Luminescence properties of reddish orange emitting BaLa2ZnO5:Sm3+ phosphor prepared by citric based sol-gel synthesis

Journal Pre-proof Luminescence properties of reddish orange emitting BaLa2ZnO5:Sm3+ phosphor prepared by citric based sol-gel synthesis

Vijay Singh, G. Lakshminarayana, Dong-Eun Lee, Jonghun Yoon, Taejoon Park PII:

S1293-2558(19)30826-X

DOI:

https://doi.org/10.1016/j.solidstatesciences.2019.106042

Reference:

SSSCIE 106042

To appear in:

Solid State Sciences

Received Date:

12 July 2019

Accepted Date:

14 October 2019

Please cite this article as: Vijay Singh, G. Lakshminarayana, Dong-Eun Lee, Jonghun Yoon, Taejoon Park, Luminescence properties of reddish orange emitting BaLa2ZnO5:Sm3+ phosphor prepared by citric based sol-gel synthesis, Solid State Sciences (2019), https://doi.org/10.1016/j. solidstatesciences.2019.106042

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Luminescence properties of reddish orange emitting BaLa2ZnO5:Sm3+ phosphor prepared by citric based sol-gel synthesis Vijay Singh a, 1, G. Lakshminarayana b, 1, Dong-Eun Lee c, Jonghun Yoon d, Taejoon Park e a Department

of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea Construction Automation Center, Kyungpook National University, 80, Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea cSchool of Architecture and Civil Engineering, Kyungpook National University, 80, Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea dDepartment of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Ansan, Gyeonggi-do, 15588, Republic of Korea eDepartment of Robotics Engineering, Hanyang University, 55 Hanyangdaehak-ro, Ansan, Gyeonggi-do, 15588, Republic of Korea bIntelligent

Abstract A series of BaLa2ZnO5:Sm3+ phosphors were synthesized with the sol-gel technique. The crystalline structure of each sample was characterized by X-ray diffraction (XRD). Phase formation was investigated using scanning electron microscope (SEM). Photoluminescence studies revealed an efficient excitation of phosphors at near blue light (~ 410 nm), providing emission in the bright reddish-orange region (603 nm). Concentration quenching through multipolar interaction was observed in BaLa2ZnO5 phosphor system doped with 0.015 mol of Sm3+ concentration. The optimum phosphor showed prominent characteristics for application in white LEDs.

Keywords: Sol-gel; XRD; Sm3+ ions; BaLa2ZnO5; Photoluminescence *Corresponding author: E-mail: [email protected] (V.Singh) 1Both authors equally contributed to the paper 1

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1. Introduction Recently, growing suitable host material search for display devices such as field emission displays, plasma display panels, and white light-emitting diodes (WLEDs) has led the researchers’ interest in studying relevant rare-earth (RE) doped luminescent materials. Especially, in the furtherance of phosphor-based materials by replacing conventional fluorescent and incandescent lamps. These new breed light emitters have various advantages such as dominant luminescence efficiency, low energy consumption, longer lifetime, flexible emission colors with different dopants, low cost, and environmental friendliness [1], [2]. InGaN blue LED chip topped by Ce3+:YAG powder phosphor encapsulated by epoxy resins is a common and well-established standard technology for WLEDs fabrication in the current working period. However, these phosphor-converted WLEDs have low luminescence intensity, color degradation as a result of high LED junction temperature, short lifetime due to low color rendering index (CRI < 80), high correlated color temperature (CCT > 7000K), missing of orange to red color emitting constituents, and light produced only in blue to yellow region of the visible spectrum. Hence, devices can emit dim and colorless light [3]. To eradicate drawbacks of white light quality, devices must require emitting phosphors that can be excited by InGaN blue LEDs and possess a long lifetime with high chemical stability. Blue LED/ near UV LED chips (380-420 nm) with a combination of red, green, and blue (tricolor) emitting phosphors are known to have quality output because of their improved CCT and CRI values. The rationale is that CCT, CRI, and CIE chromaticity values of LEDs can be tuned by altering the tri-color ratio. The development of such efficient new phosphors is the new vision with pressing need [4].

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In such devices, materials that absorb energy and emit energy with radiation have more scope. Hence, phosphors are popularly used to provide display devices with high resolution and brightness. Unlike solid-state reaction method, the sol-gel technique is a soft chemical method for synthesizing phosphor particles with regular morphology, chemical homogeneity, and low reaction temperature. Regular phosphors incorporating alkoxides as raw materials that are synthesized through so-called sol-gel technique must tolerate high toxicity and high cost. Thus, we followed a simplified sol-gel method using standard solvable metal salts such as acetates, nitrates, and chlorides as precursors and citric acid (C.A.) as chelating ligands of metal ions to reduce the segregation of metal ions and ensure compositional homogeneity [5], [6]. According to Dahiya et al. [1], oxide phosphors with promising practical applications are attracting attention, especially BaLn2ZnO5 base matrix. Ln is a RE ion owing to its tetragonal lattice with tetrahedral ZnO4 structure, 10 folds of BaO10 polyhedral C2 symmetry axis, and octacoordinated LaO8 polyhedral D4 crystal field symmetry. BaLa2ZnO5 lattice has two alternate layers of Zn-Ba-O and La-O with activator ions trapped in the C2 symmetry site [1]. In the present work, Sm3+ doped BaLa2ZnO5 phosphors were prepared using the sol-gel process. Effects of Sm3+ concentrations on the structural and luminescence properties of BaLa2xZnO5:xSm

phosphors were investigated. Excitation of Sm3+ ions in the UV-visible regions

triggers green to reddish-orange emission even with other phosphors in the blend as it is least affected by crystal field due to its partially filled 4f shells that are shielded by 5s2 and 5p6 electrons. Sm3+ ions (4f5) are not detrimental to any inorganic lattices or ligand fields. However, they produce strong reddish-orange emission due to 4G5/2→6HJ (J = 5/2, 7/2, 9/2) transitions. Sm3+ ions doped materials have the highest advantages in distinct fluorescent devices, color

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displays, laser, and so on, making Sm3+ ions the motivation for the present study to examine their capabilities for novel reddish-orange emitting phosphors [7], [8]. Performances of different base phosphors such as Ba2CaZn2Si6O17:Sm3+ [8], La6Ba4Si6O24F2:Sm3+ [9], Na2NbAlO5:Sm3+ [10], and Sr2ScGaO5:Sm3+ [11] have been evaluated by different researchers with pros and cons of the materials. However, to the best of our knowledge, oxide phosphors, especially BaLa2ZnO5:Sm3+ combination as a novel piece of work has not been reported earlier.

2. Experimental procedure A series of BaLa2-xZnO5:xSm (x=0.005≤x≤0.09) phosphors were prepared using the sol-gel method. All raw materials taken were of the highest purity available. They were used without further purification. Details of sample composition and starting materials are given in Table I. In a typical synthesis, stoichiometric quantity of metal salts such as Ba(NO3)2, La(NO3)3∙6H2O, Zn(NO3)2∙6H2O, Sm(NO3)3∙6H2O and the citric acid (citric acid/metal ion 2:1, molar ratio) were first dissolved in 10 mL deionized water in a 150 mL glass beaker under constant stirring for 1h. A transparent aqueous solution was obtained after stirring. The resultant transparent solution was kept at 110 oC in an oven until a homogeneously dried gel was formed. The dried gel obtained was then pre-fired at 400 oC for 2h in air. Finally, the resultant brown residual sample was fully ground and fired at 1100 oC for 3h in air. XRD patterns of all the prepared samples were recorded using a RIGAKU Miniflex-II diffractometer with Cu-Kα radiation (λ = 1.5406 Å) at a scan rate of 5°/minute in 2 range of 10o to 80o. Morphology details were obtained using Scanning electron microscopy (SEM) (type: S3400, Hitachi, Japan). Diffuse reflectance spectrum was recorded using a UV-VIS-NIR spectrophotometer (type: Cary-5000). Photoluminescence measurements were carried out on a 4

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Shimadzu RF-5301PC equipped with a Xenon flash lamp. All experiments were carried out at room temperature.

3. Results and discussion 3.1 XRD analysis The crystal structure of BaLa2ZnO5 is given in Fig. 1. It indicates that the coordination number of La3+ ions is 8. Since Ba2+ ions have different valence state (2+), larger ionic radii (r = 1.52 Å), and coordination number (10), La3+ ions (r = 1.16 Å) can be replaced by Sm3+ ions (r = 1.079 Å) into BaLa1-xZnO5:xSm tetragonal structure with a space group of I4/mcm [2], [13], [14], [15]. Sm3+ ions are about 7% smaller than the La3+ ions, leading to small reductions in the lattice parameters with an increase in doping. Moreover, charge compensation concerns were none for Sm3+ ions substituting La3+ ions in the BaLa2ZnO5 lattices. The X-ray diffraction patterns of BaLa2-xZnO5:xSm (x=0.005≤x≤0.09) phosphors and standard peak index of BaLa2ZnO5 with JCPDS file no. 80-1882 are shown in Fig. 2. All diffraction peaks of samples were isostructural with a single-phase when compared with JCPDS no. 80-1882. The diffraction peaks slightly shifted to a higher diffraction angle with increasing content of Sm3+ in the samples. The stick chart in XRD patterns confirmed that Sm3+ ions were doped into BaLa2ZnO5 lattice by replacing La3+ ions forming solid solutions [1], [5], [6], [13], [15]–[18], [19]. 3.2 SEM analysis Guo et al. [5] have predicted that in the application point of view phosphors must possess narrow size distribution and regular shape morphology. Optimum characteristics are achieved when particle sizes are in the order of 1 μm and spherical in shape. Representative SEM images of BaLa1.985ZnO5:0.015Sm phosphor at different magnification (x5.0k and x15.0k) synthesized through sol-gel technique are shown in Fig. 3. The sample exhibited good crystallinity in the 5

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form of 2D flaky state. The identified irregular shape and large-sized grains could be due to agglomeration during the high sintering temperature. The SEM images showed many cracks, pores, and voids on the surface due to the release of some gaseous products during the sintering process. Gases escape due to high pressure, leading to the formation of pores and resulting in micrometer range particles that are appropriate for the fabrication of lighting devices [4], [6], [8], [10], [17], [20]. 3.3 UV-VIS-NIR reflectance spectrum analysis UV-VIS-NIR reflectance spectrum of BaLa1.985ZnO5:0.015Sm phosphor is shown in Fig. 4. The broad host absorption in the range of 250-350 nm was attributed to the charge transfer band (CTB) from host ligand oxygen (O) atoms to positive ions in the group. A strong absorption was observed at ~410 nm region in the phosphor, which was used for an effective excitation. Weak absorption bands at 400-500 nm and strong absorptions in NIR regions could be assigned to 4f intra-configurational transitions from Sm3+ ions. The reflectance spectrum showed an optical transmittance up to ~60-70% in the 400-700 nm visible spectral range, making this host amiable for applications in WLEDs [9–11], [20], [21]. 3.4 Excitation spectra analysis Excitation spectra of all the synthesized BaLa2-XZnO5:xSm3+ phosphors monitored at 603 nm emission wavelength due to the 4G5/2→6H7/2 transition are presented in Fig. 5 (a). All the spectra exhibit a broadband between 200 and 300 nm, centered at 250 nm, which can be ascribed to host absorption band. The overlap of ligand to metal charge transfer, i.e., filled 2p orbitals of O atoms to half-filled 4f orbitals of Sm3+ ions, resulted in this broadband. The excitation spectra displayed characteristic 4f-4f transitions of Sm3+ ions from 6H5/2 ground state to higher excited states 6P5/2, 4H

9/2,

4P

3/2,

4D

3/2,

6L 6 4 6 4 4 4 4 15/2, P7/2, F7/2, P5/2, G9/2, I13/2, I11/2, G7/2

6

and 4F3/2 at 310, 320, 350, 370,

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380, 390, 410, 430, 450, 460, 470, 510 and 540 nm, respectively. The highest intensity excitation peak was noticed at 410 nm, indicating that the present phosphors could be excited by blue LED chips, efficiently. The 410 nm wavelength was also used as an excitation source for the emission spectra measurement to comprehend reflectance spectral data. It was revealed that the intensity of excitation peaks reached maximal at x = 0.015 without any changes in its peak positions, illustrating the uniform distribution of Sm3+ ions in BaLa2ZnO5 host [7], [11], [22]. 3.5 Photoluminescence spectra analysis Emission spectra of BaLa2-xZnO5:xSm3+ phosphors upon 410 nm excitation are presented in Fig. 5(b). The integral emission intensity at 603 nm for different Sm3+ content is given in Fig. 6. Fig. 5(b) signifies an increase in the emission intensity up to x = 0.015 followed by a decrease in the intensity due to concentration quenching with further Sm3+ ion concentration increment. Variation in the 603 nm emission intensity as a function of Sm3+ concentration revealed similar results as shown in Fig. 6. At 410 nm excitation, Sm3+ ions are excited first to 4F7/2 level and then non-radiatively decay to 4G5/2 state due to a small energy gap between these levels. From 4G5/2 excited level, ions radiatively de-excited to 6HJ (J = 5/2, 7/2, 9/2) levels giving rise to emissions at 570, 603 and 655 nm, respectively. In Sm3+ ions, a very small chance of multiphonon relaxation (MPR) is possible because of the large energy gap between 4G5/2 excited state to the immediate next 6H11/2 state. Therefore, the luminescence process can occur through non-radiative energy transfer influenced by cross-relaxation between Sm-Sm ions. The highest emission intensity was ascribed to 4G

6H

5/2→

7/2

transition satisfying the selection rule of ΔJ = ±1. The 4G5/2→6H5/2 peak was due to

pure magnetic dipole (MD) transition. The peak 4G5/2→6H9/2 originated from electric dipole (ED) transition, satisfying selection rule for magnetic (ΔJ=0 and ±1) and electric dipole (ΔJ=±2).

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However, the peak 4G5/2→6H7/2 satisfies both magnetic dipole transition selection rule, as well as electric dipole transition selection rule. Pure ED transition is sensitive to the crystal field. In general, it is presumed that the greater its intensity, the more asymmetric nature of the lattice. Consequently, the intensity ratio of ED to MD transitions was used to analyze the symmetric nature of the local environment of 4f trivalent ions. Presently, 4G5/2→6H5/2 MD transition is more intense than ED 4G5/2→6H9/2 transition of Sm3+ ions that possess more symmetric nature in the host matrix. Recently, Li et al. [23‒26] have reported on the photoluminescence of Sm3+ and other rare-earth ions doped/co-doped various kinds of phosphors for their W-LEDs application. Quenching effects occur when the average distance between Sm3+ ions decreases with an increase in its concentration, leading to an energy transfer (ET) process between neighboring ions through cross-relaxation (CR). RE ion non-radiative energy transfer occurs between acceptor and donor ions through radiative re-absorption, exchange interaction, and multipolar interactions. 4G

5/2,

6H

5/2→

Sm3+

For 6F

7/2,

6F

9/2;

4G

ions, 5/2,

6H

possible 6F

5/2→

9/2,

6F

CR

7/2;

channels

are

4G

5/2,

6H

5/2 → 

6F

5/2,

6F

11/2;

and 4G5/2, 6H5/2→6F11/2, 6F5/2, respectively. In the

present BaLa2ZnO5:xSm3+ phosphors, there was no broad overlap between excitation and emission spectra observed. Besides, the critical distance between nearby Sm ions is large at lower concentrations, leading to multipole-multipole interaction phenomenon [4], [7], [8], [20], [22]. 3.6 CIE chromaticity coordinates analysis Fig. 7 depicts the color chromaticity coordinates diagram of the Commission International delÉclairage (CIE) for the optimized BaLa1.985ZnO5:0.015Sm phosphor sample monitored at an emission wavelength of 603 nm and excitation wavelength of 410 nm. The obtained CIE coordinates are marked with a red circle. CIE coordinates are mainly used to locate the emission

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color and color purity of samples. Chromaticity coordinates of BaLa1.985ZnO5:0.015Sm phosphor were (0.58, 0.41), indicating its emission in the reddish-orange region and enforcing its usage as a reddish-orange emitting candidate for W-LED application. Hence, the phosphor can compensate the reddish-orange deficiency to some extent in the Ce3+:YAG based WLEDs. Fig. 8 conveys typical photographs of BaLa1.985ZnO5:0.015Sm phosphor under (a) room light and (b) UV-365 nm lamp. The sample illuminated under 365 nm UV lamp showed a strong reddishorange light while the one illuminated under room light appeared in light brown color. Thus, the present optimum phosphor can serve as a reddish-orange emitting candidate. It can be a significant contender for WLED applications [4], [8–10], [20–22]. 4. Conclusions In summary, a series of BaLa2-xZnO5:xSm3+ phosphors were successfully synthesized adopting the sol-gel technique. XRD profiles revealed crystallinity for the phosphor lattice. Surface morphology of samples through SEM data revealed agglomeration of grains as a result of gaseous byproducts formed at the high temperature. Concentration quenching through multipolar interactions was observed beyond Sm3+ concentration of 0.015 mol. The prepared phosphors exhibited intense emission corresponding to 4G5/2→6H7/2 transition at 603 nm wavelength upon blue excitation at 410 nm, which was close to emission wavelengths of blue LEDs and UV LEDs chips (380-420 nm). The CIE coordinates of BaLa1.985ZnO5:0.015Sm phosphor possessed strong reddish-orange emission. Further the present results, though preliminary in nature, suggest that BaLa1.985ZnO5:0.015Sm phosphors have the potential to be used as reddish-orange emitting phosphors for white light emitting diodes.

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Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018M2B2A9065656).

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Figure caption Figure 1. General view of BaLa2ZnO5 crystal structure. Figure 2. Powder XRD pattern of BaLa2-xZnO5:xSm (x=0.005≤x≤0.09) phosphor. Figure 3. SEM images of BaLa1.985 ZnO5:0.015Sm phosphor. Figure 4. UV-VIS-NIR spectra of BaLa1.985 ZnO5:0.015Sm phosphor. Figure 5. Photoluminescence spectra of the BaLa2-xZnO5:xSm (x=0.005≤x≤0.09) phosphor (a) Excitation spectrum of BaLa2-xZnO5:xSm (x=0.005≤x≤0.09) (λem=603 nm) and (b) Emission spectrum of BaLa2-xZnO5:xSm (x=0.005≤x≤0.09) (λexc=410 nm) Figure 6. Variation in the emission intensity of strong emission (603 nm) as a function of Sm3+ concentration. Figure 7. CIE chromaticity diagram of BaLa1.985ZnO5:0.015Sm phosphor. Figure 8. Typical photographs of BaLa1.985ZnO5:0.015Sm (a) phosphor sample under room light (appearance: light brown powder) and (b) under UV-365 nm (appearance: reddish-orange powder).

Table caption Table I. Chemical composition of BaLa2-xZnO5:xSm3+ phosphors

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Table I. Chemical composition of BaLa2-xZnO5:xSm3+ phosphor Samples BaLa1.995 ZnO5:0.005Sm BaLa1.985 ZnO7:0.015Sm BaLa1.97 ZnO7:0.03Sm BaLa1.95 ZnO5:0.05Sm BaLa1.93 ZnO7:0.07Sm BaLa1.91 ZnO7:0.09Sm

Ba=0.5226g Ba=0.5226g Ba=0.5226g Ba=0.5226g Ba=0.5226g Ba=0.5226g

La=1.7276g La=1.7190g La=1.7060g La=1.6887g La=1.6713g La=1.6540g

Starting materials Zn=0.5948g C.A=3.0738g Zn=0.5948g C.A=3.0738g Zn=0.5948g C.A=3.0738g Zn=0.5948g C.A=3.0738g Zn=0.5948g C.A=3.0738g Zn=0.5948g C.A=3.0738g

Sm=0.0044g Sm=0.0133g Sm=0.0266g Sm=0.0444g Sm=0.0622g Sm=0.0799g

Ba= Ba(NO3)2, La= La(NO3)3∙6H2O, Zn= Zn(NO3)2∙6H2O, C.A=Citric acid, Sm=Sm(NO3)3∙6H2O

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Fig. 1

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BaLa ZnO :0.09Sm 1.89 5

Intesity (a.u.)

BaLa ZnO :0.07Sm 1.93 5 BaLa ZnO :0.05Sm 1.95 5

BaLa ZnO :0.03Sm 1.97 5

336 523

406

217

415

BaLa ZnO :0.005Sm 1.995 5

215 314 206 402 330 332 413

224 312 006

213 204 310

220

004

002

112 200

202

BaLa ZnO :0.015Sm 1.985 5

JCPDS File No:-80-1882

10

20

30

40

50

2 (Degrees)

Fig. 2

17

60

70

80

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Fig. 3

18

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BaLa1.985ZnO5:0.015Sm

Reflection (%)

6 F11/2

6 F9/2

6 F7/2 6 F5/2

4

6

P3/2

CTB

400

400

4

I13/2 I11/2

6

P5/2

440

480

800

1200

Wavelength (nm)

Fig. 4

19

1600

2000

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6 P 3/2

em = 603 nm 6 H5/2

BaLa ZnO :0.015Sm 1.985 5 BaLa ZnO :0.03Sm 1.97 5

Intensity (a.u.)

CTB

BaLa ZnO :0.005Sm 1.995 5

BaLa ZnO :0.05Sm 1.95 5 BaLa ZnO :0.07Sm 1.93 5 BaLa ZnO :0.09Sm 1.91 5

6 P 7/2 4 L 15/2

4 4 P 5/2 P3/2

250

300

4 I 11/2 6 P 5/2

4 D 4 3/2 D 7/2

4 I 13/2 4 G 7/2

4 G 9/2

350

400

450

500

4 F 3/2

550

Wavelength (nm) (a)

exi = 410 nm

6 H

4 G5/2

BaLa ZnO :0.005Sm 1.995 5

7/2

BaLa ZnO :0.015Sm 1.985 5

Intensity (a.u.)

BaLa ZnO :0.03Sm 1.97 5 BaLa ZnO :0.05Sm 1.95 5 BaLa ZnO :0.07Sm 1.93 5

6 H

BaLa ZnO :0.09Sm 1.91 5

5/2

6 H

9/2 6 H

500

550

600

650

Wavelength (nm) (b) Fig. 5 20

700

11/2

750

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240 220

Intensity (a.u.)

200 180 160 140 120 100 80 0.005

0.015

0.03

0.05

Sm3+ concentration (mol) Fig. 6

21

0.07

0.09

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Fig. 7

22

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Fig. 8

23

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Conflict of Interest REF: Luminescence properties of reddish orange emitting BaLa2ZnO5:Sm3+ phosphor prepared by citric based sol-gel synthesis, submitted by Vijay Singh, G. Lakshminarayana, Dong-Eun Lee, Jonghun Yoon, Taejoon Park,

The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

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Highlights ►The reddish-orange emitting BaLa2ZnO5 phosphor obtained via sol-gel method. ► Phosphors exhibited intense emission corresponding to 4G5/2→6H7/2 transition. ► The Sm3+ emission is quenched at 0.015 mol. ► Synthesized phosphor could be a potential material for white LEDs application. .