DOI: 10.1002/chem.201301758

Spiral-Type Heteropolyhedral Coordination Network Based on Single-Crystal LiSrPO4: Implications for Luminescent Materials

Chun Che Lin,[a] Chin-Chang Shen,[b] and Ru-Shi Liu*[a]

Introduction

Light-emitting diodes (LEDs) are rapidly replacing tradi-tional incandescent and modern fluorescent lamps becausethey save energy, have a long lifespan, and have largemarket demand.[1–5] Down-converted phosphors are regard-ed as an important segment of near-ultraviolet (NUV)/blue-pumped devices for solid-state lighting, such as illumination,paints for sign boards, multicolor three-dimensional (3D)displays, and back lighting.[6] Many of the intrinsic qualitiesof commercial phosphors are not suitable for phosphor-con-version (pc) white-light-emitting diodes (WLEDs). MostWLEDs on the market are made of a combination of blueInGaN chips and yellow phosphor (Y3Al5O12:Ce3+); howev-er, they possess the problems of thermal quenching, whitecolor alteration with input power, poor color-renderingindex (Ra�80), and high correlated color temperature(CCT�7700 K) because of the absence of a red component.These problems produce an uncomfortable glare with exces-

sive luminance increase in small emission areas comparedwith traditional light sources; this glare is harmful to thehuman eye.[7,8] Many reports describe white devices manu-factured by coating a NUV device with RGB phosphors,which provides color uniformity with high Ra values and ex-cellent light qualities.[9,10] To solve the aforementioned prob-lems, the science of materials synthesis involves continuous-ly exploring new compounds. In recent years, numerousnovel crystalline structures have been observed with the de-velopment of ameliorated syntheses and advanced identifi-cation equipment. The chemical and physical properties ofproduced materials can be regulated by tuning chemicalconstituents or by structural modification, which result in in-novative applications.[11–13] Hosono et al.[14] presented a newyellow phosphor (Ca1�x�ySrxEuy)7ACHTUNGTRENNUNG(SiO3)6Cl2, which can beused to decrease glare in WLEDs. This phosphor has an in-ternal quantum efficiency of >90 % upon excitation withviolet light. Wang et al.[15] reported a series of zincgallo ACHTUNGTRENNUNGphosACHTUNGTRENNUNGphates (H2tmdp)2ACHTUNGTRENNUNG[Zn3Ga6OACHTUNGTRENNUNG(HPO4) ACHTUNGTRENNUNG(PO4)8]·5 H2O(NTHU-4; tmdp =4,4’-trimethylenedipyridine), includingNTHU-4Y and NTHU-4W, which emit yellow and white lu-minescence upon exposure to l=365 nm UV light.

Orthophosphates, an outstanding family of luminescenthosts with the general composition ABPO4 (A= monovalentcation; B= divalent cation) are attracting attention becauseof their high thermal stability, simple synthesis conditions,and good luminescent properties when doped with rare-earth activators. For example, the luminescent intensity andthermal stability of LiSrPO4 and KSrPO4 doped with Eu2+

ions are better than the commercially available BaMg ACHTUNGTRENNUNGAl10-

Abstract: Novel structures of lumines-cent materials, which are used as lightsources for next-generation illumina-tion, are continuously being improvedfor use in white-light-emitting diodes.Activator-doped known structures arereported as habitual down-conversionphosphors in solid-state lightings anddisplays. Consequently, the intrinsicqualities of the existent compoundsproduce deficiencies that limit their ap-plications. Herein we report a spiral-network single-crystal orthophosphate(LiSrPO4) prepared in a platinum cru-cible with LiCl flux through crystal-

growth reactions of SrCl2 and Li3PO4

in air. It crystallizes in a hexagonalsystem with a=5.0040(2) and c=

24.6320(16) �, V=534.15(5) �3, andZ=6 in the space group P65. The unitcell is comprised of LiO4 and PO4 tet-rahedrons that form a three-dimension-al LiPO4

2� anionic framework with ahelical channel structure along the caxis in which the Sr2+ cation is accom-

modated. The optical band gap of thiscomposition is about 3.65 eV, as deter-mined by using UV/Vis absorption anddiffuse reflection spectra. We used thecrystal-growth method to synthesizeblue- and red-emitting crystals that ex-hibited pure color, low reabsorption, alarge Stokes shift, and efficient conver-sion of ultraviolet excitation light intovisible light. Emphasis was placed onthe development of gratifying struc-ture-related properties of rare-earth lu-minescent materials and their applica-tions.

Keywords: crystal engineering ·lithium · luminescence · phosphates ·solid-state structures

[a] Dr. C. C. Lin, Prof. R. S. LiuDepartment of Chemistry, National Taiwan UniversityNo. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C)Fax: (+886) 2-33668671E-mail : rsliu@ntu.edu.tw

[b] Dr. C. C. ShenInstitute of Nuclear Energy Research, Atomic Energy CouncilNo. 1000, Wenhua Rd., Jiaan Village, Longtan TownshipTaoyuan County 32546, Taiwan (R.O.C.)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201301758.

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ACHTUNGTRENNUNGO17:Eu2+ blue phosphor for potential applications inWLEDs.[16,17] In addition, Li ACHTUNGTRENNUNG(Sr1�xBax)PO4:Sm3+ ,Eu3+ , Na-CaPO4:Eu2+ , and LiBaPO4 are widely used luminescent ma-terials for potential applications in solid-state lighting.[18–20]

Wanmaker et al.[21] first obtained the X-ray diffraction(XRD) patterns and luminescent properties of copper-doped orthophosphates. However, no study on the crystalstructures of LiSrPO4 and LiBaPO4 compounds has beenpublished. Neither the crystal structure nor the structuralparameters have been accurately determined in databaseJCPDS cards 14-0202 and 53-1238.[22–27] Herein, we success-fully prepared single-crystal LiSrPO4 by using a convention-al solution-growth approach and established novel structuralparameters by using single-crystal XRD analysis. The charg-es of europium ions and vacancies partially occupied byrare-earth ions show distinctive phenomena in the opticalproperties of LiSrPO4:RE (RE =Eu2+ and Eu3+). This find-ing means that they are candidates for NUV WLEDs. Ther-mal quenching was discussed by investigating the local struc-tures and electronic constituents. Moreover, Raman spec-ACHTUNGTRENNUNGtros ACHTUNGTRENNUNGcopy was used to analyze variations in local structuresaround different charge activators.

Results and Discussion

Structure determination and description : The starting mate-rials and reaction conditions were found to have extreme ef-fects on the structure of the orthophosphates, as revealed byXRD and EDS. For example, the pattern of JCPDS 14-0202(Figure 1a) was established from a LiSrPO4 (LSP) powder

sample that Wanmaker et al.[21] synthesized from Li2CO3,SrHPO4, and NH4H2PO4 by using a solid-state method withsintering at 1000 8C. However, no information was providedabout the crystal structure and lattice parameters, apartfrom the spectral energy distribution. In 2002, Mazza[27] re-ported another XRD pattern of JCPDS 53-1238 (Figure 1b)that revealed that LiSrPO4 has a monoclinic crystal structurewith lattice parameters a=16.2062, b=11.854, c=

13.2418 �. The sample was made by using mixtures of

LiOH, Sr ACHTUNGTRENNUNG(NO3)2, and NH4H2PO4 precipitates, and was sin-tered in oxygen at 600 to 1000 8C. To date, there are onlytwo standard XRD patterns of LSP in the JCPDS database.

The results of the XRD profile and detailed crystal infor-mation obtained by single-crystal XRD and crystallographicdata of crystalline LSP are shown in Figure 1c and Tables 1and 2. As shown in Table 1, Rietveld structure refinementanalysis results revealed sufficiently low R factors, whichsuggested a favorable fit. As mentioned above, the XRDpattern of the LSP single crystal differed from those in theJCPDS 14-0202 and 53-1238 databases. The diffraction peakintensity ratios (Ihkl/I016) of the samples were also calculatedand the results are shown in Table 3. The structures werefound to be similar but not isotypic in this system. The crys-talline compound had a hexagonal structure with the spacegroup P65 (no. 170) and cell parameters a=5.0040(2) andc=24.6320(16) �, V=534.15(5) �3, and Z=6 (Table 1). Thechemical components of the single crystal were verified by

Figure 1. X-ray powder diffraction patterns of versatile LiSrPO4 phos-phate compounds obtained from databases for JCPDS card nos. a) 14-0202 and b) 53-1238 as references. c) The presynthesized single crystal.

Table 1. Crystal data and structural refinement of LiSrPO4.

formula LiSrPO4

Mr 189.53T [K] 200(2)l 0.71073crystal system hexagonalspace group P65 (no. 170)a [�] 5.0040(2)b [�] 5.0040(2)c [�] 24.6320(16)a [8] 90b [8] 90g [8] 120V 534.15(5) �3

Z 61calcd [gcm�3] 3.535absorption coefficient [mm�1] 15.436F ACHTUNGTRENNUNG(000) 528size 0.3 � 0.2� 0.1 mm3

q range [8] 4.70–27.438reflns collected 2718limiting indices �5�h�5

�6�k�4�31� l�31

independent reflns 815 [Rint =0.0719]completeness to q=27.438 [%] 99.3data/restraints/parameters 815/1/59R,[a] wR[b] (I>2 s(I) 0.0571, 0.1453R,[a] wR[b] (all data) 0.0669, 0.1518GOF on F2 1.079

[a] R=� j jFo j� jFc j j /� jFo j . [b] wR= {�[w(F2o�F2

c)2]/�w(F2

o)2]}1/2

Table 2. Selected atomic coordinates and isotropic displacement parame-ters for LiSrPO4.

X Y Z Occupancy Uiso � 100 [�2]

Li 0.329(4) 0.611(4) 0.0792(8) 1.0000 2.50(4)Sr 0.9860(2) 0.9962(2) 0.1404(3) 1.0000 2.05(4)P 0.6770(5) 1.2915(5) 0.0502(2) 1.0000 1.95(6)O(1) 0.6648(14) 0.9784(14) 0.0550(4) 1.0000 2.28(14)O(2) 1.3509(14) 1.2462(14) 0.0580(4) 1.0000 2.51(16)O(3) 0.9005(14) 0.5036(14) 0.0936(4) 1.0000 2.56(16)O(4) 0.4223(16) 0.6190(16) 0.1603(4) 1.0000 2.81(16)

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FULL PAPER

EDS, as shown in the Supporting Information (Figure S1).The stoichiometric mole ratio of elemental Sr to P wasalmost unity.

The LSP crystal structure was investigated next. Theoxygen atoms and coordinated neighboring ions were foundto form two [PO4]

3� and [LiO4]7� tetrahedrons and one

[SrO7]12� polyhedron. Figure 2a portrays the structural dif-

ferences between three fractions (I, II, and III) of the unitcell, highlighted by the tilted [PO4]

3� and [LiO4]7� tetrahe-

drons in the LSP structure, which lead to the lower symme-try space group of this material. The first part (I) is com-prised of two [SrO7]

12� polyhedrons that share one face with[LiO4]

7� groups connected with two [PO4]3� groups and the

third part. The second part (II) is composed of linked[SrO7]

12� polyhedrons that have two sides integrated withother parts (I and III) by [PO4]

3� groups. Each O(4) or O(2)atom is shared by Sr(1), Li(1), and P(1) polyhedrons, andeach O(1) or O(3) atom is shared by Sr(1) and P(1) polyhe-drons (Figure 2b). Therefore, each Sr(1) polyhedron andLi(1) tetrahedron share a face, whereas each P(1) tetrahe-dron shares two apices with every Li(1) tetrahedron andSr(1) polyhedron. Each polyhedral [SrO7]

12� and tetrahedral[LiO4]

7� share a common edge, and the sharing of O–O

edges contributes to the stability of the bridging arrange-ments.[25]

Table 4 shows that the average M�O lengths in [PO4]3�,

[LiO4]7�, and [SrO7]

12� polyhedrons were 1.537, 1.958, and2.617 �, respectively, whereas the corresponding meanlengths of the O�O bridges were 2.469, 3.092, and 2.954 �,respectively. These values deviate from the ideal in the poly-hedral structure, which reveals a slight distortion in tetrahe-dral structures in the crystal structure depending on the realstructure and the mean M�O length. One of the important

Table 3. Ihkl/I016 ratio for single-crystal LiSrPO4 and JCPDS standards.

Ihkl/I016 Single-crystal LiSrPO4 JCPDS 14-0202 JCPDS 53-1238

I010/I016 0.42 0.90 0.66I013/I016 0.32 0.80 0.34I120/I016 0.60 1.00 0.45

Figure 2. a) Crystal structure of LiSrPO4 unit cell viewed in the ab direction. Indigo and pink tetrahedrons are [PO4]3� and [LiO4]

7� groups, whereasyellow polyhedrons are [SrO7]

12� groups. b) Coordination environment of Li, Sr, and P with O atoms in LiSrPO4. Purple, blue, green, and red spheresrepresent Li, Sr, P, and O atoms, respectively. c) The spiral channel consists of crisscross [LiO4]

7� and [PO4]3� coordination units around the [001] direc-

tion. Polyhedral structures were omitted for clarity. d) Structure perspective of LiSrPO4 viewed along [001].

Table 4. Average M�O lengths in LiSrPO4 for polyhedral structures.

M�O[�]

M�O (av)[�]

O�O (av)[�]

O�O (ideal)[�]

[PO4]3�

P�O(1) 1.542 1.537 2.469 2.502P�O(2) 1.543P�O(3) 1.527P�O(4) 1.535[LiO4]

7�

Li�O(1) 1.863 1.958 3.092 3.187Li�O(2) 1.954Li�O(3) 1.965Li�O(4) 2.050[SrO7]

12�

Sr�O(1-1) 2.567 2.617 2.954 3.081Sr�O(1-2) 2.622Sr�O(2-1) 2.595Sr�O(2-2) 2.630Sr�O(3-1) 2.556Sr�O(4-1) 2.813Sr�O(4-2) 2.537

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R.-S. Liu et al.

structural features of LSP is the [LiO4]7� and [PO4]

3� coordi-nation units connected in the form of a spiral-type channelaround the [001] direction (Figure 2c). Furthermore, a 3DLiPO4

2� anionic framework composed of [LiO4]7� and

[PO4]3� tetrahedrons with a spiral-type channel structure

along the c axis, within which Sr2+ cations are located (Fig-ure 2d), was also observed. This phenomenon affected theluminescence spectra, as discussed later.

Photoluminescence (PL) properties of LiSrPO4:M materials(M= Eu2+ and Eu3+): Studies on the reasonability and vari-ability of doped base structures by Raman spectra are limit-ed. Herein, representative vibrational spectroscopy was at-tempted for hexagonal LSP samples with different chargeactivators (Figure 3). The point symmetry of free (PO4)

3�

ion units was Td, and four of the most normal modes of vi-bration in phosphate groups were n1 (P�O symmetric

stretching), n2 (O�P�O symmetric bending), n3 (P�O asym-metric stretching), and n4 (O�P�O asymmetric bend-ing).[28–30] The Raman spectra of phosphate oxyanions in allsamples simultaneously showed n1(Ag) at 955 cm�1, n2(Eg) at416 cm�1, n3(Eg), and n3(Ag) at 1015 and 1075 cm�1, as wellas n4(Eg) and n4(Ag) at 570 and 620 cm�1. Notably, no signifi-cant influence of Eu substitution on strontium was observedbecause of the small differences in ionic radii. The dopedmaterials exhibited lower resonant frequency at n1(Ag),which we infer to be caused by the change in lattice parame-ters and particle size during europium doping.

The excitation spectrum of LSP:Eu2+ , moni-tored with emission at l=445 nm, showed a broadband from l=250 to 420 nm and a strong intensityat 400 nm, which matched well with NUV chips.The broad band was assigned to the 4f7 (8S7/2)!4f6 5d1 transition (Figure 4a). Upon excitation atl=396 nm, a prominent PL emission peak ataround l= 445 nm was observed, and was ascribedto the electron dipole allowed transition from thelowest level of the excited 5d state to the 4f

ground state of Eu2+ . The ex-citation spectrum of LSP:Eu3+

ranges from l= 350 to 500 nm,with the maximum at l=

393 nm attributed to the7F0–

5L6 transition within the4f6 configuration (f–f transi-tions) of Eu3+ ions (Fig-ure 4b).[31, 32] The main emis-sion peak is at l=614 nm,which was assigned to the5D0!7F2 transition. The minorpeaks at l= 591, 652, and695 nm were attributed to the5D0!7FJ (J=1, 3, 4) transi-tions of Eu3+ ion. Under UVlamp excitation, LSP:Eu2+ andLSP:Eu3+ single crystals

showed bright blue and red luminescence to the naked eye,respectively (Figure 4, insets). The performance of thesecrystal samples is summarized in Table 5.

The optical absorption coefficient and band gap of pureLSP compound were determined from the UV/Vis absorp-tion spectrum. The optical absorption coefficient (al) of thesingle crystal was calculated as shown in Equation (1):[33]

al ¼1t

ln1� Rlð Þ2

Tl

� �ð1Þ

Figure 3. Raman spectra of hexagonal LiSrPO4 samples with different charge activators: a) pure, b) Eu2+ , andc) Eu3+ . Right: An enlarged picture of the n1(Ag) peak.

Figure 4. a) Excitation and emission spectra of a LiSrPO4:Eu2+ singlecrystal (lem =445 nm, lex =396 nm). b) Excitation and emission spectra ofa LiSrPO4:Eu3+ single crystal (lem = 614 nm, lex =393 nm). Insets: Imagesof doped crystals excited at l= 365 nm in a UV box.

Table 5. Summary of the PL properties of LiSr0.9PO4:M0.1 (M=Eu2+ and Eu3+) sam-ples.

lex [nm],transition

lem [nm],transition

Quantumyield [%]

CIE Color

Eu2+ 396, 4f7–4f6 5d1 445, 4f6 5d1–4f7 38 (0.151, 0.037) blueEu3+ 292, Eu3+–O2� CTB 591, 5D0–

7F1 25 (0.613, 0.387) red360, 7F0–

5D4 614, 5D0–7F2

380, 7F0–5G2 652, 5D0–

7F3

393, 7F0–5L6 695, 5D0–

7F4

413, 7F0–5D3

464, 7F0–5D2

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FULL PAPERSingle-Crystal LiSrPO4

in which t is the path length measured in centimeters, Tl isthe transmittance, and Rl is the reflectance. The optical ab-sorption coefficient (al) of the LSP single crystal was calcu-lated to be approximately 103 cm�1. Therefore, pure single-crystal LSP exhibited a linear dependence that allowed asatisfactory direct transition [Eq. (2)]:[34,35]

ahn ¼ Aðhv�EgÞ1=2 ð2Þ

The gap energy (Eg) was estimated to be 3.65 eV by ex-trapolating a plot of (ahv)2 versus hv to zero, and is equal tothe energy of absorption of an excited photon of l=340 nm(Figure 5a). The pale clear crystal was separated from other

light platelets with a crystal diameter of 5–10 mm (Figure 5a,inset). The reflectance spectrum of the LSP host showedenergy absorption in the l<360 nm region, which agreedwell with the absorption spectrum (Figure 5b). The result in-dicated no host absorption/emission in the NUV region.However, after Eu2+ ions were introduced into the LSPhost, two absorption bands at around l=200 to 400 nmbecame evident. The first part (l= 200–300 nm) was as-signed to the charge transfer band (CTB) of O2�–Eu2+ andthe other part (l= 300–400 nm) consisted of the f–d absorp-tion of Eu2+ ions. An analogous condition occurred in thediffuse spectrum of Eu3+ ions, as expected for the weakerabsorption signal (l=300–400 nm) because of the forbidden

f–f transition. Thus, doped LSP materials can be used forNUV WLEDs and PL (Figure 4).

Thermal characteristics and devices : The mechanisms under-lying color-change and quenching behavior were observedby using temperature-dependent luminescence to determinean excellent route to designing novel compositions of phos-phors that allow the manufacture of efficient WLEDs. In-tense electron transitions of the activators influenced thethermal quenching of PL for LSP:M (M =Eu2+ and Eu3+)materials, which was important in the performance of practi-cal WLED devices (Figure 6). In the LSP:Eu2+ system,some changes in color distribution in the contour maps wereobserved with increased temperature. These changes arosefrom the broader emission transitions of the lowest-lying 5dto 4f7 (8S7/2) states of europium ion with decreased emissionintensity.

However, no change in color distribution in the contourmaps was observed for the LSP:Eu3+ system, although theemission intensity decreased with increased temperature.This finding is due to the fact that the narrow f–f transitionlines of Eu3+ ions were forbidden. The emission band of thetwo compositions hardly shifted with increased temperature,which indicates the CIE chromaticity coordinates forLSP:M materials (M=Eu2+ and Eu3+), as shown in theSupporting Information (Figure S2 and Table S1). The acti-vation energy (Ea) is interpreted as the nonradiative transi-tion probability of the luminescent center through the cross-ing between the excited and ground states. Figure 6e showsthat the curves (fit line) fitted Equation (3):[36, 37]

IT=I0 ¼ ½1þDexpð�Ea=kTÞ��1 ð3Þ

in which I0 is the intensity at T=0, D is a constant, and Ea isthe activation energy, all of which are defined variables. TheLSP:Eu2+ material had a quenching activation energy of ap-proximately 0.147 eV, but Ea was slightly decreased to0.126 eV in LSP:Eu3+ . This finding clearly indicates that theStokes shift in LSP:Eu3+ increased with increased nonirra-diative transitions and decreased activation energy.

The excitation spectra of green-emitting Ba2.89Si6O12N2:Eu0.11 and presynthesized LSP phosphors were recorded si-multaneously by using an NUV chip (lmax = 380–400 nm;Figure 7a). To evaluate the applicability of LSP phosphors,we fabricated three LED devices (blue, red, and white)composed of a mixture of various phosphors and translucentresin applied to an NUV chip (Figure 7b). The CIE chroma-ticity coordinates calculated from the emission spectra ofBa2.89Si6O12N2:Eu0.11, LSP:Eu3+ , and LSP:Eu2+ phosphorswere (0.2703, 0.6307), (0.613, 0.387), and (0.151, 0.037), re-spectively. Furthermore, the intensities of the blue and redemission increased from 30 to 50 mA with increased for-ward-bias current (Figure 7b, inset). The differences in theemission color distribution in the emission area of the RGBphosphor-based device were determined from the photo-graph of the package for white light. The performance (di-verse hue) resulted from the intrinsic qualities of the synthe-

Figure 5. a) Variation in the absorption coefficient of LiSrPO4 crystal (a)with wavelength. The plot of (ahv)2 vs. hv for single-crystal LiSrPO4 isshown. Inset: An image of the pure crystal in daylight. b) Diffuse reflec-tion spectra of host (black), Eu2+- (blue), and Eu3+-doped (red) materi-als.

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R.-S. Liu et al.

sized phosphors, such as the color of the bodies, the artificialapproach, and progression of the package. The decrease inluminous efficacy of these devices from 10 to 20 lm W�1

compared with a commercial YAG:Ce-based blue chipdevice was probably due to reabsorption, a large Stokesshift, and nonoptimization. LSP phosphors also have strongpotential applications in the development of NUV WLEDswith RGB pure color, thermal stability, and high brightness.

Conclusion

In summary, single crystals of LiSrPO4:M (M =Eu2+ , Eu3+)for an NUV chip were grown by using a self-flux method.By using spiral-type orthophosphate as the initial structuremodel, the crystal structure of heteropolyhedral-based LSPwas successfully identified as a hexagonal structure withspace group P65 and cell parameters a=5.0040(2) and c=

Figure 6. Thermal behavior of photoluminescence measured for LiSrPO4:M materials (M =Eu2+ and Eu3+). Temperature-dependent of emission spectrafor a,b) LiSrPO4:Eu2+ and c,d) LiSrPO4:Eu3+ at 298 and 473 K, respectively. e) PL intensity ratios IT/I0 measured at T =298–573 K for M =Eu2+ (blue)and Eu3+ (red).

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FULL PAPERSingle-Crystal LiSrPO4

24.6320(16) �, V=534.15(5) �3, and Z= 6. The crystal struc-ture (unit cell) was a 3D LiPO4

2� anionic framework com-posed of [LiO4]

7� and [PO4]3� tetrahedrons with a spiral-

type channel structure along the c axis, in which Sr2+ cationswere located. The LSP materials in combination with NUVchips were used to fabricate pc-LEDs with pure color andgood thermal stability. These pc-LEDs were thus suitablefor general illumination and back lighting by modifying theluminous efficiency. In addition, the self-flux growth methodwas found to enable the development of novel phosphorsfor WLEDs.

Experimental Section

Synthesis : Single crystals of undoped LiSrPO4 were grown by using aself-flux method, in which LiCl acted as both a precursor and a fluxduring the reaction process. For solution growth, a stoichiometric mixtureof high-purity SrCl2 (99 % Aldrich) and Li3PO4 (99 % Aldrich) weremixed with excess LiCl (98 %, Aldrich; 1.5 times the stoichiometricamount). The molar ratio of SrCl2/Li3PO4/LiCl in the starting mixturewas optimized at 1:1:1.5. After thorough grinding, the mixture wasplaced in a platinum crucible inside an alumina crucible and then movedto a furnace. The temperature was initially increased to 1240 8C for 10 h

in air and then cooled to 900 8C at a rate of 1 8C h�1, followed by a cool-ing rate at 5 8C min�1 to RT. We used the method for pure phosphatewith EuCl3 as the Eu source to study the optical properties of Eu2+- andEu3+-doped LiSrPO4 phosphors. Eu2+-doped single-crystal LiSrPO4 wasformed at 1000 8C for 3 h in a reducing atmosphere of H2/N2 (5/95).

Package : A mixture of thermally curable silicone resin (OE-6630 B, DowCorning) was blended with various phosphors, and then the hardener(OE-6630 A) was added to the mixture. The mixture of phosphors andtransparent epoxy resin (25:75) was dropped onto a UV LED chip (spec-trum peak l=390 nm, optical power 2 mW). Subsequently, the chip washeated in a two-step thermal curing process at 100 8C for 1 h and then at150 8C for 2 h in an oven.

Characterization : The phase purity of the obtained products was ana-lyzed by powder X-ray diffractometry by using a Bruker AXS D8 ad-vanced automatic diffractometer with CuKa radiation (l=1.5405 �).Single XRD investigation was performed to analyze and elucidate thecrystal structure of a grown LiSrPO4 crystal with approximate dimensionsof 0.3� 0.2 � 0.1 mm3.[38] Data were collected by using a Bruker SMARTCCD-based X-ray diffractometer with 1.5 kW graphite-monochromatedMoKa radiation (l=0.71073 �) at 200 K. The structure of LiSrPO4 wasdetermined by using direct methods and refined by using the full-matrixleast-squares method with the SHELXL 97 program package.[39] The 3Dfigures were produced by a visualization system for electronic and struc-tural analysis.[40] The Raman spectra of the prepared samples were re-corded by using a Jobin–Yvon LabRAM H800 spectrometer equippedwith a 1800 grooves mm�1 grating monochromator and with a l=325 nmHe–Ne laser as the excitation source. Diffuse reflection spectra were ob-tained by using a UV/Vis spectrophotometer (Thermo Scientific evolu-tion 220) attached to an integral sphere. PL spectra were obtained byusing a FluoroMax-3 spectrophotometer at RT. Thermal quenching wasperformed by using a heating apparatus (THMS-600) in combinationwith PL equipment. The surface features and composition of LiSrPO4

were examined by using field-emission scanning microscopy (Hitachi S-4800) and elemental analysis was conducted by using energy-dispersivespectroscopy (EDS). The optical properties were measured by using aVarian Cary 100 Conc instrument equipped with a DRA-CA 3300 spec-trophotometer.

Acknowledgements

The authors thank the National Science Council of Taiwan (contract no.NSC 101-2113-M-002-014-MY3) for financially supporting this research.

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Figure 7. a) Excitation and emission spectra of green-emittingBa2.89Si6O12N2:Eu0.11 (top), pre-synthesized LSP:Eu3+ ACHTUNGTRENNUNG(middle), and pre-synthesized LSP:Eu2+ ACHTUNGTRENNUNG(bottom) on excitation at l= 390 nm. b) CIE chro-maticity coordinates for all as-prepared phosphors. Inset: Images of emit-ting fabricated pc-LEDs with different phosphors and chips.

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Received: May 7, 2013Published online: September 20, 2013

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FULL PAPERSingle-Crystal LiSrPO4