Luminescence and decay behavior of divalent europium activated barium borophosphate polycrystalline ceramics in the temperature regime 10–525 K

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 11726–11732 www.elsevier.com/locate/ceramint

Luminescence and decay behavior of divalent europium activated barium borophosphate polycrystalline ceramics in the temperature regime 10–525 K Fangui Menga, Jiyao Zhangb, Zhongfeng Zhangc, Hyo Jin Seod,n, Xinmin Zhanga,nn a

School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China b School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China c Department of Furniture, Central South University of Forestry and Technology, Changsha 410004, China d Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea Received 5 May 2015; received in revised form 25 May 2015; accepted 25 May 2015 Available online 3 June 2015

Abstract The luminescence properties of Ba3BP3O12 doped with divalent Eu2 þ are investigated in the temperature range 10–525 K. Excitation and emission bands corresponding to f–d transition of Eu2 þ from two crystallographic sites are identified. The PL spectra of Ba2.99Eu0.01BP3O12 sample at low temperatures further imply that the Eu2 þ emission emanates from two different crystallographic sites for Ba2 þ ions. The temperature dependence of emission intensity and lifetimes are studied. The luminescence intensity of Eu II does not have drastic decrease in the temperature range between 10 and 300 K, while that of Eu I decreases dramatically. Unfortunately, Ba2.99Eu0.01BP3O12 sample has poor thermal stability at temperatures higher than 300 K. The quantum yield, activation energy and thermal quenching mechanism of the luminescence are also discussed. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Powders: solid state reaction; B. Spectroscopy; C. Optical properties

1. Introduction In the spectroscopy involving transitions between the 4fn ground state and the 4fn  15d excited state of rare earth ions, divalent europium occupies a special position. The f–d transition of Eu2 þ has been studied in a lot of hosts and the energy level structure of the ion is well understood [1]. Usually, broad band emission spectrum due to 4f65d-4f7 transitions can be observed depending on the host lattice, and the peak position depends strongly on the ligand field around Eu2 þ ions [2–6]. Moreover, host lattices with Eu2 þ ions on different crystallographic sites usually show more than one emission band. For example, Ca9NaMg(PO4)7:Eu2 þ phosphor n

Corresponding author. Corresponding author. Tel./fax: þ86 731 85623303. E-mail addresses: [email protected] (H.J. Seo), [email protected] (X. Zhang). nn

http://dx.doi.org/10.1016/j.ceramint.2015.05.138 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

shows a broad emission band with three obvious peaks centered at 415, 458, and 615 nm corresponding to different Eu2 þ emission centers in Ca crystallographic sites [7]. Much research has been done in the past to explain the diverse luminescence behavior of Eu2 þ in different host lattices [8–12]. Recently, divalent europium-doped alkaline earth borophosphates phosphors have attracted considerable attention for use in white light-emitting-diodes (white LEDs) [13,14]. White LEDs are expected to replace conventional incandescent and fluorescent lamps due to their long lifetime and energy saving. The borophosphates based phosphors, with characteristic of low synthesis temperature, easy preparation and high quantum efficiency, act as a large family of luminescent materials. In this paper the luminescence properties of Eu2 þ in Ba3BP3O12 in the temperature range 10–525 K are reported and discussed. Ba3BP3O12 has the orthorhombic structure [15]. In this crystal two different crystallographic sites are available

F. Meng et al. / Ceramics International 41 (2015) 11726–11732

(a)

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excitation source (Shimadzu, RF 5301PC). The temperature dependent emission spectra (10–525 K) and the lifetime measurements were recorded by the 500 MHz digital storage oscilloscope (LeCroy 9350A) in which the signal was fed from PMT. Both types of 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 a cold finger equipped with a helium flow cryostat. To study thermal quenching between 300 and 525 K, the setup was equipped with a homemade heating cell connected to a temperature controller. The quantum efficiency was measured by a 2 Port 150 mm BaSO4 coated integrating sphere which fits directly into the FSP920 (Edinburgh Instruments) sample chamber.

Two Theta (degrees)

Fig. 1. (a) Powder XRD patterns of Ba2.86Eu0.14BP3O12; (b) powder XRD patterns of Ba2.96Eu0.04BP3O12; (c) powder XRD patterns of Ba2.99Eu0.01BP3O12 and (d) the reference pattern of orthorhombic Ba3BP3O12 (JCPDS: 85-0258) is included for comparison.

for Ba2 þ ions. On both sites the Ba2 þ is coordinated by eight oxygen ions. The difference between the two sites is their size. For the larger site the average Ba–O distance is 2.848 Å; while the smaller site the average Ba–O distance is 2.775 Å. The luminescence properties of Sm2 þ , Sm3 þ , Eu3 þ , Gd3 þ , Tb3 þ and Ce3 þ doped Ba3BP3O12 have been reported in the literatures [16–18]. The scintillation properties of Eu2 þ doped Ba3BP3O12 single crystal and powder samples have been investigated under X-ray excitation by Zhang and Duan [19,20]. Though the luminescence properties of rare earths ions doped Ba3BP3O12 have been studied by some groups [21], until now there are no reports on the luminescence properties of Ba3BP3O12:Eu2 þ at low temperature. Sometimes, it is impossible to distinguish the emissions from Eu2 þ on the two different sites. As a phosphor used for white LEDs, thermal stability is also one of the important technical indexes to measure the quality of phosphors. For these reasons we studied the luminescence properties of Ba3BP3O12:Eu2 þ in detail in the temperature range 10–525 K. 2. Experimental Samples were synthesized using a conventional solid-state reaction. As starting materials were used: BaCO3 (Aldrich, 99.9%), H3BO3 (Aldrich, 99.9%), NH4H2PO4 (Aldrich, 99.9%), and Eu2O3 (Aldrich, 99.99%). Mixtures of these materials were thoroughly ground in an agate motor and subsequently fired at 600 1C for 2 h. After grounding for a second time, the samples were fired for 5 h at 900 1C in a weak CO reducing atmosphere. Working in a reducing atmosphere and being doped to a divalent lattice site, europium will be built into the host lattice as Eu2 þ . XRD analysis was carried out on a Beijing Puxi XD-2 diffractometer operating at 36 kV, 25 mA at room temperature, using Cu Kα radiation. Luminescence spectra at room temperature were recorded using a fluorescence meter with a 150 W Xe lamp as an

3. Results and discussion Crystal structure and phase purity of Ba3  xEuxBP3O12 samples synthesized in the present study were identified by powder XRD analysis and the results of samples with x ¼ 0.01, 0.04 and 0.14 are given in Fig. 1. As seen in Fig. 1, the XRD data demonstrate that all diffraction peaks of the assynthesized samples match well the reference pattern of Ba3BP3O12 (JCPDS card no. 85-0258). Neither borates nor phosphates were identified, indicating that the prepared samples are single phase. Ba3BP3O12 crystallizes in orthorhombic structure and has a space group of Ibca (Nr. 73) with lattice parameters of a¼ 7.0656 Å, b ¼ 14.2680 Å and c¼ 22.1590 Å. PL and PLE spectra of Ba2.99Eu0.01BP3O12 sample measured at room temperature are presented in Fig. 2a. The PLE spectra of Ba2.99Eu0.01BP3O12 show a broad band ranging from 220 to 400 nm, which is attributed to the electric-dipole transition from the 8S7/2 ground state of the [Xe]4f7 configuration of Eu2 þ to the [Xe]4f65d1 excited states. Close examination on the spectra, discrepancy between them is observed when excitation spectra were recorded for different emission wavelengths, which confirms the existence of different Eu2 þ luminescent centers. Among the PLE spectra, the spectra are similar to each other when 410, 430 and 450 nm emission wavelengths are monitored; and the remaining ones are also similar to each other when longer emission wavelengths are monitored. The PL spectra of Ba2.99Eu0.01BP3O12 sample show broad band emission upon UV excitation at room temperature. The PL spectra correspond to the allowed electric-dipole transition [Xe]4f65d1-[Xe]4f7 of divalent europium. Moreover, the PL spectra exhibit different band shapes under different UV light excitations. There is a shoulder peak at the high energy side, which could result from the overlap of more than one emission band. The influence of the Eu2 þ concentration on the luminescence properties under 355-nm UV excitation was investigated by varying the Eu2 þ concentration between 1% and 14%. The results are illustrated in Fig. 2b. As can be seen from Fig. 2b, for both series of powders the intensity increases gradually with increasing the Eu2 þ content. No quenching behavior takes place when the Eu2 þ content is up to 14%. The photoluminescence quantum yield of Ba2.92Eu0.08BP3O12 is

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first experiment second experiment 0.00

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Eu content (x) Fig. 2. (a) PLE and PL spectra of Ba2.99Eu0.01BP3O12 sample at room temperature. The excitation and monitoring wavelengths are marked at corresponding curves; (b) the concentration dependence of integrated intensity of Ba3  xEuxBP3O12 samples under 355-nm excitation.

measured to be 0.65 at room temperature. Energy transfer can take place between different Eu2 þ centers [22,23]. As a result, the emission color depends strongly on the Eu2 þ concentration. In this case, no change is observed for the PL spectra of samples with different dopant concentrations under 254-nm excitation. As can be seen from Fig. 2, the spectral overlap between the emission bands from high energy center and the excitation bands from low energy center is small. Therefore, the resonance energy transfer from the high energy center to the low energy center could be inefficiency. Crystal structure of Ba3BP3O12 has been determined by a single crystal XRD method [15]. In this crystal, boron and phosphorus are tetrahedral coordinated by oxygen. The polyhedra form one-dimensional infinite helices that contain two additional PO4 groups connected to the BO4 unit. Barium is eightfold coordinated by oxygen; however, Ba2 þ ions are located in two different environments. The crystal structure of Ba3BP3O12 viewed from the [100] direction and barium coordination environments are shown in Fig. 3. Based on the fact that Ba2 þ and Eu2 þ have same charges and similar ionic

radii (rBa(II) ¼ 1.42 Å, rEu(II) ¼ 1.25 Å)[24], it is believable that the dopant Eu2 þ ions should replace Ba2 þ ions. In order to confirm the existence of different Eu2 þ luminescent centers, the PL spectra under 254 and 320 nm excitation are deconvoluted by using Gaussian profile and the results are shown in Fig. 4a and b. The emission spectra can be well deconvoluted into two Gaussian components that peaked at 439 (Eu I) and 504 nm (Eu II), which belongs to the typical emission of Eu2 þ ions ascribed to 4f–5d transitions. Usually, PL spectrum at lower temperature is used to support of existing different optically active ion sites in a specific host. The PL spectra of Ba2.99Eu0.01BP3O12 sample measured between T ¼ 10 K and 300 K in 25 K increments are shown in Fig. 5. The PL spectra exhibit two well-separated bands peaking at 425 and 505 nm at lower temperatures (10, 25, 50 and 77 K). The PL spectrum at 10 K is also shown in Fig. 4 as a comparison. The emission peak position of Eu II at room temperature is consistent with the result at low temperature; while that of Eu I at room temperature has a red-shift ( 14 nm) compared to that at low temperature (see detail in the inset of Fig. 4). This kind of red-shift have reported in SrSiO4:Eu2 þ and YAG:Ce3 þ [25,26]. The red-shift behavior can be explained by the viewpoint supposed by Varshini [27]. The two obvious emission bands at 10 K further supports our conclusion that two different Ba sites are available for the doped Eu2 þ to be substituted. The luminescence intensity of Eu II does not have drastic decrease in the temperature range between 10 and 300 K; while that of Eu I decreases dramatically. The emission band of Eu I nearly quenches at room temperature. Thermal stability is one of the important technical indexes to measure the quality of phosphors used in white LEDs. This is especially important in high power LEDs where the phosphor temperature can increase up to 450 K [28]. The higher temperature will cause the thermal degradation of phosphors

Fig. 3. (a) The crystal structure of Ba3BP3O12 viewed from the [100] direction and barium coordination environments.

F. Meng et al. / Ceramics International 41 (2015) 11726–11732 Wavelength (nm) 400

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PLE spectrum matches well with the emission wavelength of near-UV chip. For an allowed f–d transition, the decay time shortens as temperature quenching takes place, which is due to an additional non-radiative contribution to the decay process. We measured the PL decay curves as a function of emission and 535 nm) for the wavelengths (λem ¼ 425 Ba2.99Eu0.01BP3O12 sample at temperatures between 10 K and 525 K. Fig. 7 shows the luminescent decay curves for the emissions at 425 nm and 535 nm (λex ¼ 355 nm) at 10 K. The decay curves at 10 K show purely single exponential decay. The calculated lifetimes are  0.5 and 1.0 μs for 425 and 535 nm emissions, respectively. Eq. (1) can be used to

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Fig. 4. (a) PL spectrum of Ba2.99Eu0.01BP3O12 sample (λex ¼320 nm) at room temperature; (b) PL spectrum of Ba2.99Eu0.01BP3O12 sample (λex ¼ 254 nm) at room temperature; (c) PL spectrum of Ba2.99Eu0.01BP3O12 sample (λex ¼355 nm) at 10 K.

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and reduce the luminescent properties such as chromaticity stability and brightness. The PL spectra for Ba2.99Eu0.01BP3O12 were measured at higher temperatures in the range of 300–525 K, and the results are shown in Fig. 6. The luminescent intensity decreases rapidly 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. 6] accounting for the interaction between the dopant ion and the vibrating lattice of the host material [24]. Unfortunately, Ba2.99Eu0.01BP3O12 sample has poor thermal stability at higher temperatures, which restrict the application of it as a phosphor for white LEDs, although the

Experimental data (425 nm) Fitted crve Experimental data (535 nm) Fitted crve

1

Fig. 5. PL spectra of Ba2.99Eu0.01BP3O12 (λex ¼ 355 nm) for various temperatures (10–300 K).

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Fig. 7. Luminescence decay curves of Eu2 þ d–f emission for Ba2.99Eu0.01BP3O12 (λem ¼425 and 535 nm) under 355 nm excitation at 10 K.

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Fig. 8. Luminescence decay curves of Eu2 þ (II) d–f emission for Ba2.99Eu0.01BP3O12 under 355 nm excitation at 10–525 K.

analyze the radiative lifetimes [29]:
D

E
2 1



¼ 5:06  10  8
5d
r
4f
χσ 3 ð1Þ τ D

E

where τ is the decay time, 5d
r
4f is the radial integral and σ is the wavenumber of the emitted light. The radial integral is in the same range in all Eu2 þ -doped lattices. Although the compositions of the excited state are different for the compounds, it does not have huge influence on the decay [30]. Thus, the longer the emission wavelength, the longer the radiative lifetime becomes, which is consistent with the result shown in Fig. 7. Fig. 8 shows the luminescent decay curves of Ba2.99Eu0.01BP3O12 for the emission at 535 nm (λex ¼ 355 nm) at 10–525 K. In Fig. 8, the decay curves of the 535 nm emission demonstrate an almost single exponential behavior with decay constant τ ¼  1.0 μs in the temperature range 10– 348 K. The lifetime is a typical value found for green Eu2 þ emission. Significant temperature quenching is observed above 373 K; at T ¼ 525 K the decay constant reaches 0.11 μs. The calculated lifetime of 535 nm emission for Ba2.99Eu0.01BP3O12 sample as a function of temperature is plotted in Fig. 9. The decay time of 4f65d-4f7 transition is usually in the range from 0.2 to 2 μs among the different Eu2 þ -activated host lattices [30]. In most cases, the lifetime of excited state decreases gradually with increasing temperature owing to an increase in non-radiative relaxation rate [31]. The activation energy (ΔE) for thermal quenching of the Eu2 þ emission in Ba2.99Eu0.01BP3O12 sample can also determined by fitting the temperature dependence of the Eu2 þ emission life times. The Arrhenius equation was employed to calculate the respective activation energy as follows [32,33]: τ   r ð2Þ τ¼ 1 þ τr =τnr expð ΔE=kTÞ where τr is the radiative life time of activator; τnr is the nonradiative life time of activator; ΔE is activation energy of thermal quenching; k is the Boltzmann constant. The fitted

0

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Temperature (K) Fig. 9. Temperature dependence of the Eu2 þ (II) d–f emission lifetime for Ba2.99Eu0.01BP3O12.

results are illustrated in Fig. 9. In this material, the radiative and non-radiative lifetimes of the Eu (II) excited state is found to be  1.1  10–6 and 9.1  10–12 s. The activation energy is  0.43 eV. This value can be compared with the thermal activation energy of the SrGa2S4:Eu2 þ phosphor (ΔE ¼ 0.6 7 0.1 eV) [31]. Three types of quenching models have been suggested to explain thermal quenching processes of the luminescence: (I) energy transfer up to impurities, (II) a large displacement between the ground and excited state in the configuration coordinate diagram and (III) electron or hole transfer between the activator states and the band structure of the lattice [31]. Model (I) is a concentration dependent mechanism. As discussed above (see detail in Fig. 3), the Eu2 þ content up to 14% has no influence on the quenching behavior. Thus, we draw a conclusion that energy migration among Eu2 þ ions up to impurities is not the preponderant mechanism involved in thermal quenching. For the SrGa2S4:Eu2 þ , Chartier et al. have estimated the cross-point location at  10 eV above the bottom of the excited state [31]. This value is considerably higher than the activation energy (0.6 7 0.1 eV). Thus, thermal quenching cannot be explained using model (II). Ba2.99Eu0.01BP3O12 sample has similar activation energy to SrGa2S4:Eu2 þ ; so the thermal quenching could not be due to the relaxation to the ground state in a non-radiative manner via the crossing between the fundamental and excited parabolas. Dorenbos demonstrates that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism [34]. Especially for host lattices having small Stokes shift and low quenching temperature, it is clear that thermally induced ionization from the fd state to the conduction band is responsible for temperature quenching of the luminescence. In this case, the Stokes shift is small and the luminescence quenching temperature is low. Therefore, we regard the thermal activation of an electron from the Eu2 þ excited state to the conduction band can be used to explain the thermal quenching behavior in Ba3  xEuxBP3O12 system.

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4. Conclusions The temperature dependence of luminescence properties and life times are reported for the f–d luminescence of Eu2 þ in Ba3BP3O12. The PL spectra exhibit asymmetric broad band due to d–f transition of Eu2 þ at room temperature; while the PL spectra exhibit two well-separated bands peaking at 425 and 505 nm at lower temperatures. The two broad bands can be assigned to two different Eu2 þ luminescent centers, which is consistent with the crystal structure of Ba3BP3O12 that there are two different Ba2 þ site in the crystal lattice. The luminescence intensity of Eu II does not have drastic decrease in the temperature range 10–300 K, while that of Eu I decreases dramatically. The luminescence intensity of Ba2.99Eu0.01BP3O12 decreases rapidly at temperatures higher than 300 K. The photoluminescence quantum yield of Ba2.92Eu0.08BP3O12 is measured to be 0.65 at room temperature. The activation energy is 0.43 eV for Eu (II). The thermal quenching in Ba2.99Eu0.01BP3O12 system can be attributed to the thermal activation of an electron from the Eu2 þ excited state to the conduction band. Acknowledgments The research was supported by the Technology Program of Environmental Protection Department of Hunan (No. 2013312). The project was also supported by 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] H. Ji, Z. Huang, Z. Xia, M.S. Molokeev, V.V. Atuchin, M. Fang, Y.G. Liu, Discovery of new solid solution phosphors via cation substitution dependent phase transition in M3(PO4)2:Eu2 þ (M¼ Ca/Sr/ Ba) quasi-binary sets, J. Phys. Chem. C 119 (2015) 2038–2045. [2] F.C. Lu, L.J. Bai, W. Dang, Z.P. Yang, P. Lin, Structure and photoluminescence of Eu2 þ doped Sr2Al2SiO7 cyan–green emitting phosphors, ECS J. Solid State Sci. Technol. 4 (2015) R27–R30. [3] D. Geng, M. Shang, Y. Zhang, H. Lian, J. Lin, Temperature dependent luminescence and energy transfer properties of Na2SrMg(PO4)2:Eu2 þ , Mn2 þ phosphors, Dalton Trans. 42 (2013) 15372–15380. [4] S. Schmiechen, H. Schneider, P. Wagatha, C. Hecht, P.J. Schmidt, W. Schnick, Toward new phosphors for application in illumination-grade white pc-LEDs: the nitridomagnesosilicates Ca[Mg3SiN4]:Ce3 þ , Sr [Mg3SiN4]:Eu2 þ , and Eu[Mg3SiN4], Chem. Mater. 26 (2014) 2712–2719. [5] T. Suehiro, R.J. Xie, N. Hirosaki, Gas-reduction-nitridation synthesis of CaAlSiN3:Eu2 þ fine powder phosphors for solid-state lighting, Ind. Eng. Chem. Res. 53 (2014) 2713–2717. [6] W.Y. Huang, F. Yoshimura, K. Ueda, Y. Shimomura, H.S. Sheu, T.S. Chan, C.Y. Chiang, W. Zhou, R.S. Liu, Chemical pressure control for photoluminescence of MSiAl2O3N2:Ce3 þ /Eu2 þ (M¼ Sr, Ba) oxynitride phosphors, Chem. Mater. 26 (2014) 2075–2085. [7] Z. Xia, H. Liu, X. Li, C. Liu, Identification of the crystallographic sites of Eu2 þ in Ca9NaMg(PO4)7: structure and luminescence properties study, Dalton Trans. 42 (2013) 16588–16595.

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