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Eu2+-doped Ba2GaB4O9Cl blue-emitting phosphor with high color purity for near-UV-pumped white light-emitting diodes
Optics and Laser Technology 98 (2018) 61–66
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Eu2+-doped Ba2GaB4O9Cl blue-emitting phosphor with high color purity for near-UV-pumped white light-emitting diodes Zhiwen Gao a, Huajuan Deng a, Na Xue a, Jung Hyun Jeong b, Ruijin Yu a,⇑ a b
College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shaanxi 712100, PR China Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 September 2016 Received in revised form 19 April 2017 Accepted 28 July 2017
Keywords: Haloborate Luminescence Eu2+ ion
a b s t r a c t Eu2+-doped borate fluoride Ba2GaB4O9Cl was synthesized by the conventional high-temperature solidstate reaction. The crystal structure and luminescence properties of the phosphors, as well as their thermal luminescence quenching capabilities and CIE chromaticity coordinates were systematically investigated. Under the excitation at 340 nm, the phosphor exhibited an asymmetric broad-band blue emission with a peak at 445 nm, which is ascribed to the 4f–5d transition of Eu2+. It was further proved that energy transfer among the nearest neighbor ions is the major mechanism for concentration quenching of Eu2+ in Ba2xGaB4O9Cl:xEu2+ phosphors. The luminescence quenching temperature is 432 K. The CIE color coordinates are very close to those of BaMgAl10O17:Eu2+ (BAM). All the properties indicated that the blueemitting Ba2GaB4O9Cl:Eu2+ phosphor has potential application in white LEDs. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Over the decades, white light emitting diodes (WLEDs), as a promising solid-state lighting source, has aroused interest as part of the revolution to overtake conventional incandescent or fluorescence lamps for illumination, considering their fascinating advantages such as energy saving, small size, eco-friendliness, brightness and long lifetime [1–3]. Two schemes are mainly used to obtain the phosphor-converted white LEDs. One scheme is the combination of the blue LEDs chip and the yellow-emitting phosphor, and the other is the combination of the near ultraviolet (nUV) LEDs chip and the red, green and blue emission phosphor [4]. In order to fulfill the corresponding requirements, many research papers on developing new phosphor systems used as light-conversion phosphors for white LEDs have been reported [5–10]. As hosts to accommodate RE ions, halo-containing haloborates have some excellent properties such as low synthesis temperature, high physical and chemical stability, and excellent thermal and hydrolytic stabilities [11,12]. The haloborate phosphor is widely studied for its tunable optical properties and potential application in lighting, display, scintillation, thermal neutron image plates, etc. [13,14]. For example, Ba2Ln(BO3)2Cl (Ln = Y, Gd and Lu):Eu2+ [15], Sr5(BO3)3Cl:Ce3+ [16], Ba2B5O9Cl:Bi [17], Ca2BO3Cl:Sm3+,Eu3+ [18], ⇑ Corresponding author. E-mail address: [email protected] (R. Yu). http://dx.doi.org/10.1016/j.optlastec.2017.07.055 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
Ca2BO3Cl:Ce3+,Eu2+ [19], M2B5O9X:Eu2+ (M = Ca, Sr, Ba; X = Cl, Br) [20–22], etc. Ba2GaB4O9Cl is the representative of a new noncentrosymmetric tetragonal structure type related to hilgardite and M2B5O9X. The compound is compositional via substitution of one framework of B atom by Ga in the Ba2B5O9Cl pentaborate halides. The efficiency of second harmonic generation for Ba2GaB4O9Cl was evaluated and found to be similar to that of KH2PO4 (KDP) [23]. In 2010, luminescence properties of Ba2AlB4O9Cl:Eu2+ were firstly reported and considered to be the promising violet-blueemitting phosphor excited by a UV InGaN chip for white LEDs [24]. Ba2TB4O9Cl (T = Al, Ga) are isomorphous with only minor differences in bond distance [23]. However, to the best of our knowledge, few research on the luminescence and thermal properties of Eu2+-doped Ba2GaB4O9Cl for optical materials has been reported thus far. In the present work, a novel blue-emitting phosphor of Eu2+doped borate halide Ba2GaB4O9Cl was synthesized, and the luminescence and thermal quenching properties of Eu2+ in Ba2GaB4O9Cl were investigated to develop new materials with potential application for white LEDs.
2. Experiments Polycrystalline powders with the general formula Ba2(1x)Eu2xGaB4O9Cl (x = 0.01, 0.02, 0.05, 0.08, 0.10, 0.15, and 0.20) were prepared by conventional solid-state reaction technique. The starting
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materials were BaCO3 (A.R.), BaCl2 (A.R.), Ga2O3 (A.R.), H3BO3 (A.R.), and Eu2O3 (99.99%). With BaCO3, BaCl2, Ga2O3, and H3BO3 mole ratio of 1:1:0.5:4, the raw materials were thoroughly mixed by grinding in an agate mortar, and reduced for 3 h under a 10% H2/90% N2 gas mixture at 800 °C. Some excess NH4Cl (5 wt%) are necessary to compensate for loss of Cl source at high temperature. The phase purity of the as-prepared phosphors was investigated by X-ray powder diffraction (XRD) using Cu Ka with a Rigaku D/Max 2200 VPC. The photoluminescence (PL) and PL excitation (PLE) spectra were recorded using a Photon Technology International (PTI, USA) fluorimeter with a 60-W Xe arc lamp. The temperature-dependent PL spectra of the phosphor Ba2GaB4O9Cl: Eu2+ were recorded using an FLS920-combined fluorescence lifetime and steady state spectrometer (Edinburgh Instruments). 3. Results
Fig. 2. XRD patterns of Ba2GaB4O9Cl:xEu2+ (x = 0.02, x = 0.20) samples.
3.1. Phase composition The structure sketch map of Ba2GaB4O9Cl is shown in Fig. 1, which was modeled using the Diamond Crystal and Molecular Structure Visualization software on the basis of the atomic coordinate [23]. The Ba2GaB4O9Cl structure consists of a zeolitic [GaB4O9]3 framework with Ba2+ and Cl ions accommodated in tunnels parallel to the c axis. The framework is based on a novel GaB4O12 fundamental building block built of a tetraborate cluster of two BO3 triangles and two BO4 tetrahedra sharing a corner with a GaO4 tetrahedron. These fundamental building blocks are linked together via corner sharing to form [0 0 1] chains which condense by sharing corners with adjacent chains to build the threedimensional framework. The chains retain their polar character as shown by the common orientation of the GaO4 and BO4 tetrahedra along the c direction. As a result, the [GaB4O9]3 framework as a whole is non-centrosymmetric and polar. There are two types of barium sites Ba(1) and Ba(2) shown in the inset of Fig. 1. Ba(1) atom has a ten-coordinated environment of [7O + 3Cl], and Ba(2) atom has an eleven-coordinated environment of [9O + 2Cl] [23]. The XRD patterns of the representative Ba2(1x)Eu2xGaB4O9Cl samples (x = 0.02, and 0.20) are shown in Fig. 2. The simulated XRD pattern in Fig. 2 was calculated using the atomic coordinates of Ba2GaB4O9Cl crystal reported by Barbier [23]. The host lattice Ba2GaB4O9Cl has a monoclinic structure with space group P42nm, and its lattice parameters are a = b = 12.1508 Å, c = 6.8618 Å, and V = 1013.09 Å3. All peaks agree well with the simulated pattern, indicating that a single-phase phosphor is obtained, and the doped Eu2+ ions do not change the host crystal lattice. Because of the sim-
Fig. 1. (a) Schematic views of the structure of Ba2GaB4O9Cl along the c-direction; (b) Coordination environment around Ba(1) and Ba(2), respectively. Purple ball: Ba; green ball: Cl; cyan ball: Ga; blue ball: B; and red ball: O. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ilarities of ionic radii of Eu2+ and Ba2+ ions, the Eu2+ ions are expected to occupy Ba2+ ion sites in the Ba2GaB4O9Cl:Eu2+ crystal structure. To confirm the crystalline morphology of Eu2+-doped polycrystals, the SEM images were investigated. Fig. 3 shows the representative SEM images of the Ba2GaB4O9Cl:xEu2+ (a, x = 0.02; b, x = 0.20) samples at 5000 magnification. The sample crystallized with the irregular shapes and aggregated for the high-temperature solidstate reaction. The morphologies of powders did not vary much for the doping concentration from 0.02 to 0.20 mol. To break up the agglomerations and improve the quality of the powders, a long ball-milling step is necessary to make them suitable for use in typical screening processes used in the construction of WLEDs.
3.2. Optical properties of Ba2GaB4O9Cl:Eu2+ Fig. 4 shows the typical photoluminescence excitation (kem = 445 nm) and emission spectra (kex = 340 nm) in 10 mol% Eu2+-doped Ba2GaB4O9Cl phosphor. All of the phosphors present bright blue color under a UV lamp. The PLE spectra have a broad band between 250 and 430 nm, and there are four excitation peaks around 293, 343, 370, and 397 nm, which is due to the 4f7(8S7/2) ? 4f6(7F)5d1 transitions of the Eu2+ ion. Because the broad excitation matches well with the near UV chips (350–400 nm), Ba2GaB4O9Cl: Eu2+ phosphors are suitable for near-UV excited solid-state lighting. As can be seen in Fig. 4(b), the Ba2GaB4O9Cl:Eu2+ phosphor shows an asymmetric blue band peaking at 445 nm with the fullwidth half-maximum (FWHM) of 48 nm under 340 nm excitations. The emission of Ba2GaB4O9Cl:Eu2+ is ascribed to the dipole-allowed transition from the lower 4f6(7F)5d1state to the 4f7(8S7/2) ground state of the Eu2+ ions. No other characteristic emission peaks from Eu3+ are observed in the PL spectra, indicating that Eu3+ ions have been reduced to Eu2+ completely in our experiments. It is worth noting that the luminescence wavelength position is similar to the other Eu2+-doped alkaline-earth haloborates. These emission peaks of the haloborates usually locate in the blue wavelength region, for example, Ba2AlB4O9Cl:Eu2+ (420 nm), Ca2B5O9Cl:Eu2+ (452 nm), Sr2B5O9Cl:Eu2+ (425 nm), and Ba2B5O9Cl:Eu2+ (401 nm) [11,24,25]. Compared with the emission peak of Ba2AlB4O9Cl:Eu2+ (420 nm) [24], the Ba2GaB4O9Cl:Eu2+ exhibits an obvious red shift. It is mainly due to the centroid shift of the 5d excited level of Eu2+ ion. Al3+ and Ga3+ ions are the so-called counter-cations that polarize or even bind the anion ligands. A strong polarization or bonding goes at the expense of the interaction between the anion and the 5d electron of Eu2+ [26]. Replacing Al3+ by the same valency and bigger Ga3+ in the Ba2MB4O9Cl:Eu2+ (M = Al,Ga) crystal leads to
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Fig. 3. Typical SEM morphology of Ba2GaB4O9Cl:xEu2+ (a, x = 0.02; b, x = 0.20) samples.
The energy of the f–d absorption and the d–f emission can be written according to the formalism of Dorenbos [26] as
Eabs ¼ Efree D and Eemi ¼ Efree D Ds
Fig. 4. The excitation (kem = 445 nm) and emission spectra (kex = 340 nm) ofBa2GaB4O9Cl:0.10Eu2+ phosphor. [(e), the fitted curve of curve (b); (c) and (d) the deconvolution curves of curve (a)].
the decrease in effective electronegativity of the counter-cations and the binding of anion ligands [27]. Therefore, both increase of anion polarizability and covalency between anion and Eu2+ will lead to bigger centroid shift. A bigger centroid shift tends to result in the red-shift behavior of the emission peaks. The asymmetric emission band can be deconvoluted into two Gaussian components peaked at 443 nm [referred to as Eu(II)] and 466 nm [referred to as Eu(I)], respectively, as shown in Fig. 4 (c) and (d). This indicates that there are two Eu2+ luminescence centers in Ba2GaB4O9Cl lattice. The coordination polyhedra for the barium ions in the Ba2GaB4O9Cl crystal have been taken to be [BaCl3O7] [Ba(1)] and [BaCl2O9] [Ba(2)], as indicated in the inset of Fig. 1. Considering the Eu2+ environment in the doped host, the energy position of the lowest Eu2+ 4f65d1 level can be modified by the covalency and polarizability of Eu2+ ligand bonds, incorporating ligands Cl[v(Cl) 3.16] with a lower electronegativity compared to O2 [v(O) 3.44] would lower the energy of the 4f65d1 levels and would lead to distinct Eu2+ sites with redder emission [19,28]. In terms of the different polyhedra in the [Ba (1)] and [Ba(2)] sites, in case Eu(I) occupied the site of Ba(1), where more Cl anions coordinated, its emission should show a redshift in comparison with that of Eu(II)。Therefore, we can propose that the emission peak centered at 466 nm is attributed to Eu2+ ions which occupy the Ba(1) site with [BaCl3O7] polyhedra. The other peak centered at 443 nm is ascribed to Eu2+ ions occupying the Ba(2) site with [BaCl2O9] polyhedra.
ð1Þ
where Efree is the energy difference between the lowest 4f7 level and the 4f6(7F0)5d level for free or gaseous ions. D is the energy lowering, also called the red shift, and Ds is the Stokes shift. Because the excitation spectrum is not well resolved, the position of the lowest 5d excited level of Eu2+ (Eabs) is generally estimated by using the mirror-image relationship between the emission and the excitation spectra [29]. The Stokes shift (Ds) can be estimated as twice the energy difference between the zero-phonon line and the energy of the emission maximum [30]. The position of the zero-phonon line is considered to be the intersection point of the excitation and emission spectra. In the present case, the lowest absorption energy, Eabs, is about 3.12 eV (397 nm) and the Stokes shift (Ds) is calculated to be 0.32 eV (2616 cm1). Knowing the value of Eabs allows us to determine the red shift D of the f–d transition with respect to the free ion (Efree = 4.19 eV for Eu2+) [31]. We found D = Efree Eabs = 4.19 3.12 = 1.07 eV (8618 cm1). As demonstrated by Dorenbos [26] in an extensive review on the position of 5d transitions of lanthanides, the influence of the crystal field and covalency of the host lattice on the red shift (D) of 5d levels are approximately equal for all lanthanides in the same compound. It is possible to use the position of the 5d levels of Eu2+ to predict that of all other lanthanides. As the first report of Eu2+-activated Ba2GaB4O9Cl phosphor, we calculated the red shift (D) value of Eu2+ in this Ba2GaB4O9Cl host. Fig. 5 presents the emission spectra of the Ba2(1x)Eu2xGaB4O9Cl phosphors with different Eu2+ concentrations (x = 0.01, 0.02, 0.05, 0.08, 0.10, 0.15, and 0.20) upon 340 nm near UV excitation. As shown in the inset, the emission intensity of Eu2+ firstly increased with the increase in its concentration, and reached the maximum when the concentration of Eu2+ is 0.10, and then the emission intensity decreased with further increasing concentration, which is caused by the concentration quenching effect. It can be concluded that the optimum doping concentration of Eu2+ is 0.10. Furthermore, with increasing Eu2+ concentration, the PL spectra have shifted to long wavelength region (peak positions: 439.0 nm for x = 0.01 and 446.3 nm for x = 0.20). According to the previous reports [32,33], the red-shift of the emission peak is frequently observed in rare-earth-doped phosphors with the doping concentration increasing due to the variations of crystal field strength surrounding the activators. Due to the increase in doping concentration of Eu2+, it is accepted that the inter-atomic distance between the two activators become shorter, and the interaction is enhanced [22]. Thus, the crystal field strength surrounding Eu2+ is increased, and it results in the red-shift of the emission peak.
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Fig. 5. Emission spectra of Ba2(1x)Eu2xGaB4O9Cl (x = 0.01, 0.02, 0.05, 0.08, 0.10, 0.15, and 0.20) phosphors with different Eu2+ concentrations. The inset shows the dependence of intensity on the Eu2+-doping concentration.
nearest neighbor ions is the main mechanism of concentration quenching of Ba2GaB4O9Cl:Eu2+. The similar phenomenon has been reported in the Ca5(PO4)2(SiO4):Eu2+, Ca4(PO4)2O:Eu2+, and Ca4P2O9:Ce3+ phosphor [36–38]. The thermal stability of Eu2+-doped Ba2GaB4O9Cl phosphor was evaluated by the temperature-dependent PL spectra. Fig. 7 shows the dependence of the PL spectra of Ba2GaB4O9Cl:0.10Eu2+ on temperature upon excitation under 340 nm light. As temperature increases from 300 K to 500 K, the PL intensity obviously decreases. As shown in the inset of Fig. 7, the quenching temperature (T1/2) is 432 K. After heating the sample up to 420 K, at which the LEDs usually work, the emission intensity decreases by approximately 54% of that measured at room temperature. The value is found to be 65% for Ca2BO3Cl:Eu2+, and 90% for Ba2Gd(BO3)2Cl:Eu2+ [15,39]. The phosphor does not have very good enough thermalquenching property compared with them. Furthermore, the emission band is a little blue-shifted with raising the temperature. It was ascribed to the thermally active phonon-assisted tunneling from the excited states of the low-energy emission band to the excited states of the high-energy emission band in the configuration coordinate diagram [40]. As discussed before, there are two
Van Uitert [34] reported that the intensity of multipolar interaction can be determined based on the change in the emission intensity from the emitting level that has multipolar interaction. The emission intensity (I) per activator ion follows the equation:
h i1 I ¼ K 1 þ bðxÞQ=3 x
ð2Þ
where v is the activator concentration; Q is a constant of multipolar interaction and equals 3, 6, 8, or 10 for the nearest-neighbor ions, dipole–dipole, dipole–quadrupole or quadrupole–quadrupole interaction, respectively; and K and b are constants under the same excitation condition for the given host crystal [34,35]. Eq. (2) can be approximately reduced to Eq. (3) for bvQ/3 1 as follows:
I Q ¼ A lg x lg x 3
ðA ¼ lg k lg bÞ
ð3Þ
The relationship of lg(I/x) versus lg(x) in Ba2GaB4O9Cl:xEu2+, shown in Fig. 6(b), is linear and the slope is 0.93. The value of h is found to be 2.79 using the above Eq. (3), which is approximately equal to 3. This result indicates that the energy transfer among the
Fig. 7. Dependence of the PL emission of Ba2GaB4O9Cl:0.10Eu2+ (kex = 340 nm) on temperature. Inset shows the relation between the integral intensity of the emission band (380–600 nm) and temperature.
Fig. 6. Fitting curve of lg(I/x) versus lg(x) in Ba2GaB4O9Cl:xEu2+ (x = 0.01–0.20) phosphors.
Fig. 8. The Arrhenius plot of the temperature dependence of the PL emission intensity of Ba2GaB4O9Cl:Eu2+.
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Fig. 9. (a) Comparison of PL (kex = 340 nm) spectra of Ba2GaB4O9Cl:0.10Eu2+ with that of commercial blue BAM:Eu2+ phosphor; (b) CIE coordinates of the Ba2GaB4O9Cl:0.10Eu2+ phosphor. The inset shows the image of the Ba2GaB4O9Cl:Eu2+ phosphor excited at 365 nm in a UV box.
emission centers for the Ba2GaB4O9Cl:Eu2+ phosphor. At higher temperature, the thermal back-transfer from lower energy centers to higher energy centers is more possible; consequently, the higher energy emission is strengthened. Thus, the blue-shift phenomenon is observed with the increase of temperature. And similar blueshifts were also observed elsewhere, such as CaLaGa3S7:Eu2+, Ca2B5O9Br:Eu2+ and K2Al2B2O7:Eu2+ [22,41,42]. The temperature dependence of luminescence intensity is described by the following Arrhenius equation (4) [43]:
IðTÞ ¼
I0 I0 Ea 1 þ ln c or ln 1 ¼ 1 þ c expðEa =kTÞ IðTÞ k T
ð4Þ
where I0 is the initial intensity; I(T) is the intensity at a given temperature T; c is a constant, k is Boltzmann’s constant (8.62 105 eV/K); and Ea is the activation energy, which represents the energy difference between the lowest excited state and the bottom of the host lattice conduction band. Fig. 8 shows the plot of ln[(I0/I) 1] versus 1/T for Ba2GaB4O9Cl:Eu2+. Linear regression shows that the thermal activation energy Ea for quenching is 0.23 eV. Compared with the reported activation energy of thermal quenching in haloborate phosphors such as Ca2BO3Cl:Eu2+ (0.43 eV) [44] and Mg3B7O13Cl:Eu2+ (0.32 eV) [12], Eu2+-doped Ba2GaB4O9Cl has lower activation energy when compared with the references. Furthermore, the comparison of PL spectra between the Ba2GaB4O9Cl:0.10Eu2+ phosphor and the commercial one BAM:Eu2+ blue-emitting phosphor is shown in Fig. 9(a). Upon excitation at a wavelength of 340 nm, the sample presented a very similar curve to that of the BAM:Eu2+ phosphor, and the integrated area of the PL spectra was measured to be 52% of that of the commercial BAM: Eu2+ phosphor, although Ba2GaB4O9Cl:Eu2+ is yet to be further optimized. As key parameters, the CIE coordinates for the Ba2GaB4O9Cl:0.10Eu2+ sample were calculated to be (x = 0.151, y = 0.044), very close to those of the BAM:Eu2+, indicating that this phosphor has a high color purity as shown in Fig. 9(b), which origins from the similar peak profiles and FWHM values with BAM. A digital photograph of the sample under 365 nm UV lamps is shown in the inset, revealing an intense blue emission. The initial results suggest that this novel phosphor may serve as a promising blue emitting component for w-LED application.
4. Conclusions Eu2+-doped Ba2GaB4O9Cl polycrystalline were prepared by conventional high-temperature solid-state reaction. The excitation of Ba2GaB4O9Cl:Eu2+ with a broad band from 340 to 430 nm could match the near UV chips well. It shows a narrow (FWHM = 48 nm) blue luminescence band centered at 445 nm. The luminescence spectra show that Eu2+ ions have two emission centers in the Ba2GaB4O9Cl lattice. The critical quenching concentration of Eu2+ in Ba2GaB4O9Cl phosphor is about 10 mol%. This study demonstrates that energy transfer among the nearest neighbor ions is the major mechanism for concentration quenching of Eu2+ in Ba2xGaB4O9Cl: xEu2+ phosphors. The luminescence quenching temperature is 432 K. The CIE color coordinates are (x = 0.151, y = 0.044). The above obtained results indicated that this novel phosphor was promising as the blue component of light-conversion phosphors for white LEDs. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21201141), the Chinese Universities Scientific Fund (Grant No. QN2011119 and QN2452015424), and the Young Faculty Study Abroad Program of Northwest A&F University. This work was also supported by Research Grant of Pukyong National University (2016). References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer-Verlag, Berlin, 1997. [2] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Mater. Sci. Eng. B 71 (2010) 1–34. [3] R. Yu, S. Zhong, N. Xue, H. Li, H. Ma, Dalton Trans. 43 (2014) 10969–10976. [4] E. Radkov, R. Bompiedi, A.M. Srivastava, A.A. Setlur, C.A. Becker, Proc. SPIE 5187 (2004) 171–177. [5] R. Yu, C. Guo, T. Li, Y. Xu, Curr. Appl. Phys. 13 (2013) 880–884. [6] J. Yan, L. Ning, Y. Huang, C. Liu, D. Hou, B. Zhang, Y. Huang, Y. Tao, H. Liang, J. Mater. Chem. C 2 (2014) 8328–8332. [7] N. Zhang, C. Guo, L. Yin, J. Zhang, M. Wu, J. Alloys Compd. 635 (2015) 66–72. [8] H.K. Yang, H.M. Noh, B.K. Moon, J.H. Jeong, S.S. Yi, Ceram. Int. 40 (2014) 12503– 12508. [9] W. Zhou, J. Han, X. Zhang, Z. Qiu, Q. Xie, H. Liang, S. Lian, J. Wang, Opt. Mater. 39 (2015) 173–177. [10] M. Peng, X. Yin, P.A. Tanner, M.G. Brik, P. Li, Chem. Mater. 27 (2015) 2938– 2945. [11] A. Meijerink, G. Blasse, J. Lumin. 43 (1989) 283–289. [12] X. Qiao, H.J. Seo, J. Am. Ceram. Soc. 98 (2015) 594–600.
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Eu2+-doped Ba2GaB4O9Cl blue-emitting phosphor with high color purity for near-UV-pumped white light-emitting diodes Zhiwen Gao a, Huajuan Deng a, Na Xue a, Jung Hyun Jeong b, Ruijin Yu a,⇑ a b
College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shaanxi 712100, PR China Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 September 2016 Received in revised form 19 April 2017 Accepted 28 July 2017
Keywords: Haloborate Luminescence Eu2+ ion
a b s t r a c t Eu2+-doped borate fluoride Ba2GaB4O9Cl was synthesized by the conventional high-temperature solidstate reaction. The crystal structure and luminescence properties of the phosphors, as well as their thermal luminescence quenching capabilities and CIE chromaticity coordinates were systematically investigated. Under the excitation at 340 nm, the phosphor exhibited an asymmetric broad-band blue emission with a peak at 445 nm, which is ascribed to the 4f–5d transition of Eu2+. It was further proved that energy transfer among the nearest neighbor ions is the major mechanism for concentration quenching of Eu2+ in Ba2xGaB4O9Cl:xEu2+ phosphors. The luminescence quenching temperature is 432 K. The CIE color coordinates are very close to those of BaMgAl10O17:Eu2+ (BAM). All the properties indicated that the blueemitting Ba2GaB4O9Cl:Eu2+ phosphor has potential application in white LEDs. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Over the decades, white light emitting diodes (WLEDs), as a promising solid-state lighting source, has aroused interest as part of the revolution to overtake conventional incandescent or fluorescence lamps for illumination, considering their fascinating advantages such as energy saving, small size, eco-friendliness, brightness and long lifetime [1–3]. Two schemes are mainly used to obtain the phosphor-converted white LEDs. One scheme is the combination of the blue LEDs chip and the yellow-emitting phosphor, and the other is the combination of the near ultraviolet (nUV) LEDs chip and the red, green and blue emission phosphor [4]. In order to fulfill the corresponding requirements, many research papers on developing new phosphor systems used as light-conversion phosphors for white LEDs have been reported [5–10]. As hosts to accommodate RE ions, halo-containing haloborates have some excellent properties such as low synthesis temperature, high physical and chemical stability, and excellent thermal and hydrolytic stabilities [11,12]. The haloborate phosphor is widely studied for its tunable optical properties and potential application in lighting, display, scintillation, thermal neutron image plates, etc. [13,14]. For example, Ba2Ln(BO3)2Cl (Ln = Y, Gd and Lu):Eu2+ [15], Sr5(BO3)3Cl:Ce3+ [16], Ba2B5O9Cl:Bi [17], Ca2BO3Cl:Sm3+,Eu3+ [18], ⇑ Corresponding author. E-mail address: [email protected] (R. Yu). http://dx.doi.org/10.1016/j.optlastec.2017.07.055 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
Ca2BO3Cl:Ce3+,Eu2+ [19], M2B5O9X:Eu2+ (M = Ca, Sr, Ba; X = Cl, Br) [20–22], etc. Ba2GaB4O9Cl is the representative of a new noncentrosymmetric tetragonal structure type related to hilgardite and M2B5O9X. The compound is compositional via substitution of one framework of B atom by Ga in the Ba2B5O9Cl pentaborate halides. The efficiency of second harmonic generation for Ba2GaB4O9Cl was evaluated and found to be similar to that of KH2PO4 (KDP) [23]. In 2010, luminescence properties of Ba2AlB4O9Cl:Eu2+ were firstly reported and considered to be the promising violet-blueemitting phosphor excited by a UV InGaN chip for white LEDs [24]. Ba2TB4O9Cl (T = Al, Ga) are isomorphous with only minor differences in bond distance [23]. However, to the best of our knowledge, few research on the luminescence and thermal properties of Eu2+-doped Ba2GaB4O9Cl for optical materials has been reported thus far. In the present work, a novel blue-emitting phosphor of Eu2+doped borate halide Ba2GaB4O9Cl was synthesized, and the luminescence and thermal quenching properties of Eu2+ in Ba2GaB4O9Cl were investigated to develop new materials with potential application for white LEDs.
2. Experiments Polycrystalline powders with the general formula Ba2(1x)Eu2xGaB4O9Cl (x = 0.01, 0.02, 0.05, 0.08, 0.10, 0.15, and 0.20) were prepared by conventional solid-state reaction technique. The starting
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materials were BaCO3 (A.R.), BaCl2 (A.R.), Ga2O3 (A.R.), H3BO3 (A.R.), and Eu2O3 (99.99%). With BaCO3, BaCl2, Ga2O3, and H3BO3 mole ratio of 1:1:0.5:4, the raw materials were thoroughly mixed by grinding in an agate mortar, and reduced for 3 h under a 10% H2/90% N2 gas mixture at 800 °C. Some excess NH4Cl (5 wt%) are necessary to compensate for loss of Cl source at high temperature. The phase purity of the as-prepared phosphors was investigated by X-ray powder diffraction (XRD) using Cu Ka with a Rigaku D/Max 2200 VPC. The photoluminescence (PL) and PL excitation (PLE) spectra were recorded using a Photon Technology International (PTI, USA) fluorimeter with a 60-W Xe arc lamp. The temperature-dependent PL spectra of the phosphor Ba2GaB4O9Cl: Eu2+ were recorded using an FLS920-combined fluorescence lifetime and steady state spectrometer (Edinburgh Instruments). 3. Results
Fig. 2. XRD patterns of Ba2GaB4O9Cl:xEu2+ (x = 0.02, x = 0.20) samples.
3.1. Phase composition The structure sketch map of Ba2GaB4O9Cl is shown in Fig. 1, which was modeled using the Diamond Crystal and Molecular Structure Visualization software on the basis of the atomic coordinate [23]. The Ba2GaB4O9Cl structure consists of a zeolitic [GaB4O9]3 framework with Ba2+ and Cl ions accommodated in tunnels parallel to the c axis. The framework is based on a novel GaB4O12 fundamental building block built of a tetraborate cluster of two BO3 triangles and two BO4 tetrahedra sharing a corner with a GaO4 tetrahedron. These fundamental building blocks are linked together via corner sharing to form [0 0 1] chains which condense by sharing corners with adjacent chains to build the threedimensional framework. The chains retain their polar character as shown by the common orientation of the GaO4 and BO4 tetrahedra along the c direction. As a result, the [GaB4O9]3 framework as a whole is non-centrosymmetric and polar. There are two types of barium sites Ba(1) and Ba(2) shown in the inset of Fig. 1. Ba(1) atom has a ten-coordinated environment of [7O + 3Cl], and Ba(2) atom has an eleven-coordinated environment of [9O + 2Cl] [23]. The XRD patterns of the representative Ba2(1x)Eu2xGaB4O9Cl samples (x = 0.02, and 0.20) are shown in Fig. 2. The simulated XRD pattern in Fig. 2 was calculated using the atomic coordinates of Ba2GaB4O9Cl crystal reported by Barbier [23]. The host lattice Ba2GaB4O9Cl has a monoclinic structure with space group P42nm, and its lattice parameters are a = b = 12.1508 Å, c = 6.8618 Å, and V = 1013.09 Å3. All peaks agree well with the simulated pattern, indicating that a single-phase phosphor is obtained, and the doped Eu2+ ions do not change the host crystal lattice. Because of the sim-
Fig. 1. (a) Schematic views of the structure of Ba2GaB4O9Cl along the c-direction; (b) Coordination environment around Ba(1) and Ba(2), respectively. Purple ball: Ba; green ball: Cl; cyan ball: Ga; blue ball: B; and red ball: O. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ilarities of ionic radii of Eu2+ and Ba2+ ions, the Eu2+ ions are expected to occupy Ba2+ ion sites in the Ba2GaB4O9Cl:Eu2+ crystal structure. To confirm the crystalline morphology of Eu2+-doped polycrystals, the SEM images were investigated. Fig. 3 shows the representative SEM images of the Ba2GaB4O9Cl:xEu2+ (a, x = 0.02; b, x = 0.20) samples at 5000 magnification. The sample crystallized with the irregular shapes and aggregated for the high-temperature solidstate reaction. The morphologies of powders did not vary much for the doping concentration from 0.02 to 0.20 mol. To break up the agglomerations and improve the quality of the powders, a long ball-milling step is necessary to make them suitable for use in typical screening processes used in the construction of WLEDs.
3.2. Optical properties of Ba2GaB4O9Cl:Eu2+ Fig. 4 shows the typical photoluminescence excitation (kem = 445 nm) and emission spectra (kex = 340 nm) in 10 mol% Eu2+-doped Ba2GaB4O9Cl phosphor. All of the phosphors present bright blue color under a UV lamp. The PLE spectra have a broad band between 250 and 430 nm, and there are four excitation peaks around 293, 343, 370, and 397 nm, which is due to the 4f7(8S7/2) ? 4f6(7F)5d1 transitions of the Eu2+ ion. Because the broad excitation matches well with the near UV chips (350–400 nm), Ba2GaB4O9Cl: Eu2+ phosphors are suitable for near-UV excited solid-state lighting. As can be seen in Fig. 4(b), the Ba2GaB4O9Cl:Eu2+ phosphor shows an asymmetric blue band peaking at 445 nm with the fullwidth half-maximum (FWHM) of 48 nm under 340 nm excitations. The emission of Ba2GaB4O9Cl:Eu2+ is ascribed to the dipole-allowed transition from the lower 4f6(7F)5d1state to the 4f7(8S7/2) ground state of the Eu2+ ions. No other characteristic emission peaks from Eu3+ are observed in the PL spectra, indicating that Eu3+ ions have been reduced to Eu2+ completely in our experiments. It is worth noting that the luminescence wavelength position is similar to the other Eu2+-doped alkaline-earth haloborates. These emission peaks of the haloborates usually locate in the blue wavelength region, for example, Ba2AlB4O9Cl:Eu2+ (420 nm), Ca2B5O9Cl:Eu2+ (452 nm), Sr2B5O9Cl:Eu2+ (425 nm), and Ba2B5O9Cl:Eu2+ (401 nm) [11,24,25]. Compared with the emission peak of Ba2AlB4O9Cl:Eu2+ (420 nm) [24], the Ba2GaB4O9Cl:Eu2+ exhibits an obvious red shift. It is mainly due to the centroid shift of the 5d excited level of Eu2+ ion. Al3+ and Ga3+ ions are the so-called counter-cations that polarize or even bind the anion ligands. A strong polarization or bonding goes at the expense of the interaction between the anion and the 5d electron of Eu2+ [26]. Replacing Al3+ by the same valency and bigger Ga3+ in the Ba2MB4O9Cl:Eu2+ (M = Al,Ga) crystal leads to
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Fig. 3. Typical SEM morphology of Ba2GaB4O9Cl:xEu2+ (a, x = 0.02; b, x = 0.20) samples.
The energy of the f–d absorption and the d–f emission can be written according to the formalism of Dorenbos [26] as
Eabs ¼ Efree D and Eemi ¼ Efree D Ds
Fig. 4. The excitation (kem = 445 nm) and emission spectra (kex = 340 nm) ofBa2GaB4O9Cl:0.10Eu2+ phosphor. [(e), the fitted curve of curve (b); (c) and (d) the deconvolution curves of curve (a)].
the decrease in effective electronegativity of the counter-cations and the binding of anion ligands [27]. Therefore, both increase of anion polarizability and covalency between anion and Eu2+ will lead to bigger centroid shift. A bigger centroid shift tends to result in the red-shift behavior of the emission peaks. The asymmetric emission band can be deconvoluted into two Gaussian components peaked at 443 nm [referred to as Eu(II)] and 466 nm [referred to as Eu(I)], respectively, as shown in Fig. 4 (c) and (d). This indicates that there are two Eu2+ luminescence centers in Ba2GaB4O9Cl lattice. The coordination polyhedra for the barium ions in the Ba2GaB4O9Cl crystal have been taken to be [BaCl3O7] [Ba(1)] and [BaCl2O9] [Ba(2)], as indicated in the inset of Fig. 1. Considering the Eu2+ environment in the doped host, the energy position of the lowest Eu2+ 4f65d1 level can be modified by the covalency and polarizability of Eu2+ ligand bonds, incorporating ligands Cl[v(Cl) 3.16] with a lower electronegativity compared to O2 [v(O) 3.44] would lower the energy of the 4f65d1 levels and would lead to distinct Eu2+ sites with redder emission [19,28]. In terms of the different polyhedra in the [Ba (1)] and [Ba(2)] sites, in case Eu(I) occupied the site of Ba(1), where more Cl anions coordinated, its emission should show a redshift in comparison with that of Eu(II)。Therefore, we can propose that the emission peak centered at 466 nm is attributed to Eu2+ ions which occupy the Ba(1) site with [BaCl3O7] polyhedra. The other peak centered at 443 nm is ascribed to Eu2+ ions occupying the Ba(2) site with [BaCl2O9] polyhedra.
ð1Þ
where Efree is the energy difference between the lowest 4f7 level and the 4f6(7F0)5d level for free or gaseous ions. D is the energy lowering, also called the red shift, and Ds is the Stokes shift. Because the excitation spectrum is not well resolved, the position of the lowest 5d excited level of Eu2+ (Eabs) is generally estimated by using the mirror-image relationship between the emission and the excitation spectra [29]. The Stokes shift (Ds) can be estimated as twice the energy difference between the zero-phonon line and the energy of the emission maximum [30]. The position of the zero-phonon line is considered to be the intersection point of the excitation and emission spectra. In the present case, the lowest absorption energy, Eabs, is about 3.12 eV (397 nm) and the Stokes shift (Ds) is calculated to be 0.32 eV (2616 cm1). Knowing the value of Eabs allows us to determine the red shift D of the f–d transition with respect to the free ion (Efree = 4.19 eV for Eu2+) [31]. We found D = Efree Eabs = 4.19 3.12 = 1.07 eV (8618 cm1). As demonstrated by Dorenbos [26] in an extensive review on the position of 5d transitions of lanthanides, the influence of the crystal field and covalency of the host lattice on the red shift (D) of 5d levels are approximately equal for all lanthanides in the same compound. It is possible to use the position of the 5d levels of Eu2+ to predict that of all other lanthanides. As the first report of Eu2+-activated Ba2GaB4O9Cl phosphor, we calculated the red shift (D) value of Eu2+ in this Ba2GaB4O9Cl host. Fig. 5 presents the emission spectra of the Ba2(1x)Eu2xGaB4O9Cl phosphors with different Eu2+ concentrations (x = 0.01, 0.02, 0.05, 0.08, 0.10, 0.15, and 0.20) upon 340 nm near UV excitation. As shown in the inset, the emission intensity of Eu2+ firstly increased with the increase in its concentration, and reached the maximum when the concentration of Eu2+ is 0.10, and then the emission intensity decreased with further increasing concentration, which is caused by the concentration quenching effect. It can be concluded that the optimum doping concentration of Eu2+ is 0.10. Furthermore, with increasing Eu2+ concentration, the PL spectra have shifted to long wavelength region (peak positions: 439.0 nm for x = 0.01 and 446.3 nm for x = 0.20). According to the previous reports [32,33], the red-shift of the emission peak is frequently observed in rare-earth-doped phosphors with the doping concentration increasing due to the variations of crystal field strength surrounding the activators. Due to the increase in doping concentration of Eu2+, it is accepted that the inter-atomic distance between the two activators become shorter, and the interaction is enhanced [22]. Thus, the crystal field strength surrounding Eu2+ is increased, and it results in the red-shift of the emission peak.
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Fig. 5. Emission spectra of Ba2(1x)Eu2xGaB4O9Cl (x = 0.01, 0.02, 0.05, 0.08, 0.10, 0.15, and 0.20) phosphors with different Eu2+ concentrations. The inset shows the dependence of intensity on the Eu2+-doping concentration.
nearest neighbor ions is the main mechanism of concentration quenching of Ba2GaB4O9Cl:Eu2+. The similar phenomenon has been reported in the Ca5(PO4)2(SiO4):Eu2+, Ca4(PO4)2O:Eu2+, and Ca4P2O9:Ce3+ phosphor [36–38]. The thermal stability of Eu2+-doped Ba2GaB4O9Cl phosphor was evaluated by the temperature-dependent PL spectra. Fig. 7 shows the dependence of the PL spectra of Ba2GaB4O9Cl:0.10Eu2+ on temperature upon excitation under 340 nm light. As temperature increases from 300 K to 500 K, the PL intensity obviously decreases. As shown in the inset of Fig. 7, the quenching temperature (T1/2) is 432 K. After heating the sample up to 420 K, at which the LEDs usually work, the emission intensity decreases by approximately 54% of that measured at room temperature. The value is found to be 65% for Ca2BO3Cl:Eu2+, and 90% for Ba2Gd(BO3)2Cl:Eu2+ [15,39]. The phosphor does not have very good enough thermalquenching property compared with them. Furthermore, the emission band is a little blue-shifted with raising the temperature. It was ascribed to the thermally active phonon-assisted tunneling from the excited states of the low-energy emission band to the excited states of the high-energy emission band in the configuration coordinate diagram [40]. As discussed before, there are two
Van Uitert [34] reported that the intensity of multipolar interaction can be determined based on the change in the emission intensity from the emitting level that has multipolar interaction. The emission intensity (I) per activator ion follows the equation:
h i1 I ¼ K 1 þ bðxÞQ=3 x
ð2Þ
where v is the activator concentration; Q is a constant of multipolar interaction and equals 3, 6, 8, or 10 for the nearest-neighbor ions, dipole–dipole, dipole–quadrupole or quadrupole–quadrupole interaction, respectively; and K and b are constants under the same excitation condition for the given host crystal [34,35]. Eq. (2) can be approximately reduced to Eq. (3) for bvQ/3 1 as follows:
I Q ¼ A lg x lg x 3
ðA ¼ lg k lg bÞ
ð3Þ
The relationship of lg(I/x) versus lg(x) in Ba2GaB4O9Cl:xEu2+, shown in Fig. 6(b), is linear and the slope is 0.93. The value of h is found to be 2.79 using the above Eq. (3), which is approximately equal to 3. This result indicates that the energy transfer among the
Fig. 7. Dependence of the PL emission of Ba2GaB4O9Cl:0.10Eu2+ (kex = 340 nm) on temperature. Inset shows the relation between the integral intensity of the emission band (380–600 nm) and temperature.
Fig. 6. Fitting curve of lg(I/x) versus lg(x) in Ba2GaB4O9Cl:xEu2+ (x = 0.01–0.20) phosphors.
Fig. 8. The Arrhenius plot of the temperature dependence of the PL emission intensity of Ba2GaB4O9Cl:Eu2+.
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Fig. 9. (a) Comparison of PL (kex = 340 nm) spectra of Ba2GaB4O9Cl:0.10Eu2+ with that of commercial blue BAM:Eu2+ phosphor; (b) CIE coordinates of the Ba2GaB4O9Cl:0.10Eu2+ phosphor. The inset shows the image of the Ba2GaB4O9Cl:Eu2+ phosphor excited at 365 nm in a UV box.
emission centers for the Ba2GaB4O9Cl:Eu2+ phosphor. At higher temperature, the thermal back-transfer from lower energy centers to higher energy centers is more possible; consequently, the higher energy emission is strengthened. Thus, the blue-shift phenomenon is observed with the increase of temperature. And similar blueshifts were also observed elsewhere, such as CaLaGa3S7:Eu2+, Ca2B5O9Br:Eu2+ and K2Al2B2O7:Eu2+ [22,41,42]. The temperature dependence of luminescence intensity is described by the following Arrhenius equation (4) [43]:
IðTÞ ¼
I0 I0 Ea 1 þ ln c or ln 1 ¼ 1 þ c expðEa =kTÞ IðTÞ k T
ð4Þ
where I0 is the initial intensity; I(T) is the intensity at a given temperature T; c is a constant, k is Boltzmann’s constant (8.62 105 eV/K); and Ea is the activation energy, which represents the energy difference between the lowest excited state and the bottom of the host lattice conduction band. Fig. 8 shows the plot of ln[(I0/I) 1] versus 1/T for Ba2GaB4O9Cl:Eu2+. Linear regression shows that the thermal activation energy Ea for quenching is 0.23 eV. Compared with the reported activation energy of thermal quenching in haloborate phosphors such as Ca2BO3Cl:Eu2+ (0.43 eV) [44] and Mg3B7O13Cl:Eu2+ (0.32 eV) [12], Eu2+-doped Ba2GaB4O9Cl has lower activation energy when compared with the references. Furthermore, the comparison of PL spectra between the Ba2GaB4O9Cl:0.10Eu2+ phosphor and the commercial one BAM:Eu2+ blue-emitting phosphor is shown in Fig. 9(a). Upon excitation at a wavelength of 340 nm, the sample presented a very similar curve to that of the BAM:Eu2+ phosphor, and the integrated area of the PL spectra was measured to be 52% of that of the commercial BAM: Eu2+ phosphor, although Ba2GaB4O9Cl:Eu2+ is yet to be further optimized. As key parameters, the CIE coordinates for the Ba2GaB4O9Cl:0.10Eu2+ sample were calculated to be (x = 0.151, y = 0.044), very close to those of the BAM:Eu2+, indicating that this phosphor has a high color purity as shown in Fig. 9(b), which origins from the similar peak profiles and FWHM values with BAM. A digital photograph of the sample under 365 nm UV lamps is shown in the inset, revealing an intense blue emission. The initial results suggest that this novel phosphor may serve as a promising blue emitting component for w-LED application.
4. Conclusions Eu2+-doped Ba2GaB4O9Cl polycrystalline were prepared by conventional high-temperature solid-state reaction. The excitation of Ba2GaB4O9Cl:Eu2+ with a broad band from 340 to 430 nm could match the near UV chips well. It shows a narrow (FWHM = 48 nm) blue luminescence band centered at 445 nm. The luminescence spectra show that Eu2+ ions have two emission centers in the Ba2GaB4O9Cl lattice. The critical quenching concentration of Eu2+ in Ba2GaB4O9Cl phosphor is about 10 mol%. This study demonstrates that energy transfer among the nearest neighbor ions is the major mechanism for concentration quenching of Eu2+ in Ba2xGaB4O9Cl: xEu2+ phosphors. The luminescence quenching temperature is 432 K. The CIE color coordinates are (x = 0.151, y = 0.044). The above obtained results indicated that this novel phosphor was promising as the blue component of light-conversion phosphors for white LEDs. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21201141), the Chinese Universities Scientific Fund (Grant No. QN2011119 and QN2452015424), and the Young Faculty Study Abroad Program of Northwest A&F University. This work was also supported by Research Grant of Pukyong National University (2016). References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer-Verlag, Berlin, 1997. [2] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Mater. Sci. Eng. B 71 (2010) 1–34. [3] R. Yu, S. Zhong, N. Xue, H. Li, H. Ma, Dalton Trans. 43 (2014) 10969–10976. [4] E. Radkov, R. Bompiedi, A.M. Srivastava, A.A. Setlur, C.A. Becker, Proc. SPIE 5187 (2004) 171–177. [5] R. Yu, C. Guo, T. Li, Y. Xu, Curr. Appl. Phys. 13 (2013) 880–884. [6] J. Yan, L. Ning, Y. Huang, C. Liu, D. Hou, B. Zhang, Y. Huang, Y. Tao, H. Liang, J. Mater. Chem. C 2 (2014) 8328–8332. [7] N. Zhang, C. Guo, L. Yin, J. Zhang, M. Wu, J. Alloys Compd. 635 (2015) 66–72. [8] H.K. Yang, H.M. Noh, B.K. Moon, J.H. Jeong, S.S. Yi, Ceram. Int. 40 (2014) 12503– 12508. [9] W. Zhou, J. Han, X. Zhang, Z. Qiu, Q. Xie, H. Liang, S. Lian, J. Wang, Opt. Mater. 39 (2015) 173–177. [10] M. Peng, X. Yin, P.A. Tanner, M.G. Brik, P. Li, Chem. Mater. 27 (2015) 2938– 2945. [11] A. Meijerink, G. Blasse, J. Lumin. 43 (1989) 283–289. [12] X. Qiao, H.J. Seo, J. Am. Ceram. Soc. 98 (2015) 594–600.
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