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Luminescence properties of Eu2+ in T–phase Ba1.3Ca0.7SiO4 lattice from multiple crystallographic sites at different temperatures
Materials Chemistry and Physics 177 (2016) 538e542
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Luminescence properties of Eu2þ in Tephase Ba1.3Ca0.7SiO4 lattice from multiple crystallographic sites at different temperatures Xinmin Zhang a, Fangui Meng a, Jiyao Zhang b, Peiqing Cai c, Hyo Jin Seo c, * a
School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China c Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea b
h i g h l i g h t s The PL can be attributed to three different Eu2þ sites emission. The low temperature PL is consistence with that at room temperature. The lifetimes of Eu I, Eu II and Eu III sites at 10 K are 0.23, 0.28 and 1.00 ms. Ba1.3Ca0.7SiO4:Eu2þ has a thermal quenching at 235 C. Ba1.3Ca0.7SiO4:Eu2þ is a candidate blue-emitting phosphor for white LEDs.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2015 Received in revised form 14 March 2016 Accepted 15 April 2016 Available online 23 April 2016
Eu2þeactivated Ba1.3Ca0.7SiO4 phosphor has been synthesized by high temperature solid state reaction method. The optical properties of (Ba1.29Eu0.01)Ca0.7SiO4 are investigated using photoluminescence (PL), photoluminescence excitation (PLE) spectra and luminescence decay time measurement between 10 and 523 K. The blue emission band peaking at ~448 nm is observed to be asymmetric. It can be divided into three Gaussian bands with maxima at ~440 (Eu I), ~464 (Eu II) and ~503 nm (Eu III), which is in good agreement with that measured from low temperature at 10 K. The temperature dependence of the PL intensity is also measured. The thermal quenching of Ba1.3Ca0.7SiO4:Eu2þ is about 235 C. The PL decay times of 425, 455 and 570 nm emission at 10 K are 0.23, 0.28 and 1.00 ms. They can be attributed to the lifetimes of Eu I, Eu II and Eu III site, respectively. The decay times of the 425 and 455 nm emission almost remain constant at 10 K and 523 K. The decay times of the emission at 570 nm do not change a lot with increasing temperature up to 300 K, then decrease to 0.2 ms at 523 K Ba1.3Ca0.7SiO4:Eu2þ is a candidate blue-emitting phosphor for white LEDs. © 2016 Elsevier B.V. All rights reserved.
Keywords: A. Optical properties B. Sintering C. Photoluminescence spectroscopy D. Luminescence
1. Introduction The luminescence of the Eu2þ ion is well known and has been investigated widely. The emission and excitation spectra of them usually consist of broad bands due to transitions between the 4f ground state and the 4f5d excited state. The peak position depends strongly on the ligand field around Eu2þ ion. For example, the Eu2þ emission is at 665 nm in Ca4(PO4)2O:Eu2þ [1], while in Ca3Mg3(PO4)4:Eu2þ the Eu2þ emission is at 450 nm [2]. In BaFBr the
* Corresponding author. E-mail address: [email protected] (H.J. Seo). http://dx.doi.org/10.1016/j.matchemphys.2016.04.066 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Eu2þ emission is at UV region, viz., at 391 nm [3]. In addition, host lattices with Eu2þ ion on different crystallographic sites show more than one emission band. Recently, many authors have reported that there are two or more sites available for Eu2þ in some matrixes [4,5]. For example, in LiMgPO4:Eu2þ luminescence spectra consist of two distinct broad emission bands peaking at 380 nm and 490 nm related to 4f65 d1 / 4f7 (8S7/2) luminescence of Eu2þ and to europiumetrapped exciton, respectively [6]. The emission bands centered at 490 and 540 nm observed in Ba4(Si3O8)2:Eu2þ could be attributed to Eu2þ occupied in different Ba2þ sites [7]. Eu2þ luminescence from 5 different crystallographic sites has been reported in a novel red phosphor, Ca15Si20O10N30:Eu2þ [8]. White light-emitting-diodes (white LEDs) are one of the
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promising illumination sources and could replace conventional incandescent and fluorescent lamps due to their low environmental impact (Hgefree), high efficiency and long lifetime. The silicate matrix is an excellent luminescent host with good chemical and thermal stability. Alkalineeearth silicates doped with rare earth and/or transition metals work as phosphors by neareUV excitation [9e11]. They have an ideal characteristic for RGBeLEDs that the color changes from red to blue through green according to the concentration of dopants and the composition of alkalineeearth metals. Phase stability in the binary system Ba2SiO4eCa2SiO4 has been extensively studied because of its importance in cement chemistry as well as in silicate mineralogy. This system has five recognized phases a, a0 H, a0 L, X and T. In RE doped Tephases Ba1.2Ca0.8SiO4 and Ba1.3Ca0.7SiO4, the luminescence properties of Ce3þ, Eu2þ, Ce3þ/ Mn2þ and Eu2þ/Mn2þ have been reported by some groups [12e16]. In this contribution, we add to this series Tephase Ba1.3Ca0.7SiO4 with three inequivalent Ba2þ sites. Further we present a more qualitative evaluation than in the earlier papers. PL spectra at low/ high temperature and lifetimes of various Eu2þ sites are reported.
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JCPDS: 48-0210, Ba1.3Ca0.7SiO4
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2 theta (degree) Fig. 1. XRD pattern of Ba1.3Ca0.7SiO4:0.01Eu2þ. The ASTM card pattern for Ba1.3Ca0.7SiO4 (No: 48-0210) is also shown for comparison.
2. Experimental 2.1. Preparation The measurements were performed on powder samples with and composition Ba1.3Ca0.7SiO4:0.01Eu2þ Ba1.3Ca0.7SiO4:0.01Ce3þ,0.01Liþ. The starting materials were BaCO3 (Aldrich), CaCO3 (Aldrich), SiO2 (Aldrich), Eu2O3 and CeO2 (Aldrich, 99.9%). Sample was prepared by solid state reactions. Stoichiometric amounts of the starting materials were thoroughly mixed in an agate motor and subsequently fired at 600 C for 2 h. After grounding for a second time, the sample was fired at 1200 C for 2 h. The second firing procedure took place in a weak CO reducing atmosphere. 2.2. Characterization The sample was confirmed to be single phase by Xeray power diffraction using Cu Ka radiation (a Beijing Puxi XDe2 diffractometer operating at 36 kV, 25 mA). The PL and PLE spectra at room temperature were measured using a fluorescence meter with a 150 W Xe lamp as an excitation source (Shimadzu, Japan, RF 5301PC). The excitation and emission bandwidths of 1.5 nm were used throughout the experiments, and the excitation and emission spectra were recorded with a 1 nm step size. The temperature dependent luminescence spectra (10e523 K) and the decay curves were recorded by the 500 MHz digital storage oscilloscope (LeCroy 9350A) in which the signal was fed from PMT. The decay and temperature-dependent luminescence measurements used the third harmonic (355 nm) of a Quanta-ray DCR YAG:Nd laser as the excitation source. For the lower temperature measurement, the sample was placed at cold finger in a He gas recycled cryostat. For the higher temperature measurement, the setup was equipped with a homemade heating cell connected to a temperature controller.
than that of Ca2þ and smaller than that of Ba2þ [17]. In this case, the doped Eu2þ ions should substitute the Ba sites. No peaks coming from Eu2O3 are observed at the sample, which indicates that the Eu dopant is certainly incorporated in the Ba1.3Ca0.7SiO4 matrix. Fig. 2 shows PL and PLE spectra of the Ba1.3Ca0.7SiO4:0.01Eu2þ sample measured at 300 K. The PL spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ exhibit blue emission bands with a peak at about 448 nm, which can be attributed to the 4f65d / 4f7 transition of Eu2þ. No clear difference in the PL spectra is observed except for emission intensity when excited by different excitation wavelengths. The CIE (Commission International de L'Eclairage) coordinates are calculated from the emission spectra to be (x ¼ 0.15, y ¼ 0.16) and it is shown in the inset of Fig. 2. Moreover, it can be seen that on an energy scale the PL spectra appear to be asymmetric, which could result from the overlap of two or more emission bands. In fact, these emission bands can be divided into three
3. Results and discussion Fig. 1 shows XRD pattern of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor. The XRD pattern of a trigonal Ba1.3Ca0.7SiO4 crystal [space group:P3m1(164)], taken from the American Society for Testing and Materials (ASTM) card, is shown on the bottom of Fig. 1 for comparison. All the peaks can be assigned to Ba1.3Ca0.7SiO4. Under the same coordination environment, the ionic radius of Eu2þ is larger
Fig. 2. Room-temperature PL and PLE spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor. The excitation and monitoring wavelengths are marked at corresponding curves. The insets show the deconvoluted results of broad emission band into three Gaussian peaks and CIE chromaticity coordinates.
X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
Gaussian bands with maxima at ~440, ~464 and ~503 nm. An example of this deconvolution result is shown in the inset of Fig. 2 (lex ¼ 282 nm). All the PLE spectra are broad bands extending from 220 to 450 nm and exhibit two obvious absorption bands with peaks at about 280 and 340 nm when monitored by different emission wavelengths. In addition, a shoulder peak at 405 nm is observed for all the PLE spectra. These absorption bands can be assigned to the 4f / 5d transitions in Eu2þ. No clear dependence of the PLE spectra on the emission wavelength is observed. Similar results are reported in literatures. In BaAl2O4 there are two different barium sites. The emission spectrum of BaAl2O4:Eu2þ at 4.2 K is asymmetric and can be divided into two Gaussian bands with maxima at 510 and 540 nm; while the excitation spectra of BaAl2O4:Eu2þ at room temperature as well as at 4.2 K are broad and structureless [18]. In system Ba4(Si3O8)2:Eu2þ, there are two non-equivalent barium ions of Ba (I) and Ba (II) which can be substituted by Eu2þ ions, which leads to a asymmetric emission band peaking at about 495 nm. The emission band can be divided into two sub-bands centered at 490 and 540 nm. However, the excitation spectrum monitored at 540 nm exhibits no difference from which monitored at 490 nm [7]. It is well known that the wavelength position of the Eu2þ emission band depends very much on hosts, changing from neareUV to red. This change is attributed to the crystal field splitting of the 5d level. The emission bands shift to longer wavelength (redshift) with increasing crystal field strength. If there are two or more different cation sites available for Eu2þ ions in a matrix, the shape of emission band will become more complicate. For example, two different crystallographic sites are available for Eu2þ in the cell of Li4SrCa(SiO4)2. Analysis suggests that the 433 nm emission belongs to Eu2þ on Sr2þ site, and the 584 nm emission originates from Eu2þ on isolated Ca2þ site [19]. The luminescence spectra of Na2BaSi2O6:Eu2þ exhibiting asymmetric band indicates that Eu2þ ions occupy more than one site in the lattice [20]. BaSrMg(PO4)2:Eu2þ phosphor is a typical example for Eu2þ solely doped white light emitting phosphors, and shows two emission bands at around 447 and 536 nm originating from Eu2þ ions substituting Sr2þ and Ba2þ sites, respectively [21]. The crystal structure of Tephase Ba1.3Ca0.7SiO4 is made of five Ba/Ca sites [one M(2)O6 octahedron and four M(1, 3, 4, 5)O10 or 12 polyhedra] and two SiO4 tetrahedral sites [16,22]. It is reported that the Ba/Ca sites with same coordination number have similar Ba/ CaeO crystal coordination environment [16]. Therefore, two sites having 10-fold coordination have similar Ba/CaeO crystal coordination environment and two other sites having 12-fold coordination also have similar Ba/CaeO crystal coordination environment. According to the crystal field theory, luminescence centers with similar coordinate environment are spectrally undistinguishable [16]. So the Eu2þ ions substituting the two (Ba/Ca)O10 and two (Ba/ Ca)O12 can be regarded as two independent luminescence centers. Three different emission bands are expected, which is in agreement with the experimental result (see the inset of Fig. 2). In addition, the crystal field strength, presented as Dq, is inversely proportional to the 5th power of the bondelength R [2,20,21,23].
Dq ¼
1 2 r4 Ze 5 6 R
length; and the higheenergy emission originates from the Eu2þ ions which occupy loose crystal circumstance with larger Ba/CaeO bond length. Therefore, the Eu III feels stronger crystal field strength than Eu I and Eu II, and the emission position shifts toward longer wavelength. So the bands peaking at ~440, ~464 and ~503 nm can be assigned to the Eu2þ occupied (Ba/Ca)O12, (Ba/Ca) O10 and (Ba/Ca)O6, respectively. Optical spectrum at lower temperature can be used to produce evidence in support of having different optically active ion sites in specific host. Fig. 3 shows the temperature dependence of the PL spectra of the Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor measured between T ¼ 10 K and 300 K in 25 K increments. At 10 K the PL spectrum shows a broad band peaking at 445 nm and an obvious shoulder peak at the low energy side. It can be divided into three Gaussian bands peaking at ~438, ~453 and ~500 nm (see detail in the inset of Fig. 3), which is consistent with the result at room temperature. The three obvious emission band at 10 K further supports our conclusion that there are three different Ba/Ca sites to be replaced by doped Eu2þ ions in the Ba1.3Ca0.7SiO4 lattice. No remarkable decrease is observed in the PL intensity in the temperature range between 25 and 300 K. In addition, a weak spectral broadening with increasing T can be found at the high energy side (400e425 nm). Thermal stability is one of the important technological parameters for phosphors used in white LEDs. The high thermal quenching will lead to variation of chromaticity of the white LEDs when it is used. The PL spectra were measured at higher temperatures in the range of 300e523 K, and the results are shown in Fig. 4. The luminescent intensity decreases slowly with increasing temperature due to thermal quenching. This decrease of the emission intensity with increasing temperature can be explained by the configuration coordinate diagram [see the inset of Fig. 4] accounting for the interaction between the dopant ion and the vibrating lattice of the host material [24]. The quenching temperature, T50, is defined as the temperature at which the intensity is half of the maximum intensity. It is clear that from Fig. 4 the T50 of Ba1.3Ca0.7SiO4:Eu2þ can be evaluated at about 235 C. For high power LEDs, the chips can reach temperatures of nearly 200 C under operation. Based on the fact that Ba1.3Ca0.7SiO4:Eu2þ has low
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The shorter the bondelength is, the stronger the crystal field strength becomes. In this case, for the three different Eu2þ sites, the bond length of EueO increases with the number of coordination (CN) increasing. So the bondelength of REu-O (CN ¼ 12) > REu-O (CN ¼ 10) > REu-O (CN ¼ 6). Thus, the loweenergy emission, as shown in the inset of Fig. 2, originates from the Eu2þ ions which occupy compact crystal circumstance with shorter Ba/CaeO bond
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Wavelength (nm) Fig. 3. Temperature dependence of PL spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor measured at T ¼ 10e300 K. The inset shows the deconvoluted results of broad emission band (T ¼ 10 K) into three Gaussian peaks.
X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
Wavenumber (cm-1)
t þb IðtÞ ¼ I0 exp
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Wavelength (nm) Fig. 4. Temperature dependence of PL spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor measured at T ¼ 300e523 K. The inset shows the Configurational coordinate diagram.
thermal quenching at 200 C, Ba1.3Ca0.7SiO4:Eu2þ is a good candidate phosphor for white LEDs application. For an allowed transition, the radiative lifetime usually does not change strongly with temperature. Therefore, a welleestablished method to determine the temperature quenching is to measure the luminescence lifetime of an emission band as a function of temperature. As quenching takes place, the luminescence lifetime shortens due to an additional noneradiative contribution to the decay process. We measured the PL decay time t as a function of emission wavelengths (lem ¼ 425, 455 and 570 nm) for the Ba1.3Ca0.7SiO4:0.01Eu2þ sample at temperatures between 10 K and 523 K. Fig. 5 shows the luminescent decay curves for the emission at 425, 455 and 570 nm and an excitation at 355 nm at 10 K. The luminescent decays are represented by exponential curves, and they were fitted with the following expression:
Fig. 5. Luminescence decay curves for the Ba1.3Ca0.7SiO4:0.01Eu2þ sample at 10 K measured at 425, 455 and 570 nm. The decay curves were fitted using Eq. (1). The inset shows the luminescence decay curves for the Ba1.3Ca0.7SiO4:0.01Eu2þ sample at 523 K measured at 425, 455 and 570 nm.
541
(2)
where b is a constant. The solid lines with cyan color in Fig. 5 show the results calculated using Eq. (1). The bestefit values are 0.23, 0.28 and 1.00 ms for 425, 455 and 570 nm emission, respectively. The expected decay times are of 1e1.4 ms for Eu2þ emission around 560 nm [25]. The experimental decay times are in good agreement with those reported in the literature and they can be attributed to lifetimes of Eu I, Eu II and Eu III, respectively. It is well known that the lifetime of the Eu2þ luminescence is in the order of microesecond, which is relatively long for an allowed transition. This can be explained as follows. The ground state of 4f7 is 8S, and the multiplicity of the excited state 4f65 d1 is 6 or 8; the sextet portion of the excited state contributes to the spin-forbidden character of the transition. There is a wide variation in the emission wavelengths and decay times among the different Eu2þeactivated host lattices. To analyze these radiative lifetimes, Eq (3) can be used [26]:
1
t
¼ 5:06 108 j〈5djrj4f〉j2 cs3
(3)
where t is the decay time in units of second (s), the radial integral 〈5djrj4f 〉 in units of Angstron (Å) and s is the wavenumber of the emitted light in unit of wavenumber (cm1). The fact that the radial integral is in the same range in all Eu2þedoped lattices is remarkable. It means that although the compositions of the excited state are different for the compounds, this has no large influence on the decay [27]. Thus, the longer the emission wavelength is, the longer the radiative lifetime becomes, which is consistent with the result showing in Fig. 5. The decay curves at 523 K are presented in the inset of Fig. 5. At higher temperature the decay curves are composed of a faster initial part followed by an exponential one. The faster initial part could be attributed to thermally enhanced energy transfer to defects [28]. The temperature dependence of decay time is illustrated in Fig. 6. The decay times of the 425 and 455 nm emission almost remain constant at 10 K and 523 K. In most cases, the lifetime of excited state decreases gradually with increasing temperature as a result of a substantial increase in noneradiative relaxation rate [28]. Contrary to this change, we can see a gradual increase in the
Fig. 6. Luminescence decay times of the Ba1.3Ca0.7SiO4:0.01Eu2þ (for lex ¼ 355 nm and lem ¼ 425, 455 and 570 nm) as a function of temperature.
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X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
decay time for the emission at 425 and 455 nm with increasing temperature up to 400 K. The abnormal vary of lifetime being similar to Eu I and Eu II in Tephase Ba1.3Ca0.7SiO4:0.01Eu2þ sample have also been reported in many system [29e35]. The reason for this unusual temperature dependence of Eu2þ fluorescence lifetime in these crystals could be explained by the trapping effect [28]. The decay times of the emission at 570 nm do not change a lot with increasing temperature up to 300 K, then decrease to 0.2 ms at 523 K. The decay times becoming shorter with increasing temperature is typical for the situation where the lifetime is shortened by a faster noneradiative decay from the excited state at higher temperature [24]. The decay time of 570 nm emission exhibits obvious decrease when the temperature is above 300 K, while that of 425 and 455 nm emissions do not show remarkable change. As discussed above, 425, 455 and 570 nm emission bands are assigned to three different Eu2þ centers. It is well known that the quenching temperature for Eu2þ emission is lower for longer wavelength emission due to a smaller energy barrier in the configuration coordinate diagram for the same Stokes shift [24,36]. 4. Conclusions We synthesized Eu2þeactivated Ba1.3Ca0.7SiO4 phosphor using solid state reaction method. The three Gaussian bands at ~440, ~464 and ~503 nm at room temperature can be assigned to the Eu2þ occupied (Ba/Ca)O12, (Ba/Ca)O10 and (Ba/Ca)O6, respectively. The low temperature PL is consistence with that at room temperature. The lifetimes of Eu I, Eu II and Eu III sites at 10 K are 0.23, 0.28 and 1.00 ms, respectively. Ba1.3Ca0.7SiO4:Eu2þ has a thermal quenching at 235 C. Ba1.3Ca0.7SiO4:Eu2þ is a good blue-emitting phosphor for white LEDs application. Acknowledgements The project was sponsored by the Technology Program of Environmental Protection Department of Hunan (No. 2013-312) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013-R1A1A2009154). References [1] D. Deng, H. Yu, Y. Li, Y. Hua, G. Jia, S. Zhao, H. Wang, L. Huang, Y. Li, C. Li, S. Xu, Ca4(PO4)2O: Eu2þ red-emitting phosphor for solid-state lighting: structure, luminescent properties and white light emitting diode application, J. Mater. Chem. C 1 (2013) 3194e3199. [2] X. Zhang, Q. Pan, S.I. Kim, Y.M. Yu, H.J. Seo, Temperature dependence of the luminescence of calcium-magnesium phosphate Ca3Mg3(PO4)4:Eu2þ, a blueemitting material for white light-emitting diodes, Mater. Res. Bull. 51 (2014) 28e34. [3] W. Chen, S.P. Wang, S.L. Westcott, J. Zhang, Structure and luminescence of BaFBr:Eu2þ and BaFBr:Eu2þ, Tb3þ phosphors and thin films, J. Appl. Phys. 97 (2005) 083506. [4] M.G. Brik, C.G. Ma, H. Liang, H. Ni, G. Liu, Theoretical analysis of optical spectra of Ce3þ in multi-sites host compounds, J. Lumin. 152 (2014) 203e205. [5] H.H. Lin, H.B. Liang, B. Han, J.P. Zhong, Q. Su, P. Dorenbos, M.D. Birowosuto, G.B. Zhang, Y.B. Fu, W.Q. Wu, Luminescence and site occupancy of Ce3þ in Ba2Ca(BO3)2, Phys. Rev. B 76 (2007) 035117. [6] H. Xie, L. Xu, L. Qin, Y. Huang, D. Wei, S.I. Kim, H.J. Seo, Orange-emitting Ba4R2ZrWO12:Eu3þ (R¼La, Gd, Y) with the strongest 5D0/7F0 transition, Mater. Lett. 115 (2014) 18e20. [7] X. Zhang, X. Xu, Q. He, J. Qiu, X. Yu, Significant improvement of photostimulated luminescence of Ba4(Si3O8)2:Eu2þ by co-doping with Tm3þ, ECS J, Solid State Sci. Tech. 2 (2013) R225eR229.
[8] W.B. Park, S.P. Singh, C. Yoon, K.-S. Sohn, Eu2þ luminescence from 5 different crystallographic sites in a novel red phosphor, Ca15Si20O10N30:Eu2þ, J. Mater. Chem. 22 (2012) 14068e14075. [9] Y. Sato, H. Kato, M. Kobayashi, T. Masaki, D.H. Yoon, M. Kakihana, Tailoring of deep-red luminescence in Ca2SiO4:Eu2þ, Angew. Chem. Int. Ed. 53 (2014) 7756e7759. [10] J. Qiao, J. Zhang, X. Zhang, Z. Hao, Y. Liu, Y. Luo, The energy transfer and effect of doped Mg2þ in Ca3Sc2Si3O12:Ce3þ, Pr3þ phosphor for white LEDs, Dalton Trans. 43 (2014) 4146e4150. [11] C. Liu, Z. Qi, C.G. Ma, P. Dorenbos, D. Hou, S. Zhang, X. Kuang, J. Zhang, H. Liang, High light yield of Sr8(Si4O12)Cl8:Eu2þ under X-ray excitation and its temperature-dependent luminescence characteristics, Chem. Mat. 26 (2014) 3709e3715. [12] K. Park, J. Lee, J. Park, J. Kim, P. Kung, S.M. Kim, G. Kim, White-light generation through Ce3þ/Mn2þ-codoped and Eu2þ-doped Ba1.2Ca0.8SiO4 T-phase phosphors, J. Lumin. 130 (2010) 2442e2445. [13] K. Park, J. Kim, P. Kung, S.M. Kim, Thermally stable deep-blue Ba1.2Ca0.8SiO4: Ce3þ phosphor for white-light-emitting diode, J. Lumin. 130 (2010) 1292e1294. [14] N.S. Choi, K.W. Park, B.W. Park, X.M. Zhang, J.S. Kim, P. Kung, S.M. Kim, Eu2þMn2þ energy transfer in white-light-emitting T-phase (Ba,Ca)2SiO4:Eu2þ, Mn2þ phosphor, J. Lumin. 130 (2010) 560e566. [15] K. Park, N. Choi, J. Kim, P. Kung, S.M. Kim, Temperature and excitation powerresistant white-light emission of the T-phase (Ba,Ca)2SiO4:Eu2þ, Mn2þ phosphor, Solid State Commun. 150 (2010) 329e332. [16] W. Lv, M. Jiao, Q. Zhao, B. Shao, W. Lü, H. You, Ba1.3Ca0.7SiO4:Eu2þ,Mn2þ: a promising single-phase, color-tunable phosphor for near-ultraviolet whitelight-emitting diodes, Inorg. Chem. 53 (2014) 11007e11014. [17] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A 32 (1976) 751e767. [18] S.H.M. Poort, W.P. Blokpoel, G. Blasse, Luminescence of Eu2þ in barium and strontium aluminate and gallate, Chem. Mat. 7 (1995) 1547e1551. [19] J. Zhang, W. Zhang, Y. He, W. Zhou, L. Yu, S. Lian, Z. Li, M. Gong, Site-occupancy on the luminescence properties of a single-phase Li4SrCa(SiO4)2:Eu2þ phosphor, Ceram. Int. 40 (2014) 9831e9834. [20] Z. Tao, Y. Cheng, Y. Huang, P. Cai, H.J. Seo, Luminescence performance and structural characteristic of Eu2þ-activated Na2BaSi2O6 phosphor, ECS J. Solid State Sci. Tech. 3 (2014) R71eR78. [21] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host, Chem. Soc. Rev. 43 (2014) 1372e1386. [22] K. Fukuda, M. Ito, T. Iwata, Crystal structure and structural disorder of (Ba0.65Ca0.35)2SiO4, J. Solid State Chem. 180 (2007) 2305e2309. [23] L. Chen, C.C. Lin, C.W. Yeh, R.S. Liu, Light Converting inorganic phosphors for white light-emitting diodes, Mater 3 (2010) 2172e2195. [24] D. Dutczak, C. Ronda, A. Meijerink, T. Jüstel, Red luminescence and persistent luminescence of Sr3Al2O5Cl2:Eu2þ,Dy3þ, J. Lumin. 141 (2013) 150e154. [25] N. Kunkel, A. Meijerink, M. Springborg, H. Kohlmann, Eu(II) luminescence in the perovskite host lattices KMgH3, NaMgH3 and mixed crystals LiBaxSr1-xH3, J. Mater. Chem. C 2 (2014) 4799e4804. [26] T. Hoshina, 5d/4f radiative transition probabilities of Ce3þ and Eu2þ in crystals, J. Phys. Soc. Jpn. 48 (1980) 1261e1268. [27] S.H.M. Poort, A. Meyerink, G. Blasse, Lifetime measurements in Eu2þ-doped host lattices, J. Phys. Chem. Solids 58 (1997) 1451e1456. [28] C. Chartier, C. Barthou, P. Benalloul, J.M. Frigerio, Photoluminescence of Eu2þ in SrGa2S4, J. Lumin. 111 (2005) 147e158. [29] X. Zhang, F. Meng, W. Li, H. Jin Seo, Investigation of Eu2þ luminescence in barium tetraphosphate Ba3P4O13 polycrystalline ceramics, Ceram. Int. 39 (2013) 8975e8978. [30] J. Gan, Y. Huang, L. Shi, X. Qiao, H.J. Seo, Luminescence properties of Eu2þactivated Sr5(PO4)2(SiO4) for green-emitting phosphor, Mater. Lett. 63 (2009) 2160e2162. [31] C. Qin, Y. Huang, L. Shi, G. Chen, X. Qiao, H.J. Seo, Thermal stability of luminescence of NaCaPO4:Eu2þ phosphor for white-light-emitting diodes, J. Phys. D. 42 (2009) 185105e185109. [32] J. Lü, Y. Huang, L. Shi, H. Seo, The luminescence properties of Eu2þ doped Na2CaMg(PO4)2 phosphor, Appl. Phys. A 99 (2010) 859e863. [33] C.K. Duan, A. Meijerink, R.J. Reeves, M.F. Reid, The unusual temperature dependence of the Eu2þ fluorescence lifetime in CaF2 crystals, J. Alloy. Compd. 408e412 (2006) 784e787. [34] P. Kisliuk, H.H. Tippins, C.A. Moore, S.A. Pollack, Optical spectrum and zeeman effect of CaF2:Eu2þ, Phys. Rev. 171 (1968) 336e342. [35] A. Meijerink, G. Blasse, Luminescence and temperature-dependent decay behavior of europium in Ba5SiO4Cl6, Ba5SiO4Br6, J. Lumin. 47 (1990) 1e5. [36] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer Berlin etc, 1994.
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Luminescence properties of Eu2þ in Tephase Ba1.3Ca0.7SiO4 lattice from multiple crystallographic sites at different temperatures Xinmin Zhang a, Fangui Meng a, Jiyao Zhang b, Peiqing Cai c, Hyo Jin Seo c, * a
School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China c Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea b
h i g h l i g h t s The PL can be attributed to three different Eu2þ sites emission. The low temperature PL is consistence with that at room temperature. The lifetimes of Eu I, Eu II and Eu III sites at 10 K are 0.23, 0.28 and 1.00 ms. Ba1.3Ca0.7SiO4:Eu2þ has a thermal quenching at 235 C. Ba1.3Ca0.7SiO4:Eu2þ is a candidate blue-emitting phosphor for white LEDs.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2015 Received in revised form 14 March 2016 Accepted 15 April 2016 Available online 23 April 2016
Eu2þeactivated Ba1.3Ca0.7SiO4 phosphor has been synthesized by high temperature solid state reaction method. The optical properties of (Ba1.29Eu0.01)Ca0.7SiO4 are investigated using photoluminescence (PL), photoluminescence excitation (PLE) spectra and luminescence decay time measurement between 10 and 523 K. The blue emission band peaking at ~448 nm is observed to be asymmetric. It can be divided into three Gaussian bands with maxima at ~440 (Eu I), ~464 (Eu II) and ~503 nm (Eu III), which is in good agreement with that measured from low temperature at 10 K. The temperature dependence of the PL intensity is also measured. The thermal quenching of Ba1.3Ca0.7SiO4:Eu2þ is about 235 C. The PL decay times of 425, 455 and 570 nm emission at 10 K are 0.23, 0.28 and 1.00 ms. They can be attributed to the lifetimes of Eu I, Eu II and Eu III site, respectively. The decay times of the 425 and 455 nm emission almost remain constant at 10 K and 523 K. The decay times of the emission at 570 nm do not change a lot with increasing temperature up to 300 K, then decrease to 0.2 ms at 523 K Ba1.3Ca0.7SiO4:Eu2þ is a candidate blue-emitting phosphor for white LEDs. © 2016 Elsevier B.V. All rights reserved.
Keywords: A. Optical properties B. Sintering C. Photoluminescence spectroscopy D. Luminescence
1. Introduction The luminescence of the Eu2þ ion is well known and has been investigated widely. The emission and excitation spectra of them usually consist of broad bands due to transitions between the 4f ground state and the 4f5d excited state. The peak position depends strongly on the ligand field around Eu2þ ion. For example, the Eu2þ emission is at 665 nm in Ca4(PO4)2O:Eu2þ [1], while in Ca3Mg3(PO4)4:Eu2þ the Eu2þ emission is at 450 nm [2]. In BaFBr the
* Corresponding author. E-mail address: [email protected] (H.J. Seo). http://dx.doi.org/10.1016/j.matchemphys.2016.04.066 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Eu2þ emission is at UV region, viz., at 391 nm [3]. In addition, host lattices with Eu2þ ion on different crystallographic sites show more than one emission band. Recently, many authors have reported that there are two or more sites available for Eu2þ in some matrixes [4,5]. For example, in LiMgPO4:Eu2þ luminescence spectra consist of two distinct broad emission bands peaking at 380 nm and 490 nm related to 4f65 d1 / 4f7 (8S7/2) luminescence of Eu2þ and to europiumetrapped exciton, respectively [6]. The emission bands centered at 490 and 540 nm observed in Ba4(Si3O8)2:Eu2þ could be attributed to Eu2þ occupied in different Ba2þ sites [7]. Eu2þ luminescence from 5 different crystallographic sites has been reported in a novel red phosphor, Ca15Si20O10N30:Eu2þ [8]. White light-emitting-diodes (white LEDs) are one of the
(110)
2+
20
30
40
50
(118)
(024)
(006) (022) 10
(116) (122) (108) (124) (300)
Ba1.3Ca0.7SiO4:0.01Eu
(102)
Intensity (a.u.)
promising illumination sources and could replace conventional incandescent and fluorescent lamps due to their low environmental impact (Hgefree), high efficiency and long lifetime. The silicate matrix is an excellent luminescent host with good chemical and thermal stability. Alkalineeearth silicates doped with rare earth and/or transition metals work as phosphors by neareUV excitation [9e11]. They have an ideal characteristic for RGBeLEDs that the color changes from red to blue through green according to the concentration of dopants and the composition of alkalineeearth metals. Phase stability in the binary system Ba2SiO4eCa2SiO4 has been extensively studied because of its importance in cement chemistry as well as in silicate mineralogy. This system has five recognized phases a, a0 H, a0 L, X and T. In RE doped Tephases Ba1.2Ca0.8SiO4 and Ba1.3Ca0.7SiO4, the luminescence properties of Ce3þ, Eu2þ, Ce3þ/ Mn2þ and Eu2þ/Mn2þ have been reported by some groups [12e16]. In this contribution, we add to this series Tephase Ba1.3Ca0.7SiO4 with three inequivalent Ba2þ sites. Further we present a more qualitative evaluation than in the earlier papers. PL spectra at low/ high temperature and lifetimes of various Eu2þ sites are reported.
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X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
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JCPDS: 48-0210, Ba1.3Ca0.7SiO4
10
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2 theta (degree) Fig. 1. XRD pattern of Ba1.3Ca0.7SiO4:0.01Eu2þ. The ASTM card pattern for Ba1.3Ca0.7SiO4 (No: 48-0210) is also shown for comparison.
2. Experimental 2.1. Preparation The measurements were performed on powder samples with and composition Ba1.3Ca0.7SiO4:0.01Eu2þ Ba1.3Ca0.7SiO4:0.01Ce3þ,0.01Liþ. The starting materials were BaCO3 (Aldrich), CaCO3 (Aldrich), SiO2 (Aldrich), Eu2O3 and CeO2 (Aldrich, 99.9%). Sample was prepared by solid state reactions. Stoichiometric amounts of the starting materials were thoroughly mixed in an agate motor and subsequently fired at 600 C for 2 h. After grounding for a second time, the sample was fired at 1200 C for 2 h. The second firing procedure took place in a weak CO reducing atmosphere. 2.2. Characterization The sample was confirmed to be single phase by Xeray power diffraction using Cu Ka radiation (a Beijing Puxi XDe2 diffractometer operating at 36 kV, 25 mA). The PL and PLE spectra at room temperature were measured using a fluorescence meter with a 150 W Xe lamp as an excitation source (Shimadzu, Japan, RF 5301PC). The excitation and emission bandwidths of 1.5 nm were used throughout the experiments, and the excitation and emission spectra were recorded with a 1 nm step size. The temperature dependent luminescence spectra (10e523 K) and the decay curves were recorded by the 500 MHz digital storage oscilloscope (LeCroy 9350A) in which the signal was fed from PMT. The decay and temperature-dependent luminescence measurements used the third harmonic (355 nm) of a Quanta-ray DCR YAG:Nd laser as the excitation source. For the lower temperature measurement, the sample was placed at cold finger in a He gas recycled cryostat. For the higher temperature measurement, the setup was equipped with a homemade heating cell connected to a temperature controller.
than that of Ca2þ and smaller than that of Ba2þ [17]. In this case, the doped Eu2þ ions should substitute the Ba sites. No peaks coming from Eu2O3 are observed at the sample, which indicates that the Eu dopant is certainly incorporated in the Ba1.3Ca0.7SiO4 matrix. Fig. 2 shows PL and PLE spectra of the Ba1.3Ca0.7SiO4:0.01Eu2þ sample measured at 300 K. The PL spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ exhibit blue emission bands with a peak at about 448 nm, which can be attributed to the 4f65d / 4f7 transition of Eu2þ. No clear difference in the PL spectra is observed except for emission intensity when excited by different excitation wavelengths. The CIE (Commission International de L'Eclairage) coordinates are calculated from the emission spectra to be (x ¼ 0.15, y ¼ 0.16) and it is shown in the inset of Fig. 2. Moreover, it can be seen that on an energy scale the PL spectra appear to be asymmetric, which could result from the overlap of two or more emission bands. In fact, these emission bands can be divided into three
3. Results and discussion Fig. 1 shows XRD pattern of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor. The XRD pattern of a trigonal Ba1.3Ca0.7SiO4 crystal [space group:P3m1(164)], taken from the American Society for Testing and Materials (ASTM) card, is shown on the bottom of Fig. 1 for comparison. All the peaks can be assigned to Ba1.3Ca0.7SiO4. Under the same coordination environment, the ionic radius of Eu2þ is larger
Fig. 2. Room-temperature PL and PLE spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor. The excitation and monitoring wavelengths are marked at corresponding curves. The insets show the deconvoluted results of broad emission band into three Gaussian peaks and CIE chromaticity coordinates.
X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
Gaussian bands with maxima at ~440, ~464 and ~503 nm. An example of this deconvolution result is shown in the inset of Fig. 2 (lex ¼ 282 nm). All the PLE spectra are broad bands extending from 220 to 450 nm and exhibit two obvious absorption bands with peaks at about 280 and 340 nm when monitored by different emission wavelengths. In addition, a shoulder peak at 405 nm is observed for all the PLE spectra. These absorption bands can be assigned to the 4f / 5d transitions in Eu2þ. No clear dependence of the PLE spectra on the emission wavelength is observed. Similar results are reported in literatures. In BaAl2O4 there are two different barium sites. The emission spectrum of BaAl2O4:Eu2þ at 4.2 K is asymmetric and can be divided into two Gaussian bands with maxima at 510 and 540 nm; while the excitation spectra of BaAl2O4:Eu2þ at room temperature as well as at 4.2 K are broad and structureless [18]. In system Ba4(Si3O8)2:Eu2þ, there are two non-equivalent barium ions of Ba (I) and Ba (II) which can be substituted by Eu2þ ions, which leads to a asymmetric emission band peaking at about 495 nm. The emission band can be divided into two sub-bands centered at 490 and 540 nm. However, the excitation spectrum monitored at 540 nm exhibits no difference from which monitored at 490 nm [7]. It is well known that the wavelength position of the Eu2þ emission band depends very much on hosts, changing from neareUV to red. This change is attributed to the crystal field splitting of the 5d level. The emission bands shift to longer wavelength (redshift) with increasing crystal field strength. If there are two or more different cation sites available for Eu2þ ions in a matrix, the shape of emission band will become more complicate. For example, two different crystallographic sites are available for Eu2þ in the cell of Li4SrCa(SiO4)2. Analysis suggests that the 433 nm emission belongs to Eu2þ on Sr2þ site, and the 584 nm emission originates from Eu2þ on isolated Ca2þ site [19]. The luminescence spectra of Na2BaSi2O6:Eu2þ exhibiting asymmetric band indicates that Eu2þ ions occupy more than one site in the lattice [20]. BaSrMg(PO4)2:Eu2þ phosphor is a typical example for Eu2þ solely doped white light emitting phosphors, and shows two emission bands at around 447 and 536 nm originating from Eu2þ ions substituting Sr2þ and Ba2þ sites, respectively [21]. The crystal structure of Tephase Ba1.3Ca0.7SiO4 is made of five Ba/Ca sites [one M(2)O6 octahedron and four M(1, 3, 4, 5)O10 or 12 polyhedra] and two SiO4 tetrahedral sites [16,22]. It is reported that the Ba/Ca sites with same coordination number have similar Ba/ CaeO crystal coordination environment [16]. Therefore, two sites having 10-fold coordination have similar Ba/CaeO crystal coordination environment and two other sites having 12-fold coordination also have similar Ba/CaeO crystal coordination environment. According to the crystal field theory, luminescence centers with similar coordinate environment are spectrally undistinguishable [16]. So the Eu2þ ions substituting the two (Ba/Ca)O10 and two (Ba/ Ca)O12 can be regarded as two independent luminescence centers. Three different emission bands are expected, which is in agreement with the experimental result (see the inset of Fig. 2). In addition, the crystal field strength, presented as Dq, is inversely proportional to the 5th power of the bondelength R [2,20,21,23].
Dq ¼
1 2 r4 Ze 5 6 R
length; and the higheenergy emission originates from the Eu2þ ions which occupy loose crystal circumstance with larger Ba/CaeO bond length. Therefore, the Eu III feels stronger crystal field strength than Eu I and Eu II, and the emission position shifts toward longer wavelength. So the bands peaking at ~440, ~464 and ~503 nm can be assigned to the Eu2þ occupied (Ba/Ca)O12, (Ba/Ca) O10 and (Ba/Ca)O6, respectively. Optical spectrum at lower temperature can be used to produce evidence in support of having different optically active ion sites in specific host. Fig. 3 shows the temperature dependence of the PL spectra of the Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor measured between T ¼ 10 K and 300 K in 25 K increments. At 10 K the PL spectrum shows a broad band peaking at 445 nm and an obvious shoulder peak at the low energy side. It can be divided into three Gaussian bands peaking at ~438, ~453 and ~500 nm (see detail in the inset of Fig. 3), which is consistent with the result at room temperature. The three obvious emission band at 10 K further supports our conclusion that there are three different Ba/Ca sites to be replaced by doped Eu2þ ions in the Ba1.3Ca0.7SiO4 lattice. No remarkable decrease is observed in the PL intensity in the temperature range between 25 and 300 K. In addition, a weak spectral broadening with increasing T can be found at the high energy side (400e425 nm). Thermal stability is one of the important technological parameters for phosphors used in white LEDs. The high thermal quenching will lead to variation of chromaticity of the white LEDs when it is used. The PL spectra were measured at higher temperatures in the range of 300e523 K, and the results are shown in Fig. 4. The luminescent intensity decreases slowly with increasing temperature due to thermal quenching. This decrease of the emission intensity with increasing temperature can be explained by the configuration coordinate diagram [see the inset of Fig. 4] accounting for the interaction between the dopant ion and the vibrating lattice of the host material [24]. The quenching temperature, T50, is defined as the temperature at which the intensity is half of the maximum intensity. It is clear that from Fig. 4 the T50 of Ba1.3Ca0.7SiO4:Eu2þ can be evaluated at about 235 C. For high power LEDs, the chips can reach temperatures of nearly 200 C under operation. Based on the fact that Ba1.3Ca0.7SiO4:Eu2þ has low
-1
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Eu I 438
Eu II 453 Eu III 500
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Experimental Data Gauss fit of Data Gauss fit peak 1 Gauss fit peak 2 Gauss fit peak 3
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The shorter the bondelength is, the stronger the crystal field strength becomes. In this case, for the three different Eu2þ sites, the bond length of EueO increases with the number of coordination (CN) increasing. So the bondelength of REu-O (CN ¼ 12) > REu-O (CN ¼ 10) > REu-O (CN ¼ 6). Thus, the loweenergy emission, as shown in the inset of Fig. 2, originates from the Eu2þ ions which occupy compact crystal circumstance with shorter Ba/CaeO bond
400
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Wavelength (nm) Fig. 3. Temperature dependence of PL spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor measured at T ¼ 10e300 K. The inset shows the deconvoluted results of broad emission band (T ¼ 10 K) into three Gaussian peaks.
X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
Wavenumber (cm-1)
t þb IðtÞ ¼ I0 exp
t
Intensity (arb. units)
100 %
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498 K
50 %
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e
Dis tanc e
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Wavelength (nm) Fig. 4. Temperature dependence of PL spectra of Ba1.3Ca0.7SiO4:0.01Eu2þ phosphor measured at T ¼ 300e523 K. The inset shows the Configurational coordinate diagram.
thermal quenching at 200 C, Ba1.3Ca0.7SiO4:Eu2þ is a good candidate phosphor for white LEDs application. For an allowed transition, the radiative lifetime usually does not change strongly with temperature. Therefore, a welleestablished method to determine the temperature quenching is to measure the luminescence lifetime of an emission band as a function of temperature. As quenching takes place, the luminescence lifetime shortens due to an additional noneradiative contribution to the decay process. We measured the PL decay time t as a function of emission wavelengths (lem ¼ 425, 455 and 570 nm) for the Ba1.3Ca0.7SiO4:0.01Eu2þ sample at temperatures between 10 K and 523 K. Fig. 5 shows the luminescent decay curves for the emission at 425, 455 and 570 nm and an excitation at 355 nm at 10 K. The luminescent decays are represented by exponential curves, and they were fitted with the following expression:
Fig. 5. Luminescence decay curves for the Ba1.3Ca0.7SiO4:0.01Eu2þ sample at 10 K measured at 425, 455 and 570 nm. The decay curves were fitted using Eq. (1). The inset shows the luminescence decay curves for the Ba1.3Ca0.7SiO4:0.01Eu2þ sample at 523 K measured at 425, 455 and 570 nm.
541
(2)
where b is a constant. The solid lines with cyan color in Fig. 5 show the results calculated using Eq. (1). The bestefit values are 0.23, 0.28 and 1.00 ms for 425, 455 and 570 nm emission, respectively. The expected decay times are of 1e1.4 ms for Eu2þ emission around 560 nm [25]. The experimental decay times are in good agreement with those reported in the literature and they can be attributed to lifetimes of Eu I, Eu II and Eu III, respectively. It is well known that the lifetime of the Eu2þ luminescence is in the order of microesecond, which is relatively long for an allowed transition. This can be explained as follows. The ground state of 4f7 is 8S, and the multiplicity of the excited state 4f65 d1 is 6 or 8; the sextet portion of the excited state contributes to the spin-forbidden character of the transition. There is a wide variation in the emission wavelengths and decay times among the different Eu2þeactivated host lattices. To analyze these radiative lifetimes, Eq (3) can be used [26]:
1
t
¼ 5:06 108 j〈5djrj4f〉j2 cs3
(3)
where t is the decay time in units of second (s), the radial integral 〈5djrj4f 〉 in units of Angstron (Å) and s is the wavenumber of the emitted light in unit of wavenumber (cm1). The fact that the radial integral is in the same range in all Eu2þedoped lattices is remarkable. It means that although the compositions of the excited state are different for the compounds, this has no large influence on the decay [27]. Thus, the longer the emission wavelength is, the longer the radiative lifetime becomes, which is consistent with the result showing in Fig. 5. The decay curves at 523 K are presented in the inset of Fig. 5. At higher temperature the decay curves are composed of a faster initial part followed by an exponential one. The faster initial part could be attributed to thermally enhanced energy transfer to defects [28]. The temperature dependence of decay time is illustrated in Fig. 6. The decay times of the 425 and 455 nm emission almost remain constant at 10 K and 523 K. In most cases, the lifetime of excited state decreases gradually with increasing temperature as a result of a substantial increase in noneradiative relaxation rate [28]. Contrary to this change, we can see a gradual increase in the
Fig. 6. Luminescence decay times of the Ba1.3Ca0.7SiO4:0.01Eu2þ (for lex ¼ 355 nm and lem ¼ 425, 455 and 570 nm) as a function of temperature.
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X. Zhang et al. / Materials Chemistry and Physics 177 (2016) 538e542
decay time for the emission at 425 and 455 nm with increasing temperature up to 400 K. The abnormal vary of lifetime being similar to Eu I and Eu II in Tephase Ba1.3Ca0.7SiO4:0.01Eu2þ sample have also been reported in many system [29e35]. The reason for this unusual temperature dependence of Eu2þ fluorescence lifetime in these crystals could be explained by the trapping effect [28]. The decay times of the emission at 570 nm do not change a lot with increasing temperature up to 300 K, then decrease to 0.2 ms at 523 K. The decay times becoming shorter with increasing temperature is typical for the situation where the lifetime is shortened by a faster noneradiative decay from the excited state at higher temperature [24]. The decay time of 570 nm emission exhibits obvious decrease when the temperature is above 300 K, while that of 425 and 455 nm emissions do not show remarkable change. As discussed above, 425, 455 and 570 nm emission bands are assigned to three different Eu2þ centers. It is well known that the quenching temperature for Eu2þ emission is lower for longer wavelength emission due to a smaller energy barrier in the configuration coordinate diagram for the same Stokes shift [24,36]. 4. Conclusions We synthesized Eu2þeactivated Ba1.3Ca0.7SiO4 phosphor using solid state reaction method. The three Gaussian bands at ~440, ~464 and ~503 nm at room temperature can be assigned to the Eu2þ occupied (Ba/Ca)O12, (Ba/Ca)O10 and (Ba/Ca)O6, respectively. The low temperature PL is consistence with that at room temperature. The lifetimes of Eu I, Eu II and Eu III sites at 10 K are 0.23, 0.28 and 1.00 ms, respectively. Ba1.3Ca0.7SiO4:Eu2þ has a thermal quenching at 235 C. Ba1.3Ca0.7SiO4:Eu2þ is a good blue-emitting phosphor for white LEDs application. Acknowledgements The project was sponsored by the Technology Program of Environmental Protection Department of Hunan (No. 2013-312) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013-R1A1A2009154). References [1] D. Deng, H. Yu, Y. Li, Y. Hua, G. Jia, S. Zhao, H. Wang, L. Huang, Y. Li, C. Li, S. Xu, Ca4(PO4)2O: Eu2þ red-emitting phosphor for solid-state lighting: structure, luminescent properties and white light emitting diode application, J. Mater. Chem. C 1 (2013) 3194e3199. [2] X. Zhang, Q. Pan, S.I. Kim, Y.M. Yu, H.J. Seo, Temperature dependence of the luminescence of calcium-magnesium phosphate Ca3Mg3(PO4)4:Eu2þ, a blueemitting material for white light-emitting diodes, Mater. Res. Bull. 51 (2014) 28e34. [3] W. Chen, S.P. Wang, S.L. Westcott, J. Zhang, Structure and luminescence of BaFBr:Eu2þ and BaFBr:Eu2þ, Tb3þ phosphors and thin films, J. Appl. Phys. 97 (2005) 083506. [4] M.G. Brik, C.G. Ma, H. Liang, H. Ni, G. Liu, Theoretical analysis of optical spectra of Ce3þ in multi-sites host compounds, J. Lumin. 152 (2014) 203e205. [5] H.H. Lin, H.B. Liang, B. Han, J.P. Zhong, Q. Su, P. Dorenbos, M.D. Birowosuto, G.B. Zhang, Y.B. Fu, W.Q. Wu, Luminescence and site occupancy of Ce3þ in Ba2Ca(BO3)2, Phys. Rev. B 76 (2007) 035117. [6] H. Xie, L. Xu, L. Qin, Y. Huang, D. Wei, S.I. Kim, H.J. Seo, Orange-emitting Ba4R2ZrWO12:Eu3þ (R¼La, Gd, Y) with the strongest 5D0/7F0 transition, Mater. Lett. 115 (2014) 18e20. [7] X. Zhang, X. Xu, Q. He, J. Qiu, X. Yu, Significant improvement of photostimulated luminescence of Ba4(Si3O8)2:Eu2þ by co-doping with Tm3þ, ECS J, Solid State Sci. Tech. 2 (2013) R225eR229.
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