简介概要

Synthesis and enhanced luminescence properties of double perovskite NaLa_0.95-xY_xEu_0.05MgWO₆ phosphors

来源期刊：Rare Metals2020年第8期

论文作者：Jing Zhang Jia-Lin Shen Nai-Cen Xu

文章页码：895 - 901

摘要：Double perovskite Na（La,Y）MgWO₆:Eu³⁺phosphors were obtained by the sol-gel method.X-ray diffractometer（XRD） patterns,scanning electron microscopy（SEM） images and photoluminescence properties were carried out to study the effect of polyethylene glycol（PEG）.Enhanced optical properties were obtained by adding 3 wt% PEG under the excitation of 395 nm.Y³⁺with smaller radius was selected to substitute La³⁺ in this host to decrease the symmetry and further enhance the luminescence properties.With Y³⁺concentration increasing,Na₂WO₄ and Y₃₀W₈O₆₉ phases came up.The emission spectra showed different shapes,and the emission intensity was enhanced by doping Y³⁺.Under the excitation of 395 and 465 nm,the emission intensity reached the maximum.The Commission Internationale de L’Eclairage（CIE）coordinates of the phosphors were on the edge of the CIE diagram,illustrating that this double perovskite phosphor showed good color purity.Above luminescence properties showed that the synthesized phosphors had great potential in solid-state lighting.

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稀有金属(英文版) 2020,39(08),895-901

Synthesis and enhanced luminescence properties of double perovskite NaLa_0.95-xY_xEu_0.05MgWO₆ phosphors

Jing Zhang Jia-Lin Shen Nai-Cen Xu

Nanjing Center,China Geological Survey

Supervision and Testing Center of East China,Mineral Resources of the Ministry of Land and Resources

摘要：

Double perovskite Na(La,Y)MgWO₆:Eu³⁺phosphors were obtained by the sol-gel method.X-ray diffractometer(XRD) patterns,scanning electron microscopy(SEM) images and photoluminescence properties were carried out to study the effect of polyethylene glycol(PEG).Enhanced optical properties were obtained by adding 3 wt% PEG under the excitation of 395 nm.Y³⁺with smaller radius was selected to substitute La³⁺ in this host to decrease the symmetry and further enhance the luminescence properties.With Y³⁺concentration increasing,Na₂WO₄ and Y₃₀W₈O₆₉ phases came up.The emission spectra showed different shapes,and the emission intensity was enhanced by doping Y³⁺.Under the excitation of 395 and 465 nm,the emission intensity reached the maximum.The Commission Internationale de L'Eclairage(CIE)coordinates of the phosphors were on the edge of the CIE diagram,illustrating that this double perovskite phosphor showed good color purity.Above luminescence properties showed that the synthesized phosphors had great potential in solid-state lighting.

作者简介：*Jia-Lin Shen,e-mail:njcgssjl@163.com;

收稿日期：23 July 2018

基金：financially supported by the Opening Project of State Key Laboratory of High-Performance Ceramics and Superfine Microstructure (No.SKL201309SIC);

Synthesis and enhanced luminescence properties of double perovskite NaLa_0.95-xY_xEu_0.05MgWO₆ phosphors

Jing Zhang Jia-Lin Shen Nai-Cen Xu

Nanjing Center,China Geological Survey

Supervision and Testing Center of East China,Mineral Resources of the Ministry of Land and Resources

Abstract：

Double perovskite Na(La,Y)MgWO₆:Eu³⁺phosphors were obtained by the sol-gel method.X-ray diffractometer(XRD) patterns,scanning electron microscopy(SEM) images and photoluminescence properties were carried out to study the effect of polyethylene glycol(PEG).Enhanced optical properties were obtained by adding 3 wt% PEG under the excitation of 395 nm.Y³⁺with smaller radius was selected to substitute La³⁺ in this host to decrease the symmetry and further enhance the luminescence properties.With Y³⁺ concentration increasing,Na₂WO₄ and Y₃₀W₈O₆₉ phases came up.The emission spectra showed different shapes,and the emission intensity was enhanced by doping Y³⁺.Under the excitation of 395 and 465 nm,the emission intensity reached the maximum.The Commission Internationale de L'Eclairage(CIE)coordinates of the phosphors were on the edge of the CIE diagram,illustrating that this double perovskite phosphor showed good color purity.Above luminescence properties showed that the synthesized phosphors had great potential in solid-state lighting.

Keyword：

Double perovskite; Structure evolution; Phosphors; White light emitting diodes;

Received： 23 July 2018

1 Introduction

White light emitting diodes (W-LEDs) have been widely used as the fourth lighting source due to the excellent properties,such as environment friendliness,low power consumption,long lifetimes,high efficiency,and high brightness [ 1, 2, 3, 4] .The commercial ways to get white light are phosphor conversion methods [ 5, 6, 7] .One of them is combining a blue LED chip with YAG:Ce³⁺yellow phosphors.But the color rendering index of the devices is low due to the lack of red emission component [ 8, 9] .The other way is combining a near ultraviolet (NUV) LED chip with tricolor phosphors.However,the intensity of red light is much lower than those of blue and green phosphors under the same excitation.Therefore,phosphors with strong emission red light attracted researchers'attention [ 10, 11, 12, 13, 14, 15, 16, 17] .

Perovskite oxides ABO₃ have been widely used as the host due to the micro-structures and performances [ 18, 19, 20, 21] .Especially,double perovskite AA'BB'O₆ and A₂BB'O₆ have kept booming in recent years [ 2, 11, 22, 23, 24] .A-and/or B-site ordering in these double perovskites would extend the original framework to super-lattice.The representative phosphors are AA'BB'O₆:Eu³⁺(BaLaMgSbO₆,NaLaMgWO₆) and A₂BB'O₆:Eu³⁺(Sr₂CaMoO₆,Ca₂LaSbO₆) [ 9, 11, 24, 25] .Our research work focused on red-emitting NaLaMgW0₆:Eu³⁺phosphor with high quenching concentration.NaLaMgWO₆ shows doubly ordering,rock-salt B-site cation ordering and layered A-site cation ordering.Li+(0.092 nm,coordination number (CN)=8) with smaller radius had been chosen to substitute Na⁺(0.139 nm,CN=12) and Gd³⁺(0.111 nm,CN=9) to substitute La³⁺(0.136 nm,CN=12) [ 22] .The doping of both Li+and Gd³⁺could immensely enhance the luminescence intensity.As known,A-site ions and A'-site ions show layered ordering in this double perovskite.Doping ions with smaller radius on both A site and A'site would decrease the tolerance factor and further affect the properties of the phosphor.Based on above ideas,smallerradius Y³⁺was selected to replace La³⁺to enhance the luminescence intensity.The structural evolution and the luminescence properties were studied in detail.Besides,the modifier polyethylene glycol (PEG) was added during the synthesis period and the effect of PEG was discussed in detail.

2 Experimental

Starting materials were La₂O₃,Eu₂O₃,Y₂O₃,NaNO₃,Mg(NO₃)2,(NH₄)₁₀W₁₂O₄₁,HNO₃,NH₃·H₂O,citric acid(CA) and PEG.All the materials were analytically pure and directly used without purification.The amount of CA was3 mol times of the metal ions,and the weight ratio of PEG was 2 wt%of the target powders.Rare earth oxides were dissolved in HNO₃ to prepare nitrate.Then stoichiometric amount of all nitrates was mixed.The CA and PEG were added into the mixed solution,and the pH value of the solution was modulated to 2.0-4.0.Then (NH₄)10W₁₂O₄₁was added into the above solution,after which the mixed solution was heated to 80-90℃for 1 h to get gel.After then,the transparent gel was heated to 180-200℃to obtain spumous and fluffy precursor.Finally,the precursor was pre-calcined at 1100℃for 6 h.Series of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤x≤0.95) red phosphors were obtained.

The crystalline phases were determined by X-ray diffractometer (XRD,smartlabTM 9 kW,Rigaku,Japan)using Cu Kαradiation.Microstructural evolution of obtained powders was investigated by a scanning electron microscope (SEM,SU8010,Hitachi,Japan).The optical properties were measured by the fluorescent spectrophotometer (Lumina,America).

3 Results and discussion

3.1 Effect of addition of PEG

XRD patterns of the as-obtained NaLaMgWO₆:Eu³⁺phosphors using different amounts of PEG (0 wt%,1 wt%,2 wt%,3 wt%,and 4 wt%) are shown in Fig.1.All the spectra have the same shape and can be indexed by NaLaMgWO₆ (JCPDS No.88-1761).Only some small peaks at 17.1°and 43.2°can be indexed by Na₂WO₄ as the impurity phase.Typically,the peaks at 11.4°and 25.7°belong to the layered ordering of Na⁺and La³⁺.The peaks at 19.2°and 37.9°are observed in the patterns,indicating that MgO₆ and WO₆ exist in this double perovskite as a rock-salt ordering [ 26, 27] .

Fig.1 XRD patterns of NaLaMgWO₆:Eu³⁺phosphors using differ-ent amounts of PEG

SEM images of the obtained powders are shown in Fig.2.The particles exhibit irregular shape with blurry edges while no PEG was used.With PEG addition,the edges become clear and the particles become polyhedrons gradually.On the other hand,with the PEG amount increasing,the size of the particles increases firstly and reaches the maximum while 2 wt%PEG was used.Continuing adding PEG,the size of the particles reduces and the edges of the particles become clearer.Clearer edges mean better crystalline of the particles,indicating that adding PEG benefits the crystalline.With PEG addition,the ions in the solution could be modified more uniformly,which benefits the formation of the gel.During the calcination period,the uniform particles would benefit the grain growth,resulting in better crystalline.

Figure 3 shows the excitation and emission spectra of NaLaMgWO₆:Eu³⁺and the energy level diagram of Eu³⁺.The excitation spectrum is composed of a strong wide band and several weak sharp peaks.The strong wide band belongs to charge transfer band (CTB) and the sharp peaks belong to characteristic.f-f transition absorption peaks of Eu³⁺.The CTB centered at about 320 nm is mainly attributed to W-O absorption [ 28] .Figure 3b shows the simplified energy level diagram of Eu³⁺.The sharp peaks centered at 395 and 465 nm are assigned to the ⁷F₀-⁵L₆ and⁷F₀-⁵D₂ transitions of Eu³⁺ [ 29] .The emission spectrum shows several sharp peaks,among which the peak at615 nm plays the dominant role.The peaks at 590,615,650 and 698 nm are corresponding to the transitions of⁵D₀-⁷F₁,⁵D₀-⁷F₂,⁵D₀-⁷F₃ and ⁵D₀-⁷F₄,respectively.The excitation spectra of NaLaMgWO₆:Eu³⁺using different amounts of PEG are shown in Fig.4.All the spectra show the same shape,a strong wide band and several weak sharp peaks.Among the sharp peaks,the peak centered at465 nm shows the strongest intensity,indicating that this phosphor is more suitable for blue chips.With the amount of PEG increasing,the absorption intensity reaches the maximum at 2 wt%,1 wt%,and 3 wt%for CTB,395 nm and 465 nm,respectively.

Fig.2 SEM images of NaLaMgWO₆:Eu³⁺phosphors using different amounts of PEG:a 0 wt%,b 1 wt%,c 2 wt%,d 3 wt%,and e 4 wt%

Fig.3 a Excitation and emission spectra of NaLaMgWO₆:Eu³⁺and b simplified energy level diagram of Eu³⁺

The emission spectra of the samples are shown in Fig.5.All the spectra show a strong electric dipole transition and a weak magnetic dipole transition,indicating that Eu³⁺occupies a low symmetry site in the lattice [ 1, 30] .With PEG addition,the luminescence intensities of the phosphors reach the maximum at 2 wt%,1 wt%and 3 wt%PEG while the phosphors are excited by CTB,395 nm and465 nm,respectively.As known,one side of PEG contacts the clathrate and the other side stretches sufficiently in solution.Little amount of PEG could not contact the clathrate well and excessive PEG would twine around each other,which both count against the luminescence intensities.

Fig.4 Excitation (λ_em=615 nm) spectra of NaLaMgWO₆:Eu³⁺phosphors using different amounts of PEG

Fig.5 Emission spectra (λ_ex=CTB,395 nm and 465 nm) of NaLaMgWO₆:Eu³⁺phosphors using different amounts of PEG

3.2 Structural evolution and luminescence properties of NaLa0.95-xYxEu0.05MgWO6

NaLa_0.95-xY_xMgWO₆:Eu³⁺(0≤x≤0.95) phosphors are exhibited in Fig.6.For the samples prepared by sol-gel method,the Na₂WO₄ and Y₃₀W₈O₆₉ phases are observed as impurity phases.When x≤0.30,double perovskite NaLaMgWO₆ is the main phase and Na₂WO₄ is the impurity phase.Obviously,the diffraction peaks of A-site ordering and B-site ordering still exist.With Y³⁺concentration increasing,the Na₂WO₄ phase amount increases. When x is between 0.30 and 0.70,the phase is mainly composed of Na₂WO₄ and NaLaMgWO₆.With the x value further increasing,double perovskite phase disappears and Y₃₀W₈O₆₉ turns up as the main phase,which might result from the big radius difference between Y³⁺and La³⁺.Schematic illustration of Na(La,Y)MgWO₆ is shown in Fig.7.Obviously,this double perovskite shows double ordering of A-site and B-site ions.Na⁺and La³⁺/Y³⁺show unique layered ordering among the double perovskite structures while Mg²⁺and W⁶⁺show rock-salt ordering.

Fig.6 XRD patterns of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤x≤0.95)

Figure 8a shows the excitation spectra (λ_em=594 nm)of Eu³⁺-doped Na(La,Y)MgWO₆.A strong charge transfer band in the region of 250-350 nm and a series of sharp excitation ships above 350 nm could be clearly observed.With the x value increasing,the CTB shifts to the high wavelength number.The absorption intensity reaches the maximum at x=0.90,x=0.90,and x=0.70 for CTB,395 nm and 465 nm,respectively.Figure 8b shows the excitation spectra (λ_em=615 nm) of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤x≤0.95).Differently,the spectra show different shapes due to the phase transition.When x is between 0 and 0.30,the CTB centered at 325 nm is strong.Upon increasing the x value,CTB shifts to lower wavelength number and is centered at 280 nm,which might result from the increasing phase of Na₂WO₄.The absorption intensity reached the maximum at x=0.70 for395 and 465 nm.When x is bigger than 0.7,double perovskite phase disappears and the CTB weakens.

Fig.7 Schematic illustration of Na(La,Y)MgWO₆

Fig.8 Excitation spectra of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤x≤0.95) withλ_em of a 594 nm and b 615 nm

Figure 9 shows the emission spectra (λ_ex=CTB,395 nm and 465 nm) of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤ x≤0.95).The spectra show different shapes for different doping concentrations when the phosphors were excited by CTB.The electric dipole transition shows the dominant role when x is smaller than 0.70.Differently,the peaks split into two peaks at 612 and 618 nm while x is between 0 and0.30.The transition for x=0.50 and x=0.70 shows a single peak at 615 nm.The emission spectra show a strong peak at 615 nm while x<0.90,indicating that Eu³⁺occupies the La³⁺site in the double perovskite phase [ 22] .When x≥0.90,the Y₃₀W₈O₆₉ phase shows as the main phase and then the emission spectra show different shapes.Because of the large radius difference between Eu³⁺and W⁶⁺,Eu³⁺mainly occupied the Y³⁺site in Y₃₀W₈O₆₉.The spectra contain two strong bands at 605 and 628 nm and some weak bands.By doping Y³⁺,the emission intensities decrease under the excitation of CTB,which is resulted from the damage of the host lattice.The spectra show different regular under the excitation of 395 and465 nm.The emission intensity reaches the maximum at x=0.7.The intensity at 0.70 is about 4 times that at 0.The intensities for 0.90 and 0.95 are much lower than that for0.70 under the excitation of 465 nm.

Luminescence decay curves of the selected NaLa_0.95Eu_0.05MgWO₆ and NaLa_0.25Y_0.7Eu_0.05MgWO₆phosphors are shown in Fig.10.Lifetimes of the phosphors were calculated based on the decay curves.Both the two curves could be fitted by the single experimental function,as shown in Eq.(1).The lifetimes were calculated to be484 and 442μs for NaLa_0.95Eu_0.05MgWO₆ and NaLa_0.25Y_0.7Eu_0.05MgWO₆,respectively.The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates were calculated based on the emission spectra,and the results are shown in Table 1 and Fig.11.The coordinates are in the range of (0.654,0.345) and (0.660,0.340)while x≤0.70.The coordinates of NaLa_0.95Eu_0.05MgWO₆are (0.660,0.340),which is rather close to the“ideal red”(0.67,0.33).Upon increasing the x value,the coordinates come away from (0.67,0.33).In addition,the chromaticity coordinates,which are close to the border of CIE diagram,indicate that the phosphors show good color purity.

Fig.9 Emission spectra of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤x≤0.95) withλ_ex of a CTB,b 395 nm and c 465 nm

Fig.10 Luminescence decay curves (λ_ex=465 nm) of NaLa_0.95Eu_0.05MgWO₆ and NaLa_0.25Y_0.7Eu_0.05MgWO₆

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Table 1 CIE chromaticity coordinates of NaLa_0.95-xY_x-Eu_0.05MgWO₆ (0≤x≤0.95)

Fig.11 CIE chromaticity coordinates of NaLa_0.95Eu_0.05MgWO₆phosphors

where I andI₀ represent the emission intensity at the time of t and 0,respectively,τis the decay lifetime.

4 Conclusion

A series of NaLa_0.95-xY_xEu_0.05MgWO₆ (0≤x≤0.95)phosphors were successfully synthesized.The luminescence properties were enhanced by adding PEG,and the phosphors showed better luminescence properties while adding 3 wt%PEG under the excitation of 395 nm.The particles transferred from irregular shape to polyhedron by adding PEG.With the concentration of Y³⁺increasing,the Na₂WO₄ and Y₃₀W₈O₆₉ phase came up and the double perovskite phase gradually disappeared.Both the excitation spectra and emission spectra showed different shapes because of the structural evolution.The coordinates of NaLa_0.95Eu_0.05MgWO₆ were (0.660,0.340),which was rather close to the“ideal red”(0.67,0.33).The normal phosphors showed good color purity in solid-state lighting.