Photoluminescence properties of M2Mg(BO3)2:Sm3+ (M:Sr and Ba)

Journal of Luminescence 134 (2013) 8–13

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Photoluminescence properties of M2Mg(BO3)2:Sm3 þ (M:Sr and Ba) _Ilhan PEKGO ¨ ZLU ¨n Bartin University, Faculty of Engineering, Department of Environmental Engineering, Bartin 74100, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 April 2012 Received in revised form 3 August 2012 Accepted 21 September 2012 Available online 11 October 2012

M2Mg(BO3)2:Sm3 þ (M:Sr and Ba) phosphors were synthesized by a solution combustion synthesis method followed by heating of the precursor combustion ash at 900 1C in air. The synthesized materials were characterized by using the powder XRD. The emission and excitation spectra of these materials were measured at room temperature with a spectrofluorometer. Both Sr2Mg(BO3)2:Sm3 þ and Ba2Mg(BO3)2:Sm3 þ phosphors emit a strong in orange-red region. It was observed that the optimum concentration of Sm3 þ in Sr2Mg(BO3)2 and Ba2Mg(BO3)2 are 0.01 and 0.04 mol, respectively. Finally, the relation between the photoluminescence properties of Sm3 þ and host compositions was discussed in detail. & 2012 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence Chemical synthesis XRD Inorganic borate

1. Introduction Trivalent samarium with 4f5 configuration has complicated energy levels and various possible transitions between 4f levels. The transitions between these 4fn energy levels are highly selective and offer sharp line spectra [1,2]. Inorganic borates have long been a focus of research for their variety of structure types, transparency to a wide range of wavelengths, high laser damage tolerance, and high optical quality. A variety of BOn atomic groups are considered to be dominant for the physical properties, in particular the optical properties, of inorganic borates [3]. It is well-known that the photoluminescent properties of Sm3 þ doped inorganic borates have been widely investigated because of the varied optical energy level structures of Sm3 þ which result in light emission from blue to red regions [4–9]. The phases of Sr2Mg(BO3)2 and Ba2Mg(BO3)2 are an example of alkaline-earth borates and they are characterized by having an association of BO3 triangles, MO9 (M¼Sr, Ba) polyhedra, and MgO6 octahedra. The crystal structures of M2Mg(BO3)2 (M¼Sr, Ba) have been studied in detail [10–12]. Both Sr2Mg(BO3)2 and Ba2Mg(BO3)2 are a useful photoluminescence hosts for rare earth ions. For example, the luminescent properties of Eu2 þ , Ce3 þ , Tb3 þ , Tm3 þ , Dy3 þ , Pr3 þ and Pb2 þ doped M2Mg(BO3)2 (M¼Sr, Ba) have been reported up to now [11–22], but, photoluminescence properties of Sm3 þ doped M2Mg(BO3)2 (M¼Sr, Ba) has not yet been studied. In the present work, M2Mg(BO3)2 (M¼Sr, Ba) materials with different mol ratios of Sm3 þ were prepared by a solution combustion synthesis method. The combustion process to prepare the

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precursor powders, however, is very facile and only takes a few minutes, which has been extensively applied to the preparation of inorganic phosphors. The method makes use of the heat produced in exothermic reactions between metal nitrates and urea or other fuels. Furthermore, the process is also safe, instantaneous and energy saving in comparison with other methods [23,24]. The synthesized materials were characterized by using the powder X Ray Diffraction. After synthesis and characterization of the phosphors, the photoluminescence properties of the synthesized materials were studied using a spectrofluorometer.

2. Experimental Pure and Sm3 þ doped M2Mg(BO3)2 (M¼Sr, Ba) materials were prepared by a solution combustion synthesis method. Sr(NO3)2 (Merck Z99%), Ba(NO3)2 (Merck Z99%), Mg(NO3)2 (Merck Z99%), H3BO3(Merck Z99.8%), Sm2O3 (Alfa Aesar Z99.99%) and CO(NH2)2 (Fluka Z99.5%) were used as starting materials. The stoichiometric amounts of M(NO3)2 (M¼Sr, Ba), Mg(NO3)2, H3BO3 and CO(NH2)2 were dissolved in a minimum amount of distilled water and placed in porcelain containers, separetely. The stoichiometric amounts of Sm2O3 was also dissolved in a minimum amount of concentrated HNO3. And then, this solution were added to porcelain containers. The precursor solutions were introduced into a muffle furnace and maintained at 500 1C for 10 min. The precursor powders were removed from furnace. The voluminous and foamy combustion ashes were easily milled to obtain a precursor powder of Sr2  xSmxMg(BO3)2 (x ¼0, 0.005, 0.01, 0.02 and 0.03) and Ba2  xSmxMg(BO3)2 (x ¼0, 0.0075, 0.01, 0.015, 0.02, 0.03, 0.04 and 0.05). The well-mixed precursor powders were thoroughly mixed and then heated up to 600 1C

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9

311 111

220 111

200

202

110

001

201 002

112

Sr 1.99 Sm0.01Mg(BO3)2

402 310

222

401

400 112

221 313

113

003

Sr2Mg(BO3)2

reference

10

15

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25

30

35

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60

2 Theta Degree Fig. 1. XRD pattern obtained for Sr2  xSmxMg(BO3)2 (x ¼0 and 0.01) prepared by a solution combustion synthesis.

for 6 h in air. After milling, the samples were slowly heated at 900 1C for 12 h in air. The XRD structural analysis of the synthesized materials were performed on an X-ray Bruker AXS D8 Advance equipped with Cu Ka (30 kV, 15 mA, l ¼ 1.54051 A1) radiation at room temperature. Scanning was generally performed between 101 and 901 2y. Measurement was made with 0.051 steps and a 0.0021/sec scan rate. The photoluminescence excitation and emission spectra were measured at room temperature with a Thermo Scientific Lumina fluorescence spectrometer equipped with a 150 W Xenon lamp.

3. Results and discussion 3.1. X- ray powder diffraction analysis The XRD pattern of pure and Sm3 þ doped Sr2Mg(BO3)2 is presented in Fig. 1, which is in agreement with the XRD data of Sr2Mg(BO3)2 in Ref. [18–20]. The XRD pattern of pure and Sm3 þ doped Ba2Mg(BO3)2 is presented in Fig. 2, which is in agreement with the JCPDS (82–1883). From Figs. 1 and 2, the XRD results indicate that all of the diffraction peaks can be attributed to the Sr2Mg(BO3)2 and Ba2Mg(BO3)2 phases. 3.2. Photoluminescence of Sr2Mg(BO3)2:Sm3 þ The compound of Sr2Mg(BO3)2 contains layers built up from isolated BO3 triangles and MgO6 octahedra, interleaved with SrO9 polyhedra to form a three-dimensional framework. Each Sr atom is nine-coordinate in a distorted tricapped trigonal prismatic geometry [10]. The two possible sites available for incorporating Sm3 þ in Sr2Mg(BO3)2 lattice are either the Mg2 þ sites or the Sr2 þ sites. The Sm3 þ (1.132 A1 for CN¼9) ion has a much larger ionic radius, compared with that of Mg2 þ (0.72 A1 for CN ¼6) ion. However, the ionic radius of Sr2 þ (1.31 A1 for CN¼9) is larger

than that of Sm3 þ ion. So it would be expected that Sm3 þ would replace Sr2 þ in Sr2Mg(BO3)2 lattice; this is confirmed by XRD analysis. The excitation and emission spectra of Sr2Mg(BO3)2:Sm3 þ are demonstrated in Figs. 3 and 4. The excitation spectrum of Sr2Mg(BO3)2:Sm3 þ (Fig. 3) monitored with 597 nm emissions of Sm3 þ (4G5/2-6H7/2). The excitation spectrum of Sm3 þ is composed of several bands at 231, 341, 358, 371, 399, 458 and 477 nm, respectively. The strong broadband around 231 nm is due to charge transfer band (CTB) of Sm3þ – O2 . And, the other excitation peaks at 341, 358, 371, 399, 458 and 477 nm have been assigned to the transition from 6H5/2 to 4H9/2, 4D3/2, 6P7/2, 6L13/2, 4I13/2, and 4I11/2 of Sm3 þ , respectively [8,9,25,26]. The emission spectrum of Sr2Mg(BO3)2: Sm3 þ (Fig. 4) has been measured upon 231 nm excitation. It is composed of four bands corresponding to the transitions from the 4G5/2 to the 6H5/2 (565 nm), 6H7/2 (597 nm), 6 H9/2(647 nm) and 6H11/2 (707 nm). The strongest one is located at 597 nm due to 4G5/2 to 6H7/2 transition of Sm3 þ [27–31]. In addition, the emission and excitation spectra of the synthesized material Sr2Mg(BO3)2 with different Sm3 þ doping concentrations were analyzed at room temperature. As seen in Fig. 5, with different Sm3 þ doping concentrations, the shapes and positions of the emission peaks have exhibited no obvious changes. The dependence of the emission intensity on the Sm3 þ concentration for the Sr2 xSmxMg(BO3)2 (0.005rxr0.03) is shown in Fig. 5. With increasing Sm3 þ concentration in Sr2Mg(BO3)2, the emission intensity of the synthesized phosphors increases and reaches a maximum at 0.01 mol. And then, when the mole concentration of Sm3 þ ion exceeds this concentration level, the emission intensity decreases, due to concentration quenching. The concentration quenching of Sm3 þ in phosphors is that the interaction of Sm3 þ –Sm3 þ also increases with increasing Sm3 þ concentration; consequently, emission intensity becomes lower [5,32,33]. Therefore, it can be seen that the optimum concentration of Sm3 þ in Sr2Mg(BO3)2:Sm3 þ phosphor is 0.01 mol.

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10 110

012

202 104

Ba 1.96Sm 0.04Mg (BO3)2

015

003

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024

006 113

101

107

205 116

122

009 211

214

125

119

027

217 0111

1013

128 220

312

134

223

315

226

1211

039

0213 404

Ba 2Mg (BO 3) 2

JCPDS Card No: 82-1883

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80

90

2 Theta Degree Fig. 2. XRD pattern obtained for Ba2  xSmxMg(BO3)2 (x¼ 0 and 0.04) prepared by a solution combustion synthesis. 7500

CTB

Relative Intensity

6000

4500

3000

4f → 4f transitions

1500

0 150

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250

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350

400

450

500

550

Wavelength (nm)

Fig. 3. The excitation spectra of Sr2  xSmxMg(BO3)2 (x¼ 0.01) at room temperature (lems ¼597 nm).

3.3. Photoluminescence of Ba2Mg(BO3)2:Sm3 þ The crystal structure of Ba2Mg(BO3)2 is that of the mineral buetschliite. And, the Ba atom is bound by parallel bases of distorted hexagons and triangles, while the Mg atom occupies a distorted octahedral environment [12]. The two possible sites available for incorporating Sm3 þ in Ba2Mg(BO3)2 lattice are either the Mg2 þ sites or the Ba2 þ sites. The Sm3 þ (1.132 A1 for CN ¼9) ion has a much larger ionic radius, compared with that of Mg2 þ (0.72 A1 for CN ¼6) ion. However, the ionic radius of Ba2 þ (1.57 A1 for CN¼ 9) is larger than that of Sm3 þ ion. So it would be expected

that Sm3 þ would replace Ba2 þ in Ba2Mg(BO3)2 lattice; this is confirmed by XRD analysis. The excitation and emission spectra of Ba2Mg(BO3)2:Sm3 þ are demonstrated in Figs. 6 and 7. The excitation spectrum of Ba2Mg(BO3)2:Sm3 þ (Fig. 6) was monitored with 600 nm emissions of Sm3 þ (4G5/2-6H7/2). The excitation spectrum of Sm3 þ is composed of several strong bands at 240, 343, 360, 374, 402, and 467 nm, respectively. The broadband around 240 nm is due to charge transfer band (CTB) of Sm3 þ –O2  . And, the other strong peaks at 343, 360, 374, 402, and 467 nm have been assigned to the transition from 6H5/2 to 4H9/2, 4D3/2, 6P7/2, 6L13/2, and 4I13/2 of

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7500

Relative Intensity

6000

4500

3000

1500

0 450

500

550

600

650

700

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800

Wavelength (nm)

Fig. 4. The emission spectra of Sr2  xSmxMg(BO3)2 (x ¼0.01) at room temperature (lexc ¼231 nm).

Fig. 5. Emission spectra of Sm3 þ (0.50, 1, 2, and 3 mol%):Sr2Mg(BO3)2 phosphors (lexc ¼231 nm).

Sm3 þ , respectively [8,9,25,26]. The emission spectrum of Ba2Mg(BO3)2: Sm3 þ (Fig. 7) has been measured upon 402 nm excitation. It is composed of four bands corresponding to the transitions from the 4G5/2 to the 6H5/2 (563 nm), 6H7/2 (600 nm), 6 H9/2 (647 nm) and 6H11/2 (706 nm). The strongest one is located at 600 nm due to 4G5/2 to 6H7/2 transition of Sm3 þ [27–31]. In addition, the emission and excitation spectra of the synthesized material Ba2Mg(BO3)2 with different Sm3 þ doping concentrations were analyzed at room temperature. As seen in Fig. 8, with different Sm3 þ doping concentrations, the shapes and positions of the emission peaks have exhibited no obvious changes. The dependence of the emission intensity on the Sm3 þ concentration for the Ba2  xSmxMg(BO3)2 (0.0075rx r0.05) is shown in Fig. 8. With increasing Sm3 þ concentration in Ba2Mg(BO3)2, the emission intensity of the synthesized phosphors increases and reaches a maximum at 0.04 mol. And then, when the mole concentration of Sm3 þ ion exceeds this concentration level, the emission intensity decreases, due to concentration quenching. The concentration quenching of Sm3 þ in phosphors

is that the interaction of Sm3 þ –Sm3 þ also increases with increasing Sm3 þ concentration and consequently, emission intensity becomes lower [5,32,33]. Therefore, it can be seen that the optimum concentration of Sm3 þ in Ba2Mg(BO3)2: Sm3 þ phosphor is 0.04 mol. Additionally, in this study, the relation between the photoluminescence properties of Sm3 þ and host compositions were studied. References [34–36] indicate that the hypersensitive (dominant) transitions of RE3 þ is quite sensitive to the covalency of the RE3 þ –O2  bond. The transition 4G5/2-6H7/2 of Sm3 þ belongs to a dominant transition, which is particularly sensitive to the change of crystal structure. When considering the bond structure of Sm3 þ –O2  –M2 þ (M ¼Sr, Ba), the degree of covalency of the Sm3 þ –O2  bond is weaker in the Sr2Mg(BO3)2 than in the Ba2Mg(BO3)2. Because Sr2 þ atoms attracts the electrons of O2  most strongly due to the fact that it has a larger electronegativity and smaller radius than Ba2 þ atoms [21]. Therefore, electrons could more easily transfer from O2  orbitals to the Sm3 þ ion in the Ba2Mg(BO3)2 than the other one. Thus, the energy of charge

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12 25000

4f → 4f transitions

Relative Intensity

20000

15000 CTB

10000

5000

0 150

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Wavelength (nm)

Fig. 6. The excitation spectra of Ba2  xSmxMg(BO3)2 (x ¼0.04) at room temperature (lems ¼600 nm).

25000 4G 5/2

6H 7/2

Relative Intensity

20000

15000

10000

4G 5/2

6H 5/2

4G 5/2

6H 9/2

5000 4G 6 5/2→ H11/2

0 450

500

550

600 650 Wavelength (nm)

700

750

800

Fig. 7. The emission spectra of Ba2  xSmxMg(BO3)2 (x ¼0.04) at room temperature (lexc ¼402 nm).

Fig. 8. Emission spectra of Sm3 þ (0.75, 1, 1.5, 2, 3, 4, and 5 mol%):Ba2Mg(BO3)2 phosphors (lexc ¼402 nm).

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Table 1 Positions of CTB of Sm3 þ and ionic radius of M2 þ in M2Mg(BO3)2 (M ¼Sr, Ba). Compound

CTB (nm)

ionic radius of M2 þ (A) ˚

CN

Sr2Mg(BO3)2:Sm3 þ Ba2Mg(BO3)2:Sm3 þ

231 240

1.31 1.57

9 9

13

excitation with 231 nm. And, Ba2Mg(BO3)2:Sm3 þ phosphor shows four emission bands at 563, 600, 647 and 706 nm under excitation with 402 nm. So, both the synthesized phosphors exhibit a strong orange-red photoluminescence in the visible region. Additionally, photoluminescence properties of M2Mg(BO3)2 (M¼Sr, Ba) materials with different mol ratios of Sm3 þ were analyzed at room temperature. It was observed that the optimum concentration of Sm3 þ in Sr2Mg(BO3)2 and Ba2Mg(BO3)2 are 0.01 and 0.04 mol, respectively. Based on these investigations, it was concluded that the position of the CTB and the transition 4G5/2-6H7/2 of Sm3 þ in both Sr2Mg(BO3)2 and Ba2Mg(BO3)2 depend strongly on the cation M2 þ (M¼ Sr, Ba) site occupied by Sm3 þ ions and the degree of covalency of the Sm3 þ –O2 bond. Consequently, M2Mg(BO3)2:Sm3 þ (M:Sr and Ba) phosphors could be considered as an ideal optical material for the development of new optical display systems.

References

Fig. 9. Energy level scheme and emission bands of Sm3 þ doped Sr2Mg(BO3)2 (a) and Ba2Mg(BO3)2 (b).

transfer band (CTB) of Sm3 þ decreases in the series of Sr2 þ to Ba2 þ phosphors (Table 1) [37]. As a result, it can be explained that the charge transfer band (CTB) of Sm3 þ in both Sr2Mg(BO3)2 and Ba2Mg(BO3)2, are 231 and 240 nm, respectively, shift to longer wavelength depending on the degree of covalency of the Sm3 þ – O2  bond in M2Mg(BO3)2 (M:Sr, Ba) and the ionic radius of the M2 þ (M:SroBa). Additionally, when M2 þ is changed from Sr2 þ to Ba2 þ , the degree of covalency of the Sm3 þ –O2  bond rises with decreasing electronegativity and increasing radius of the M2 þ ions. As a result (Fig. 9), it was observed that with increasing the radius of the M2 þ (M:SroBa) ion in M2Mg(BO3)2 (M:Sr, Ba), the dominant transition 4G5/2-6H7/2 of Sm3 þ in Ba2Mg(BO3)2 (600 nm) shift to longer wavelength compared with Sr2Mg(BO3)2 (597 nm).

4. Conclusion M2Mg(BO3)2: Sm3 þ (M: Sr and Ba) phosphors were synthesized and characterized by using the powder XRD. The photoluminescence properties of the synthesized phosphors were studied using a spectrofluorometer at room temperature. Sr2Mg(BO3)2:Sm3 þ phosphor shows four emission bands at 565, 597, 647 and 707 nm under

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