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Luminescence properties of red-emitting Mn2+-Activated Na2Mg5Si12O30 phosphors
Materials Research Bulletin 118 (2019) 110494
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Luminescence properties of red-emitting Mn2+-Activated Na2Mg5Si12O30 phosphors Lin Qina,b, Cuili Chenb, Jing Wangb, Shala Bib, Yanlin Huangc, Hyo Jin Seob,
T
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a
School of Electronics and Information, Nantong University, Jiangsu, China Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea c College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Luminescence Optical materials and properties Phosphors
Na2Mg5(1-x)Si12O30:5xMn2+(x = 0.005, 0.01, 0.03,0.05,0.07 and 0.09) red phosphors were synthesized by solid state reactions. The formation of a single phase compound is verified through the X-ray diffraction (XRD) studies. The excitation and emission spectra, and decay curves of Na2Mg5Si12O30:Mn2+ are investigated by optical and laser excitation spectroscopy in the temperature range 7–520 K. The strong red emission peaking at 600 nm is observed due to the spin forbidden transition 4T1 → 6A1 of Mn2+ ions. At high Mn2+ concentration, new side bands appear at lower energy side at around 670 nm which can be ascribed to the appearance of the formation of new Mn2+ centers. In addition, the thermal quenching behavior is investigated to evaluate the commercial value of Na2Mg5Si12O30:Mn2+ as red-emitting materials.
1. Introduction Mn2+ doped into many inorganic hosts is used as a luminescent center with colorful emission ranging from 490 to 750 nm [1]. From the Tanabe-Sugano diagram, it can be deduced that the Mn2+ ions can be excited into several bands in the wavelength range 400–520 nm related to the 6A1→4A1, 4E, 6A1→4T1 and 6A1→4T2 transitions. The emission wavelength of the Mn2+ ion depends strongly on the strength of crystal field and the coordination number (CN) in a host lattice [2]. The Mn2+ ion usually gives a green to yellow emission in a lattice with tetrahedrally coordinated (weak crystal-field) site, whereas it shows an orange to deep red emission with octahedrally coordinated (strong crystal-field) site [3]. Great efforts to investigate the Mn2+-doped compounds have been made and some Mn2+-doped phosphors with good luminescence properties have been synthesized successfully [4,5]. Wu et al. reported the luminescence of single Mn2+-doped KMgBO3 red phosphors [6]. Mn2+-activated CaZnOS shows the great potential for application as an alternative red-emitting LED conversion phosphor due to its high absorption and strong excitation bands [7]. However, few works pay attentions to high thermal stability of Mn2+-activated oxides, which is one of the important parameters for obtained phosphors in potential application due to a considerable influence on the color rendering index (CRI) and light output [8,9].
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In this work, the Na2Mg5Si12O30 host is selected as a lattice to explore a novel Mn2+ -activated red emitting phosphors with high thermal stability. The phosphors are characterized by some measurements such as X-ray diffraction (XRD), emission and excitation spectra and luminescence decay curves. The temperature dependent emission behaviors are further investigated. 2. Experimental A series of Na2Mg5(1-x)Si12O30:5xMn2+ (x = 0.005, 0.01, 0.03, 0.05, 0.07 and 0.09) phosphors were prepared by convenient solid state reaction. The raw reactants are Na2CO3 (99.9%), 4MgCO3-Mg(OH)25H2O(99.9%), SiO2 (99.9%), and MnCO3 (99.9%). The stoichiometric mixtures were thoroughly ground in an agate mortar and heated at 850 °C for 8 h in a crucible in atmosphere. The obtained mixtures were reground carefully and heated to 1150 °C for 8 h in a crucible in atmosphere. The crystal structure of the Na2Mg5Si12O30:Mn2+ powders was examined by the X-ray diffraction (XRD) on the Rigaku D/Max 2000 diffractometer with operating parameters set to 40 kV and 30 mA. A 450 W Xe lamp dispersed by a 25 cm monochromator (Acton Research Corp. Pro-250) was used as a light source for excitation and emission spectra. The luminescence signal was detected using a photomultiplier tube (Hamamatsu, R928, Shizuoka, Japan) mounted on a 75 cm
Corresponding author. E-mail address: [email protected] (H.J. Seo).
https://doi.org/10.1016/j.materresbull.2019.110494 Received 27 January 2019; Received in revised form 5 May 2019; Accepted 19 May 2019 Available online 20 May 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 118 (2019) 110494
L. Qin, et al.
Table 1 Refined Crystallographic Parameters of Na2Mg5Si12O30:Mn2+. Formula
Refined-Na2Mg5Si12O30:Mn2+
radiation 2θ range(degree) symmetry space group# a/Å b/Å c/Å α/° β/° γ/° Z Rp Rwp X2 V /Å3
Cu Ka 10-70 hexagonal P 6/m c c(192) 10.1617(2) 10.1617(2) 14.2377(4) 90 90 120 2 0.0254 0.0174 1.250 1273.21(4)
refinement profiles of the sample. It is in good agreement between the observed and calculated inter-planar spacing without any trace of extra peaks, further suggesting the formation of a single-phase compound. The detailed refinement parameters and atom positions are presented in Tables 1 and 2, respectively. According to the obtained parameters and atom positions, the structural sketch map of Na2Mg5Si12O30 drawn with the Diamond Crystal and Molecular Structure Visualization software is displayed in Fig. 3. Na2Mg5Si12O30 crystallizes in the single hexagonal phase with space-group P6/mcc, z = 2 and cell parameters: α = 90°, γ = 120°, a = 13.3891(7) Å, c = 30.5032(18) Å, and V = 4567.09(60) Å3. In the three-dimensional framework of the Na2Mg(I)2(II)3Si12O30 compound, there are two types of chains. One consists of Si12O30 rings formed through edge-sharing SiO4 and another consists of two types of Mg coordination. The Mg(I) site has tetrahedral coordination and the Mg (II) site has octahedral coordination. The two types of Mg forms chain through edge-sharing symmetric tetrahedral MgO4 and octahedral MgO6. The Na elements with larger sizes occupy two types of sites: one is 12 coordinated sites in the tunnels bounded by the Si12O30 rings and another is 9 coordinated sites between these rings. Fig. 4a and b shows the SEM micrographs of Mn2+ doped Na2Mg5Si12O30 phosphors synthesized using the solid state method, which result in a non-uniform morphology with larger particle size distribution. The average sizes of the particles as depicted in the micrograph are in the range of 2–3 microns. Fig. 4c shows the EDS spectrum of Na2Mg5Si12O30:0.05 Mn2+ Phosphors. The inset of (c) shows the elemental composition of Na2Mg5Si12O30:0.05 Mn2+ determined using EDS. Element mapping analysis (Fig. 5) indicates that Na2Mg5Si12O30:0.05 Mn2+ are composed of sodium, magnesium, silicon, oxygen and manganese with a molar ratio of Na/Mg/Si/O/Mn of ≈ 2:4.75:12:30:0.25, which gives an approximate stoichiometric formula of Na2Mg5Si12O30:0.05 Mn2+. The peak due to C in the spectrum is attributed to the latex in the SEM sample holder.
Fig. 1. XRD patterns of Na2Mg5Si12O30:Mn2+ phosphors with different Mn2+ concentration sintered at 1150 °C for 6 h.
monochromator (Acton Research Corp. Pro-750). The third harmonic (355 nm) of a pulsed Nd:YAG laser was introduced as an excitation source for luminescence decays which were digitized and saved by means of a 500 MHz Tektronix DPO 3054 oscilloscope. For low temperature measurements, the samples were placed in a closed-cycle helium cryostat in the variable-temperature range (10–300 K).
3. Results and discussion 3.1. The phase formation and structural refinement Fig. 1 shows the XRD patterns of Mn2+-doped Na2Mg5Si12O30 as functions of Mn2+ concentration along with the standard PDF card. Diffraction peaks of all the samples are well indexed to the structural results for Na2Mg5Si12O30 single crystal reported by Nguyen et al (JCPDS card 20–1124) [10]. No traces of impurities are detected, especially diffraction peaks belonging to MnO in the samples, which suggests that Mn2+ ions are well substituted for the Mg2+ ions in the Na2Mg5Si12O30 lattice. The structure refinement of 7 mol % Mn2+-doped Na2Mg5Si12O30 was carried out with the GSAS program. Fig. 2 shows the Rietveld
Table 2 The Crystallographic Parameters Obtained from Rietveld Refinement for Na2Mg5Si12O30:Mn2+.
Fig. 2. Rietveld refinement of the powder XRD patterns of Na2Mg5Si12O30:Mn2+. The observed counts and calculated pattern are indicated by crosses and a smooth line, respectively. The vertical marks indicate the position of Bragg peaks, and the bottom trace indicates the difference between the observed and calculated values. 2
Atom
Wyck.
Site
x/a
y/b
z/c
U [Å2]
Na1 Na2 Mg1 Mg2 Si1 O1 O2 O3
2a 4d 4c 6f 24m 12l 24m 24m
622 −6. 3.2 222 1 m. 1 1
0 1/3 1/3 1/2 0.76772(23) 0.73000 0.94500 0.65200
0 2/3 2/3 0 0.11942(30) 0.12300 0.21300 0.15600
1/4 0 1/4 1/4 0.11056(13) 0 0.13500 0.17100
0.099(10) 0.027(4) 0.053(4) 0.0290(24) 0.0224(11) 0.0025 0.0025 0.0025
Materials Research Bulletin 118 (2019) 110494
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Fig. 3. Schematic illustration of the Na2Mg5Si12O30:Mn2+ crystal structure based on the Rietveld refinement, and the coordination environment of the [MgO6]/ [MgO4].
3.2. Luminescence properties of Mn2+-doped Na2Mg(I)2(II)3Si12O30
show the excitation and emission spectra of Na2Mg5Si12O30:Mn2+ at room temperature. The excitation spectra were obtained by monitoring the 600 nm emission and the emission spectra were measured under excitation at 355 nm. The excitation spectra show the absorption in the spectral range 330–530 nm, and the emission spectra exhibit broad emission
Fig. 6a shows the Tanabe-Sugano diagram for the Mn2+ ion with 3d5 electronic configuration. The optical spectra of Na2Mg5Si12O30:Mn2+ can be explained by the Tanabe-Sugano diagram containing the two empirical parameters of Dq and B. Fig. 6b and c
Fig. 4. (a–b) SEM micrographs and (c) EDS spectrum of Na2Mg5Si12O30:0.05 Mn2+ Phosphors. The inset of (c) shows the elemental composition of Na2Mg5Si12O30:0.05 Mn2+ determined using EDS. 3
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Fig. 5. Element mapping of (a) Mn, (b) Si, (c) O, (d) Mg, (e) Na and integration of (a–e).
band in the range of 520–750 nm. The emission band peaking at 600 nm is due to the spin forbidden transition of 4T1 → 6A1. In the Na2Mg(I)2(II)3Si12O30 lattice, the Mn4+ would substitute Mg2+ in expectation (Mg2+: 0.57 A˚ in 4 coordination; 0.72 A˚ in 6 coordination and Mn2+: 0.66 A˚ in 4 coordination; 0.83 A˚ in 6 coordination). Actually, besides the commonly equivalent doping, the non-equivalent doping can also be realized in Mn2+ activated phosphors, for example, the Mn2+ ions would occupy Na+ sites (1.18 A˚ in 8 coordination). Hence, there is great potential for Mn2+ replacing Mg2+ due to their same valance state and similar radius [11]. As mentioned above, two types of Mg sites for the Mn2+ ions are present in the Na2Mg5Si12O30 lattices. Therefore, the Mn2+ ions can substitute for tetrahedrally coordinated Mg(I) sites and/or octahedrally coordinated Mg(II) sites. It is known that the tetrahedrally coordinated Mn2+ (weak crystal-field) usually gives a green emission and the octahedrally coordinated Mn2+ (stronger crystal field) gives an orange to red emission [2]. However, only the red emission occurs from the Mn2+ ions suggesting that the Mn2+ ions replace the single octahedral Mg(II)2+ ions in Na2Mg5Si12O30 lattice. The excitation bands centered at 353, 392, 420, 460 and 512 nm correspond to the transitions from the 6A1(6S) ground state to the 4E (4D), 4T2(4D), 4A1,4E(4G), 4T2(4G) and 4T1(4G) excited states, respectively [12]. It is noted that the 6A1(6S) → 4E(4D) and 4A1(4G) transitions exhibit narrow lines, while the 6A1(6S) → 4T2(4D), 4T2(4G) and 4T1(4G) transitions are rather broad in accordance with the Tanabe-Sugano diagram in which the 4E(4D) and 4A1(4G) levels are relatively less influenced by the crystal field than others [13]. The energy parameters presented in this work are the crystal field Dq, Racah parameter B and C. The first parameter (Dq) indicates the symmetry of impurity ion sites, the second parameter B is related to the covalence of impurity-ligand binding and the Racah C parameter is related to B through C = 4B [14,15]. The crystal field Dq and Racah B parameter are usually obtained from Tanabe-Sugano matrices for d5 electronic configurations as shown in Fig. 6a. In this work, we used the energy matrices including Trees correction (α) given by Mehra. And the Trees correction (α) was introduced due to the configuration 3d54s of
Fig. 6. (a) The Tanabe-Sugano diagram of Mn2+ ions in octahedral crystal fields. (b) and (c) Excitation and emission spectra of the Mn2+-doped Na2Mg5Si12O30 phosphor.
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B=
94α +
49(T2 − T1 )2 − 768α 2 49
(1) −1
where α is the Trees correction with free ion value of 76 cm , T1 and T2 are the energies of 6A1(6S) → 4E(4G) and 6A1(6S) → 4E(4D) transitions which can be obtained from the excitation spectra. Then the value of crystal field parameter B can be calculated to be 788 cm−1. Since the emission energy E of 4T1(G) → 6A1(6S) is 600 nm (16667 cm−1), then the value of E/B is 21.15. Based on the Tanabe-Sugano diagram shown in Fig. 6a, the crystal field parameter value of Dq is 1063 cm−1 and Dq/ B is equal to 1.35 corresponding to the strong crystal field environment. 3.3. Concentration dependent luminescence properties of Mn2+-doped Na2Mg5Si12O30 The emission spectra and decay curves as functions of Mn2+ concentration under excitation at 355 nm at room temperature are presented in Fig. 7. The integrated emission intensity depends on the Mn2+ concentration. The emission reaches a maximum intensity at 7 mol %, and then quenches by further increase in Mn2+ concentration as shown in the inset of Fig. 7b [18]. Normalized emission spectra to the peak intensity at 600 nm are shown in Fig. 7c. The band shapes of the emission spectra for different Mn2+ concentration maintain well at higher energy side including peak maximum at 600 nm. However, new side bands appear at lower energy side at around 670 nm (longer wavelength than 600 nm) and the relative intensities of the new bands increase with increasing Mn2+ concentration. Fig. 7d and e show the luminescence decay curves of the emitting state 4T1 obtained by monitoring the 600 nm emission and the emission of the longer wavelength side at 670 nm as functions of Mn2+-concentration, respectively. The decay curves monitored 600 nm by can be fitted by single-exponential function as I = Aexp(t/τ) [19], and the value of lifetime for all the Mn2+ concentration from 0.5 to 9 mol % (Fig. 7d) are calculated to be around 19.5 ms at 300 K. The decays of the 670 nm emission for 0.5 and 1.0 mol % are single exponential with the decay time of 19.5 ms which are identical to that of the 600 nm emission (Fig. 7e). However, the decays of the 670 nm emission for the Mn2+ concentration higher than 1.0 mol % deviate from single exponential and the deviations are larger with increasing Mn2+ concentration from 3 to 9 mol % (Fig. 7e). The decay time for the Mn2+ concentration of 9 mol % was calculated to be 15 ms. The new 670 nm band (Fig. 7c) and its decay behavior (Fig. 7e) for higher Mn2+ concentration (3–9 mol %) seem to be due to the formation of new Mn2+ centers such as Mn(II)-Mn(II) pairs and Mn(II)-defect centers with stronger crystal-field strength [20,21]. At lower concentration of 0.5 and 1.0 mol %, the decays of the 670 nm emission are consistent with those of 600 nm. This means that the emission in the whole spectral region comes from the identical Mn2+ centers at octahedral Mg(II) sites for low Mn2+ concentration. 3.4. Temperature dependent luminescence properties The temperature dependent emission spectra of Na2Mg5Si12O30:0.07 Mn2+ under excitation at 355 nm are presented in Fig. 8a. The anti-Stokes emission appears at higher energy side with the band broadening from 44 to 70 nm with increasing temperature from 10 to 520 K. The integrated emission intensity increases unexpectedly about 20% from 10 to 300 K and then thermal quenching occurs with further increase in temperature as shown in the inset of Fig. 8a. Luminescence decay curves as functions of temperature are shown in Fig. 8b. Decay times are calculated and displayed in the inset of Fig. 8b. The decay times do not change significantly in the temperature range 10–300 K. From about 300 K, in which the thermal quenching sets, the decay times shorten because of a nonradiative contribution to the decay process. The excited state of electrons can be depopulated by both radiative
Fig. 7. (a) Emission spectra of the Na2Mg5Si12O30:Mn2+ phosphors as functions of Mn2+ concentration under excitation at 355 nm. (b) Integrated emission intensity as a function of Mn2+ Concentration. (c) Concentration dependent emission spectra normalized to the peak intensity at 600 nm. Decay curves of Na2Mg5Si12O30:Mn2+ obtained by monitoring the 600 nm emission (d) and the 670 nm emission (e).
Mn2+ interacts with other configurations such as 3d6 and 3d44s2, generating a mean deviation between experimental and theoretical values for energy terms up to 200 cm−1 [16]. Here, the expression for crystal field parameter B can be taken by the following equation [17]:
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maintains the intensity about 84% at 150 °C and 75% at 200 °C. It has been reported that the luminescence intensities of the commercial red nitride phosphors (Sr0.82Ba0.15Eu0.03)2Si5N8:Eu2+ and (Sr0.75Ca0.25)0.98 SiAlN3:Eu2+ at 150 °C are 87% of the value at 25 °C [24]. The Na2Mg5Si12O30:0.07 Mn2+ phosphor shows similar thermal stabilities the reported phosphors. 4. Conclusions In this work, a series of Na2Mg5Si12O30:Mn2+ red phosphors were synthesized by high temperature solid state reactions. The emission of Na2Mg5Si12O30:Mn2+ occurs in the red region peaking at 600 nm and the spectral feature and emission intensity depends on Mn2+ concentration. There exist two different Mg2+ sites, tetrahedral and octahedral, in the Na2Mg5Si12O30 lattice. Of the two the Mn2+ ion substitutes for the octahedral Mg2+ site. The spectral features of the emission bands are well maintained at lower Mn2+ concentration less than 1.0 mol%. However, new side bands appear at around 670 nm for the Mn2+ concentration higher than 1.0 mol% and the relative intensities of the new bands increase with increasing Mn2+ concentration due to the formation of new Mn2+ centers such as Mn(II)-Mn(II) pairs and Mn(II)-defect centers with stronger crystal-field strength. Temperature dependent emission spectra and decay curves indicate that the synthesized Na2Mg5Si12O30:Mn2+ red phosphor show excellent thermal quenching behavior. The phosphor maintains the intensity about 84% at 150 °C and 75% at 200 °C, due to the radiative rate which increases in this system. The results indicate that the Na2Mg5Si12O30:Mn2+ red phosphor is a promising candidate applied in solid-state lighting and display. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) (2017R1D1A1B03029432). References Fig. 8. (a) Temperature dependent emission spectra of the Na2Mg5Si12O30:Mn2+ phosphor under excitation at 355 nm. the inset shows integrated emission intensity as a function of temperature. (b) Decay curves of the Na2Mg5Si12O30:Mn2+ phosphor as functions of temperature. The inset shows decay times as a function of temperature.
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and nonradiative transition processes which is expressed by the following equation [22]. −1 τ −1 = τR−1 + τNR
(2)
τ −1
is decay rate, i.e., the transition probability from the excited Where, −1 state to the ground state, τR−1 is radiative rate and τNR is nonradiative rate. The emission intensity is related to the ratio of radiative rate to the decay rate:
I = I0
τR−1 τ −1
(3)
Where, I0 is a constant and proportional to the density of excited states. In the Na2Mg5Si12O30:Mn2+ system in the temperature range 10–300 K, the decay rate remains constant (inset of Fig. 8b) and the emission intensity increases (inset of Fig. 8a). The constant decay time means that the sum of the radiative rate and the nonradiative rate is constant from Eq. 2. Thus, according to Eq. 3, the radiative rate in this system depends on temperature and should increase at the expensive of nonradiative decay rate with increasing temperature. The thermal quenching of a certain phosphor is an important parameter which influences considerably on the light output [23]. The Na2Mg5Si12O30:Mn2+ phosphor shows excellent thermal stability. Compared to the intensity at room temperature, the phosphor 6
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L. Qin, et al. [17] R. Kripal, D.K. Singh, ESR and optical study of Mn2+ doped diglycine barium chloride monohydrate, J. Phys. Soc. Jpn. 75 (2006) 114711–114715. [18] A.B. Ageeth, M. Andries, Luminescence quantum efficiency of nanocrystalline ZnS:Mn2+. 1. Surface passivation and Mn2+ concentration, J. Phys. Chem. B 105 (2001) 10197–10202. [19] M. Daldosso, D. Falcomer, A. Speghini, P. Ghigna, M. Bettinelli, Synthesis, EXAFS investigation and optical spectroscopy of nanocrystalline Gd3Ga5O12 doped with Ln3+ ions (Ln = Eu, Pr), Opt. Mater. (Amst) 30 (2008) 1162–1167. [20] A.P. Vink, M.A. de Bruin, S. Roke, P.S. Peijzel, A. Meijerink, Luminescence of exchange coupled pairs of transition metal ions, J. Electrochem. Soc. 148 (2001) E313–E320.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Luminescence properties of red-emitting Mn2+-Activated Na2Mg5Si12O30 phosphors Lin Qina,b, Cuili Chenb, Jing Wangb, Shala Bib, Yanlin Huangc, Hyo Jin Seob,
T
⁎
a
School of Electronics and Information, Nantong University, Jiangsu, China Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea c College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Luminescence Optical materials and properties Phosphors
Na2Mg5(1-x)Si12O30:5xMn2+(x = 0.005, 0.01, 0.03,0.05,0.07 and 0.09) red phosphors were synthesized by solid state reactions. The formation of a single phase compound is verified through the X-ray diffraction (XRD) studies. The excitation and emission spectra, and decay curves of Na2Mg5Si12O30:Mn2+ are investigated by optical and laser excitation spectroscopy in the temperature range 7–520 K. The strong red emission peaking at 600 nm is observed due to the spin forbidden transition 4T1 → 6A1 of Mn2+ ions. At high Mn2+ concentration, new side bands appear at lower energy side at around 670 nm which can be ascribed to the appearance of the formation of new Mn2+ centers. In addition, the thermal quenching behavior is investigated to evaluate the commercial value of Na2Mg5Si12O30:Mn2+ as red-emitting materials.
1. Introduction Mn2+ doped into many inorganic hosts is used as a luminescent center with colorful emission ranging from 490 to 750 nm [1]. From the Tanabe-Sugano diagram, it can be deduced that the Mn2+ ions can be excited into several bands in the wavelength range 400–520 nm related to the 6A1→4A1, 4E, 6A1→4T1 and 6A1→4T2 transitions. The emission wavelength of the Mn2+ ion depends strongly on the strength of crystal field and the coordination number (CN) in a host lattice [2]. The Mn2+ ion usually gives a green to yellow emission in a lattice with tetrahedrally coordinated (weak crystal-field) site, whereas it shows an orange to deep red emission with octahedrally coordinated (strong crystal-field) site [3]. Great efforts to investigate the Mn2+-doped compounds have been made and some Mn2+-doped phosphors with good luminescence properties have been synthesized successfully [4,5]. Wu et al. reported the luminescence of single Mn2+-doped KMgBO3 red phosphors [6]. Mn2+-activated CaZnOS shows the great potential for application as an alternative red-emitting LED conversion phosphor due to its high absorption and strong excitation bands [7]. However, few works pay attentions to high thermal stability of Mn2+-activated oxides, which is one of the important parameters for obtained phosphors in potential application due to a considerable influence on the color rendering index (CRI) and light output [8,9].
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In this work, the Na2Mg5Si12O30 host is selected as a lattice to explore a novel Mn2+ -activated red emitting phosphors with high thermal stability. The phosphors are characterized by some measurements such as X-ray diffraction (XRD), emission and excitation spectra and luminescence decay curves. The temperature dependent emission behaviors are further investigated. 2. Experimental A series of Na2Mg5(1-x)Si12O30:5xMn2+ (x = 0.005, 0.01, 0.03, 0.05, 0.07 and 0.09) phosphors were prepared by convenient solid state reaction. The raw reactants are Na2CO3 (99.9%), 4MgCO3-Mg(OH)25H2O(99.9%), SiO2 (99.9%), and MnCO3 (99.9%). The stoichiometric mixtures were thoroughly ground in an agate mortar and heated at 850 °C for 8 h in a crucible in atmosphere. The obtained mixtures were reground carefully and heated to 1150 °C for 8 h in a crucible in atmosphere. The crystal structure of the Na2Mg5Si12O30:Mn2+ powders was examined by the X-ray diffraction (XRD) on the Rigaku D/Max 2000 diffractometer with operating parameters set to 40 kV and 30 mA. A 450 W Xe lamp dispersed by a 25 cm monochromator (Acton Research Corp. Pro-250) was used as a light source for excitation and emission spectra. The luminescence signal was detected using a photomultiplier tube (Hamamatsu, R928, Shizuoka, Japan) mounted on a 75 cm
Corresponding author. E-mail address: [email protected] (H.J. Seo).
https://doi.org/10.1016/j.materresbull.2019.110494 Received 27 January 2019; Received in revised form 5 May 2019; Accepted 19 May 2019 Available online 20 May 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 118 (2019) 110494
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Table 1 Refined Crystallographic Parameters of Na2Mg5Si12O30:Mn2+. Formula
Refined-Na2Mg5Si12O30:Mn2+
radiation 2θ range(degree) symmetry space group# a/Å b/Å c/Å α/° β/° γ/° Z Rp Rwp X2 V /Å3
Cu Ka 10-70 hexagonal P 6/m c c(192) 10.1617(2) 10.1617(2) 14.2377(4) 90 90 120 2 0.0254 0.0174 1.250 1273.21(4)
refinement profiles of the sample. It is in good agreement between the observed and calculated inter-planar spacing without any trace of extra peaks, further suggesting the formation of a single-phase compound. The detailed refinement parameters and atom positions are presented in Tables 1 and 2, respectively. According to the obtained parameters and atom positions, the structural sketch map of Na2Mg5Si12O30 drawn with the Diamond Crystal and Molecular Structure Visualization software is displayed in Fig. 3. Na2Mg5Si12O30 crystallizes in the single hexagonal phase with space-group P6/mcc, z = 2 and cell parameters: α = 90°, γ = 120°, a = 13.3891(7) Å, c = 30.5032(18) Å, and V = 4567.09(60) Å3. In the three-dimensional framework of the Na2Mg(I)2(II)3Si12O30 compound, there are two types of chains. One consists of Si12O30 rings formed through edge-sharing SiO4 and another consists of two types of Mg coordination. The Mg(I) site has tetrahedral coordination and the Mg (II) site has octahedral coordination. The two types of Mg forms chain through edge-sharing symmetric tetrahedral MgO4 and octahedral MgO6. The Na elements with larger sizes occupy two types of sites: one is 12 coordinated sites in the tunnels bounded by the Si12O30 rings and another is 9 coordinated sites between these rings. Fig. 4a and b shows the SEM micrographs of Mn2+ doped Na2Mg5Si12O30 phosphors synthesized using the solid state method, which result in a non-uniform morphology with larger particle size distribution. The average sizes of the particles as depicted in the micrograph are in the range of 2–3 microns. Fig. 4c shows the EDS spectrum of Na2Mg5Si12O30:0.05 Mn2+ Phosphors. The inset of (c) shows the elemental composition of Na2Mg5Si12O30:0.05 Mn2+ determined using EDS. Element mapping analysis (Fig. 5) indicates that Na2Mg5Si12O30:0.05 Mn2+ are composed of sodium, magnesium, silicon, oxygen and manganese with a molar ratio of Na/Mg/Si/O/Mn of ≈ 2:4.75:12:30:0.25, which gives an approximate stoichiometric formula of Na2Mg5Si12O30:0.05 Mn2+. The peak due to C in the spectrum is attributed to the latex in the SEM sample holder.
Fig. 1. XRD patterns of Na2Mg5Si12O30:Mn2+ phosphors with different Mn2+ concentration sintered at 1150 °C for 6 h.
monochromator (Acton Research Corp. Pro-750). The third harmonic (355 nm) of a pulsed Nd:YAG laser was introduced as an excitation source for luminescence decays which were digitized and saved by means of a 500 MHz Tektronix DPO 3054 oscilloscope. For low temperature measurements, the samples were placed in a closed-cycle helium cryostat in the variable-temperature range (10–300 K).
3. Results and discussion 3.1. The phase formation and structural refinement Fig. 1 shows the XRD patterns of Mn2+-doped Na2Mg5Si12O30 as functions of Mn2+ concentration along with the standard PDF card. Diffraction peaks of all the samples are well indexed to the structural results for Na2Mg5Si12O30 single crystal reported by Nguyen et al (JCPDS card 20–1124) [10]. No traces of impurities are detected, especially diffraction peaks belonging to MnO in the samples, which suggests that Mn2+ ions are well substituted for the Mg2+ ions in the Na2Mg5Si12O30 lattice. The structure refinement of 7 mol % Mn2+-doped Na2Mg5Si12O30 was carried out with the GSAS program. Fig. 2 shows the Rietveld
Table 2 The Crystallographic Parameters Obtained from Rietveld Refinement for Na2Mg5Si12O30:Mn2+.
Fig. 2. Rietveld refinement of the powder XRD patterns of Na2Mg5Si12O30:Mn2+. The observed counts and calculated pattern are indicated by crosses and a smooth line, respectively. The vertical marks indicate the position of Bragg peaks, and the bottom trace indicates the difference between the observed and calculated values. 2
Atom
Wyck.
Site
x/a
y/b
z/c
U [Å2]
Na1 Na2 Mg1 Mg2 Si1 O1 O2 O3
2a 4d 4c 6f 24m 12l 24m 24m
622 −6. 3.2 222 1 m. 1 1
0 1/3 1/3 1/2 0.76772(23) 0.73000 0.94500 0.65200
0 2/3 2/3 0 0.11942(30) 0.12300 0.21300 0.15600
1/4 0 1/4 1/4 0.11056(13) 0 0.13500 0.17100
0.099(10) 0.027(4) 0.053(4) 0.0290(24) 0.0224(11) 0.0025 0.0025 0.0025
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Fig. 3. Schematic illustration of the Na2Mg5Si12O30:Mn2+ crystal structure based on the Rietveld refinement, and the coordination environment of the [MgO6]/ [MgO4].
3.2. Luminescence properties of Mn2+-doped Na2Mg(I)2(II)3Si12O30
show the excitation and emission spectra of Na2Mg5Si12O30:Mn2+ at room temperature. The excitation spectra were obtained by monitoring the 600 nm emission and the emission spectra were measured under excitation at 355 nm. The excitation spectra show the absorption in the spectral range 330–530 nm, and the emission spectra exhibit broad emission
Fig. 6a shows the Tanabe-Sugano diagram for the Mn2+ ion with 3d5 electronic configuration. The optical spectra of Na2Mg5Si12O30:Mn2+ can be explained by the Tanabe-Sugano diagram containing the two empirical parameters of Dq and B. Fig. 6b and c
Fig. 4. (a–b) SEM micrographs and (c) EDS spectrum of Na2Mg5Si12O30:0.05 Mn2+ Phosphors. The inset of (c) shows the elemental composition of Na2Mg5Si12O30:0.05 Mn2+ determined using EDS. 3
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Fig. 5. Element mapping of (a) Mn, (b) Si, (c) O, (d) Mg, (e) Na and integration of (a–e).
band in the range of 520–750 nm. The emission band peaking at 600 nm is due to the spin forbidden transition of 4T1 → 6A1. In the Na2Mg(I)2(II)3Si12O30 lattice, the Mn4+ would substitute Mg2+ in expectation (Mg2+: 0.57 A˚ in 4 coordination; 0.72 A˚ in 6 coordination and Mn2+: 0.66 A˚ in 4 coordination; 0.83 A˚ in 6 coordination). Actually, besides the commonly equivalent doping, the non-equivalent doping can also be realized in Mn2+ activated phosphors, for example, the Mn2+ ions would occupy Na+ sites (1.18 A˚ in 8 coordination). Hence, there is great potential for Mn2+ replacing Mg2+ due to their same valance state and similar radius [11]. As mentioned above, two types of Mg sites for the Mn2+ ions are present in the Na2Mg5Si12O30 lattices. Therefore, the Mn2+ ions can substitute for tetrahedrally coordinated Mg(I) sites and/or octahedrally coordinated Mg(II) sites. It is known that the tetrahedrally coordinated Mn2+ (weak crystal-field) usually gives a green emission and the octahedrally coordinated Mn2+ (stronger crystal field) gives an orange to red emission [2]. However, only the red emission occurs from the Mn2+ ions suggesting that the Mn2+ ions replace the single octahedral Mg(II)2+ ions in Na2Mg5Si12O30 lattice. The excitation bands centered at 353, 392, 420, 460 and 512 nm correspond to the transitions from the 6A1(6S) ground state to the 4E (4D), 4T2(4D), 4A1,4E(4G), 4T2(4G) and 4T1(4G) excited states, respectively [12]. It is noted that the 6A1(6S) → 4E(4D) and 4A1(4G) transitions exhibit narrow lines, while the 6A1(6S) → 4T2(4D), 4T2(4G) and 4T1(4G) transitions are rather broad in accordance with the Tanabe-Sugano diagram in which the 4E(4D) and 4A1(4G) levels are relatively less influenced by the crystal field than others [13]. The energy parameters presented in this work are the crystal field Dq, Racah parameter B and C. The first parameter (Dq) indicates the symmetry of impurity ion sites, the second parameter B is related to the covalence of impurity-ligand binding and the Racah C parameter is related to B through C = 4B [14,15]. The crystal field Dq and Racah B parameter are usually obtained from Tanabe-Sugano matrices for d5 electronic configurations as shown in Fig. 6a. In this work, we used the energy matrices including Trees correction (α) given by Mehra. And the Trees correction (α) was introduced due to the configuration 3d54s of
Fig. 6. (a) The Tanabe-Sugano diagram of Mn2+ ions in octahedral crystal fields. (b) and (c) Excitation and emission spectra of the Mn2+-doped Na2Mg5Si12O30 phosphor.
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B=
94α +
49(T2 − T1 )2 − 768α 2 49
(1) −1
where α is the Trees correction with free ion value of 76 cm , T1 and T2 are the energies of 6A1(6S) → 4E(4G) and 6A1(6S) → 4E(4D) transitions which can be obtained from the excitation spectra. Then the value of crystal field parameter B can be calculated to be 788 cm−1. Since the emission energy E of 4T1(G) → 6A1(6S) is 600 nm (16667 cm−1), then the value of E/B is 21.15. Based on the Tanabe-Sugano diagram shown in Fig. 6a, the crystal field parameter value of Dq is 1063 cm−1 and Dq/ B is equal to 1.35 corresponding to the strong crystal field environment. 3.3. Concentration dependent luminescence properties of Mn2+-doped Na2Mg5Si12O30 The emission spectra and decay curves as functions of Mn2+ concentration under excitation at 355 nm at room temperature are presented in Fig. 7. The integrated emission intensity depends on the Mn2+ concentration. The emission reaches a maximum intensity at 7 mol %, and then quenches by further increase in Mn2+ concentration as shown in the inset of Fig. 7b [18]. Normalized emission spectra to the peak intensity at 600 nm are shown in Fig. 7c. The band shapes of the emission spectra for different Mn2+ concentration maintain well at higher energy side including peak maximum at 600 nm. However, new side bands appear at lower energy side at around 670 nm (longer wavelength than 600 nm) and the relative intensities of the new bands increase with increasing Mn2+ concentration. Fig. 7d and e show the luminescence decay curves of the emitting state 4T1 obtained by monitoring the 600 nm emission and the emission of the longer wavelength side at 670 nm as functions of Mn2+-concentration, respectively. The decay curves monitored 600 nm by can be fitted by single-exponential function as I = Aexp(t/τ) [19], and the value of lifetime for all the Mn2+ concentration from 0.5 to 9 mol % (Fig. 7d) are calculated to be around 19.5 ms at 300 K. The decays of the 670 nm emission for 0.5 and 1.0 mol % are single exponential with the decay time of 19.5 ms which are identical to that of the 600 nm emission (Fig. 7e). However, the decays of the 670 nm emission for the Mn2+ concentration higher than 1.0 mol % deviate from single exponential and the deviations are larger with increasing Mn2+ concentration from 3 to 9 mol % (Fig. 7e). The decay time for the Mn2+ concentration of 9 mol % was calculated to be 15 ms. The new 670 nm band (Fig. 7c) and its decay behavior (Fig. 7e) for higher Mn2+ concentration (3–9 mol %) seem to be due to the formation of new Mn2+ centers such as Mn(II)-Mn(II) pairs and Mn(II)-defect centers with stronger crystal-field strength [20,21]. At lower concentration of 0.5 and 1.0 mol %, the decays of the 670 nm emission are consistent with those of 600 nm. This means that the emission in the whole spectral region comes from the identical Mn2+ centers at octahedral Mg(II) sites for low Mn2+ concentration. 3.4. Temperature dependent luminescence properties The temperature dependent emission spectra of Na2Mg5Si12O30:0.07 Mn2+ under excitation at 355 nm are presented in Fig. 8a. The anti-Stokes emission appears at higher energy side with the band broadening from 44 to 70 nm with increasing temperature from 10 to 520 K. The integrated emission intensity increases unexpectedly about 20% from 10 to 300 K and then thermal quenching occurs with further increase in temperature as shown in the inset of Fig. 8a. Luminescence decay curves as functions of temperature are shown in Fig. 8b. Decay times are calculated and displayed in the inset of Fig. 8b. The decay times do not change significantly in the temperature range 10–300 K. From about 300 K, in which the thermal quenching sets, the decay times shorten because of a nonradiative contribution to the decay process. The excited state of electrons can be depopulated by both radiative
Fig. 7. (a) Emission spectra of the Na2Mg5Si12O30:Mn2+ phosphors as functions of Mn2+ concentration under excitation at 355 nm. (b) Integrated emission intensity as a function of Mn2+ Concentration. (c) Concentration dependent emission spectra normalized to the peak intensity at 600 nm. Decay curves of Na2Mg5Si12O30:Mn2+ obtained by monitoring the 600 nm emission (d) and the 670 nm emission (e).
Mn2+ interacts with other configurations such as 3d6 and 3d44s2, generating a mean deviation between experimental and theoretical values for energy terms up to 200 cm−1 [16]. Here, the expression for crystal field parameter B can be taken by the following equation [17]:
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maintains the intensity about 84% at 150 °C and 75% at 200 °C. It has been reported that the luminescence intensities of the commercial red nitride phosphors (Sr0.82Ba0.15Eu0.03)2Si5N8:Eu2+ and (Sr0.75Ca0.25)0.98 SiAlN3:Eu2+ at 150 °C are 87% of the value at 25 °C [24]. The Na2Mg5Si12O30:0.07 Mn2+ phosphor shows similar thermal stabilities the reported phosphors. 4. Conclusions In this work, a series of Na2Mg5Si12O30:Mn2+ red phosphors were synthesized by high temperature solid state reactions. The emission of Na2Mg5Si12O30:Mn2+ occurs in the red region peaking at 600 nm and the spectral feature and emission intensity depends on Mn2+ concentration. There exist two different Mg2+ sites, tetrahedral and octahedral, in the Na2Mg5Si12O30 lattice. Of the two the Mn2+ ion substitutes for the octahedral Mg2+ site. The spectral features of the emission bands are well maintained at lower Mn2+ concentration less than 1.0 mol%. However, new side bands appear at around 670 nm for the Mn2+ concentration higher than 1.0 mol% and the relative intensities of the new bands increase with increasing Mn2+ concentration due to the formation of new Mn2+ centers such as Mn(II)-Mn(II) pairs and Mn(II)-defect centers with stronger crystal-field strength. Temperature dependent emission spectra and decay curves indicate that the synthesized Na2Mg5Si12O30:Mn2+ red phosphor show excellent thermal quenching behavior. The phosphor maintains the intensity about 84% at 150 °C and 75% at 200 °C, due to the radiative rate which increases in this system. The results indicate that the Na2Mg5Si12O30:Mn2+ red phosphor is a promising candidate applied in solid-state lighting and display. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) (2017R1D1A1B03029432). References Fig. 8. (a) Temperature dependent emission spectra of the Na2Mg5Si12O30:Mn2+ phosphor under excitation at 355 nm. the inset shows integrated emission intensity as a function of temperature. (b) Decay curves of the Na2Mg5Si12O30:Mn2+ phosphor as functions of temperature. The inset shows decay times as a function of temperature.
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and nonradiative transition processes which is expressed by the following equation [22]. −1 τ −1 = τR−1 + τNR
(2)
τ −1
is decay rate, i.e., the transition probability from the excited Where, −1 state to the ground state, τR−1 is radiative rate and τNR is nonradiative rate. The emission intensity is related to the ratio of radiative rate to the decay rate:
I = I0
τR−1 τ −1
(3)
Where, I0 is a constant and proportional to the density of excited states. In the Na2Mg5Si12O30:Mn2+ system in the temperature range 10–300 K, the decay rate remains constant (inset of Fig. 8b) and the emission intensity increases (inset of Fig. 8a). The constant decay time means that the sum of the radiative rate and the nonradiative rate is constant from Eq. 2. Thus, according to Eq. 3, the radiative rate in this system depends on temperature and should increase at the expensive of nonradiative decay rate with increasing temperature. The thermal quenching of a certain phosphor is an important parameter which influences considerably on the light output [23]. The Na2Mg5Si12O30:Mn2+ phosphor shows excellent thermal stability. Compared to the intensity at room temperature, the phosphor 6
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