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Controlling The Activator Site To Tune Europium Valence inOxyfluoride PhosphorsKuan-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 phosphorCa12Al14O32F2:Eu

3+ (CAOF:Eu3+) was synthesized by a solidstate reaction. Commonly red line emission was detected inthe range of 570−700 nm. To achieve the requirement ofillumination, this study revealed a crystal chemistry approachto reduce Eu ions from 3+ to 2+ in the lattice. Replacing Al3+−F− by the appreciate dopant Si4+−O2− is adopted to enlargethe activator site that enables Eu3+ to be reduced. Thecrystallization of samples was examined by powder X-raydiffraction (XRD) and high resolution transmission electronmicroscopy (HRTEM). Photoluminescence results indicatedthat as-synthesized phosphors Ca12Al14‑zSizO32+zF2−z:Eu (z = 0−0.5, CASOF:Eu) display an intense blue emission peaking at 440nm that was produced by 4f−5d transition of Eu2+, along with the intrinsic emission of Eu3+ under UV excitation. Moreover, theeffect of Si4+−O2− substitution involved in the coordination environment of the activator site was investigated by furthercrystallographic data from Rietveld refinements. The 19F solid-state nuclear magnetic resonance (NMR) data were in agreementwith 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 ofCa12Al14−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 fordeveloping phosphors for white light emitting diodes (WLEDs). Using an activator that is valence tunable by controlling the sizeof the activator site represents a hitherto unreported structural motif for designing phosphors in phosphor converted lightemitting diodes (pc-LEDs).

KEYWORDS: phosphor, mixed valence, solid-state NMR, XANES, Rietveld refinement, crystal chemistry

■ INTRODUCTIONLight-emitting diodes (LEDs) have received wide attention inthe recent decade owing to their high brightness, long lifetime,material hardness, and environmental friendliness.1−4 Theconventional means of generating white light in white LEDsis combining a phosphor layer with UV- or blue-LEDs thatconverts the initial radiation into a complementary color.Among all rare earth ions, Eu is the most commonly usedactivator because both Eu2+ and Eu3+ can function as anemission center in the host lattice. Since the line emissions viathe 4f−4f parity-forbidden transition in Eu3+ activatedphosphor leads to a low color rendering index (CRI) andlow efficiency, 4f−5d transitions in Eu2+, which produceintensely broad band photoluminescence, are more applicablefor LED-pumped white light (i.e., Ca−α−SiAlON andSr2SiO4:Eu

2+).5,6 However, the coordination environment andcrystal site size determine the valence state of activator ions andinfluence the photoluminescence properties of phosphors,explaining why dopant control of emission bands by modifying

the covalency and polarizability of activator−ligand bonds inphosphors has received considerable attention.7−10 Thephenomenon was exhibited by incorporating Si4+−N3− in(Sr,Ba,Ca)Al2O4:Eu

2+, subsequently leading a red shift in the4f−5d emission owing to the lower electronegativity of N3−than O2−.7 Similarly, incorporating Si4+−N3− into Ce3+ dopedgarnet phosphors leads to the low energy Ce3+ emission bandand is applicable in warm white LED.8 However, changing thevalence state of activator in Eu3+-activated phosphors bymodifying the coordination environment of an activator site hasscarcely been investigated. Clearly, developing an approach toreduce Eu3+ is an alternative means of designing phosphors,which is of priority concern in phosphors-related research.The diverse particle size of different phosphors causes

inhomogeneous suspension in epoxy resin, resulting in self-

Received: April 11, 2012Revised: May 17, 2012Published: May 21, 2012

Article

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© 2012 American Chemical Society 2220 dx.doi.org/10.1021/cm3011327 | Chem. Mater. 2012, 24, 2220−2227

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absorption and a complex packaging process. A single-composition phosphor co-doped sensitizer and activatorproduces multiband emissions via an energy transfer mecha-nism that can alleviate the above limitations.11−18 Thosephosphors are seriously limited in adjusting photolumines-cence, and the energy is consumed during the transfer process.Therefore, an alternative means, mixed valence activatedphosphor, has been reported, where the optical combinationof different valences directly achieves white light.19,20 A notableexample is LaAlO3:Eu, which exhibits white light emission byadopting the strategy of coexistence of Eu2+ and Eu3+.20 Thepackaging process may therefore be simplified, demonstratingits potential applications in LED industry. Unfortunately, themixed valence of europium appears only in a few host lattices;fewer examples are suitable for phosphors.21,22 Developing anapproach for tuning the valence of europium obviously makesdesigning mixed valence Eu activated phosphors more feasible.To develop mixed valence europium phosphors, inserting Li

into EuIII0.33Zr2(PO4)3 would reduce the valence state of Eufrom 3+ to 2+, as in a previous study.23−26 Mixed valenceeuropium in Eu0.33Zr2(PO4)3 shows white light emission bymixing both Eu2+ (blue) and Eu3+ (red) emission bands.26

However, the approach is specific for a NaZr2(PO4)3-typestructure, which can provide a vacant site for lithiumoccupation. This complex synthetic route and insertion processare difficult for practical LED-driven applications.23,24,26

In this work, we report an approach based on crystalchemistry that an appropriate dopant tunes the valence state ofEu via controlling the activator site in a novel phosphorCa12Al14O32F2:Eu

3+(CAOF:Eu3+), demonstrating the transformfeasibility of Eu3+-activated phosphor into Eu2+ or mix valenceEu activated phosphor. We also point out how the dopantaffects the crystal structure, photoluminescence, and valencestate of Eu in phosphors. The proposed approach overcomesthe limitation of Eu3+ activated phosphors, and the results ofthis study significantly contribute to future research indesigning phosphors.

■ EXPERIMENTAL SECTIONSynthesis. Ca12Al14−zSizO32+zF2−z:Eu (z = 0.1−0.5) powders were

prepared by a solid-state reaction from CaCO3, Al2O3, SiO2, CaF2, andEu2O3. For each compound, 1.2 g of starting materials were weighedout and mixed together in an agate mortar according to differentvalues of z. The powder mixtures were then transferred to aluminacrucibles, with subsequently firing at 1250 °C for 6 h in an electrictube furnace under a reducing atmosphere (N2/H2 = 95:5). Afterfiring, the sample were gradually cooled to room temperature in thefurnace and ground into powder form for subsequent analysis.Characterization. The crystal structure and phase purity of the as-

synthesized samples were studied by using high energy (λ = 0.774901Å) XRD at a beamline BL01C2 of National Synchrotron RadiationResearch Center (NSRRC) in Hsinchu, Taiwan. Structural refine-ments of X-ray diffractograms used the Rietveld method asimplemented in a general structure analysis system (GSAS).28 Thecrystal structures were also examined by high resolution transmissionelectron microscopy (HRTEM, JEM-2000EX, operating at 200 kV).Photoluminescence (PL) and PL of excitation (PLE) spectra wererecorded using a FluoroMax-3 spectrophotometer at room temper-ature. The vacuum ultraviolet (VUV) PL and PLE spectra wereobtained using a beamline BL03A at NSRRC. The PLE spectra wereobtained by scanning a 6 m cylindrical grating monochromator with agrating of 450 grooves/mm, which is capable of spanning wavelengthrange of 100−350 nm. A CaF2 plate was used as a filter to remove thehigh-order light from the synchrotron. Next, the PL spectra wereevaluated in a photon-counting mode with a 0.32 m monochromator.

X-ray absorption near edge structure (XANES) of Eu L3 edge wasrecorded with a wiggler beamline BL17C at NSRRC. Solid-statenuclear magnetic resonance (NMR) spectra were acquired on a 500MHz Varian Unity Inova wide bore NMR spectrometer equipped witha 4 mm rotors. The Larmor frequencies for 19F and were 470.2 MHz.19F chemical shifts were externally referenced to tetramethylsilane(TMS) at 0.0 ppm.

■ RESULTS AND DISCUSSIONFigure 1a shows the results of Rietveld refinement forCa12Al14O32F2:Eu

3+ (CAOF:Eu) implemented with the crys-

tallographic information files identified by previous reports.29,30

The black crosses and red line depict the observed andcalculated patterns, respectively; the as-obtained goodness of fitparameter χ2 = 2.89 and Rwp (10.3%) can ensure the samplephase purity. The compound exhibits a cubic crystal systemwith space group I4̅3d, and its cell parameter is a = b = c =11.9937(5) Å, which matches the literature data (11.981 Å)reported by Qijun et al.30 Table 1 lists the crystallographic dataof CAOF:Eu3+. Figure 1b presents the crystal structure ofCAOF as viewed from [010]. Al3+ forms two kinds of

Figure 1. (a) Observed (crosses) and calculated (solid line) XRDpatterns of the Rietveld refinement of Ca11.9Al14O32F2:Eu0.1. Blackvertical lines represent the position of Bragg reflection. The differenceprofile is plotted on the same scale in the bottom. (b) Crystal structureof Ca11.9Al14O32F2 unit cell viewed in b-direction. (c) The coordinationgeometry of Ca2+ site in Ca11.9Al14O32F2.

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tetrahedra and builds up to an AlO4 ring. Three-dimensionalframeworks of the CAOF structure are further formed bysharing the O2− between AlO4 rings. Notably, the Ca

2+ has onecrystal site surrounded by AlO4 and coordinated by six O

2− andone F−; all of the O2− is shared by AlO4 tetranedra. We caninfer that Eu3+ replaces Ca2+ owing to the similar ionic radiibetween Ca2+ (7r = 1.06 Å) and Eu3+ (7r = 1.01 Å).27 We notedthat a slight excess of positive charge is formed due to thesubstitution of Ca2+ by Eu3+. The structure offers manymechanisms to compensate this excess charge from Eu3+, themost possible being slight off-stoichiometry between F− andO2−.31 Moreover, according to the Kröger-Vink defect notation,cation vacancies and oxygen interstitials are also possiblyformed to maintain electrical neutrality.32−35 Figure 2 describesthe VUV excitation and emission spectra of CAOF:Eu3+ withthe schematic energy state. The VUV excitation spectramonitored at 613 nm reveals a broad band in 210−270 nmrange. We tentatively assigned this band to O2p → Eu4f CTband because the charge transfer (CT) band of O2p → Eu4f and

F2p → Eu4f were observed around 250 and 150 nm,respectively.36,37 Jørgensen has formulated an expression toestimate the CT band position:38

σ χ χ= − × −[ (X) (M)]30 10 cmopt uncorr3 1

where σ is the energy of the CT band and χopt(X) is the opticalelectronegativity of the ligand ion, which is approximately thePauling’s electronegativity. χuncorr(M) is the optical electro-negativity of the central cation. With χopt(O) = 3.2 and theenergy of O2p → Eu4f CT experimentally observed, theχuncorr(Eu) value is calculated to be 1.78, which is close toreported study.39 The F2p → Eu4f CT band wavelength inCAOF lattice can therefore be estimated as 151 nm. In FigureS1 (Supporting Information), we measured the VUV excitationspectrum (λem = 613 nm) from 300 to 125 nm, and a weaklybroad band is observed around 125−150 nm, close to theestimated value. The intensity of the F2p → Eu4f CT band isweaker than O2p → Eu4f one because the coordinated F

− ion isless than O2− in the Ca(Eu) site. Several weak peaks in therange of 350 to 500 nm are related to the 4f−4f transitions ofEu3+ ions as shown in Figure S2 (Supporting Information). Thesharp emission lines within the red range under 236 nmexcitation can be assigned to Judd-Ofelt transition (5D0 →

7FJ)of Eu3+ arising from electrical dipole (J = 2−4) and magneticdipole transitions (J = ± 1) as depicted in Figure 2, indicatingthat an efficient phonon-assisted process leads to the relaxationfrom charge transition state to Eu3+ levels. Moreover, the 5D0→ 7F2 emission peak at 613 nm is the strongest one, indicatingthat Eu3+ which occupies a site without inversion symmetrycorrelates well with the coordination environment of Ca2+ inFigure 1c.40−42 However, Eu3+ activated phosphors are difficultto apply in pc-LED because line emission yields a rather lowCRI 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 importantfactor due to the following considerations. When the localstructure of Eu3+ is considered, Eu2+ (7r = 1.2 Å) has a largersize than Ca2+ (7r = 1.06 Å). Moreover, the Ca2+ site issurrounded compactly by AlO4 tetrahedra on one side, leadingto a distorted coordination environment of Ca2+ as shown inFigure 3. A previous study incorporated Si4+−N3− in structure,in which the red shift of 5d energy position of activators wasattributed to a higher covalency and polarizability of activator−N3− bonds versus activator−O2− bonds. Although thereplacement of Al3+ by Si4+ is normally attributed to chargecompensation,7−10 the shrinkage of AlO4 tetrahedra by smallerSi4+ substitution has seldom been discussed. While attemptingto expand the compact side of the Ca2+ site, this studyintroduced Si4+−O2− into the CAOF structure to replace Al3+−F− resulting in Ca12Al14−zSizO32+zF2−z:Eu (CASOF:Eu), andthe substitution of Al3+ by Si4+ is herein assumed to shrink theAlO4 tetrahedra; because Si

4+ has a smaller radius than Al3+ andO2−, replacing F− can achieve charge compensation in thewhole structure. Figure 4 shows the synchrotron X-ray powderdiffraction patterns of the Ca12Al14−zSizO32+zF2−z:Eu withincreasing z, which matched with Joint Committee on PowderDiffraction 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 finestructure of CASOF:Eu (z = 0.5) is further examined by highresolution transmission electron microscopy (HRTEM) asshown in Figure 5. Figure 5b shows the related selected area

Table 1. Crystallographic Data of Ca11.9Al14O32F2:Eu0.1, AsDetermined by the Rietveld Refinement of Power XRD Dataat Room Temperaturea

atom site x y z occu. U (Å2)

Ca1 24d 0.0975(3) 0 1/4 0.99 0.0397Al1 12a 3/8 0 1/4 1.00 0.0117Al2 16c 0.2312(3) 0.2312(3) 0.2312(3) 1.00 0.0283O1 16c 0.0630(4) 0.0630(4) 0.0630(4) 1.00 0.0269O2 48e 0.1917(0) 0.2844(8) 0.0989(0) 1.00 0.0232F1 12b 1/4 1/8 1/2 0.33 0.0592Eu1 24d 0.0975(3) 0 1/4 0.01 0.0397

aSpace group: I4̅3d (No. 220), Z = 2, V = 1725.30(4) Å3, a = b = c =11.9937(5) Å, Rp = 7.64%, Rwp = 10.27%, χ

2 = 2.89.

Figure 2. VUV excitation (λem = 613 nm, blue part) and emission (λex= 236 nm, red part) spectrum with schematic energy state of Eu3+.



electron diffraction (SAED) pattern, in which diffraction spotscorrespond to the [110] zone axis of cubic structure with spacegroup I4̅3d. This result reveals that the particle has a singlecrystal structure. Figure 5c displays the HRTEM image of theselected area. In the SAED pattern, the d-spacing can becalculated by the following equation:43,44

λ × = ×L d Rwhere λ is the wavelength of TEM accelerating voltage; L is thecamera length; and R is the measured distance of the spots. Thed-spacing of indexed spots (2̅20) and (11 ̅2) are calculated to be4.35 Å and 4.91 Å, respectively, which are constituent with themeasured distance in Figure 5c, and correlate well to thetheoretical value of 4.24 Å for (2 ̅20) and 4.89 Å for (11 ̅2).

To investigate how Si4+−O2− substitution affects theCAOF: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 theCASOF samples, in which all of the observed peaks consistwith the Bragg reflections that verify the formation of a singlephase. During the refinement procedure, the occupancyparameters of all atoms are referenced by stoichiometry; inaddition, the temperature factors are fixed for all substitutedions. Table 2 summarizes lattice parameters and reliabilityfactors of CASOF:Eu (z = 0−0.5) samples that crystallized in acubic structure with space group I4 ̅3d. A gradual change in thelattice 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 shrinkthree-dimensional frameworks, according to Vegard’s rule,45 alarger O2− (2r = 1.35 Å) occupying the F− (2r = 1.285 Å) sitecauses a slight increase in the lattice as shown in Figure 6a. Theunit cell expansion by Si4+−O2− replacing Al3+−F− is alsoobserved by Im et al.46 To elucidate the site feature of Ca2+, theeffect of Si4+−O2− incorporation involved in the Ca2+ site canbe analyzed by the bond lengths of (Al,Si)−O and Ca−O.Figure 6b,c plots the average bond lengths of (Al,Si)−O andCa−O as obtained by the refinement results. The (Al,Si)−Obond length decreases with increasing amount of Si4+−O2−. Itcan be assigned to the Si4+ replacement of Al3+. Consequently,the Ca−O bond length is further elongated due to shrinking ofthe (Al,Si)O4 tetrahedra when incorporating Si

4+, thusloosening the crystal site of Ca2+. The Ca−(F,O) length isalso elongated by incorporating Si4+−O2− as shown in Figure6d. However, because the F− originally occupied in a cage-likesite is surrounded by the AlO4 framework and coordinated toCa2+,29 the incorporated O2− generates another Ca2+ site in acrystal, which is coordinated to seven O2− ions.Adding Si4+−O2− affects both the crystal structure and the

photoluminescence properties. Figure 7 illustrates the indis-pensable effect of incorporating Si4+−O2− in the CASOF hostlattice. Figure 7a displays the emission spectra of CASOF withz = 0.0−0.5 at room temperature under 254 nm excitation.Besides the intrinsic line emission of Eu3+ within the red rangeas discussed above, a surprising observation which is an

Figure 3. Coordination environment of the Ca2+ site in theCa11.9Al14O32F2:Eu0.1 structure.

Figure 4. Powder XRD patterns of Ca12Al14−zSizO32+zF2−z:Eu0.1 (z =0−0.5), which are compared with JCPDs Card.

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).



apperence of a broadband peak at 440 nm with increasing z, itcan be assigned to 5d → 4f emission of Eu2+. Interestingly, theemission of Eu3+ within 570−700 nm and broadband at 440 nmdisappear and emerge simultaneously. This result can betherefore attributed to the increase of z value, suggesting thatEu3+ is transformed to Eu2+ in the lattice. With regard to thetendency in refined bond length of Al−O, Ca−O, and Ca−F,the expanded site of Ca2+ could be demonstrated bysubstitution of Al3+−F− by Si4+−O2−. Eu3+ can therefore bereduced to Eu2+ in the CASOF lattice. The excitation spectra ofCa12Al14−zSizO32+zF2−z:Eu (z = 0−0.5) in Figure 7b bymonitoring 440 nm reveals a broad peak from 250 to 410nm, which can be ascribed to 4f−5d transition of Eu2+. Thepeak intensity gradually increases with increasing value of z,which correlates well with the peak intensity of Eu2+ in Figure7a. The emission spectra upon excitation of 334 nm shows blueluminescence of the CASOF phosphors, and the intensity iscorrelated to the increasing level of Si4+−O2− incorporation, asshown in Figure S4 (Supporting Information). Interestingly, nored line luminescence is detected upon 334 nm excitation thatis consonant with no absorption of Eu3+ in this range as shownin Figure S2 (Supporting Information). The CommissionInternational de I’Eclairage (CIE) chromaticity coordinate of

Ca11.9Al13.5Si0.5O32.5F1.5:Eu0.1 under 334 nm excitation is plottedin Figure S5 (Supporting Information). In the excitationspectra in Figure 7c monitored at 613 nm, which is producedby 5D0 →

7F2 of Eu3+, a peak appears around 234 nm, which

can be assigned to O2p → Eu4f charge transfer band. Asmentioned above, a decreased intensity with an increasing z isalso compatible with the results in Figure 7a. Since thereplacement of F− by O2− implies the variation of firstcoordination layer of the activation site, the asymmetricemission spectra in Figure 7a can be deconvoluted into twoGaussian components at 440 and 473 nm, revealing that Eu2+

has two centers in the CASOF lattice. Features of the emissionposition are discussed via the change in the Eu2+−ligandcovalency. Previous investigations have studied the config-uration of Eu2+ with the anion polarizability and cationelectronegativity in various hosts.47−50 Two factors influencethe energy position of the 5d band: gravity shift and crystal fieldsplitting. Gravity shift is associated with the nephelauxetic effectcaused by the interaction between cation and electron cloudy ofligands. The [6O1F]Ca site is original coordinated with sixoxygen atoms and one fluorine atom. Due to the replacementof F− by O2−, the first coordination layer of the [6O1F]Ca site ischanged to a [7O]Ca site. The two distinct Eu2+ emissions aretherefore observed. Because the incorporated ligand O2− with alower electronegativity than that of F− would lower the energyof the 5d1 level, the shoulder emission band with a longwavelength at 473 nm can thus be assigned to the [7O]Eu2+ dueto the gravity shift and crystal field splitting. The main factor forchanging photoluminescence respects the first coordinationlayer activator, so the second layer of Al3+/Si4+ replacementwith negligible influence in photoluminescence just changes thesteric structure. We can also conclude that replacing Al3+ by Si4+

cannot influenced the first coordination sphere; it mainlychanges the crystal structure as the results in XRD refinement.During valence transfer of Eu, the CIE coordinates upon 254nm excitation of CASOF:Eu are regularly shifted from (0.6101,0.3513) to (0.1629, 0.0649) in relation to an increasing value ofz as depicted in Figure 7d, and the inset schematically depictsthe proposed crystal variation and photographs of eachcomposition irradiated under a 254 nm UV lamp. Thechromaticity coordinates of CASOF:Eu (z = 0−0.5) aresummarized in Table 3.Solid-state NMR measurement, which is atom specific and

sensitive to the local order around the nucleus, can beperformed to complete the crystal chemistry experimentsbeyond XRD analysis.51−54 Owing to the fact that F− iscoordinated with Ca2+, this study performs 19F solid state NMRto further investigate how incorporating Si4+−O2− affects theCa2+ site. Figure 8 describes the 19F NMR spectra, in which thepeak area correlates well with the amount of F in study samples.Only one peak is obtained in CAOF (z = 0) at −120 ppm,which can be assigned to the ligand atom to Ca.55−57 However,

Table 2. Crystallographic Data and Reliability Factor of Ca12Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5) Phosphorsa

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 Factorχ2 2.898 2.911 3.176 3.199 3.59 4.295Rwp 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: I4̅3d (No. 220).

Figure 6. (a) Lattice constant of Ca11.9Al14−zSizO32+zF2−z:Eu0.1 withdifferent amounts of dopants (z = 0−0.5). The average bond length of(b) Al−O, (c) Ca−O, and (d) Ca−F of Ca11.9Al14−zSizO32+zF2−z:Eu0.1(z = 0−0.5).



a shoulder peak appears at −173 ppm in z = 0.1, 0.3, and 0.5.The NMR spectra are decovoluted into two peaks in thebottom of Figure 8, in which the peaks appearing at −120 ppmand −173 ppm decrease and increase in correlation with thelevel of Si4+−O2− incorporation and the peak area ratio ofI(−173 ppm)/I(−120 ppm) correlated well with the level ofSi4+−O2− incorporation as shown in Figure 8b. The peak at−173 ppm can be attributed to F−, which is coordinated withthe loose Ca2+ site, capable of maintaining higher electrondensity at F− than the original site, resulting in an upfield inchemical shift.47 The results imply the average Ca−F bond iselongated by incorporating Si4+−O2, which is supported byRietveld refinement, indicating the increased amount of looseCa2+ site which is suitable for Eu2+ occupation.XANES spectroscopy can easily identify that two valence

states of Eu2+(4f7) and Eu3+(4f6), due to the different thresholdenergies around 8 eV of their white light (WL) resonance,

corresponding to the transition to the unoccupied 5dstates.58−61 The energy difference originates from the shieldingof the nuclear potential through an additional 4f electron inEu2+, subsequently lowering the binding energy of therespective core electrons.54 To further examine two valencestates of Eu in the CASOF samples, XANES was performednear Eu L3 edge with BaMgAl10O17:Eu

2+ and Eu2O3 asreference for Eu2+ and Eu3+, respectively.58 The normalizedEu L3 edge XANES spectra of the study CASOF reveal twopeaks at 6975 and 6983 eV, which are attributed to the electrontransition of 2p3/2 → 5d in Eu

2+ and Eu3+, respectively. Thisfinding suggests that two valence states of Eu coexist in theCASOF samples. The relative intensities of absorption by Eu2+

at 6975 eV and Eu3+ at 6983 eV systematically increase anddecrease, which correlates with the amount of Si4+−O2−incorporated. Owing to that the sum of the two peaks arenearly constants, the area ratio can be treated as the ratio of theamount of Eu2+ and Eu3+, as shown in Figure 9b.62−64 Theincreasing ratio of Eu2+/Eu3+ and the emission intensity of Eu2+

display a similar trend for the increased level of Si4+−O2−incorporation, demonstrating that the luminescent enhance-ment results mainly from the increasing number of Eu2+. Thisobservation also corresponds to our results in crystal structurestudies and solid state NMR.

■ CONCLUSIONSIn summary, we have revealed that by appropriate dopantincorporation, the valence state of Eu3+ can be tuned to Eu2+ inphosphors due to the enlargement of the activator site.

Figure 7. (a) VUV emission spectra (λex = 254 nm), (b) PL excitation spectra (λem = 440 nm), (c) VUV excitation spectra (λem = 613 nm) ofCa12Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5), and (d) dependence of the CIE chromaticity coordinates on varying z value inCa11.9Al14−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 Si4+−O2− incorporation and the irridated phosphor images of each composition under a 254 nm UV lamp.

Table 3. CIE Chromaticity Coordinates ofCa12Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5)

CIE chromaticity

Si4+−O2− con. (z) x y

0 0.6101 0.35130.1 0.4920 0.29540.2 0.2873 0.16530.3 0.2024 0.09290.4 0.1705 0.07130.5 0.1629 0.0649



Effectiveness of the proposed approach is demonstrated in thenew phosphor Ca12Al14O32F2:Eu

3+; Si4+−O2− are incorporatedto substituted Al3+−F− to release the geometry restriction ofthe activator site. Combinatorial studies with synchrotron XRDrefinement, XANES, HRTEM, and solid state NMR help us tounderstand how the dopant affects the crystal structure andphotoluminescence. The average bond lengths of Al−O andCa−O obtained in refinement are systematically shortened andelongated, respectively, indicating that the enlargement ofactivator site corresponds to the amount of Si4+−O2−.Incorporating Si4+−O2− in CAOF:Eu3+ phosphor leads to arise in broadband emission at 440 nm that can be ascribed tothe 4f−5d transition of Eu2+. The emission intensity of Eu2+and Eu3+ increases and decreases systematically with theamount of dopant. XANES results further confirm that Eu3+ istransferred 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 featureand efficient radiation in Eu2+ based phosphor are more usefulin illumination. The proposed approach is also characterized bythe fact that only a single activator, Eu, generates the multiband,even a white light by optical combination of different valencesof europium. This approach does not just limited in presentstudy materials but could be general to other related Eu system.Thus, our approach may lead to opportunities for moresuccessful development of phosphors in LED applications.

■ ASSOCIATED CONTENT*S Supporting InformationFigures S1 and S2 show the VUV excitation and photo-luminescence excitation measurements of Ca11.9Al14O32F2:Eu0.1.Figure S3 shows Rietveld refinement of synchrotron PXRDdata of CASOF samples (z = 0.1−0.5). Figures S4 shows thephotoluminescence emission spectra of CASOF samples.Figure S5 plots the CIE chromaticity coordinate ofCa11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0.5) under 334 nmexcitation. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to thank the National Science Councilof the Republic of China, Taiwan, for financially supporting thisresearch under Contract Nos. NSC 97-2113-M-002-012-MY3and 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.(3) Pimputkar, S.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Nat.Photonics 2009, 3, 180−182.(4) Schubert, E. F.; Kim, J. K. Science 2005, 308, 1274−1278.(5) Xie, R. J.; Hirosaki, N.; Sakuma, K.; Yamamoto, Y.; Mitomo, M.Appl. Phys. Lett. 2004, 84, 5404−5406.

F i g u r e 8 . ( a ) 1 9 F s o l i d s t a t e NMR s p e c t r a o fCa11.9Al14−zSizO32+zF2−z:Eu0.1 with z = 0, 0.1, 0.3, 0.5. The bottomrepresents the deconvolution components used for the spectralanalysis. (b) The ratio of the deconvoluted peak area at −173 ppm and−120 ppm in 19F NMR, which is a function of the amount of Si4+−O2− incorporation.

Figure 9. (a) Normalized Eu L3-edge XANES spectra ofCa11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5). (b) Dependence ofIEu2+/Eu3+ on the amount of Si

4+−O2− incorporation inCa11.9Al14−zSizO32+zF2−z:Eu0.1 (z = 0−0.5).



http://pubs.acs.orgmailto:[email protected]

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