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Luminescence properties, local symmetry and Judd–Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor
Accepted Manuscript Luminescence Properties, local symmetry and Judd-Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor
Yang Yang, Lei Wang, Ping Huang, Qiufeng Shi, Yue Tian, Cai'e Cui PII: DOI: Reference:
S0277-5387(17)30160-2 http://dx.doi.org/10.1016/j.poly.2017.02.038 POLY 12505
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
12 December 2016 23 February 2017 27 February 2017
Please cite this article as: Y. Yang, L. Wang, P. Huang, Q. Shi, Y. Tian, C. Cui, Luminescence Properties, local symmetry and Judd-Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor, Polyhedron (2017), doi: http://dx.doi.org/ 10.1016/j.poly.2017.02.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Luminescence Properties, local symmetry and Judd-Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor YangYang,1 Lei Wang,1,2 Ping Huang,1,2 Qiufeng Shi,1,2 Yue Tian,1,2 and Cai’e Cui1,2,* 1
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR China
2
Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education
and Shanxi Province, Taiyuan University of Technology
* Corresponding authors: E-mail: [email protected]
Abstract A series of red phosphors Sr2CaWO6:Eu3+ and Sr2CaWO6:Eu3+, M+ (M=Li, Na, K) were synthesized by conventional solid-state reactions. The optical band gap was calculated through the UV-diffuse reflectance spectrum and strong CT band absorption was observed in compounds in the near ultraviolet region. The optimum doped concentration of Eu3+ was determined to be 20 mol%, and the concentration quenching mechanism was found to be electric dipole-dipole interaction. After the introduction of charge compensators Na+, the intensity of the red emission band at 615nm was almost three times than before and shows the strongest emission. Finally, the
Ω λ ( λ =2 and 4) intensity parameters was calculated according to Judd-Ofelt theory. The CIE colour co-ordinates of the Sr2CaWO6:Eu3+, Na+ was calculated (x=0.661, y=0.339), which are close to NTSC standard values of primary red.
1. Introduction White light-emitting diodes (LEDs) have attracted much interest due to its advantages in light
efficiency, energy consumption, service longevity and environmental conservation [1-2]. Currently, the most common commercial white LEDs employ a blue LED coated with YAG: Ce3+ yellow phosphors. However, for lack of sufficient red emission components, these white LEDs produce low color rendering index (CRI) [3-4]. Another approach to produce white light is combining near-UV LEDs with red/green/blue tricolor phosphors [5-7]. Unfortunately, compared with blue and green phosphor, the luminescence efficiency of red-emitting phosphor excited by near ultraviolet is still unsatisfied [8]. Hence, it is an urgent task to develop novel, stable and highly-efficient red phosphors. Recently, tungstate compounds with ordered perovskite structure attract great attention due to their broad and intense ligand-to-metal charge transfer band in the UV or near-UV region [9-11]. V.Sivakumar et al. reported the phase transition and photoluminescence in Sr1.9-xBaxCaWO6:Eu3+, Li+[12], Sr1.9-xBaxCaMoO6:Eu3+, Li+ [13] and Ba2MgW(1−x) MoxO6:Eu3+ phosphors[14]; K.Vdabre doped Eu3+, Sm3+ and Pr3+ into Ca2ZnWO6 phosphor and obtained intense emission in yellow and orange regions[15]; In addition, double-perovskite (Ba, Sr)2CaMoO6: Eu3+,Yb3+ has been investigated which shows unique anti-Stokes luminescence properties[16]. All above papers show that the ordered perovskite structure is sensitive to Eu3+ and obtain excellent red emission. In this work, we discussed the optimal Eu3+ concentration in Sr2CaWO6, and then doped Li +, Na+, and K+ to Sr2CaWO6:Eu3+ as charge compensator, investigate the intensity of emission spectra, asymmetry ratio and color coordinates etc. The results indicated Sr2CaWO6: Eu3+, Na+ is a promising red phosphor for blue and NUV chips LEDs. Furthermore, the Judd-Ofelt theory is important for understanding the spectral properties, the J-O intensity parameters of the Eu3+ in the matrix can reveal information regarding the covalence and the surrounding of the metal ion. The
parameter Ω 2 determines the covalency, polarizability and the asymmetric behavior of the Eu3+ and the ligand whereas Ω 4 is related to long range effects.[17] Finally, J-O Parameters of Sr2CaWO6:20 mol% Eu3+ and Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ phosphors were calculated according to Judd-Ofelt theory.
2. Experimental Section 2.1 Preparation A set of red phosphors Sr2CaWO6:Eu3+ and Sr2CaWO6:Eu3+, M+ (M=Li, Na, K) were synthesized by solid-state reactions. The starting materials Sr2CO3 (99%), CaCO3 (99%), WO3 (99%), Eu2O3 (99.99%), Na2CO3 (99.8%), Li2CO3 (99.5%), K2CO3 (99%) were weighed according to the stoichiometric ratios. These powders were mixed thoroughly in an agate mortar. After 1h of grinding, the homogeneous mixture was filled into a corundum crucible. After annealing under 900℃ for 3 h, the mixture was grinded once again and continuously increased the temperature to 1200℃ for 12 h. The final product was obtained when the furnace was cooled to room temperature.
2.2 Characterization The X-ray diffraction (XRD) pattern was collected on an X-ray diffractometer (Japan, Shimadzu, XRD-6000) with Cu Kα (λ=0.15406 nm) over the angular range 10o ≤ 2θ ≤ 90o, operating at 40 kV and 30 mA. Diffuse reflection spectra were obtained by a UV-visible spectrophotometer (Shimadzu, UV-2600) using the white powder BaSO4 as a reference. The photoluminescence excitation (PLE)/emission (PL) spectra was measured using an Edinburg FLS-980 fluorescence spectrophotometer equipped with a 450 W xenon lamp.
3. Results and discussion 3.1 XRD and structure In order to investigate the phase purities, different concentrations of Eu3+-doped Sr2CaWO6 and the samples with Li+, Na+, K+ as charge compensators were examined by means of XRD. As shown in figure 1, all the diffraction peaks of the samples can be index into the standard Sr2CaWO6 (PDF NO.76-1983). This introduction of Eu3+, Li+, Na+ and K+ ions does not significantly influence the crystal structure of the phosphor.
<> 3.2 UV-vis diffuse reflectance spectroscopy analyses Figure 2 (a) gives the diffuse reflectance spectrum of Sr2CaWO6 and Sr2CaWO6:20 mol% Eu3+. It is observed that there is a strong absorption in the UV region in the Sr2CaWO6 without Eu3+ doped, which due to the electron excitation from the valence to the conduction band in Sr2CaWO6 host [18]. The spectra of Sr2CaWO6:20 mol% Eu3+ shows stronger absorption from host lattice and two sharp absorption lines located at 395 and 465 nm, deriving from 7F0 to 5 L6 and 5D2 of Eu3+, respectively. In short, the Sr2CaWO6:20 mol% Eu3+ can be excited by blue and NUV LED chip. The optical band gap energy (Eg) values can be calculated from diffuse reflectance spectrum using Kubelka-Munk equation for indirect band gap material [19-20]:
[ F ( R∞ ) hν ]1/ 2 = A( hν − E g )
(1)
Where hv is the photon energy, Eg is the value of the band gap, A is a proportional constant, and F(R∞) is a Kubelka-Munk function defined as 2 ( 1 − R) F ( R∞ ) =
2R
=
K S
(2)
In which R, K, and S are the reflection, the absorption coefficient, and the scattering coefficient,
respectively. From the linear extrapolation of [F(R∞)hv]1/2=0, the Eg value was estimated approximately to be 3.675eV as shown in Fig. 2(b) .
<> 3.3 Photoluminescence properties Figure 3 displays the photoluminescence excitation (PLE) and the photoluminescence (PL) spectra of the as-prepared 20 mol% Eu3+-doped Sr2CaWO6 phosphor. As observed in Fig. 3, the excitation spectra while monitoring 615 nm emission corresponding to the 5D0 →7F2 transition consists of two parts: One is a weak broad band; another is a series of sharp lines from 350-500 nm. The former is referred to the charge transfer bands (CTB) of the Eu3+-O2- and W6+-O2transitions. In most cases, the contribution of these two CTB components cannot be distinguished due to the spectral overlap [21]. The latter is from the characteristic f-f transitions of Eu3+ ions including the peaks with maximum at 362 nm (7F0→5D4), 395 nm (7F0→5 L6), 383 nm (7 F0→5 L7), 414 nm (7F0→5D3) and 465 nm (7 F0→5D2). The strong peaks located in 395 nm and 465nm show that this phosphor can be effectively applied with NUV and blue LED chips.
<> Emission spectra in Fig. 3 under the excitation of 465 nm exhibits the strong and dominant red emission at around 615 nm for the 5D0 → 7F2 electric-dipole transition over the 5D0 → 7F1 magnetic-dipole transition that occurs at 587 nm of Eu3+. Other minor peaks centered at 579, 655 and 701 nm comes from the 5D0→7F0, 7F3, 7F4 respectively. It is well-known that the 5D0→7F2 transition belongs to hypersensitive transition, in other words, the emission intensity is strongly influenced by ligand ions in the crystals[22]. When the Eu3+ ions are located at a low symmetry site without asymmetric inversion center, the 5D0→7F2
emission transition often dominates in the emission spectrum. While the magnetic dipole 5D0→7F1 transition regardless of environment with asymmetric inversion center. Herein, the R ratio (R=intensity of 5D0→7F2/intensity of 5 D0→7F1), the asymmetry ratio, can be used as an index to measure the site centers symmetry of Eu3+ ions. It can be seen that the strongest emission peak centered at 615 nm, suggesting that the sites without asymmetric inversion center are occupied by Eu3+ ions in the Sr2 CaWO6: Eu3+ phosphors. In the work, the value of R is calculated as 6.79 for Sr2CaWO6:20 mol% Eu3+ phosphor, which is considerably larger than 1 means the Sr2CaWO6: 20 mol% Eu3+ phosphor could be a pure red light emitting phosphor. 3.4 Concentration Quenching and Energy Transfer The doping concentration of luminescent centers is an important factor influencing the phosphor performance [23]. Figure 4 shows the emission spectra (λex=465 nm) of Sr2CaWO6:Eu3+ with different concentrations of Eu3+. With the increase in Eu3+ doping concentration, the emission intensity of Eu3+ ions obviously increases. However, when Eu3+ doping concentration reaches 20 mol%, the intensity of Eu3+ begins to decrease, namely, concentration quenching happens.
<> Concentration quenching may occur because the excitation energy migrates about a large number of centers before emitting. [24] The interaction of Eu3+ ions is limited when the concentration is lower than 20 mol%, therefore, the intensities increased with increasing the Eu3+ ions concentration until reach maximum. As no cross-relaxation channel exists to quench the 5D0 level, the luminescence intensities of the 5D0 level are not quenched obviously even if the Eu3+ concentration is as high as 20 mol%. [24] In this phenomenon, the concentration quenching behavior in the case of electric multipole interaction between luminescent centers has been
quantitatively expressed by Van Uitert’s model as follows:
I / x = k[1 + β （x)θ / 3 ]−1
(3)
Where x is the luminescent center concentration, k and β are constants for a certain system; θ represents the interaction mechanism between rare earth ions, θ=3, 6, 8 or 10 for the exchange interaction, electric dipole-dipole (D-D), electric dipole-quadrupole (D-Q), and electric quadrupole-quadrupole (Q-Q) interactions, respectively. Figure 5 shows the dependence of integrated emission intensities for 5D0→7FJ transition on Eu3+ concentrations. Finally, it was deduced to be 6.27, which is approximately equal to 6, implying that electric D-D interaction is responsible for quenching 5D0 fluorescence.
<> 4. Effect of alkali ions on PL enhancement of Sr2CaWO6:Eu3+ Taking into account the charge balance, Na+, K+ and Li + ions are co-doped to Sr2CaWO6:20%Eu3+, their emission spectra under 465 nm are shown in Fig. 6. It can be seen that all the emission intensity are increased obviously after co-doping Na+, K+ or Li+ ions into Sr2CaWO6:20% Eu3+ as charge compensators. The strongest intensity of the Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ is almost 3 times than before. Meanwhile, The statistic result of the average particle sizes of Sr2CaWO6:20% Eu3+, Sr2CaWO6: 20% Eu3+, 20% K+, Sr2CaWO6: 20% Eu3+, 20% Na+ , Sr2CaWO6: 20% Eu3+, 20% Li+ are 3.30 μm, 4.65μm,4.56μm,4.29μm, respectively. when Na+, K+ or Li + ions are co-doped to Sr2CaWO6:20% Eu3+ as charge compensators, all the grain size and emission intensity are increased obviously. It can be attributed to lager grain size reduces the density of the grain boundaries, which in turn might be responsible for reduced adsorption and/or scattered light, which favors PL enhancement.[25] Furthermore, it presents an average size
less than 10μm, exhibiting a proper particle size for uses in white LEDs. In order to more clearly evaluate this phosphor luminescence intensity, the luminescent intensity of the commercial Y2O2S:Eu3+ was used as a comparison under the same measurement condition. The comparison is as shown in Fig. 7. It can be seen that the intensity of our phosphor is about 43.4 times, 3.1 times stronger than Y2O2S:Eu3+ under the 465 nm and 395 nm, respectively. The CIE color coordinates of this phosphor were calculated to be (0.661, 0.339) which is close to the CIE coordinate of s-RGB standard red light (0.640, 0.330). The coordinates are better than commercially available Y2O2S: Eu3+ (0.622, 0.351) and Sr2CaWO6:20 mol% Eu3+ (0.657, 0.343).
<> <> In order to future investigate the luminescence site symmetry and luminescence behavior of Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+, Judd-Ofelt (J-O) theory is used for characterizing the optical transitions of it. It is well known that 5D0→7F1 is magnetic dipole transition and the magnetic dipole transition rate of the 5D0→7F1 can be expressed as follow:[19] 3
Amd =
64π 4k md 3 n Smd 3h(2 J ' + 1)
(4)
Where k md is the transition energy for 5D0→7F1 in wavenumber, h is Planck’s constant (h=6.626 -27
×10
erg s) and 2 J ' + 1 is the degeneracy of the initial state(1 for 5D0). The factor n is the
refractive index of host, and an index of n equal to 2.28 is used.[26] S md is a constant independent of the host; here it is 7.83×10-42[27]. As a consequence, the magnetic dipole transition rate of the 5D0→7F1 can be calculated to be about 141.219 s −1 according the eqn (6). The 5D0→7FJ(J=2,4,6) transition are electric dipole transition and the radiative rate can be written
as:
AJ =
64π 4e2 k 3 n(n 2 + 2)2 Ωλ ψJ U λ ψ ' J ' ∑ 3h(2 J ' + 1) 9 λ = 2, 4,6
2
(5)
Where e is the electric charge and its value is 4.80×10-10 with Gaussian units here, k is the transition energy of electric transitions
in
cm −1 , Ω λ is the intensity parameter,
2
ψJ U λ ψ ' J ' values are the squared reduced matrix elements, whose values are 0.0032 and 0.0023 for J’=2 and 4, respectively[28-29]. All parameters are used in equations with Gaussian units for convenient calculation. Therefore, eqn 7 also can be described by
AJ =
64π 4e2 k 3 n (n 2 + 2)2 Ωλ ψJ U λ ψ ' J ' 3h(2 J ' + 1) 9
2
(6)
The transition intensity ratio between electronic dipole and magnetic dipole can be expressed by
∫I
(
dk
)
2
AJ e 2 k J3 n 2 + 2 = = Ω λ ψJ U λ ψ ' J ' 3 2 ∫ I md (k )dk Amd S md kmd 9n
∫
J
2
(7)
∫
The value of ( I J ( k ) dk ) ( I md (k ) dk ) can be gained from the integral area of the emission spectra. Therefore, Ω λ can be calculated on the basis of the analysis of the emission spectra. Referring to the above description, the PL intensity of the Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ is optimal, in order to understand the effect of Na+ on Ω λ parameters, the same computational procedure may be easily adapted to obtain a new set of data. Furthermore, in the computational procedure, we assume the refractive index of host is constant and the emission spectrum of the Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ is used. (Table 1)
<> It can be seen that Ω 2 is larger than Ω 4 , which means that the site symmetry occupied by Eu3+ ions in tungstate systems do not have the character of a centrosymmetric chemical environment. It>
Yang Yang, Lei Wang, Ping Huang, Qiufeng Shi, Yue Tian, Cai'e Cui PII: DOI: Reference:
S0277-5387(17)30160-2 http://dx.doi.org/10.1016/j.poly.2017.02.038 POLY 12505
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
12 December 2016 23 February 2017 27 February 2017
Please cite this article as: Y. Yang, L. Wang, P. Huang, Q. Shi, Y. Tian, C. Cui, Luminescence Properties, local symmetry and Judd-Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor, Polyhedron (2017), doi: http://dx.doi.org/ 10.1016/j.poly.2017.02.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Luminescence Properties, local symmetry and Judd-Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor YangYang,1 Lei Wang,1,2 Ping Huang,1,2 Qiufeng Shi,1,2 Yue Tian,1,2 and Cai’e Cui1,2,* 1
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR China
2
Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education
and Shanxi Province, Taiyuan University of Technology
* Corresponding authors: E-mail: [email protected]
Abstract A series of red phosphors Sr2CaWO6:Eu3+ and Sr2CaWO6:Eu3+, M+ (M=Li, Na, K) were synthesized by conventional solid-state reactions. The optical band gap was calculated through the UV-diffuse reflectance spectrum and strong CT band absorption was observed in compounds in the near ultraviolet region. The optimum doped concentration of Eu3+ was determined to be 20 mol%, and the concentration quenching mechanism was found to be electric dipole-dipole interaction. After the introduction of charge compensators Na+, the intensity of the red emission band at 615nm was almost three times than before and shows the strongest emission. Finally, the
Ω λ ( λ =2 and 4) intensity parameters was calculated according to Judd-Ofelt theory. The CIE colour co-ordinates of the Sr2CaWO6:Eu3+, Na+ was calculated (x=0.661, y=0.339), which are close to NTSC standard values of primary red.
1. Introduction White light-emitting diodes (LEDs) have attracted much interest due to its advantages in light
efficiency, energy consumption, service longevity and environmental conservation [1-2]. Currently, the most common commercial white LEDs employ a blue LED coated with YAG: Ce3+ yellow phosphors. However, for lack of sufficient red emission components, these white LEDs produce low color rendering index (CRI) [3-4]. Another approach to produce white light is combining near-UV LEDs with red/green/blue tricolor phosphors [5-7]. Unfortunately, compared with blue and green phosphor, the luminescence efficiency of red-emitting phosphor excited by near ultraviolet is still unsatisfied [8]. Hence, it is an urgent task to develop novel, stable and highly-efficient red phosphors. Recently, tungstate compounds with ordered perovskite structure attract great attention due to their broad and intense ligand-to-metal charge transfer band in the UV or near-UV region [9-11]. V.Sivakumar et al. reported the phase transition and photoluminescence in Sr1.9-xBaxCaWO6:Eu3+, Li+[12], Sr1.9-xBaxCaMoO6:Eu3+, Li+ [13] and Ba2MgW(1−x) MoxO6:Eu3+ phosphors[14]; K.Vdabre doped Eu3+, Sm3+ and Pr3+ into Ca2ZnWO6 phosphor and obtained intense emission in yellow and orange regions[15]; In addition, double-perovskite (Ba, Sr)2CaMoO6: Eu3+,Yb3+ has been investigated which shows unique anti-Stokes luminescence properties[16]. All above papers show that the ordered perovskite structure is sensitive to Eu3+ and obtain excellent red emission. In this work, we discussed the optimal Eu3+ concentration in Sr2CaWO6, and then doped Li +, Na+, and K+ to Sr2CaWO6:Eu3+ as charge compensator, investigate the intensity of emission spectra, asymmetry ratio and color coordinates etc. The results indicated Sr2CaWO6: Eu3+, Na+ is a promising red phosphor for blue and NUV chips LEDs. Furthermore, the Judd-Ofelt theory is important for understanding the spectral properties, the J-O intensity parameters of the Eu3+ in the matrix can reveal information regarding the covalence and the surrounding of the metal ion. The
parameter Ω 2 determines the covalency, polarizability and the asymmetric behavior of the Eu3+ and the ligand whereas Ω 4 is related to long range effects.[17] Finally, J-O Parameters of Sr2CaWO6:20 mol% Eu3+ and Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ phosphors were calculated according to Judd-Ofelt theory.
2. Experimental Section 2.1 Preparation A set of red phosphors Sr2CaWO6:Eu3+ and Sr2CaWO6:Eu3+, M+ (M=Li, Na, K) were synthesized by solid-state reactions. The starting materials Sr2CO3 (99%), CaCO3 (99%), WO3 (99%), Eu2O3 (99.99%), Na2CO3 (99.8%), Li2CO3 (99.5%), K2CO3 (99%) were weighed according to the stoichiometric ratios. These powders were mixed thoroughly in an agate mortar. After 1h of grinding, the homogeneous mixture was filled into a corundum crucible. After annealing under 900℃ for 3 h, the mixture was grinded once again and continuously increased the temperature to 1200℃ for 12 h. The final product was obtained when the furnace was cooled to room temperature.
2.2 Characterization The X-ray diffraction (XRD) pattern was collected on an X-ray diffractometer (Japan, Shimadzu, XRD-6000) with Cu Kα (λ=0.15406 nm) over the angular range 10o ≤ 2θ ≤ 90o, operating at 40 kV and 30 mA. Diffuse reflection spectra were obtained by a UV-visible spectrophotometer (Shimadzu, UV-2600) using the white powder BaSO4 as a reference. The photoluminescence excitation (PLE)/emission (PL) spectra was measured using an Edinburg FLS-980 fluorescence spectrophotometer equipped with a 450 W xenon lamp.
3. Results and discussion 3.1 XRD and structure In order to investigate the phase purities, different concentrations of Eu3+-doped Sr2CaWO6 and the samples with Li+, Na+, K+ as charge compensators were examined by means of XRD. As shown in figure 1, all the diffraction peaks of the samples can be index into the standard Sr2CaWO6 (PDF NO.76-1983). This introduction of Eu3+, Li+, Na+ and K+ ions does not significantly influence the crystal structure of the phosphor.
<
[ F ( R∞ ) hν ]1/ 2 = A( hν − E g )
(1)
Where hv is the photon energy, Eg is the value of the band gap, A is a proportional constant, and F(R∞) is a Kubelka-Munk function defined as 2 ( 1 − R) F ( R∞ ) =
2R
=
K S
(2)
In which R, K, and S are the reflection, the absorption coefficient, and the scattering coefficient,
respectively. From the linear extrapolation of [F(R∞)hv]1/2=0, the Eg value was estimated approximately to be 3.675eV as shown in Fig. 2(b) .
<
<
emission transition often dominates in the emission spectrum. While the magnetic dipole 5D0→7F1 transition regardless of environment with asymmetric inversion center. Herein, the R ratio (R=intensity of 5D0→7F2/intensity of 5 D0→7F1), the asymmetry ratio, can be used as an index to measure the site centers symmetry of Eu3+ ions. It can be seen that the strongest emission peak centered at 615 nm, suggesting that the sites without asymmetric inversion center are occupied by Eu3+ ions in the Sr2 CaWO6: Eu3+ phosphors. In the work, the value of R is calculated as 6.79 for Sr2CaWO6:20 mol% Eu3+ phosphor, which is considerably larger than 1 means the Sr2CaWO6: 20 mol% Eu3+ phosphor could be a pure red light emitting phosphor. 3.4 Concentration Quenching and Energy Transfer The doping concentration of luminescent centers is an important factor influencing the phosphor performance [23]. Figure 4 shows the emission spectra (λex=465 nm) of Sr2CaWO6:Eu3+ with different concentrations of Eu3+. With the increase in Eu3+ doping concentration, the emission intensity of Eu3+ ions obviously increases. However, when Eu3+ doping concentration reaches 20 mol%, the intensity of Eu3+ begins to decrease, namely, concentration quenching happens.
<
quantitatively expressed by Van Uitert’s model as follows:
I / x = k[1 + β （x)θ / 3 ]−1
(3)
Where x is the luminescent center concentration, k and β are constants for a certain system; θ represents the interaction mechanism between rare earth ions, θ=3, 6, 8 or 10 for the exchange interaction, electric dipole-dipole (D-D), electric dipole-quadrupole (D-Q), and electric quadrupole-quadrupole (Q-Q) interactions, respectively. Figure 5 shows the dependence of integrated emission intensities for 5D0→7FJ transition on Eu3+ concentrations. Finally, it was deduced to be 6.27, which is approximately equal to 6, implying that electric D-D interaction is responsible for quenching 5D0 fluorescence.
<
less than 10μm, exhibiting a proper particle size for uses in white LEDs. In order to more clearly evaluate this phosphor luminescence intensity, the luminescent intensity of the commercial Y2O2S:Eu3+ was used as a comparison under the same measurement condition. The comparison is as shown in Fig. 7. It can be seen that the intensity of our phosphor is about 43.4 times, 3.1 times stronger than Y2O2S:Eu3+ under the 465 nm and 395 nm, respectively. The CIE color coordinates of this phosphor were calculated to be (0.661, 0.339) which is close to the CIE coordinate of s-RGB standard red light (0.640, 0.330). The coordinates are better than commercially available Y2O2S: Eu3+ (0.622, 0.351) and Sr2CaWO6:20 mol% Eu3+ (0.657, 0.343).
<
Amd =
64π 4k md 3 n Smd 3h(2 J ' + 1)
(4)
Where k md is the transition energy for 5D0→7F1 in wavenumber, h is Planck’s constant (h=6.626 -27
×10
erg s) and 2 J ' + 1 is the degeneracy of the initial state(1 for 5D0). The factor n is the
refractive index of host, and an index of n equal to 2.28 is used.[26] S md is a constant independent of the host; here it is 7.83×10-42[27]. As a consequence, the magnetic dipole transition rate of the 5D0→7F1 can be calculated to be about 141.219 s −1 according the eqn (6). The 5D0→7FJ(J=2,4,6) transition are electric dipole transition and the radiative rate can be written
as:
AJ =
64π 4e2 k 3 n(n 2 + 2)2 Ωλ ψJ U λ ψ ' J ' ∑ 3h(2 J ' + 1) 9 λ = 2, 4,6
2
(5)
Where e is the electric charge and its value is 4.80×10-10 with Gaussian units here, k is the transition energy of electric transitions
in
cm −1 , Ω λ is the intensity parameter,
2
ψJ U λ ψ ' J ' values are the squared reduced matrix elements, whose values are 0.0032 and 0.0023 for J’=2 and 4, respectively[28-29]. All parameters are used in equations with Gaussian units for convenient calculation. Therefore, eqn 7 also can be described by
AJ =
64π 4e2 k 3 n (n 2 + 2)2 Ωλ ψJ U λ ψ ' J ' 3h(2 J ' + 1) 9
2
(6)
The transition intensity ratio between electronic dipole and magnetic dipole can be expressed by
∫I
(
dk
)
2
AJ e 2 k J3 n 2 + 2 = = Ω λ ψJ U λ ψ ' J ' 3 2 ∫ I md (k )dk Amd S md kmd 9n
∫
J
2
(7)
∫
The value of ( I J ( k ) dk ) ( I md (k ) dk ) can be gained from the integral area of the emission spectra. Therefore, Ω λ can be calculated on the basis of the analysis of the emission spectra. Referring to the above description, the PL intensity of the Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ is optimal, in order to understand the effect of Na+ on Ω λ parameters, the same computational procedure may be easily adapted to obtain a new set of data. Furthermore, in the computational procedure, we assume the refractive index of host is constant and the emission spectrum of the Sr2CaWO6:20 mol% Eu3+, 20 mol% Na+ is used. (Table 1)
<