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Journal of Alloys and Compounds 492 (2010) 384–388

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Journal of Alloys and Compounds

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Combustion synthesis and luminescence investigation of Na3Al2(PO4)3:RE(RE = Ce3+, Eu3+ and Mn2+) phosphor

I.M. Nagpurea,∗, K.N. Shindeb, Vinay Kumara, O.M. Ntwaeaborwaa, S.J. Dhobleb, H.C. Swarta,∗

a Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, Free State 9300, South Africab Department of Physics, RTM Nagpur University, Nagpur 440033, India

a r t i c l e i n f o

Article history:Received 19 October 2009Received in revised form12 November 2009Accepted 16 November 2009Available online 20 November 2009

Keywords:XRDPhotoluminescenceNASICONPhosphorLamp phosphorScintillatorsTGADTA

a b s t r a c t

Rare-earth (Ce3+or Eu3+) or alkali-earth metal (Mn2+) doped Na3Al2(PO4)3 phosphor powder were pre-pared by a combustion method. Regardless of the dopants, the Na3Al2(PO4)3 was shown to crystallizeas hexagonal (rhombohedral) NASICON phase with spheroidal micron sized particles based upon X-raydiffraction (XRD) and scanning electron microscopy (SEM) data. As confirmed by thermogravimetric(TGA) and differential thermal analysis (DTA) data, the powders were stable at high temperatures upto 900 ◦C. A broad UV emission (with maximum at 328 nm) associated with the 5d → 4f transition ofCe3+ was observed from Na3Al2(PO4)3:Ce3+ while Mn2+ and Eu3+ doping showed stable green and redphotoluminescence emission at 515 and 615 nm, respectively. For different molar concentrations of rare-earth/alkali-earth metal dopants, the PL intensity was shown to increase to a certain concentration beforequenching occurred at higher concentrations. The structure and optical properties of these phosphorswere evaluated.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Phosphate compounds are known as multifunctional materials.In particular, orthophosphate have been extensively investigateddue to their structural diversity. Phosphate are used as elementsof optical devices, fluorescent lamps, plasma panel display, superionic conductors (NASICON) and matrices for long time storageof many radioactive substances waste, etc. [1–5]. Recently, it hasbeen found that NASICON also has luminescence characteristicsand there are limited reports on this effect in the literature [6–8].NASICON (Na+ super ionic conductor) is a name of compoundshas a three-dimensional framework structure and possesses ahigh Na ionic conductivity [9]. NASICON compounds are char-acterized by an anionic framework of PO4 or SiO4 tetrahedrallinked by shared oxygen ions to octahedral frame [10]. These prop-erties make it suitable for use in electrochemical devices [11].Beside this to investigate the luminescent mechanism of NASI-CON, He et al. [6] reported the PL properties of un-doped anddoped-NASICON materials. Hirayama et al. [7] and Masui et al.[12] reported Eu2+ emission in A0.5Zr2(PO4)3 (A = Ca, Sr, Ba) phos-

∗ Corresponding authors. Tel.: +27 051 401 9749; fax: +27 051 401 3507.E-mail addresses: indrajitnagpure@yahoo.co.in (I.M. Nagpure),

swarthc@ufs.ac.za (H.C. Swart).

phors with the NASICON structure. Mouline et al. [13] reportedthe copper(I) and manganese(II) luminescence in NASICON-typestructure CuI

0.5MnII0.25Zr2(PO4)3, which crystallizes in the rhom-

bohedral (hexagonal cell) system with the efficient energy transferfrom Cu+ → Mn2+ site. Chukova et al. [14] reported the possibilityof incorporation of the f-elements impurities rare-earth elements(Dy, Sm, Nd) into NASICON matrix of the phosphates. Tradition-ally, rare-earths doped phosphate phosphors with general formulassuch as MLnP2O7 [15], M3Ln(PO4)2 [16], and MLn(PO3)4 [17] (M,alkali metal; Ln, rare-earth metal) were prepared by the solid statereaction method, which demands longer reaction times and higherannealing temperatures. Today, low temperature methods suchas hydrothermal microwave [18], co-precipitation [19,20], sol–gel[21,22] and combustion [23] are used to prepare these phosphors.These methods are relatively cost-effective, quick and can be easilyexploited to prepare phosphors with enhanced optical properties.

In this study, the low cost combustion method was used to pre-pare sodium aluminium triphosphate (Na3Al2(PO4)3) doped withrare-earth (Ce3+ or Eu3+) or alkali-earth metal (Mn2+) ion activators.Na3Al2(PO4)3 belongs to a family of triphosphate materials thatcan be represented by a general formula Me3

+(Me3+)2(Me5+O4)3,where Me+ = Na, Li, Ag or K; Me3+ = Ga, Cr, Fe, Al, Sc or In;and Me5+ = As or P [24]. Both amorphous [25,26] and crystalline[27,28] Na3Al2(PO4)3 structure have been reported. Na3Al2(PO4)3can crystallize in a NASICON (sodium super ionic conductor)-

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like or garnet-like structure. The Na3Al2(PO4)3 synthesized inthis study resembles the low pressure NASICON-like polymorphwith rhombohedral (hexagonal) reported in Ref. [27]. This poly-morph was doped with different molar concentrations Ce3+,Eu3+ or Mn2+ to make phosphors that might be used in lightemitting devices of different types including ionizing radiationdetectors (Na3Al2(PO4)3:Ce3+), fluorescent lamps, light emittingdiodes and plasma display panels (PDP) (Na3Al2(PO4)3:Mn2+ andNa3Al2(PO4)3:Eu3+).

2. Experimental details

The Na3Al2(PO4)3:Ce3+/Eu3+/Mn2+ phosphors were prepared via the combustionmethod. The starting AR grade materials with 99.99% purity used for the prepara-tion were sodium nitrate (NaNO3), aluminium nitrate (Al(NO3)3·9H2O), ammoniumdihydrogen phosphate (NH4H2(PO4)), europium nitrate Eu(NO3)3, cerium nitrateCe(NO3)3, and manganese acetate ((CH3COO)2Mnx·4H2O). Urea (NH2CONH2) andammonium nitrate NH4NO3 were, respectively, used as fuel and flux for thesynthesis. Appropriate amounts of in metrological proportions these were takenstoichiometrically, were homogenized thoroughly and were ground using a mor-tar and pestle for 30 min. The mixture was stirred for about 30 min, then heatedrapidly at ∼600 ◦C, underwent dehydration followed by decomposition generat-ing combustible gases. The volatile and/or combustible gases such as N2, H2O andnascent O2 escaped from a firing container leaving behind voluminous foamingfine powder occupying the entire volume of the container. The basic combustionreaction for the formation of Na3Al2(PO4)3 can be described by the following equa-tion:

3NaNO3 + 2Al(NO3)39H2O + 3NH4H2(PO4) + 20NH2CONH2 + 20NH4(NO3)

→ Na3Al2(PO4)3 + 47N2↑ + 20CO ↑ + 85H2O ↑

The dopant concentrations were varied between 0.5 and 5 mol% for Ce3+, 0.2 and2 mol% for Eu3+ and 5 and 15 mol% for Mn2+.

Several complementary techniques were used to characterize the pre-pared samples. The crystalline structure was determined by X-ray powderdiffraction (XRD) using PAN Analytical X’pert Pro XRD diffractometer (Cu K˛radiation) at a scanning step of 0.01◦ , and counting time of 20 s in the 2�range of 10–80◦ . The morphology of the powders was examined by scan-ning electron microscopy (SEM) using a JEOL JSM 6380A microscope and thepowders were sputter coated with platinum to minimize charging. The pho-toluminescence measurement (excitation and emission) was recorded in theShimadzu RF5301PC spectrofluorophotometer fitted with a sensitive photo-multiplier tube. This spectrofluorophotometer provides corrected emission andexcitation spectra in the 220–400 and 300–700 nm ranges, respectively, withspectral slit width of 1.5 nm. The same amount of sample (2 mg) was used foreach measurement. Thermal analysis was performed with PerkinElmer thermo-gravimetry (TGA) and differential thermal analysis (DTA). The spectra wererecorded in the atmosphere of argon between 30 and 900 ◦C at a heating rate of10 ◦C min−1.

3. Results and discussion

3.1. Structure and morphology

Fig. 1 shows the XRD patterns from un-doped Na3Al2(PO4)3powder. The patterns were matched with hexagonal (rhombo-hedral) symmetry of Na3Al2(PO4)3 referenced in JCPDS card No.31-1265. These patterns are consistent with the ball and stickmodel of the rhombohedral NASICON structure of Na3Al2(PO4)3proposed by Shrivastava et al. [29]. Based upon ionic radii,Ce3+/Eu3+ [30] and Mn2+ are expected to substitute Al3+ in thelattice resulting in six and four coordination sites, respectively tobind up with PO4. Similar framework structures were reported inRefs. [30,31]. XRD crystallography data of Na3Al2(PO4)3 lattices arelisted in Table 1. The experimental values of d-spacing comparereasonably well with theoretical values as shown in Table 2. Formore profound structural elucidation of Na3Al2(PO4)3 the readeris referred to the literature cited [27,29]. The SEM images in Fig. 2show that the particles were spheroidal in shape and the averageparticle size was estimated to be 0.1–0.4 �m in diameter.

Fig. 1. X-ray diffraction pattern of the Na3Al2(PO4)3 host lattice.

3.2. Thermal analysis

The TGA (solid line) and DTA (dashed line) curves ofNa3Al2(PO4)3 host lattice are presented in Fig. 3. The TGA datashow that the weight loss occurred in three steps which includethe exothermic effect. The first significant weight loss occurred at200 ◦C and it can be attributed to evaporation of 3–4 wt% residualmoisture or solvent. The second loss at ∼500 ◦C may be due to evap-oration of moisture absorbed during the thermal treatment of thegel at 600 ◦C. The exothermic heat flow peaks at 600 and 800 ◦C maybe due to the increase in crystallinity as a minor impurity phase van-

Table 1Crystallographic data for Na3Al2(PO4)3 lattice.

Phase Na3Al2(PO4)3

Empirical formula Na3Al2P3O12

Structure Hexagonal (rhombohedral)Space group R-3c; Z = 6Formula weight 407.886 g/molDensityCal (g/cm3) 3.247

Lattice parametera (Å) 8.7920b (Å) 8.7920c (Å) 22.7751˛ = ˇ = 90◦ = � = 120◦

Table 2h k l planes and d-spacing.

h k l Calculated d-spacing (Å) Theoretical d-spacing (Å)

1 0 −2 5.0993 5.09061 0 4 4.6142 4.59951 1 0 4.1322 4.12941 1 3 3.7383 3.72822 0 −4 3.4719 3.47161 1 6 2.9718 2.97102 1 1 2.8435 2.84003 0 0 2.6379 2.63722 0 8 2.6188 2.61801 1 10 2.3452 2.34562 2 3 2.1584 2.15813 0 −6 2.0971 2.09693 0 6 2.0192 2.02003 1 2 2.0252 2.02482 0 −10 1.9249 1.92630 0 12 1.8814 1.88093 1 8 1.7081 1.7079

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Fig. 2. SEM image of Na3Al2(PO4)3 phosphor at different magnifications.

ishes at high temperatures. The DTA curve shows that endothermiceffects were observed at 50 and 300 ◦C. These peaks may be asso-ciated with the detectable change in weight loss. The dehydrationdecreased slowly and the endothermic curve appeared to be linearafter 300 ◦C suggesting that the phosphor was stable and there wasno further weight loss up to 900 ◦C. The total weight loss was nearly5%, which shows that this phosphor was thermally stable and didnot decomposing over the 30–900 ◦C temperature range.

3.3. Photoluminescence studies

Photoluminescence excitation (1) and emission (2) spectra ofNa3Al2(PO4)3:Ce3+ powder phosphor are shown in Fig. 4. The emis-sion spectra were measured for the different Ce3+ concentrationsas indicated. The emission spectrum (excited at 242 nm) has a max-imum at 328 nm and a shoulder at 350 nm due to the crystal fieldsplitting of the 4f ground state. As shown in the inset of Fig. 4, theemission occurs from the lowest component of the 5d configurationto the two-crystal field split levels (2F5/2 and 2F7/2) of the 4f groundstate. The PL excitation at 242 nm in spectrum (1) is for the 328 nmUV emission. The PL emission intensity was shown to increase withconcentration from 0.5 to 2 mol% of Ce3+ and quenching occurredat 5 mol% of Ce3+ as shown in Fig. 5. This decrease in the PL inten-sity is due to concentration quenching effects. The observed Ce3+

emission in this phosphor can be used in scintillators according toan energy transfer process explained by Lempicki et al. [32] andWojtowicz et al. [33,34]. According to this process, Ce3+ capturesprimary excitation energy (hv) and becomes Ce4+. After capturing a

Fig. 3. TGA/DTA analysis of the Na3Al2(PO4)3 phosphor.

free electron (ec) from the conduction band, Ce4+ will be convertedto an excited Ce3+ ion or (Ce3+)*. Relaxation to the ground state willbe accompanied by emission of the scintillation photon hv. Thisprocess can be summarized as follows:

Ce3+ + h� → Ce4+

Ce4+ + ec → (Ce3+)∗

(Ce3+)∗ → Ce3+ + h�.

Fig. 4. PL emission and excitation spectra of the Na3Al2(PO4)3:Ce3+ phosphor, theemission peaks were monitored at �exc = 242 nm. The PL mechanism is shown asinset.

Fig. 5. Variation in the PL intensity as function of the Ce3+ ion concentration.

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Fig. 6. PL excitation spectrum of the Na3Al2(PO4)3:Eu3+ phosphor, the excitationpeak was monitored at �emi = 615 nm.

Figs. 6 and 7 represent the PL excitation and emission spectra ofNa3Al2(PO4)3:Eu3+ powder phosphors, respectively. The emissionspectra are from the different Eu3+ concentrations as indicated. ThePL excitation spectrum is broad and maximizes at 243 nm. This exci-tation can be ascribed to the Eu3+ → O2− charge transfer transition.This transition should be followed by nonradiatiave relaxation tothe 5DJ (J = 0, 1, 2, 3, . . .) levels and then radiative transitions fromthe 5D0 to 7FJ levels of the ground state. The PL emission spec-trum of Fig. 7 consists of major line emission at 615 nm and minoremission at 593 nm which can be ascribed to the 5D0 → 7F2 and5D0 → 7F1 transitions of Eu3+, respectively. The inset of Fig. 7 is asimplified energy level diagram showing non-radiative transitionsfrom charge transfer states followed by radiative transitions fromthe 5D0 to the ground state. A plot of maximum PL intensity ver-sus concentration in Fig. 8 shows that the intensity increased withconcentration from 0.5 to 1 mol% of Eu3+and it decreased slightlywhen the concentration was increased to 2 mol%. The fact that theexcitation wavelength (�exc = 243 nm) of the Na3Al2(PO4)3:Eu3+ issimilar to the discharge wavelength of the fluorescent mercurylamp suggests that this material can be used as a lamp phosphor.

Fig. 9 shows the PL excitation (1) and emission (2) spectrafrom Na3Al2(PO4)3:Mn2+ powder phosphors. The emission spec-tra obtained from different Mn2+ concentrations, as indicated.Stable green emission was observed at 515 nm when the pow-

Fig. 7. PL emission spectra of the Na3Al2(PO4)3:Eu3+ phosphor the emission peakswere monitored at �exc = 243 nm. The PL mechanism is shown as inset.

Fig. 8. Variation in the PL intensity as function of the Eu3+ ion concentration.

der was excited at 288 nm. This emission can be ascribed to the4T1 → 6A1 transition of the Mn2+ ion shown in inset of Fig. 9. Avisible emission from Mn2+ ion may vary from green to orange-red [35,36] depending on the site occupied by the ion in a hostmatrix. The green emission is often obtained at 512 nm when Mn2+

occupies a site which is considerably larger than its radius [37].A plot of maximum PL intensity versus concentration in Fig. 10shows that the intensity increased with concentration from 5to 15 mol% of Mn2+and it decreased slightly when the concen-

Fig. 9. PL emission and excitation spectra of the Na3Al2(PO4)3:Mn2+ phosphor, theemission peaks were monitored at �exc = 288 nm. The PL mechanism is shown asinset.

Fig. 10. Variation in the PL intensity as function of theMn2+ ion concentration.

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tration was increased to 15 mol%. Photoluminescence applicationof Na3Al2(PO4)3:Mn2+ phosphate is strongly more promising atshorter wavelengths than that of the existing ZnS:Mn EL displays.Which should be a candidate to realize the advanced all solid stateextremely reliable and colour displays for multi-media use.

4. Conclusions

The present data shows that Na3Al2(PO4)3, Na3Al2(PO4)3:Ce3+,Na3Al2(PO4)3:Mn2+ and Na3Al2(PO4)3:Eu3+ phosphors can be eas-ily prepared by the combustion method. The XRD data confirmedthe formation of crystalline Na3Al2(PO4)3 host lattice having hexag-onal (rhombohedral) symmetry NASICON-type with spheroidalparticles of 0.1–0.4 �m size. TG/DTA results show the phosphorwas stable at higher temperature of up to 900 ◦C. The matrix wasshown to be chemically stable and seems to be suitable for hostingrare-earths or alkali-earth metal ions for preparation of phosphors.The PL data suggest that dominant Na3Al2(PO4)3:Ce3+ may be usedin scintillators (ionizing radiation), while Na3Al2(PO4)3:Eu3+ andNa3Al2(PO4)3:Mn2+ may be used in phosphor lamps, PDP and solidstate lighting devices.

References

[1] H. Ben Yahia, E. Gaudin, J. Darriet, J. Alloys Compd. 442 (2007) 74.[2] H. Naili, H. Ettis, S. Yahyaoui, T. Mhiri, Mater. Res. Bull. 41 (2006) 1990.[3] D. Zhao, Z. Xie, J.M. Hu, H. Zhang, W.-L. Zhang, S.-L. Yang, W.-D. Cheng, J. Mol.

Struct. 922 (2009) 127.[4] G.V. Prakash, R. Jagannathan, D.N. Rao, Mater. Lett. 57 (2002) 134.[5] R. Chourasia, O.P. Shrivastavaa, P.K. Wattal, J. Alloys Compd. 473 (2009) 579.[6] Y. He, B. Quan, Y. Wang, C. Cheng, B. Wang, Mater. Lett. 61 (2007) 4519.[7] M. Hirayama, N. Sonoyama, A. Yamada, R. Kanno, J. Solid State Chem. 182 (2009)

730.

[8] S.W. Kim, T. Masui, H. Matsushita, N. Imanaka, Chem. Lett. 38 (2009) 1010.[9] P. Pasierb, Mater. Sci. 24 (2006) 279.

[10] N. Imanaka, A. Ogura, M. Kamikawa, Electrochem. Commun. 3 (2001) 451.[11] J.P. Boilot, P. Salanie, G. Desplanches, D.L. Potier, Mater. Res. Bull. 14 (1979)

1469.[12] T. Masui, K. Koyabu, S. Tamura, N. Imanaka, J. Alloys Compd. 418 (2006) 73.[13] A. Mouline, M. Alami, R. Brochu, R. Olazcuaga, C. Parent, G. Le Flem, J. Solid State

Chem. 152 (2000) 453.[14] O.V. Chukova, R.S. Boyko, S.G. Nedilko, Funct. Mater. 11 (2004) 147.[15] A. Hamady, T. Jouini, Acta Crystallogr. C 52 (1996) 2947.[16] H.Y. Hong, S.R. Chinn, Mater. Res. Bull. 11 (1976) 421.[17] W. Rekik, H. Naili, T. Mhiri, Acta Crystallogr. C 60 (2004) 50.[18] K. Byrappa, M. Yoshimura, Handbook of Hydrothermal Technology, 2001, p.

754.[19] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129.[20] B. Yan, J. Gu, J. Non-Cryst. Solids 355 (2009) 826.[21] R.P. Rao, J. Lumin. 113 (2005) 271.[22] S. Briche, D. Zambon, D. Boyer, G. Chadeyron, R. Mahiou, Opt. Mater. 28 (2006)

615.[23] N. Arul Dhas, K.C. Patil, J. Alloys Compd. 202 (1993) 137.[24] F. Brunet, V. Brunet, V. Bonneau, T. Irifune, Am. Miner. 91 (2006) 211.[25] R.J. Angel, L.W. Finger, R.M. Hazen, M. Kanzaki, D.J. Weidner, R.C. Liebermann,

D.R. Veblen, Am. Miner. 74 (1989) 509.[26] F.C. Bishop, J.V. Smith, J.B. Dawson, African Kimberlites, Lithos 11 (1978) 155.[27] F. Brunet, N. Bagdassarov, R. Miletich, Solid State Ionics 159 (2003) 35.[28] J. Gopalakrishna, K.K. Rangan, Chem. Mater. 4 (1992) 745.[29] O.P. Shrivastava, R. Chourasia, N. Kumar, Ann. Nucl. Energy 35 (2008) 1147.[30] D.M. Bykov, E.R. Gobechiya, Y.K. Kabalov, A.I. Orlova, S.V. Tomilin, J. Solid State

Chem. 179 (2006) 3101.[31] M.P. Orlova, D.B. Kitaev, M.L. Spiridonova, N.V. Zubkova, Y.K. Kabalov, A.I.

Orlova, Struct. Inorg. Compd. Crystallogr. Rep. 50 (2005) 918.[32] A. Lempicki, E. Berman, A.J. Wojtowicz, M. Balcerzyk, L.A. Boatner, IEEE T. Nucl.

Sci. 40 (1993) 384.[33] A.J. Wojtowicz, A. Lempicki, D. Wisniewaski, L.A. Boatner, Mater. Res. Soc.

Symp., San Francisco, CA, 1994, 348.[34] A.J. Wojtowicz, Conf. Rec. EURODIM 94, Lyon, France, 1994.[35] D.T. Palumbo, J.J. Brown Jr., J. Electrochem. Soc. 117 (1970) 1184.[36] D.T. Palumbo, J.J. Brown Jr., J. Electrochem. Soc. 118 (1971) 1159.[37] T. Koskentato, M. Leskel, L. Niinisto, Mater. Ref. Bull. 20 (1985) 265.