Article

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Controlling The Activator Site To Tune Valence in Oxyfluoride † † † ‡ § Kuan-Wei Huang, Wei-Ting Chen, Cheng-I Chu, Shu-Fen Hu, Hwo-Shuenn Sheu, § § † Bing-Ming Cheng, Jin-Ming Chen, and Ru-Shi Liu*, † Department of Chemistry, National Taiwan University, Taipei 106, Taiwan ‡ Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan § National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan

*S Supporting Information

ABSTRACT: A new Eu3+-activated oxyfluoride 3+ 3+ Ca12Al14O32F2:Eu (CAOF:Eu ) was synthesized by a solid state reaction. Commonly red line emission was detected in the range of 570−700 nm. To achieve the requirement of illumination, this study revealed a crystal chemistry approach to reduce Eu ions from 3+ to 2+ in the lattice. Replacing Al3+− F− by the appreciate Si4+−O2− is adopted to enlarge the activator site that enables Eu3+ to be reduced. The crystallization of samples was examined by powder X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Photoluminescence results indicated − that as-synthesized phosphors Ca12Al14‑zSizO32+zF2−z:Eu (z =0 0.5, CASOF:Eu) display an intense blue emission peaking at 440 nm that was produced by 4f−5d transition of Eu2+, along with the intrinsic emission of Eu3+ under UV excitation. Moreover, the effect of Si4+−O2− substitution involved in the coordination environment of the activator site was investigated by further crystallographic data from Rietveld refinements. The 19F solid-state nuclear magnetic resonance (NMR) data were in agreement with refinement and photoluminescence results. Furthermore, the valence states of Eu in the samples were analyzed with the X- ray absorption near edge structure (XANES). The quantity of substituted Si4+−O2− tunes chromaticity coordinates of Ca12Al14−zSizO32+zF2−z:Eu phosphors from (0.6101, 0.3513) for z = 0 to (0.1629, 0.0649) for z = 0.5, suggesting the potential for developing phosphors for white light emitting diodes (WLEDs). Using an activator that is valence tunable by controlling the size of the activator site represents a hitherto unreported structural motif for designing phosphors in phosphor converted light emitting diodes (pc-LEDs). KEYWORDS: phosphor, mixed valence, solid-state NMR, XANES, Rietveld refinement, crystal chemistry

■ INTRODUCTION the covalency and polarizability of activator−ligand bonds in 7−10 Light-emitting diodes (LEDs) have received wide attention in phosphors has received considerable attention. The 4+− 3− the recent decade owing to their high brightness, long lifetime, phenomenon was exhibited by incorporating Si N in 1−4 2+ material hardness, and environmental friendliness. The (Sr,Ba,Ca)Al2O4:Eu , subsequently leading a red shift in the − 3− conventional means of generating white light in white LEDs 4f 5d emission owing to the lower electronegativity of N 2− 7 4+− 3− 3+ is combining a phosphor layer with UV- or blue-LEDs that than O . Similarly, incorporating Si N into Ce doped 3+ converts the initial radiation into a complementary color. garnet phosphors leads to the low energy Ce emission band 8 Among all rare earth ions, Eu is the most commonly used and is applicable in warm white LED. However, changing the 3+ activator because both Eu2+ and Eu3+ can function as an valence state of activator in Eu -activated phosphors by emission center in the host lattice. Since the line emissions via modifying the coordination environment of an activator site has the 4f−4f parity-forbidden transition in Eu3+ activated scarcely been investigated. Clearly, developing an approach to phosphor leads to a low color rendering index (CRI) and reduce Eu3+ is an alternative means of designing phosphors, low efficiency, 4f−5d transitions in Eu2+, which produce which is of priority concern in phosphors-related research. intensely broad band photoluminescence, are more applicable The diverse particle size of different phosphors causes for LED-pumped white light (i.e., Ca−α−SiAlON and inhomogeneous suspension in epoxy resin, resulting in self- 2+ 5,6 Sr2SiO4:Eu ). However, the coordination environment and crystal site size determine the valence state of activator ions and Received: April 11, 2012 influence the photoluminescence properties of phosphors, Revised: May 17, 2012 explaining why dopant control of emission bands by modifying Published: May 21, 2012

© 2012 American Chemical Society 2220 dx.doi.org/10.1021/cm3011327 | Chem. Mater. 2012, 24, 2220−2227 Chemistry of Materials Article absorption and a complex packaging process. A single- X-ray absorption near edge structure (XANES) of Eu L3 edge was composition phosphor co-doped sensitizer and activator recorded with a wiggler beamline BL17C at NSRRC. Solid-state produces multiband emissions via an energy transfer mecha- nuclear magnetic resonance (NMR) spectra were acquired on a 500 − nism that can alleviate the above limitations.11 18 Those MHz Varian Unity Inova wide bore NMR spectrometer equipped with a 4 mm rotors. The Larmor frequencies for 19F and were 470.2 MHz. phosphors are seriously limited in adjusting photolumines- 19F chemical shifts were externally referenced to tetramethylsilane cence, and the energy is consumed during the transfer process. (TMS) at 0.0 ppm. Therefore, an alternative means, mixed valence activated phosphor, has been reported, where the optical combination ■ RESULTS AND DISCUSSION of different valences directly achieves white light.19,20 A notable Figure 1a shows the results of Rietveld refinement for example is LaAlO3:Eu, which exhibits white light emission by 3+ adopting the strategy of coexistence of Eu2+ and Eu3+.20 The Ca12Al14O32F2:Eu (CAOF:Eu) implemented with the crys- packaging process may therefore be simplified, demonstrating its potential applications in LED industry. Unfortunately, the mixed valence of europium appears only in a few host lattices; fewer examples are suitable for phosphors.21,22 Developing an approach for tuning the valence of europium obviously makes designing mixed valence Eu activated phosphors more feasible. To develop mixed valence europium phosphors, inserting Li into EuIII Zr (PO ) would reduce the valence state of Eu 0.33 2 4 3 − from 3+ to 2+, as in a previous study.23 26 Mixed valence europium in Eu0.33Zr2(PO4)3 shows white light emission by mixing both Eu2+ (blue) and Eu3+ (red) emission bands.26 fi However, the approach is speci c for a NaZr2(PO4)3-type structure, which can provide a vacant site for lithium occupation. This complex synthetic route and insertion process are difficult for practical LED-driven applications.23,24,26 In this work, we report an approach based on crystal chemistry that an appropriate dopant tunes the valence state of Eu via controlling the activator site in a novel phosphor 3+ 3+ Ca12Al14O32F2:Eu (CAOF:Eu ), demonstrating the transform feasibility of Eu3+-activated phosphor into Eu2+ or mix valence Eu activated phosphor. We also point out how the dopant affects the crystal structure, photoluminescence, and valence state of Eu in phosphors. The proposed approach overcomes the limitation of Eu3+ activated phosphors, and the results of this study significantly contribute to future research in designing phosphors.

■ EXPERIMENTAL SECTION − Synthesis. Ca12Al14−zSizO32+zF2−z:Eu (z = 0.1 0.5) powders were prepared by a solid-state reaction from CaCO3,Al2O3, SiO2, CaF2, and Eu2O3. For each compound, 1.2 g of starting materials were weighed out and mixed together in an agate mortar according to different values of z. The powder mixtures were then transferred to alumina crucibles, with subsequently firing at 1250 °C for 6 h in an electric tube furnace under a reducing atmosphere (N2/H2 = 95:5). After fi ring, the sample were gradually cooled to room temperature in the Figure 1. (a) Observed (crosses) and calculated (solid line) XRD furnace and ground into powder form for subsequent analysis. fi patterns of the Rietveld re nement of Ca11.9Al14O32F2:Eu0.1. Black Characterization. The crystal structure and phase purity of the as- fl ff λ vertical lines represent the position of Bragg re ection. The di erence synthesized samples were studied by using high energy ( = 0.774901 profile is plotted on the same scale in the bottom. (b) Crystal structure Å) XRD at a beamline BL01C2 of National Synchrotron Radiation fi of Ca11.9Al14O32F2 unit cell viewed in b-direction. (c) The coordination Research Center (NSRRC) in Hsinchu, Taiwan. Structural re ne- geometry of Ca2+ site in Ca Al O F . ments of X-ray diffractograms used the Rietveld method as 11.9 14 32 2 implemented in a general structure analysis system (GSAS).28 The crystal structures were also examined by high resolution transmission tallographic information files identified by previous reports.29,30 electron microscopy (HRTEM, JEM-2000EX, operating at 200 kV). The black crosses and red line depict the observed and Photoluminescence (PL) and PL of excitation (PLE) spectra were calculated patterns, respectively; the as-obtained goodness of fit recorded using a FluoroMax-3 spectrophotometer at room temper- χ2 parameter = 2.89 and Rwp (10.3%) can ensure the sample ature. The vacuum (VUV) PL and PLE spectra were phase purity. The compound exhibits a cubic crystal system obtained using a beamline BL03A at NSRRC. The PLE spectra were ̅ obtained by scanning a 6 m cylindrical grating monochromator with a with space group I43d, and its cell parameter is a = b = c = grating of 450 grooves/mm, which is capable of spanning wavelength 11.9937(5) Å, which matches the literature data (11.981 Å) − fi 30 range of 100 350 nm. A CaF2 plate was used as a lter to remove the reported by Qijun et al. Table 1 lists the crystallographic data high-order light from the synchrotron. Next, the PL spectra were of CAOF:Eu3+. Figure 1b presents the crystal structure of evaluated in a photon-counting mode with a 0.32 m monochromator. CAOF as viewed from [010]. Al3+ forms two kinds of

2221 dx.doi.org/10.1021/cm3011327 | Chem. Mater. 2012, 24, 2220−2227 Chemistry of Materials Article → Table 1. Crystallographic Data of Ca11.9Al14O32F2:Eu0.1,As F2p Eu4f were observed around 250 and 150 nm, Determined by the Rietveld Refinement of Power XRD Data respectively.36,37 Jørgensen has formulated an expression to a at Room Temperature estimate the CT band position:38 atom site xyzoccu. U (Å2) 31− σχ=−[opt (X) χ uncorr (M)]30 × 10 cm Ca1 24d 0.0975(3) 0 1/4 0.99 0.0397 Al1 12a 3/8 0 1/4 1.00 0.0117 where σ is the energy of the CT band and χ (X) is the optical Al2 16c 0.2312(3) 0.2312(3) 0.2312(3) 1.00 0.0283 opt electronegativity of the ligand ion, which is approximately the O1 16c 0.0630(4) 0.0630(4) 0.0630(4) 1.00 0.0269 ’ χ Pauling s electronegativity. uncorr(M) is the optical electro- O2 48e 0.1917(0) 0.2844(8) 0.0989(0) 1.00 0.0232 χ negativity of the central cation. With opt(O) = 3.2 and the F1 12b 1/4 1/8 1/2 0.33 0.0592 → energy of O2p Eu4f CT experimentally observed, the Eu1 24d 0.0975(3) 0 1/4 0.01 0.0397 χ uncorr(Eu) value is calculated to be 1.78, which is close to aSpace group: I43̅d (No. 220), Z =2,V = 1725.30(4) Å3, a = b = c = 39 → χ2 reported study. The F2p Eu4f CT band wavelength in 11.9937(5) Å, Rp = 7.64%, Rwp = 10.27%, = 2.89. CAOF lattice can therefore be estimated as 151 nm. In Figure S1 (Supporting Information), we measured the VUV excitation λ tetrahedra and builds up to an AlO4 ring. Three-dimensional spectrum ( em = 613 nm) from 300 to 125 nm, and a weakly frameworks of the CAOF structure are further formed by broad band is observed around 125−150 nm, close to the 2− 2+ → sharing the O between AlO4 rings. Notably, the Ca has one estimated value. The intensity of the F2p Eu4f CT band is 2− → − crystal site surrounded by AlO4 and coordinated by six O and weaker than O2p Eu4f one because the coordinated F ion is − 2− 2− one F ; all of the O is shared by AlO4 tetranedra. We can less than O in the Ca(Eu) site. Several weak peaks in the infer that Eu3+ replaces Ca2+ owing to the similar ionic radii range of 350 to 500 nm are related to the 4f−4f transitions of 27 between Ca2+ (7r = 1.06 Å) and Eu3+ (7r = 1.01 Å). We noted Eu3+ ions as shown in Figure S2 (Supporting Information). The that a slight excess of positive charge is formed due to the sharp emission lines within the red range under 236 nm 2+ 3+ ff 5 → 7 substitution of Ca by Eu . The structure o ers many excitation can be assigned to Judd-Ofelt transition ( D0 FJ) mechanisms to compensate this excess charge from Eu3+, the 3+ − − of Eu arising from electrical dipole (J =2 4) and magnetic most possible being slight off-stoichiometry between F and dipole transitions (J = ± 1) as depicted in Figure 2, indicating − 31 O2 . Moreover, according to the Kröger-Vink defect notation, that an efficient phonon-assisted process leads to the relaxation cation vacancies and oxygen interstitials are also possibly 3+ 5 − from charge transition state to Eu levels. Moreover, the D0 32 35 → 7 formed to maintain electrical neutrality. Figure 2 describes F2 emission peak at 613 nm is the strongest one, indicating the VUV excitation and emission spectra of CAOF:Eu3+ with that Eu3+ which occupies a site without inversion symmetry the schematic energy state. The VUV excitation spectra correlates well with the coordination environment of Ca2+ in − monitored at 613 nm reveals a broad band in 210−270 nm Figure 1c.40 42 However, Eu3+ activated phosphors are difficult → range. We tentatively assigned this band to O2p Eu4f CT to apply in pc-LED because line emission yields a rather low → band because the charge transfer (CT) band of O2p Eu4f and CRI and weak absorption in UV or blue-LED. Since Eu3+ cannot be reduced in CAOF:Eu structure under reducing atmosphere, we suggest that geometry is an important factor due to the following considerations. When the local structure of Eu3+ is considered, Eu2+ (7r = 1.2 Å) has a larger size than Ca2+ (7r = 1.06 Å). Moreover, the Ca2+ site is surrounded compactly by AlO4 tetrahedra on one side, leading to a distorted coordination environment of Ca2+ as shown in Figure 3. A previous study incorporated Si4+−N3− in structure, in which the red shift of 5d energy position of activators was attributed to a higher covalency and polarizability of activator− N3− bonds versus activator−O2− bonds. Although the replacement of Al3+ by Si4+ is normally attributed to charge 7−10 compensation, the shrinkage of AlO4 tetrahedra by smaller Si4+ substitution has seldom been discussed. While attempting to expand the compact side of the Ca2+ site, this study introduced Si4+−O2− into the CAOF structure to replace Al3+− − F resulting in Ca12Al14−zSizO32+zF2−z:Eu (CASOF:Eu), and the substitution of Al3+ by Si4+ is herein assumed to shrink the 4+ 3+ AlO4 tetrahedra; because Si has a smaller radius than Al and O2−, replacing F− can achieve charge compensation in the whole structure. Figure 4 shows the synchrotron X-ray powder ff di raction patterns of the Ca12Al14−zSizO32+zF2−z:Eu with increasing z, which matched with Joint Committee on Powder Diffraction Standards (JCPDS) card No. 00-070-1353. According to the XRD results, Ca12Al14−zSizO32+zF2−z (CASOF) solid solution is formed up to z = 0.5. The fine structure of CASOF:Eu (z = 0.5) is further examined by high λ λ Figure 2. VUV excitation ( em = 613 nm, blue part) and emission ( ex resolution transmission electron microscopy (HRTEM) as = 236 nm, red part) spectrum with schematic energy state of Eu3+. shown in Figure 5. Figure 5b shows the related selected area

2222 dx.doi.org/10.1021/cm3011327 | Chem. Mater. 2012, 24, 2220−2227 Chemistry of Materials Article

Figure 5. (a) TEM image of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0.5), (b) corresponding SAED pattern along the [110] zone axis, and (c) HRTEM images of the selected area in (a).

To investigate how Si4+−O2− substitution affects the CAOF:Eu3+ crystal structure, we performed Rietveld refine- ment with GSAS program to obtain more detailed information. Figure S3 (Supporting Information) plots the experimental, calculated, and difference results from the refinements of the CASOF samples, in which all of the observed peaks consist Figure 3. Coordination environment of the Ca2+ site in the with the Bragg reflections that verify the formation of a single Ca11.9Al14O32F2:Eu0.1 structure. phase. During the refinement procedure, the occupancy parameters of all atoms are referenced by stoichiometry; in addition, the temperature factors are fixed for all substituted ions. Table 2 summarizes lattice parameters and reliability factors of CASOF:Eu (z =0−0.5) samples that crystallized in a cubic structure with space group I43̅d. A gradual change in the lattice parameter with increasing z indicates that CASOF:Eu (z =0−0.5) solid solutions are formed. Although the radius of Si4+ (4r = 0.26 Å) smaller than Al3+ (4r = 0.39 Å) appears to shrink three-dimensional frameworks, according to Vegard’s rule,45 a larger O2− (2r = 1.35 Å) occupying the F− (2r = 1.285 Å) site causes a slight increase in the lattice as shown in Figure 6a. The unit cell expansion by Si4+−O2− replacing Al3+−F− is also observed by Im et al.46 To elucidate the site feature of Ca2+, the effect of Si4+−O2− incorporation involved in the Ca2+ site can be analyzed by the bond lengths of (Al,Si)−O and Ca−O. Figure 6b,c plots the average bond lengths of (Al,Si)−O and Ca−O as obtained by the refinement results. The (Al,Si)−O 4+ 2− Figure 4. Powder XRD patterns of Ca12Al14−zSizO32+zF2−z:Eu0.1 (z = bond length decreases with increasing amount of Si −O .It − 0 0.5), which are compared with JCPDs Card. can be assigned to the Si4+ replacement of Al3+. Consequently, the Ca−O bond length is further elongated due to shrinking of 4+ electron diffraction (SAED) pattern, in which diffraction spots the (Al,Si)O4 tetrahedra when incorporating Si ,thus loosening the crystal site of Ca2+. The Ca−(F,O) length is correspond to the [110] zone axis of cubic structure with space − ̅ also elongated by incorporating Si4+−O2 as shown in Figure group I43d. This result reveals that the particle has a single − crystal structure. Figure 5c displays the HRTEM image of the 6d. However, because the F originally occupied in a cage-like selected area. In the SAED pattern, the d-spacing can be site is surrounded by the AlO4 framework and coordinated to 2+ 29 2− 2+ calculated by the following equation:43,44 Ca , the incorporated O generates another Ca site in a crystal, which is coordinated to seven O2− ions. λ ×=×LdR Adding Si4+−O2− affects both the crystal structure and the where λ is the wavelength of TEM accelerating voltage; L is the photoluminescence properties. Figure 7 illustrates the indis- camera length; and R is the measured distance of the spots. The pensable effect of incorporating Si4+−O2− in the CASOF host d-spacing of indexed spots (220)̅ and (112)̅ are calculated to be lattice. Figure 7a displays the emission spectra of CASOF with 4.35 Å and 4.91 Å, respectively, which are constituent with the z = 0.0−0.5 at room temperature under 254 nm excitation. measured distance in Figure 5c, and correlate well to the Besides the intrinsic line emission of Eu3+ within the red range theoretical value of 4.24 Å for (220)̅ and 4.89 Å for (112).̅ as discussed above, a surprising observation which is an

2223 dx.doi.org/10.1021/cm3011327 | Chem. Mater. 2012, 24, 2220−2227 Chemistry of Materials Article z − a Table 2. Crystallographic Data and Reliability Factor of Ca12Al14−zSizO32+zF2−z:Eu0.1 ( =0 0.5) Phosphors z =0 z = 0.1 z = 0.2 z = 0.3 z = 0.4 z = 0.5 a = b = c 11.9937(2) 11.9944(1) 11.9978(2) 11.9965(1) 11.9967(2) 11.9973(2) cell volume 1725.30(4) 1725.58(4) 1727.05(4) 1726.51(4) 1726.61(5) 1726.84(5) Reliability Factor χ2 2.898 2.911 3.176 3.199 3.59 4.295

Rwp 10.27 9.97% 10.02% 9.57% 10.21% 11.19%

Rp 7.64 7.35% 7.44% 7.18% 7.47% 8.06% aCrystal system: cubic, space group: I43̅d (No. 220).

Ca11.9Al13.5Si0.5O32.5F1.5:Eu0.1 under 334 nm excitation is plotted in Figure S5 (Supporting Information). In the excitation spectra in Figure 7c monitored at 613 nm, which is produced 5 → 7 3+ by D0 F2 of Eu , a peak appears around 234 nm, which → can be assigned to O2p Eu4f charge transfer band. As mentioned above, a decreased intensity with an increasing z is also compatible with the results in Figure 7a. Since the replacement of F− by O2− implies the variation of first coordination layer of the activation site, the asymmetric emission spectra in Figure 7a can be deconvoluted into two Gaussian components at 440 and 473 nm, revealing that Eu2+ has two centers in the CASOF lattice. Features of the emission position are discussed via the change in the Eu2+−ligand covalency. Previous investigations have studied the config- uration of Eu2+ with the anion polarizability and cation − electronegativity in various hosts.47 50 Two factors influence the energy position of the 5d band: gravity shift and crystal field splitting. Gravity shift is associated with the nephelauxetic effect caused by the interaction between cation and electron cloudy of ligands. The [6O1F]Ca site is original coordinated with six oxygen atoms and one fluorine atom. Due to the replacement of F− by O2−, the first coordination layer of the [6O1F]Ca site is changed to a [7O]Ca site. The two distinct Eu2+ emissions are therefore observed. Because the incorporated ligand O2− with a lower electronegativity than that of F− would lower the energy Figure 6. (a) Lattice constant of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 with 1 different amounts of (z =0−0.5). The average bond length of of the 5d level, the shoulder emission band with a long − − − wavelength at 473 nm can thus be assigned to the [7O]Eu2+ due (b) Al O, (c) Ca O, and (d) Ca FofCa11.9Al14−zSizO32+zF2−z:Eu0.1 (z =0−0.5). to the gravity shift and crystal field splitting. The main factor for changing photoluminescence respects the first coordination 3+ 4+ apperence of a broadband peak at 440 nm with increasing z,it layer activator, so the second layer of Al /Si replacement can be assigned to 5d → 4f emission of Eu2+. Interestingly, the with negligible influence in photoluminescence just changes the emission of Eu3+ within 570−700 nm and broadband at 440 nm steric structure. We can also conclude that replacing Al3+ by Si4+ disappear and emerge simultaneously. This result can be cannot influenced the first coordination sphere; it mainly therefore attributed to the increase of z value, suggesting that changes the crystal structure as the results in XRD refinement. Eu3+ is transformed to Eu2+ in the lattice. With regard to the During valence transfer of Eu, the CIE coordinates upon 254 tendency in refined bond length of Al−O, Ca−O, and Ca−F, nm excitation of CASOF:Eu are regularly shifted from (0.6101, the expanded site of Ca2+ could be demonstrated by 0.3513) to (0.1629, 0.0649) in relation to an increasing value of substitution of Al3+−F− by Si4+−O2−.Eu3+ can therefore be z as depicted in Figure 7d, and the inset schematically depicts reduced to Eu2+ in the CASOF lattice. The excitation spectra of the proposed crystal variation and photographs of each − Ca12Al14−zSizO32+zF2−z:Eu (z =00.5) in Figure 7b by composition irradiated under a 254 nm UV lamp. The monitoring 440 nm reveals a broad peak from 250 to 410 chromaticity coordinates of CASOF:Eu (z =0−0.5) are nm, which can be ascribed to 4f−5d transition of Eu2+. The summarized in Table 3. peak intensity gradually increases with increasing value of z, Solid-state NMR measurement, which is atom specific and which correlates well with the peak intensity of Eu2+ in Figure sensitive to the local order around the nucleus, can be 7a. The emission spectra upon excitation of 334 nm shows blue performed to complete the crystal chemistry experiments − − luminescence of the CASOF phosphors, and the intensity is beyond XRD analysis.51 54 Owing to the fact that F is correlated to the increasing level of Si4+−O2− incorporation, as coordinated with Ca2+, this study performs 19F solid state NMR − shown in Figure S4 (Supporting Information). Interestingly, no to further investigate how incorporating Si4+−O2 affects the red line luminescence is detected upon 334 nm excitation that Ca2+ site. Figure 8 describes the 19F NMR spectra, in which the is consonant with no absorption of Eu3+ in this range as shown peak area correlates well with the amount of F in study samples. in Figure S2 (Supporting Information). The Commission Only one peak is obtained in CAOF (z =0)at−120 ppm, − International de I’Eclairage (CIE) chromaticity coordinate of which can be assigned to the ligand atom to Ca.55 57 However,

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λ λ λ Figure 7. (a) VUV emission spectra ( ex = 254 nm), (b) PL excitation spectra ( em = 440 nm), (c) VUV excitation spectra ( em = 613 nm) of − Ca12Al14−zSizO32+zF2−z:Eu0.1 (z =00.5), and (d) dependence of the CIE chromaticity coordinates on varying z value in − 4+− Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z =0 0.5) upon 254 nm excitation. The inset shows the schematic variation of the activator site driven by Si O2− incorporation and the irridated phosphor images of each composition under a 254 nm UV lamp.

Table 3. CIE Chromaticity Coordinates of corresponding to the transition to the unoccupied 5d z − 58−61 ff Ca12Al14−zSizO32+zF2−z:Eu0.1 ( =0 0.5) states. The energy di erence originates from the shielding of the nuclear potential through an additional 4f electron in CIE chromaticity Eu2+, subsequently lowering the binding energy of the − Si4+−O2 con. (z) xyrespective core electrons.54 To further examine two valence 0 0.6101 0.3513 states of Eu in the CASOF samples, XANES was performed 2+ 0.1 0.4920 0.2954 near Eu L3 edge with BaMgAl10O17:Eu and Eu2O3 as 0.2 0.2873 0.1653 reference for Eu2+ and Eu3+, respectively.58 The normalized 0.3 0.2024 0.0929 Eu L3 edge XANES spectra of the study CASOF reveal two 0.4 0.1705 0.0713 peaks at 6975 and 6983 eV, which are attributed to the electron → 2+ 3+ 0.5 0.1629 0.0649 transition of 2p3/2 5d in Eu and Eu , respectively. This finding suggests that two valence states of Eu coexist in the a shoulder peak appears at −173 ppm in z = 0.1, 0.3, and 0.5. CASOF samples. The relative intensities of absorption by Eu2+ at 6975 eV and Eu3+ at 6983 eV systematically increase and The NMR spectra are decovoluted into two peaks in the − bottom of Figure 8, in which the peaks appearing at −120 ppm decrease, which correlates with the amount of Si4+−O2 and −173 ppm decrease and increase in correlation with the incorporated. Owing to that the sum of the two peaks are level of Si4+−O2− incorporation and the peak area ratio of nearly constants, the area ratio can be treated as the ratio of the 62−64 I(−173 ppm)/I(−120 ppm) correlated well with the level of amount of Eu2+ and Eu3+, as shown in Figure 9b. The 4+− 2− increasing ratio of Eu2+/Eu3+ and the emission intensity of Eu2+ Si O incorporation as shown in Figure 8b. The peak at − −173 ppm can be attributed to F−, which is coordinated with display a similar trend for the increased level of Si4+−O2 the loose Ca2+ site, capable of maintaining higher electron incorporation, demonstrating that the luminescent enhance- density at F− than the original site, resulting in an upfield in ment results mainly from the increasing number of Eu2+. This chemical shift.47 The results imply the average Ca−F bond is observation also corresponds to our results in crystal structure elongated by incorporating Si4+−O2, which is supported by studies and solid state NMR. Rietveld refinement, indicating the increased amount of loose Ca2+ site which is suitable for Eu2+ occupation. ■ CONCLUSIONS XANES spectroscopy can easily identify that two valence In summary, we have revealed that by appropriate dopant states of Eu2+(4f7) and Eu3+(4f6), due to the different threshold incorporation, the valence state of Eu3+ can be tuned to Eu2+ in energies around 8 eV of their white light (WL) resonance, phosphors due to the enlargement of the activator site.

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Effectiveness of the proposed approach is demonstrated in the 3+ 4+− 2− new phosphor Ca12Al14O32F2:Eu ;Si O are incorporated to substituted Al3+−F− to release the geometry restriction of the activator site. Combinatorial studies with synchrotron XRD refinement, XANES, HRTEM, and solid state NMR help us to understand how the dopant affects the crystal structure and photoluminescence. The average bond lengths of Al−O and Ca−O obtained in refinement are systematically shortened and elongated, respectively, indicating that the enlargement of activator site corresponds to the amount of Si4+−O2−. Incorporating Si4+−O2− in CAOF:Eu3+ phosphor leads to a rise in broadband emission at 440 nm that can be ascribed to the 4f−5d transition of Eu2+. The emission intensity of Eu2+ and Eu3+ increases and decreases systematically with the amount of dopant. XANES results further confirm that Eu3+ is transferred to Eu2+ by incorporating Si4+−O2−. The proposed approach is highly promising for applications involving Eu3+ phosphors. Overcoming the limitations of Eu3+ activated phosphor via valence transfer, the broadband feature and efficient radiation in Eu2+ based phosphor are more useful in illumination. The proposed approach is also characterized by the fact that only a single activator, Eu, generates the multiband, even a white light by optical combination of different valences of europium. This approach does not just limited in present study materials but could be general to other related Eu system. Thus, our approach may lead to opportunities for more successful development of phosphors in LED applications. ■ ASSOCIATED CONTENT Figure 8. (a) 19F solid state NMR spectra of *S Supporting Information Ca11.9Al14−zSizO32+zF2−z:Eu0.1 with z = 0, 0.1, 0.3, 0.5. The bottom Figures S1 and S2 show the VUV excitation and photo- represents the deconvolution components used for the spectral luminescence excitation measurements of Ca11.9Al14O32F2:Eu0.1. analysis. (b) The ratio of the deconvoluted peak area at −173 ppm and Figure S3 shows Rietveld refinement of synchrotron PXRD −120 ppm in 19F NMR, which is a function of the amount of Si4+− − 2− data of CASOF samples (z = 0.1 0.5). Figures S4 shows the O incorporation. photoluminescence emission spectra of CASOF samples. Figure S5 plots the CIE chromaticity coordinate of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z =0.5)under334nm excitation. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 97-2113-M-002-012-MY3 and NSC 97-3114-M-002. ■ REFERENCES (1) Nakamura, S.; Fasol, G. The Blue Laser Diode; Springer: Berlin, 1997. (2) Hashimoto, T.; Wu, F.; Speck, J. S.; Nakamura, S. Nat. Mater. 2007, 6, 568−571. Figure 9. (a) Normalized Eu L3-edge XANES spectra of − (3) Pimputkar, S.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Nat. Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z =00.5). (b) Dependence of − − 2+ 3+ 4+− 2 Photonics 2009, 3, 180 182. IEu /Eu on the amount of Si O incorporation in − (4) Schubert, E. F.; Kim, J. K. Science 2005, 308, 1274−1278. Ca11.9Al14−zSizO32+zF2−z:Eu0.1 (z =0 0.5). (5) Xie, R. J.; Hirosaki, N.; Sakuma, K.; Yamamoto, Y.; Mitomo, M. Appl. Phys. Lett. 2004, 84, 5404−5406.

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