Processing and Application of Ceramics 12 [1] (2018) 8–12

https://doi.org/10.2298/PAC1801008F

Synthesis and luminescent properties of novel red-emitting

M7Sn(PO4)6:Eu3+ (M = Sr, Ba) phosphors

Guo Feng1,∗, Weihui Jiang1,2,3,∗, Jianmin Liu1, Cong Li1, Quan Zhang1

1National Engineering Research Center for Domestic & Building Ceramics, Jingdezhen Ceramic Institute,

Jingdezhen 333000, China2Department of Material Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333000, China3Jiangxi Key Laboratory of Advanced Materials, Jingdezhen 333000, China

Received 17 March 2017; Received in revised form 6 October 2017; Received in revised form 20 November 2017;Accepted 18 January 2018

Abstract

Novel Eu3+-activated M7Sn(PO4)6 (where M = Sr, Ba) red-emitting phosphors were synthesized via conven-tional solid-state reaction method at 1200 °C for 2 h. The luminescence properties of the prepared samplesand quenching concentration of Sr7-xSn(PO4)6 :xEu3+ and Ba7-xSn(PO4)6 :xEu3+ were investigated. These phos-phors can be efficiently excited by UV (395 nm) and visible blue (465 nm) light nicely matching the output wave-lengths of the near-UV LEDs and InGaN blue LED chips and emit the red light. The critical concentrationsof the Eu3+ activator were found to be 0.175 mol and 0.21 mol per formula unit for Sr7-xSn(PO4)6 :xEu3+ and

Ba7-xSn(PO4)6 :xEu3+, respectively. The M7-xSn(PO4)6 :xEu3+ (M = Sr, Ba) phosphor may be a good candidatefor light-emitting diodes application.

Keywords: strontium tin phosphate, barium tin phosphate, Eu3+-doping, red phosphors, luminescence

I. Introduction

Solid state lighting based on InGaN light-emittingdiodes (LED) shows significant potential for replacingconventional lighting sources, such as incandescent andfluorescent lamps, because of their high luminous effi-ciency, energy-saving, long lifetime and environmentalprotection. In this field, there are three different methodsthat can be used to realize white light emitting: i) red-green-blue (RGB) light emitting diode chips combineddirectly, ii) blue-LED chip combined with yellow (orgreen and red) wavelength conversion phosphor and iii)near-ultraviolet LED chip combined with RGB wave-length conversion phosphor [1–5].

Inorganic phosphors typically consist of an inert hostlattice that is doped with activator ions, usually transi-tion (3d) or rare-earth (4f) metals. The host lattice istransparent for the incident radiation and the activatoris excited to emit photons [6]. In recent years, extensive

∗Corresponding author: tel: +86 798 8499000,e-mail: fengguo@jci.edu.cn (Guo Feng)whj@jci.edu.cn (Weihui Jiang)

research has been carried out on rare-earth- doped phos-phors because of several important superior properties,such as luminescent characteristics,stability in vacuum,and corrosion-free gas emission under electron bom-bardment compared with traditional cathode ray tubeused in current field emission displays [7,8]. TrivalentEu ion, as one of the promising species that provideoptical emission in red colour regions, has been dopedin various compounds [9–11]. However, to the best ofour knowledge, there is no report on the research ofM7Sn(PO4)6 (M = Sr, Ba) phosphor activated by rareearth or transition metal.

In this work, new luminescent materialM7Sn(PO4)6 : Eu3+ (M = Sr, Ba) was synthesized,its luminescence properties and the Eu3+ concen-tration dependence of the emission properties wereinvestigated.

II. Experimental

Strontium carbonate SrCO3 (A.R.), barium carbon-ate BaCO3 (A.R.), tin dioxide SnO2 (A.R.), ammo-

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nium dihydrogen phosphate NH4H2PO4 (A.R.), andeuropium oxide Eu2O3 (A.R.) were taken as start-ing materials. Stoichiometric amounts of these mate-rials were weighted as the nominal composition ofM7-xSn(PO4)6 :xEu3+ (where M = Sr, Ba and x = 0,0.035, 0.07, 0.105, 0.14, 0.175, 0.21, 0.245). Then theyare blended and mixed in agate mortar. The finelyground powders were directly placed in a high denseAl2O3 crucible and processed by conventional solidstate reactions route at 1200 °C for 2 h at ambient airatmosphere. Following reactions occurred, where M =Sr or Ba:

(7−x)MCO3 + SnO2 + 6 NH4H2PO4 +x

2Eu2O3 −−−→

M7−xSn(PO4)6 : xEu3+ + 6 NH3 + 9 H2O + (7−x)CO2 (1)

The structure of M7Sn(PO4)6:Eu3+ phosphors wasidentified by recording the powder X-ray diffraction(XRD) patterns using X’pert PRO X-ray diffractome-ter with CuKα1 radiation (λ = 1.54056 Å). Scanningelectron microscope (SEM, PHENOM Pr) was em-ployed to analyse the morphology and microstruc-ture of the typical Sr6.86Sn(PO4)6 : 0.14 Eu3+ andBa6.86Sn(PO4)6 : 0.14 Eu3+ phosphors. Excitation andemission spectra were measured at room temperature byusing Hitachi F-4500 spectrofluorometer equipped witha 60 W Xenon lamp as excitation source.

III. Results and discussion

Figure 1 shows the typical XRD patterns of theM7-xSn(PO4)6 :xEu3+ samples, where M = Sr or Baand x = 0 or 0.14. The XRD patterns of the samplesSr7Sn(PO4)6 and Sr6.86Sn(PO4)6 : 0.14 Eu3+ matchedwell with JCPDS 33-1355 card (corresponding tocubic Sr7Sn(PO4)6), and the XRD patterns of thesamples Ba7Sn(PO4)6 and Ba6.86Sn(PO4)6 :0.14 Eu3+

agreed well with JCPDF 34-0064 card (correspond-

ing to cubic Ba7Sn(PO4)6). In all the samples no char-acteristic peaks of the raw materials or other impu-rities were detected. The diffraction peak positionsand relative intensities of the samples Sr7Sn(PO4)6and Ba7Sn(PO4)6 are consistent with the JCPDS val-ues. In contrast, a slight diffraction angle shift tohigher angles are observed in the diffraction patternsof the doped samples (Sr6.86Sn(PO4)6 :0.14 Eu3+ andBa6.86Sn(PO4)6 :0.14 Eu3+) suggesting a decrease in theinterplanar distance, which is due to the two main rea-sons: i) the substitution of Sr2+ (the ionic radius of0.127 nm) and Ba2+ (the ionic radius of 0.143 nm) byEu3+ with smaller ionic radius (0.113 nm) and ii) va-cancy in structure introduced by unequal valence sub-stitution according to equation (2). In addition, the XRDpatterns also indicate that Eu3+ does not significantly in-fluence the structure of the host, and the single-phasedphosphors can be obtained successfully in our experi-mental conditions.

2 Eu3+ + 3 M2+−−−→ 2 Eu·

M + V,,M (M = Sr or Ba) (2)

SEM analyses were employed to inves-tigate the morphology and particle size ofthe samples Sr6.86Sn(PO4)6 : 0.14 Eu3+ andBa6.86Sn(PO4)6 : 0.14 Eu3+. The typical morpho-logical images, represented in Fig. 2, show that bothof these phosphors have regularly shaped individualparticles with clearcut edges, which indicates excellentcrystallinity of phosphors. The surface of the powdershas many pores and voids. They may be formed byvolatile gases exiting matrix (CO2 and NH3, equation(1)). The micrograph referring to the obtained phos-phors shows the presence of large agglomerates in theirregular rigid block form of approximately 10 µm. Theaverage grain size is on micrometric scale.

The fluorescence excitation spectra ofthe samples Sr6.86Sn(PO4)6 : 0.14 Eu3+ andBa6.86Sn(PO4)6 : 0.14 Eu3+ were shown in Fig. 3.

(a) (b)

Figure 1. X-ray diffraction patterns of: a) Sr7Sn(PO4)6 and Sr6.86Sn(PO4)6 :0.14 Eu3+ and b) Ba7Sn(PO4)6 and

Ba6.86Sn(PO4)6 :0.14 Eu3+

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(a) (b)

Figure 2. SEM images of: a) Sr6.86Sn(PO4)6 :0.14 Eu3+ and b) Ba6.86Sn(PO4)6 :0.14 Eu3+ phosphor

Figure 3. Excitation spectra of Sr6.86Sn(PO4)6 :0.14 Eu3+ and

Ba6.86Sn(PO4)6 :0.14 Eu3+ samples

They exhibit similar excitation bands positions andrelative intensities, indicating that excitation bands

stem from the same electronic state. The excitationbands located at 362 nm, 393 nm, 413 nm and 463 nmare attributed to the 7F0→

5D1, 7F0→5L6, 7F1→

5D3and 7F0→

5D2 transitions of Eu3+, respectively, andthe 7F0→

5L7 transition of Eu3+ splits into two bandslocated at 376 nm and 382 nm. These phosphors can beexcited with wavelengths of 395 nm and 465 nm nicelyin agreement with the widely applied near-UV LEDsand InGaN blue LED.

The emission spectra of the host M7Sn(PO4)6 anddoped M6.86Sn(PO4)6 :0.14 Eu3+ samples (M = Sr, Ba)were shown in Fig. 4. The emission spectra of theSr6.86Sn(PO4)6 :0.14 Eu3+ phosphor under 395 nm and465 nm excitation show roughly the same emissionbands, except for the difference in intensity (Fig. 4a).The emission bands at about 591 nm, 610 nm and647 nm are assigned to transitions of 5D0→

7FJ (J =

1–3), respectively. The emission bands at about 576 nmand 696 nm are supposed to be the host-related excita-tion, ascribing to the overlapping of the host (its emis-

(a) (b)

Figure 4. Emission spectra of: a) host Sr7Sn(PO4)6 and doped Sr6.86Sn(PO4)6 :0.14 Eu3+ samples; and b) host Ba7Sn(PO4)6 and

doped Ba6.86Sn(PO4)6 :0.14 Eu3+ samples

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sion spectrum is the yellow line in Fig. 4a) with 5D0→7F0 and 5D0→

7F4, respectively. Apart from aboveobvious emission bands, the excitation spectra of theSr6.86Sn(PO4)6 :0.14 Eu3+ phosphor contain two weakshoulder bands at about 553 nm and 565 nm, which isconsistent with results of the excitation spectrum of thehost Sr7Sn(PO4)6.

As it can be seen from Fig. 4b, similar to theSr6.86Sn(PO4)6 :0.14 Eu3+ phosphors, there is only a tinydistinction in intensity between the emission spectra ofthe Ba6.86Sn(PO4)6 :0.14 Eu3+ phosphors under 395 nmand 465 nm excitations. Nevertheless, the 5D0→

7FJ

(J = 0–4) transition of Eu3+ emission bands exhibitslight blue-shifted emissions (relative to the correspond-ing emission bands in the emission spectra of the sampleSr6.86Sn(PO4)6 :0.14 Eu3+) located at 576 nm, 586 nmand 588 nm (split into two bands), 609 nm, 649 nm and694 nm, respectively. It is because the electronegativityof Ba (0.89) is smaller than that of Sr (0.95), whichmakes the bonding strength between Ba2+ and nega-tively charged phosphate groups stronger than that ofSr2+ [12], leading to the energy transmission of theBa7Sn(PO4)6 host lattice (absorbing group [PO4]) to theactivation centre and the following electromagnetic ra-diation [13] is higher. This is also in accordance with thesplitting comparison of Eu3+ 5D0→

7F1 transition in thesamples Ba7Sn(PO4)6 and Sr7Sn(PO4)6. Definitely, theweak shoulder excitation bands below 560 nm originatefrom the host lattice excitation as it can be concludedfrom the comparison to the excitation spectrum of thehost Ba7Sn(PO4)6. The emission intensity correspond-ing to the 465 nm excitation is slightly lower than that of395 nm because of the relatively lower absorption at thiswavelength (Fig. 4). The appearance of the host latticeexcitation bands in the excitation spectra of phosphorsindicates that there exists efficient energy transfer fromthe host lattice of M7Sn(PO4)6 (M = Sr, Ba) to Eu3+

ions.It is well known that the efficient Eu3+-activated

phosphors mainly depend on the absorption of the hostand energy transfer efficiency [14]. It can be concludedfrom these results that PO 3 –

4 plays an important rolein this novel phosphor. It absorbs the energy and thentransfers it to Eu3+, which increases the excited energyof Eu3+ and enhances the emission efficiency. Accord-ingly, the novel phosphor could be regarded as an effi-cient luminescent material.

The change of emission intensity and wavelength forthe samples M7Sn(PO4)6 :Eu3+ (M = Sr, Ba) as a func-tion of Eu3+ concentration (x = 0.005, 0.01, 0.015, 0.02,0.02, 0.025, 0.03 and 0.35) was shown in Fig. 5. Forboth investigated systems lower Eu3+ doping concen-trations lead to weak luminescence, while higher dop-ing beyond an optimum causes concentration quench-ing of the Eu3+ emission. The highest integrated emis-sion intensity is noted at the Eu3+ concentration takenas the critical concentration. Generally, energy migra-tion processes increase the probability that the opticalexcitation is trapping at defects or impurity sites, en-hancing non-radiative relaxation. As the excitation en-ergy migrates among a large number of centres be-fore being emitted, the excitation energy may trans-fer between the close Eu3+ ions by the exchange in-teraction. With the increase in Eu3+ concentration, theaverage distance between Eu3+ ions decreases. Thisfavours the energy transfer, and the critical concen-tration corresponds to the sufficient reduction in theaverage distance. Further reduction leads to cross re-laxation namely above mentioned non-radiative relax-ation, which causes concentration quenching. On theother hand, a decrease in the activator concentration de-creases the energy stored by the ions. In the specificsystems such as M7Sn(PO4)6 :Eu3+ (M = Sr, Ba), thecritical concentration of the activator (Eu3+) was foundto be 0.175 mol and 0.21 mol per formula unit for theSr7-xSn(PO4)6 :xEu3+ and Ba7-xSn(PO4)6 :xEu3+, respec-tively. This result is mainly attributed to two reasons:i) the unit cell volume of Ba7Sn(PO4)6 (12299 Å3) is

(a) (b)

Figure 5. Emission spectra of: a) Sr7-xSn(PO4)6 :xEu3+ and b) Ba7-xSn(PO4)6 :xEu3+ (where x = 0.035, 0.07, 0.105, 0.14, 0.175,0.21, 0.245)

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much larger than that of Sr7Sn(PO4)6 (10599 Å3), whichcauses increase in the critical concentration to achievethe best average distance of Eu3+ ions in two extremelysimilar cubic crystal structures; ii) the stronger interac-tion between activation centres with the host structurein Ba7-xSn(PO4)6 :xEu3+ weakens the energy migrationand the cross relaxation between the activation centres.

IV. Conclusions

The novel red phosphors M7-xSn(PO4)6 : xEu3+

(where M = Sr, Ba and x = 0, 0.035, 0.07, 0.105,0.14, 0.175, 0.21, 0.245) were synthesized by theconventional solid-state reaction at 1200 °C for 2 h.The quenching concentration in the Ba7Sn(PO4)6is (0.21 mol per formula unit) higher than in theSr7Sn(PO4)6 (0.175 mol per formula unit) due to the rel-atively larger unit cell volume and crystal field effectsof the Ba7Sn(PO4)6. It is also discovered that PO 3+

4 ab-sorbs the energy and then transfers it to Eu3+, whichincreases the excited energy of Eu3+ and enhances theemission efficiency. These phosphors can be efficientlyexcited by UV (395 nm) and visible blue (465 nm) lightnicely matching the output wavelengths of the near-UVLEDs and InGaN blue LED chips and emits the redlight. The M7-xSn(PO4)6 :xEu3+ (M = Sr, Ba) phosphormay be a good candidate for light-emitting diodes ap-plication.

Acknowledgement: This work was supported bythe National Natural Science Foundation of China[grant numbers 51162013, 51362014]; the MajorDiscipline Academic and Technical Leader Train-ing Plan Project of Jiangxi Province [grant num-ber 20113BCB22009]; the Science and TechnologySupporting Plan Project of Jiangxi Province, China[grant number 20111BBE50018]; the Youth ScienceFoundation of Jiangxi Province, China [grant number20171BAB216009]; the Science Foundation of JiangxiProvincial Department of Education, China [grant num-ber GJJ150887]; the Youth Science Foundation ofJiangxi Provincial Department of Education, China[grant number GJJ150892]; the postdoctoral researcherspreferred funded projects of Jiangxi Province [grantnumber 2013KY34]; and the Jingdezhen Science andtechnology program [grant number 20161GYZD011-007].

References

1. J. Kido, M. Kimura, K. Nagai, “Multilayer white light-emitting organic electroluminescent device”, Science, 267

[5202] (1995) 1332.2. S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura,

“Prospects for led lighting”, Nat. Photonics, 3 [4] (2009)180–182.

3. S. Neeraj, N. Kijima, A.K. Cheetham, “Novelred phosphors for solid-state lighting: the systemNaM(WO4)2-x(MoO4)x : Eu3+, (M = Gd, Y, Bi)”, Chem.

Phys. Lett., 387 [1-3] (2004) 2–6.4. G. Gundiah, Y. Shimomura, N. Kijim, A.K. Cheetham,

“Novel red phosphors based on vanadate garnets for solidstate lighting applications”, Chem. Phys. Lett., 455 [4](2008) 279–283.

5. A. Potdevin, G. Chadeyron, R. Mahiou, “Tb3+-doped yt-trium garnets: promising tunable green phosphors forsolid-state lighting”, Chem. Phys. Lett., 490 [1] (2010) 50–53.

6. R.L. Toquin, A.K. Cheetham, “Red-emitting cerium-basedphosphor materials for solid-state lighting applications”,Chem. Phys. Lett., 423 [4-6] (2006) 352–356.

7. J.S. Bae, K.S. Shim, B.K. Moon, “Enhanced luminescentcharacteristics of Y2-xGdxO3 : Eu3+ ceramic phosphors byLi-doping”, J. Korean Phys. Soc., 46 [5] (2005) 1193–1197.

8. Z.W. Pei, Q. Su, S.H. Li, “Investigation on the lumines-cence properties of Dy3+ and Eu3+ in alkaline-earth bo-rates”, J. Lumin., 50 [2] (1991) 123–126.

9. X.M. Liu, C.X. Li, Z.W. Quan, Z.Y. Cheng, J. Lin, “Tun-able luminescence properties of CaIn2O4 : Eu3+ phos-phors”, J. Phys. Chem. C, 111 [44] (2007) 16601–16607.

10. S.F. Wang, K.K. Rao, Y.R. Wang, Y.F. Hsu, S.H. Chen,Y.C. Lu, “Structural characterization and luminescentproperties of a red phosphor series: Y2-xEux(MoO4)3, (x =0.4–2.0)”, J. Am. Ceram. Soc., 92 [8] (2009) 1732–1738.

11. Y.R. Do, J.W. Bae, “Application of photoluminescencephosphors to a phosphor-liquid crystal display”, J. Appl.

Phys., 88 [8] (2000) 4660–4665.12. R.C. Ropp, Luminescence and the Solid State, Elsevier,

Amsterdam, The Netherlands, 1991.13. A. Nag, N. Kutty, “Role of interface states associated

with transitional nanophase precipitates in the photolumi-nescence enhancement of SrTiO3 : Pr3+

,Al3+”, J. Mater.

Chem., 13 [9] (2003) 2271–2278.14. G. Blasse, “On the Eu3+ fluorescence of mixed metal

oxides. IV. The photoluminescent efficiency of Eu3+-activated oxides”, J. Chem. Phys., 45 [7] (1966) 2356–2360.

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