changed gradually from blue (0.17, 0.17) through white light (0.33, 0.25) toward orange (0.44, 0.32) in the visible spectral under 325 nm excitation. On the basis of the theoretical prediction and experimental preparation, a white emission LED lamp was produced using a 365 nm ultraviolet chip and Lu1.99Sm0.01WO6 phosphor. The present design method can be applied to select excellent activators from a large number of rare-earth (Re) ions like Sm3+ and Eu3+/2+ or non-Re ions like Bi3+ and Mn4+ in various matrixes.

1. INTRODUCTION The first-principle calculations have been employed widely to explain internal physical mechanism of self-activated, intrinsic and impurity defects luminescence.1,2 Generally free electrons jumped from the conduction band (CB) to the valence band (VB) generating intrinsic emission in self-activated hosts such as tungstates, molybdates, vanadates and so on.3 In sample synthesis processes, there existed various intrinsic defects like oxygen vacancy or impurity ions with different chemical valence.4,5 On the basis of density functional theory (DFT) calculation, these defects produced different energy band gap states and tunable excitation and emission can be achieved as electrons transitions among VB, CB, and local states.6 CdWO4 nanorods displayed temperature-dependent photoluminescence (PL) spectrum from 20K to room temperature and mainly contained three emission peaks at 410, 436, and 490 nm.7 Cd vacancy (VCd) and different valence states oxygen vacancies (VO) induced extra levels in the band gap by the theoretical calculations; thus, electron transitions among these local states, the lowest unoccupied molecular orbital, and the highest occupied molecular orbital caused three excitation and emission © XXXX American Chemical Society

peaks. Moreover, single and twin VO models reasonably interpreted different excitation phenomena of monoclinic Y2WO6 calcined at Air and Ar atmosphere.8 Except for ground-state calculation, the excitation-state simulation, that is, the constraint DFT, was also employed to reveal the luminescence origin of activators in hosts.9 On the basis of excited-state simulation, a necessary condition of Ce luminescence in inorganic scintillators was that occupied Ce 5d excitation energy level must be located in semiconductor band gap indicating the localization of excited-state decomposed charge density.10 Thus, the experimental results were related to the ground state and the excitation-state simulation to achieve a deeper understanding of inner luminescence mechanism.11−14 Both tungstates and molybdates were a self-activated luminescence body, which was applied widespreadly as matrixes in phosphor-converted white light-emitting diodes (LED).15,16 Usually, tungstates and molybdates were divided into three Received: November 3, 2017

A

DOI: 10.1021/acs.inorgchem.7b02787 Inorg. Chem. XXXX, XXX, XXX−XXX

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white LEDs lamp was not prepared.44 Lu2WO6 space group and crystal parameters were similar to those of Y2WO6. As we know, different synthesis conditions such as solution PH values in hydrothermal process can change monoclinic Y2WO6 luminescence intensity,45 and many activators like Sm3+, Tb3+, and Dy3+ were doped into Y2WO6 to obtain different luminescence phenomena.46 When other activator ions like Sm3+, Yb3+, and Dy3+ were doped into Lu2WO6 samples, these doping samples should show tunable luminescence phenomena. Moreover, the DFT+U or hybridized density function methods can be applied to correct the band gap underestimation problem. After correcting the band gap, the luminescence origin of activations ions in Lu2WO6 host can be explained from the ground-state or excited-state calculation.47 Moreover, the luminescence mechanism of Sm ions in self-activation hosts was rarely studied by the theoretical calculation. Though the theoretical simulation can explain well the PL results in SrZrO3:Sm, five 4f electrons in Sm3+ ion were not included in Sm_3 poseudopotential.48 When 4f electrons was treated as valence electrons, Sm3+ luminescence origin in LaSi3N5 was mainly attributed to charge transitions between VB and 4f state.49 Until now, there was no relevant investigation about Sm 4f electrons structure calculation in other crystals and the luminescence properties of Lu2WO6 doped by other activators. In this manuscript, the first-principle ground-excited-state calculation predicted Sm3+ PL behavior in self-activated monoclinic Lu2WO6, which was further confirmed by the experiment. Monoclinic Lu2WO6 crystal structures and electronic properties have been systematically studied by local and hybridization DFT. The obtained lattice parameters and band gap were consistent with the experimental values. Through the groundstate simulation, 4f and 5d energy levels of Sm ions replacing three Lu sites located in the band gap and above CB, respectively. When an electron was moved from 4f orbits to 5d orbits in the excited-state calculation, energy-band decomposed charge density of 5d states delocalized at whole lattice, indicating that 5d → 4f transition luminescence cannot be realized. Thus, red emission can be obtained in Sm doping Lu2WO6 via intra 4f → 4f transition. Further in the condition of atmosphere air, a series of Lu2WO6 powders was synthesized by the conventional solid-state reaction under different calcined temperatures. Through XRD and PL spectra analysis, the samples crystallite, excitation and emission intensity were enhanced along with temperature rising. In addition, the peak position in PL spectra showed blue and red shift phenomenon. Introducing Sm3+ into Lu sites, Lu2−xSmxWO6 white powders were synthesized. Under 325 nm UV excitation, WO66− and Sm3+ simultaneously produced PL and there was energy transfer between them. Thus, Lu2−xSmxWO6 emission spectra covered whole visible light regions and Lu2−xSmxWO6 powders can be used to prepare white LED lamp. Using the X-ray photoelectron spectrum and XRD refinement, the valence state of Sm was +3 and the occupancy number of Sm3+ in three Lu sites was concentration-dependent selective occupancy. Different occupancy number may result into PL intensity change. Along with concentration variation, the Commission Internationale de L’Eclairage (CIE) coordination changed from the blue region through white region to red region. Finally, a white light LED lamp was prepared by Lu2WO6:0.01Sm3+ phosphor and a 365 nm UV LED chip. This device exhibited an excellent color rendering index (Ra) of 87.0, a correlated color temperature of 5617 K and a CIE coordination of (x = 0.33,

types, such as normal tungstates like MeW(Mo)O4 (Me = Zn, Ca, etc.), rare earth tungstates like RexW(Mo)yOz (Re = La− Lu, including Y and Sc), and polytungstates or polymolybdates.17 Their luminescence origins mainly contained charge transfer transitions, defects luminescence, and exciton recombination. The corresponding emission wavelength covered the blue-green region in visible spectrum.18 In order to obtain warm white or other color lights, many self-activated luminescence ions were doped into these two matrixes such as Eu3+, Sm3+, Dy3+, Tb3+, Yb3+, Er3+, Tm3+, and Bi3+.19−25 For example, Bi3+-doping gadolinium tungstate phosphor showed visible emission from blue to red regions under 330 and 350 nm excitations.26 Introducing the Yb3+ in Bi3+-doping phosphor resulted into an intense near-infrared luminescence peaking at 976 nm, primarily due to cooperative energy transfer from Bi3+ 3 P1 level to Yb3+ 2F5/2 level. Using ethylenediaminetetraacetic acid-assisted hydrothermal method, Wang et al. prepared Eu3+activated NaLu(WO4)2 phosphors and found that NaLu(WO4)2:Eu3+ samples have an intense red emission and suitable near-ultraviolet excitation region.27 Therefore, this sample possessed great application potential for near-ultraviolet (n-UV) chip-based LED lighting. Except for experimental investigation, the above-mentioned first-principle ground-state calculation was also employed to reveal the luminescence origin of pure or doped tungstates and molybdates.28−31 For transition metal salts compounds, the valence band and conduction band were principally composed of oxygen 2p states and metal nd states. Thus, their intrinsic emission was ascribed to electrons jumping from nd orbits to 2p orbits. In addition, less than band gap energy emission was induced from local states in forbidden bands. Through the first-principle ground-state calculation, two emission peaks was ascribed to WO4 charge transfer in α-Ag2WO4, and the second emission peak in β-phase was originated from the oxygen vacancy local state.32 Because of the radius of rare-earth luminescence ions approximately equaling those of Re3+ in rare earth tungstates, these ions can be easily injected into Re3+ sites in RexWyOz.33,34 Thus, white light can be achieved with common emissions of activated ions and tungstate groups. Co-doping Eu3+ and Tb3+ inside Gd2W3O12 microstructures realized strong and multicolor emission.35 In addition, Re2WO6-type structures such as Y2WO6, Lu2WO6, and Gd2WO6 were studied extensively recently. Different synthesis methods36,37 and various activator ions38,39 have been employed to tune monoclinic Y2WO6 luminescence properties and obtain white light emission. By the X-ray diffraction (XRD) refinement, Lu2WO6 structure was determined first monoclinic P12/C1 phase.40 Under X-ray and UV excitation, Lu2WO6:Eu3+ showed high luminescent efficiency in the red light region because of efficient energy transfer between WO6 group and Eu3+, while Lu2WO6:Pr3+ showed less efficiency in orange light regions due to Pr3+ reabsorbing WO6 emission. Similarly, introducing Bi3+ inside Lu2WO6 powders brought new 350 nm long wavelength excitation band and Bi3+ doping enhanced emission intensity under low-temperature.41 Furthermore, high density and homogeneously micrometer-thick Lu2−2xEu2xWO6 film was prepared for the first time and possessed good luminescence properties.42 By DFT calculations, Lu2WO6 was an indirect band gap semiconductor with 3.13 eV which was smaller than optical band gap 3.60 eV.43 Though activator ion doping can enhance Lu2WO6 red light emission in the visible spectrum, the practical Lu2WO6-base B

DOI: 10.1021/acs.inorgchem.7b02787 Inorg. Chem. XXXX, XXX, XXX−XXX

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the crystal parameters a−c and β being 7.518, 5.268, and 11.225 Å and 104.61°.40 After geometry relaxation with pure PBE functional, 1 × 2 × 1 supercell Lu2WO6 lattice constants become a = 7.532, b = 10.547, c = 11.217 Å with α=γ = 90° and β = 104.86°. The calculated a, b and c were slighted overestimated and underestimated by ±0.1%, while the angle β was overestimated by 0.25° compared with the experiment values. Thus, the obtained configuration was rational and its electronic property was derived by further calculation. The corresponding optimized configuration was plotted in Figure 1a,b. As can be seen, there were six nonequivalent O positions

y = 0.20). Therefore, the Lu2−xSmxWO6 phosphors exhibited great potential application in near UV-excitation white LED devices to use single-phase phosphor pumped by long-wave UV LEDs. On the basis of method of theory guiding experiment, the experiment cost and exploration time can be greatly reduced.

2. CALCULATION AND EXPERIMENTAL DETAILS 2.1. Calculation Method. All the calculations were performed using spin-polarized plane wave pseudopotential density functional approach.50,51 The projector augmented wave method was employed to describe electrons and ions interaction. The valence electron configurations of O, Lu, W, and Sm were 2s22p4, 4f145s25p65d16s2, 5p65d46s2, and 4f65s25p66s2. A cutoff energy of 400 eV was used for the expansion of plane wave basis set in pure and Sm doping models. A 2 × 1 × 1 Monkhorst−Pack k-space sampling in reciprocal space was used for the Brillouin zone integration. The generalized gradient approximation in the Perdew−Burke−Emzerhof (PBE) functional was applied to optimize the crystal structure.52 When the total energy was converged to 10−4 eV and the residual forces on every atom were below 0.03 eV/Å, the relaxation processes were completed. After optimization, the screened-exchange hybrid functional of Heyd, Scuseria, and Ernzerhof was adopted to calculate the electronic properties. To study the Sm ions excitation state, the constraint DFT calculation was done. The occupation numbers were manually set to move an electron in Sm 4f orbits and fill the next empty 5d orbits. The band decomposed charge density was used to analyze the excitationstate localization.53 2.2. Experimental Processes. The Lu2−xSmxWO6 (0 ≤ x ≤ 0.1) samples were synthesized by high-temperature solid-state reaction technique. The initial materials were Lu2O3 (99.99%, analytical reagent) and WO3 (99.99%, AR) for pure sample synthesis. The stoichiometric reactants were individually weighed on the analytical balance and ground in an agate mortar for 15 min to mix them homogeneously. In the grinding process, a small amount of ethanol was added to grind fully. The resulting mixtures were transferred to alumina crucible and the covered aluminum crucibles were calcined at 800, 900, 1000, 1100, and 1150 °C in air for 450 min in the muffle furnace. After these steps, the furnace was cooled to room temperature by turning power off. For Sm-doped Lu2WO6 samples, the stoichiometric amounts of Lu2O3, WO3, and Sm2O3 (99.99%, AR) were weighed according to chemical composition of Lu2−xSmxWO6 (0.002 ≤ x ≤ 0.1). Repeating a similar operation procedure of pure samples synthetization, different concentrations of doping Lu2−xSmxWO6 were obtained by calcining at 1150 °C. The samples composition and phase purity were studied by XRD using a TD-3500 diffractometer with Cu Kα radiation (λ = 1.54056 Å). The scan range for all powders was set from 10 to 80° with step of 0.02°. The XRD structure refinements were made using the general structural analysis system (GSAS) program software.54 PL excitation and emission spectra was measured using F-4600 fluorescence spectrophotometer (HITACHI) with a 150W Xe lamp as excitation light source. The diffuse reflectance spectra of the samples were measured using a TU-1900 dual-beam UV−vis spectrophotometer. The chemical valence state of Sm element was analyzed by ECSALAB 250 X-ray photoelectron spectroscopy. LED was made with 365 nm UV LED chip and Lu2WO6:0.01Sm3+ phosphor. All the LED measurements were performed in Key Laboratory of Advance Display and System Application of Shanghai University. The sample electroluminescence (EL) spectra, color rendering index (CRI), CIE color coordinates, and correlated color temperature (CCT) were recorded using a HASS-2000 system.

Figure 1. (a) Frame structure of 1 × 2 × 1 supercell monoclinic Lu2WO6 with space group P12/C1, and (b) Local coordination configurations of a W and three Lu atoms after pure PBE geometry optimization.

distinguished by O1, O2, O3, O4, O5, and O6. Element W only possessed a site and was surrounded by six different O atoms consisting WO6 distorted octahedron which was described as a trigonal prism distorted toward an octahedron. Lu has three different sites labeled as Lu1, Lu2, and Lu3. The former two formed LuO8 polyhedral which was described as a cube distorted in the direction of an antiprism. Lu3 constituted LuO7 polyhedral which was described as a distorted cube with one corner missing. The energy band structure and density of states for Lu2WO6 were displayed in Figure S1. Lu2WO6 band gap value was 3.04 eV with indirect band gap, which was consistent with that of Castep software calculation.43 This value was smaller than the experiment data with 3.60 eV as displayed in Figure S2. In order to improve theoretical value, hybridization functional method with 10% nonlocal Fock exchange energy was employed to this Lu2WO6 system as shown in Figure 2. The band gap was increased to 3.64 eV, almost equaling to the experimental, indicating the effectiveness of HSE06 hybridization function. The VB was mainly O 2p states with smaller contributions of W and Lu, while W 5d states composed the CB. The above CB of 6−13 eV was contributed from Lu elements. Moreover, the Lu 4f state located insider VB and thus the Lu 14 4f electrons can be considered as core state electrons in calculation process to reduce calculation quantity. From the Pauling electronegativity, the electronegativity of W (2.36) was much higher than that of Lu (1.27), and thus W 5d states form a narrow CB with 1.5 eV width which was split by 2 eV from the Lu bands.56 As we know, Sm3+ ions usually exhibited red emission because of 4G5/2 → 6H5/2−11/2 transitions in most compounds, and the corresponding luminescence or quenching mechanism was revealed on the basis of PL and decay spectra.57 However, the theoretical explanation was seldom for Sm doping

3. RESULTS AND DISCUSSION 3.1. Theoretical Prediction. Lu2WO6 crystal structure was first investigated by Brixner et al. and its space group was centric 10-P2/m-C2h1 or 11-P21/m-C2h2.55 Space group 13-P12/C1C2h4 monoclinic Lu2WO6 was determined by Zhang et al. with C

DOI: 10.1021/acs.inorgchem.7b02787 Inorg. Chem. XXXX, XXX, XXX−XXX

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and the spin down LS3, LS6 and LS9 were also unoccupied. Moreover, Sm 5d state located above CB with 6−13 eV range. When the input photon energy was larger than Eg = 3.60 eV, the electrons in VB, LS1, LS4, and LS7, jumped to the CB, LS2, LS5, and LS8, or empty bands with higher energy like Sm 5d orbits, generating excited or absorption phenomenon. Moreover, 4f5 electrons can also jump to LS3, LS6, and LS9 due to large Jahn−Teller effect described as vertical blue arrow line in Figure 7a. Thus, the absorption spectra of Lu2WO6:Sm3+ mainly contained host absorption band and Sm 4f → 4f or 4f → 5d absorption peaks. The excited electrons in CB relaxed the bottom of CB then recombined the VB holes producing wide band host luminescence. In addition, the electrons in LSm (m = 2, 5, 8, 3, 6, and 9) jumped directly to LSn (n = 1, 4, and 7) producing Sm3+ emission peaks. Theoretically, Sm ions’ red emission can be realized in Lu2WO6 host. The 5d → 4f transitions luminescence cannot happen due to Sm 5d positions above 6 eV. In order to further confirm the impossibility of 5d → 4f transition, the excited-state calculation was executed. Figure 4 displayed the energy-band decomposed charge density of Sm3+ 5d occupied bands. The (Sm3+)ES charge density localized around Sm atoms and tungsten atoms has also charge distribution, indicating delocalization of Sm3+ 5d excited state. When an electron was promoted from Sm3+ 4f orbits to Sm3+ 5d orbits, Sm3+ excited states were not located at band gaps. The Sm 5d excited electrons relaxed to CB bottom or 4f spindown or 4f spin-up empty levels, enhancing tungstates or Sm luminescence intensity. Therefore, Sm3+ 5d → 4f translations cannot emit light in Lu2WO6 under irradiation. The detailed electrons transitions were schematically drawn in Figure 7a. 3.2. Experiment Realization. For the sake of further confirming theory prediction, the pure and Sm doped Lu2WO6 were synthesized by solid-state reaction. The relation of crystallinity and luminescence performance was first investigated. In order to check the phase purity of the as-prepared samples calcined at 800, 900, 1000, 1100, and 1150 °C, XRD measurement results were plotted in Figure S3. When the calcined temperature was higher than 1000 °C, the XRD patterns agree well with the patterns of simulated ISCD number 161694, and no other phase peaks such as Lu6WO12 and Lu2W3O12 were observed. Moreover the chemical reaction of Lu2O3 and WO3 also generated Lu2WO6 below 900 °C, while some impurity phase peaks appeared in low diffraction

Figure 2. (a) Partial and total DOS. (b) Band structure of pure Lu2WO6 using HSE06 hybridization functional with 10% Hartree− Fock exchange terms.

compounds such as Sm 3+ and Sm 2+ doping LaSi 3 N 5 phosphors.58 In order to explore Sm luminescence behavior in Lu2WO6, the ground-state and excited-state calculations of Sm ions in Lu2WO6 were carried out. When a Sm3+ was doped into three Lu sites of Lu2WO6, the lattice parameters change were displayed in Table S1. Compared with the pure in experiment and theory, some lattice constants increased and others become smaller. The angles in Lu1Sm and Lu2Sm models were almost unchanged, and the γ angle in Lu3Sm model was not 90° possibly because the symmetry of Lu3 site was different from those of Lu1 and Lu2 sites. Different symmetry may induce different local lattice deformation. The volumes in three doping systems were larger than that of the pure in experiment due to difference of Sm and Lu ions radius. Through the PBE relaxation and HSE06 hybridization calculation, the ground DOS for three Lu2WO6:Sm3+ models were plotted in Figure 3. From Figure 3a−c, Sm doping does not change the bandgap values because of low-concentration doping, which was in good agreement with the experiment results as shown in Figure S2. An obvious local state appears in band gaps for three LukSm models and are mainly contributed by Sm3+ 4f state. Trivalent Sm possesses 4f5 electrons and spin up local states near VB edges in three models are occupied by 4f5 electrons such as LS1, LS4 and LS7. The deep local states with spin up inside band gap like LS2, LS5 and LS8 were empty

Figure 3. Ground DOS for Sm atoms in three Lu sites of monoclinic Lu2WO6 through the HSE06 hybridization function calculation. D

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Figure 4. Energy-band decomposed charge densities of (Sm3+)ES occupied bands in three Lu2WO6:Sm3+ models with isosurface values 0.01 e/Buhr3.

Figure 5. (a) PL emission and (b) excitation of pure Lu2WO6 powders calcined at different temperature in air atmosphere, where λex and λem denoted excitation wavelength and detection luminescence wavelengths, respectively.

rise, the corresponding peak sites shifted to long and short wave sides. The peak site of the charge transfer bands depended on the crystal field strength of W6+ surrounding O2−, W6+−O2− binding strength, the W6+−O2− bond lengths, and the oxygen coordination number of W6+.18,61 Therefore, the blue and red shift of charge transfer band might be ascribed to coordination environment change of W6+−O2− with calcination temperature. To further obtain the crystal parameters of the prepared samples calcined at 1150 °C, the structure refinement was performed through the GSAS software package as shown in Figure 6. The calculated patterns were in good agreement with the observed XRD patterns, and all the peaks in two patterns matched to the standard diffraction peaks sites. The resulting R (reliable factor) factor Rp, weighted R factor of profile Rwp and goodness of fit χ2 in Rietveld refinement were 7.22%, 9.15% and 3.381. The atomic positions, crystal parameters and unit cell volume were tabulated in Table 1. These parameters were consistent with that of the previous.40 The unit volume reduced slightly along with calcination temperature rising and all the atomic sites changed a little. The Lu2WO6 space group was 13P12/c1-C2h4; thus, the possible site symmetries have 4Ci(2), 2C2(2), and C1(4),62 where the first Arabic numerals indicate the occurrence times of the site symmetry and the Arabic numerals in the parentheses represent atom numbers in this symmetry site. Thus, 1 W and 6 Oi (i = 1−6) sites have C1 symmetry. Luk (k = 1 and 2) and Lu3 possess C2 and C1 symmetry, respectively. According to symmetry, it was found that each unit cell contains 4 W, 24 O, 2 Luk (k = 1 and 2), and

angles ( reff‑Lu3+, the sample volumes gradually become larger with the increase of Sm3+ content. The corresponding atomic coordinate sites and lattice constants changed a little. From occupancy number analysis, Sm 3+ entered into three Lu 3+ sites simultaneously under any Sm3+ concentration doping condition. At low Sm3+ content, Lu1 (2e) and Lu3 (4g) sites were occupied first, and Lu2 (2f) site was gradually occupied with increasing Sm3+ content, which was ascribed to different neighbors type of Sm3+ in three Lu sites.77 Different occupancy numbers ratio in three Lu sites brought about the concentration-dependent PL intensity. H

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Figure 11. (a) Correlation color temperature and (b) color coordinates of Lu2WO6:xSm3+ phosphors upon 325 nm UV excitation with A: x = 0.00, B: x = 0.002, C: x = 0.006, D: x = 0.010, E: x = 0.016, F: x = 0.020, G: x = 0.040, H: x = 0.060, I: x = 0.080, and J: x = 0.100.

mA. It presented 162.4 mW low consumption and low CCT = 5617 K which was superior to those of the Y3Al5O12-based commercial available white LED pumped by blue InGaN chip (CCT = 7756 K).78 Table S4 displayed the color rendering index (CRI) of this prepared white LED lamp. The corresponding average CRI was Ra = 87.0, better than Ra = 75 in Y3Al5O12-based LED. Though the Ra = 76 value related to red luminescence was almost equal to Ra (≈77) in the Y3Al5O12 LED device,79 it was tuned by changing bias current as shown in Table S4. Moreover, optimizing the ratio of Lu1.99Sm0.01WO6 powders and epoxy resin (called AB gules), LED chip performance like changing maximum excitation wavelength and LuWO 6 matrix luminescence properties, all the Lu2−xSmxWO6-based white LED performances can be further improved.

Under 325 nm UV light excitation, the white light parameters like CCT and color coordinates were derived from PL emission spectra by Color Coordinate software, which was displayed in Figure 11. In a microconcentration doping sample like Lu2WO6:0.002Sm3+, the CCT values increased quickly, then decreased fast. As x ≥ 0.006, the CCT values maintained basically stably. From the inset of Figure 11a, the CCT values happened minor changes. The resulting color coordinates changed from blue region like points A and B, through white region like points C−G, to red light region, like points H−J in visible spectra, by adjusting the Lu3+/Sm3+ ratio as shown in Figure 11b. At suitable doping concentrations, 0.006 ≤ x ≤ 0.04, Lu2WO6:xSm3+ samples can generate the white light emission. Therefore, the white LED lamp with excellent optical parameters was prepared using these phosphors and n-UV chip. In order to obtain the thermal stability of the phosphor, the emission spectra at different temperatures were measured as shown in Figure S5, The emission intensity changed a little, and the spectral shape remained almost the same when the test temperature was lower than 90 °C, indicating that the phosphor has relatively good thermal stability. Furthermore, the quantum efficiency of sample doped with 1% Sm3+ was measured as 50.92%. These advantages can provide the basis for Lu2−xSmxWO6−WLED application. Figure 12 displayed EL spectrum of the packaged LED device under a forward bias voltage 3.236 V and current 50.20

4. CONCLUSION Using the pure PBE optimization, the lattice parameters of pure and Sm doping monoclinic Lu2WO6 were in agreement with the experiment value. Furthermore, the band gaps 3.60 eV calculated by HSE06 method was consistent with the value derived from diffuse reflectance spectra. The ground-state calculation predicted that Sm3+ absorption can be ascribed intra 4f → 4f or 4f → 5d transitions. On the basis of the constrained density function calculation, Sm3+ emission can originated from 4f → 4f transition, not 5d → 4f transition, due to delocalization of 5d excited-state energy band decomposed charge density. Theoretically, Sm3+ in three asymmetry sites of Lu2WO6 can emit strong red luminescence. Experimentally, a series of pure and Sm3+ doping Lu2WO6 were synthesized through a simple solid-phase reaction. For pure samples, the high-temperature calcination in air atmosphere increased the Lu2WO6 crystallinity, resulting into strong PL excitation and emission intensity. Using the Rietveld refinement upon the sample calcined at 1150 °C, the obtained crystal parameters such atomic coordinates and lattice constants agreed well with that of No. 161649 in Inorganic Crystal Structure Database. Under 325 nm long-wave UV excitation, the red light luminescence intensity of Lu2WO6 was relatively weak and Sm3+ doping enhanced its intensity, thus combining the host and Sm3+ emission generated white luminescence. As the Sm3+ concentration x was less than 0.006 and larger than 0.06 in Lu2WO6:xSm3+ samples, the luminescence color located at blue and red-yellow regions, respectively. The white light emission was obtained as 0.01 ≤ x ≤ 0.04. The white LED lamp was prepared using 365 nm LED-chip to pump a single-phase Lu2WO6:0.01Sm3+

Figure 12. EL spectra of the prepared Lu1.99Sm0.01WO6-based phosphor-converted LED device was obtained under a 50 mA forward bias current, where the inset illustrated the digital photos of the assembled actual product without and with power input. I

DOI: 10.1021/acs.inorgchem.7b02787 Inorg. Chem. XXXX, XXX, XXX−XXX

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phosphor. The present strategy of theory guiding experiment can be extended to screen good activator in tens of thousands of hosts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02787. DOS and energy band of Lu2WO6 using pure PBE calculation, diffuse reflectance spectra and the fitted optical band gap, comparison of calculation and experiment crystal parameters, different calcined temperature samples XRD profiles, selected bond distances derived from Rietveld refinement, Rietveld refinement results of three Sm doping Lu2WO6 powders, atomic coordinated position of three Sm doping Lu2WO6 from structure refinement, full set of 8 CRIs and average Ra of Lu1.99Sm0.01WO6-based LED lamp with different bias current, normalized emission spectra of Lu1.99Sm0.01WO6 under different test temperatures and corresponding emission peaks intensity changes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Bangfu Ding: 0000-0002-0247-7609 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by Talents of High Level Scientific Research Foundation (No.1061-005050501) and Science and technology research project of Hebei colleges and Universities (No. ZD2017008). All the calculations were performed on TianHe-1(A) at National Supercomputer Center in Tianjin and the High-Performance Computing Center of Hebei University.

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REFERENCES

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