Materials Research Bulletin 49 (2014) 469–474

Eu2+, Mn2+ co-doped Ba9Y2Si6O24 phosphors based on near-UV-excitable LED lights

Yoejin Kim, Sangmoon Park *

Center for Green Fusion Technology and Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea

A R T I C L E I N F O

Article history:

Received 25 June 2013

Accepted 22 September 2013

Available online 29 September 2013

Keywords:

A. Optical materials

A. Orthosilicate

C. X-ray diffraction

D. Luminescence

D. Energy transfer

A B S T R A C T

New single-phase and near-ultraviolet (NUV)-excitable materials composed of Ba9EumMnnY2Si6O24

(m = 0.01–0.5, n = 0–0.7) were prepared via a solid-state reaction in reducing atmosphere. X-ray

diffraction patterns of the obtained phosphors were examined to index the peak positions. After doping

the host structure with Eu2+ and Mn2+ emitters, the intense green, white, and orange emission lights that

were observed in the photoluminescence spectra under NUV excitation were monitored. The

dependence of the luminescent intensity of the Mn2+ co-doped (n = 0.1–0.7) host lattices on the fixed

Eu2+ content (m = 0.1, 0.3, 0.5) is also investigated. Co-doping Mn2+ into the Eu2+-doped host structure

enabled a high energy-transfer from Eu2+ to Mn2+ and their energy-transfer mechanism were discussed.

Using these phosphors, the desired CIE values including emissions throughout the green to orange

regions of the spectra were achieved. Efficient white-light light-emitting diodes (LEDs) were fabricated

using Eu2+ and Mn2+ co-doped phosphors based on NUV-excitable LED lights.

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jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Light-emitting diodes (LEDs) are extensively used as ‘‘smart’’light sources in applications in lighting, automobiles, transporta-tion, communication, imaging, agriculture, and medicine [1].Phosphor-conversion LEDs dominate the market in white-lightLED applications, rather than multi-chip LEDs, because of their lowcost and ease of use [2]. White-light LEDs are typically made usinga combination of a blue LED chip emitting at around 450 nm, with acrucial component, namely a yellow phosphor. A combination ofnear-ultraviolet (NUV) LED chips emitting at about 400 nm with agreen-red-emitting single-phase phosphor such as BaMg2Si2O7:E-u2+,Mn2+, Ca2MgSi2O7:Eu2+,Mn2+, CaAl2Si2O8:Eu2+,Mn2+, Ca9La(-PO4)7:Eu2+,Mn2+, KCaY(PO4)2:Eu2+,Mn2+ or Ca2P2O7:Eu2+,Mn2+ isalso a specific method for producing white-light LEDs [3–8]. Asmentioned above, white-light LEDs are obtained using an elaboratephosphor that incorporates LED chips. The market demand forsuitable phosphors for these applications has therefore signifi-cantly increased. The growth of the phosphor market for LEDlighting applications is expected to expand from �$525 million in2012 to $730 million by 2015 and $1.6 billion by 2019 [9]. The mostwidely available high-efficiency phosphors used for white-lightLEDs are YAG (Y3Al5O12:Ce) and silicon-based oxides [e.g.,

* Corresponding author. Tel.: +82 51 999 5891; fax: +82 51 999 5335.

E-mail address: spark@silla.ac.kr (S. Park).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.09.035

(Y1�p�q�rGdpCeqSmr)3(Al1�xGax)5O12, (Sr0.7Ba0.3 Eu0.02)2Si1.02

O4.08�x, and Sr2�x�y(Ba,Ca)xEuySi1�a�b�c�dPaAlbBcGddO4] [10]. Asis known, YAG and silicon-based phosphors belong to the garnet(cubic phase) and olivine (orthorhombic phase) groups, respec-tively. Furthermore, these mineral groups belong to the orthosi-licate (or nesosilicate) subclass. Orthosilicates could thereforeclearly be the predominant source of white-light LED phosphors.The unique layered orthosilicate Ba9Sc2(SiO4)6, which containslarge Ba2+ as well as octahedral Sc3+ and tetrahedral Si4+, species,has been prepared by Wang et al. as shown in Fig. 1 [11,12].Ba9Sc2Si6O24 orthosilicate host lattice comprises tetrahedral SiO4

and octahedral ScO6 moieties arranged along the c-axis, withdodeca-coordinated Ba(1), nona-coordinated Ba(2), and deca-coordinated Ba(3) cations; moreover, the isolated silicate tetrahe-drons do not share oxygen atoms, which is an importantdistinction of orthosilicate compounds [12]. The unique Ba9Sc2-

Si6O24 structure, which features a trigonal phase with an R-3Hspace group, has already been elucidated; this compound acts as anefficient luminescent host material that is activated in the NUVregion [12–14]. In this work, near-ultraviolet (NUV)-excitable andnew barium–yttrium–silicate based optical materials composed ofBa9EumMnnY2Si6O24 (m = 0.01–0.5, n = 0–0.7) were prepared andtheir X-ray diffraction patterns were characterized as well. Thephotoluminescence (PL) spectra, which exhibit an efficient greenemission assigned to the f–d transitions of a Eu2+ emitter to theorthosilicates, were analyzed. Moreover, PL spectra of Eu2+ andMn2+ co-doped orthosilicate phosphors under NUV excitation were

Fig. 1. The structure of Ba9Sc2Si6O24 [11].

Fig. 2. XRD patterns of (a) calculated Ba9Sc2Si6O24, (b)–(h) observed

Ba9�m�nEumMnnY2Si6O24 (m = 0, 0.1, 0.3, 0.5; n = 0, 0.7) orthosilicate structure.

Fig. 3. (a) The excitation and emission spectra of Ba9�mEumY2Si6O24 (m = 0.01–0.5)

phosphors and (b) the changes in the relative intensity of green PL emission in the

orthosilicate phosphors.

Y. Kim, S. Park / Materials Research Bulletin 49 (2014) 469–474470

monitored. The dependence of the luminescent intensity andenergy-transfer mechanism of the Mn2+ co-doped (n = 0.1–0.7)host lattices on each Eu2+ content (m = 0.1, 0.3, 0.5) are alsodiscussed. Using these phosphors, the desired CIE values includingemission lights throughout the green to orange regions of thespectra were achieved. Ba8.8Eu0.1Mn0.1Y2Si6O24 orthosilicate phos-phor was combined with a 365 nm LED in order to monitor NUVLEDs.

2. Experimental

Optical materials of Ba9�m�nEumMnnY2Si6O24 (m = 0.01–0.5;n = 0–0.7) were prepared by heating the appropriate stoichiomet-ric amounts of BaCO3 (Alfa 99.8%), Y2O3 (Alfa 99.9%), SiO2 (Alfa99.5%), Eu2O3 (Alfa 99.9%), and MnO (Aldrich 99%) at 950 and1100 8C for 3 h under the reducing atmosphere using 4%H2/96%Ar.In each sample, 2.5 wt% Li2CO3 (Alfa 99%) was added as a flux [11].Phase identification was established using a Shimadzu XRD-6000powder diffractometer (Cu Ka radiation). UV spectroscopy tomeasure the excitation and emission spectra of the opticalmaterials was done using spectrofluorometers (Sinco FluoromateFS-2). For the characterization of NUV-excitable orthosilicate LEDlight, Ocean Optics USB4000 spectrometer was used.

3. Results and discussion

Fig. 2(a) depicts the X-ray diffraction (XRD) patterns of theBa9Sc2Si6O24 (ICSD 50736) structure. In the previous study, anexcess 2.5 wt% Li2CO3 flux was experimentally optimized and usedat 1100 8C to generate new orthosilicate Ba9Y2Si6O24 compounds,as shown in Fig. 2(b) [11]. Fig. 2(c)–(h) shows the XRD patterns ofBa9�m�nEumMnnY2Si6O24 (m = 0.1, 0.3, 0.5; n = 0.7) orthosilicatestructure. Phase identification was performed by studying thepowder X-ray diffraction, performed after the Eu2+ and Mn2+ ionswere replaced with Ba2+ ions in the Ba9Y2Si6O24 host lattice. It isobserved that single-phase orthosilicate Ba9�m�nEumMnnY2Si6O24

phosphors were formed when the Eu2+ and Mn2+ substitution inthe Ba sites reached m = 0.5, n = 0.7. Silicon powder was added tothe phosphors to monitor clear peak shifts. When the Ba2+ ions

were substituted with smaller Eu2+ ions, the obtained XRD patternsof Ba9�mEumY2Si6O24 (m = 0.1, 0.3, 0.5) showed clear shifts in thepositions of the various Bragg reflections at higher angles, as isshown in Fig. 2(c), (e) and (g). The XRD patterns of Eu2+ and Mn2+

co-doped Ba9�m�nEumMnnY2Si6O24 (m = 0.1, n = 0.7; m = 0.3,n = 0.7; m = 0.5, n = 0.7), shown in Fig. 2(d), (f), and (h), showclear deviations from the patterns of Eu2+ doped host lattices inpeak positions of the reflections at higher angles by adding Mn2+

ions, respectively.Fig. 3(a) shows photoluminescence (PL) spectra of

Ba9�mEumY2Si6O24 (m = 0.01–0.5) phosphors that exhibit intensegreen emission. A small band around 290 nm in the excitation

Fig. 4. (a) The PL spectra of Ba8.9Eu0.1Y2Si6O24 and Ba8.5Mn0.5Y2Si6O24 phosphors, (b) the PL spectra of the Eu2+, Mn2+ co-doped (m = 0.1, 0.3, 0.5, n = 0.1–0.7) Ba9Y2Si6O24

phosphors, (c) the changes in the relative intensity of the Eu2+, Mn2+ co-doped orthosilicate phosphors.

Y. Kim, S. Park / Materials Research Bulletin 49 (2014) 469–474 471

Fig. 5. (a) The energy transfer efficiency (hT) from Eu2+ to Mn2+ in

Ba9�m�nEumMnnY2Si6O24 phosphors and (b) and (c) the plot of IS/ISO versus

CMna/3 (a = 6, 8).

Y. Kim, S. Park / Materials Research Bulletin 49 (2014) 469–474472

spectrum of Eu2+-doped Ba9Y2Si6O24 represents the host absorp-tion; moreover, two main spectral bands, centered at around 375and 420 nm, characterize the Eu2+ transition, which represents thetransition from the ground state 4f75d0 to the excited state 4f65d1.For higher concentration of Eu2+, the absorption peak that wascentered near 420 nm became stronger as compared with the otherpeak. The main green emission peak, assigned to the4f65d1! 4f75d0 transition of the Eu2+ ions in the Eu2+-substitutedBa9Y2Si6O24 phosphor, is in the wavelength range of 450–600 nmand is centered at 503 nm. Doping the Ba9Y2Si6O24 host lattice withEu2+ ions produced a clear NUV excitation and green emissionbands. These emission bands are attributed to the f–d electricdipole-allowed transitions of the Eu2+ ions [15]. In general, the d–ftransition of Eu2+ ions results in the emission of light withwavelength ranging from the UV to the visible range, and isaccompanied by a crystal field effect of the activator in the hoststructure [16,17]. The Ba9Y2Si6O24 orthosilicate host structure isknown to consist of tetrahedral silicon-oxide (SiO4) and octahedralyttrium-oxide (YO6) moieties that are arranged along the c-axis,with 12-, 9-, and 10-coordinated Ba(1), Ba(2), and Ba(3) cations[11]. After substituting the Ba2+ by Eu2+ ions in the Ba9Y2Si6O24

host lattice, the reduced Eu2+ ions (r = 1.3 A (9 CN) or r = 1.35 A (10CN)) may properly fit a single Eu2+ center at the 9 or 10-coordinated Ba2+ sites. The Eu2+ substituted silicate phosphorssuch as the Ca2Y2Si2O9:Eu and the Sr2SiO4:Eu show two wellseparated Gaussian components due to their two different Eu2+ ionsites, resulting in two Eu2+ centers located in the emission bandsthat are centered around 480 and 540 nm [18,19]. However, theemission band caused by a single Eu2+ center located around500 nm was mostly observed in the Eu2+-doped Ba9Y2Si6O24

phosphors. It appears that, in contrast to other silicate phosphors, aweak crystal-field effect of the activator occurs in the host lattice ofsuch phosphors. Fig. 3(b) shows the changes in the relativeintensity of green PL emission caused by the d–f transition of theEu2+ ions in the Ba9�mEumY2Si6O24 phosphors, achieved byincreasing the concentration of Eu2+ ions (m = 0.01–0.5). In thecase of Ba9�mEumY2Si6O24 phosphors, the green emission clearlyincreases when the Eu2+ content increases, from m = 0.01 to 0.1.Once the maximum luminescent intensity of a Ba9�mEumY2Si6O24

phosphor is reached for the Eu2+ content corresponding to m = 0.1,any further increase in the Eu2+ content in the orthosilicatephosphors leads to a slight quenching of the relative intensity ofthe green emission. As the concentration of Eu2+ ions increases, thedistance between the activators becomes smaller with increasingenergy transfer. Contrastively, a decrease in the intensity ofemission results in the nonradiative energy transfer between theactivators resulting from the electric dipole–dipole interaction[20].

Fig. 4(a) shows the PL spectra of Ba8.9Eu0.1Y2Si6O24 andBa8.5Mn0.5Y2Si6O24 phosphors. The intensity of the Mn2+-substi-tuted phosphor, which was multiplied by a factor of 50 as shown,was extremely low as compared with the intensity of the greenBa8.9Eu0.1Y2Si6O24 phosphor. The luminescent excitation centers ofMn2+ ions were assigned as 4E(D), 4T2(D), 4A1(4G)/4E(4G), and4T1(4G) between 300 and 500 nm, and their emissions were causedby d–d transition [4T1(6G) ! 6A1(6S)] at around 630 nm, consistentwith previous reports [6,7]. The Mn2+ d–d transition is difficult tomonitor because the corresponding electric dipole is forbidden.However, the red emission of Mn2+ transitions in host lattices canbe elevated by co-doping a Eu2+ or Ce3+ sensitizer through energytransfer from Eu2+ or Ce3+ to Mn2+ ions [6,7,21]. Fig. 4(b) shows thePL emission spectra of the Eu2+, Mn2+ co-doped (m = 0.1, 0.3, 0.5,n = 0.1–0.7) Ba9Y2Si6O24 phosphors. Co-doping the Mn2+ into theEu2+-doped host structure enabled a high energy transfer fromEu2+ to Mn2+. The Eu2+ green emission of Ba8.9�nEu0.1MnnY2Si6O24

phosphor centered around 500 nm abruptly decreased when the

Mn2+ content corresponded to n = 0.3. A steady increase in theMn2+ red emission of Ba8.9�nEu0.1MnnY2Si6O24 phosphors wasobserved up to the Mn2+ content corresponding to n = 0.7.In Fig. 4(c), similar to what was observed for theBa8.9�nEu0.1MnnY2Si6O24 phosphor, the Eu2+ luminescent intensi-ties of Ba9�m�nEumMnnY2Si6O24 phosphors for Eu2+ contentcorresponding to m = 0.3 and 0.5 decreased as the Mn2+ contentincreased up to the level corresponding to n = 0.3. However, themaximum Mn2+ red emission of Ba8.7�nEu0.3MnnY2Si6O24 and

Fig. 6. The chromaticity coordinates with the CIE values of Ba9�m�nEumMnnY2Si6O24 (m = 0.1, 0.3, and 0.5, n = 0.1–0.7) phosphors and photographs of the PL light emission

from green to orange in Ba8.9�nEu0.1MnnY2Si6O24 (n = 0.1–0.7) phosphors under 365 nm handheld lamps (inset). (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of the article.)

Fig. 7. The PL emission spectrum of Ba8.8Eu0.1 Mn0.1Y2Si6O24 phosphor combined

with a 365 nm LED light and their photograph of cool white light (inset).

Y. Kim, S. Park / Materials Research Bulletin 49 (2014) 469–474 473

Ba8.5�nEu0.5MnnY2Si6O24 phosphors was reached when the Mn2+

content corresponding to n = 0.5 and 0.3, respectively. Furtherincrease in the Mn2+ content in the phosphors led to the apparentquenching of the relative intensity of the red emission. In thisBa9�m�nEumMnnY2Si6O24 host structure, a transfer of energy fromEu2+ to Mn2+ occurs by the absorption of Eu2+; moreover, the Eu2+

and Mn2+ ions play a role of a sensitizer and an activator,respectively. The energy transfer efficiency (hT) between the Eu2+

and Mn2+ ions was calculated as.

hT ¼ 1 � IS

ISO;

where IS and ISO are the luminescence intensities of a sensitizer inthe absence and in the presence of an activator, respectively [5–7].Using this expression, the energy transfer efficiency from Eu2+ toMn2+ was calculated as shown in Fig. 5(a). As the Mn2+ content inBa9�m�nEumMnnY2Si6O24 phosphors (m = 0.1, 0.3, 0.5) increasedfrom the level of from n = 0.1 to 0.3, the efficiency at each level wasextensively enhanced by up to 80%. When the Mn2+ content in theorthosilicate phosphors was increased to the level of n = 0.5, theefficiency of the energy transfer was further increased to 90%.When the Eu2+ concentration corresponding to the level of m = 1in Ba9�m�nEumMnnY2Si6O24 phosphors was arrange to vary fromthe level of n = 0.3 to 0.5, the efficiency of transfer of energy toMn2+ ions became superior to those of other energy transfers.Furthermore, according to Dexter theory, the energy transfermechanism can be expressed by the linear plots of IS/ISO versusCMn

a/3, where CMn is the concentration of Mn2+ ions, with a = 6, 8,or 10, which corresponds to dipole–dipole, dipole–quadrupole,and quadrupole–quadrupole interactions, respectively. InFig. 5(b) and (c), the linear plots show that energy transfer fromthe Eu2+ to Mn2+ ions in the case of Ba9�m�nEumMnnY2Si6O24

(m = 0.1) phosphors is caused by a nonradiative dipole–quadru-pole (a = 8) interaction. Moreover, because the values of a that aredetermined by the linear plotting for Ba9�m�nEumMnnY2Si6O24

(m = 0.3, 0.5, n = 0.1 to 0.5) phosphors are close to 6, it appears thatdipole–dipole interaction is the energy transfer mechanism [5–7].

As shown in Fig. 6, the chromaticity coordinates, x and y, are inaccordance with the CIE values of the green, white, and orangeorthosilicate phosphors, corresponding to Ba9�m�nEumMnnY2Si6O24

with m = 0.1, 0.3, and 0.5, n = 0.1–0.7, respectively. Photographsof the PL light emission from green to orange inBa8.9�nEu0.1MnnY2Si6O24 (n = 0.1–0.7) phosphors under 365 nmhandheld lamps are shown as insets in Fig. 6. The CIE values aresummarized in the insets in Fig. 6, along with the values obtained forthe Eu2+- and Eu2+-, Mn2+ co-doped orthosilicate optical materials.The CIE coordinates near the green and white regions of the CIEdiagram were observed to be x = 0.209 and y = 0.510, x = 0.207 andy = 0.517, x = 0.21 and y = 0.517 for m = 0.1, 0.3, 0.5, n = 0 in theBa9�m�nEumMnnY2Si6O24 phosphors; x = 0.296 and y = 0.488,x = 0.325 and y = 0.481, x = 0.324 and y = 0.482 for Ba9Y1.8-

Si6O24:Eu0.23+; and x = 0.212 and y = 0.507 for m = 0.1, 0.3, and

0.5, and n = 0.1 in the orthosilicate phosphors, respectively. Whenthe concentration of Mn2+ ions in the Ba9�m�nEumMnnY2Si6O24

(m = 0.1, 0.3, 0.5) phosphors was further increased to n = 0.3–0.7, the

Y. Kim, S. Park / Materials Research Bulletin 49 (2014) 469–474474

CIE coordinates exhibited a significant shift from the green-white toorange-red regions.

The green Eu3+ (�503 nm) and/or red Mn2+ (� 603 nm)emitting orthosilicate phosphors were combined with a 365 nmLED in order to monitor NUV LEDs under a current of 20 mA, asshown in Fig. 7. The CIE coordinates of the Ba8.8Eu0.1 Mn0.1Y2Si6O24

phosphor under 365 nm LED light were x = x = 0.3592, y = 0.3957;the coordinates indicate that the addition of the Mn2+ activator tothe Eu2+-activated orthosilicate phosphor mixed with 365 nm LEDlight resulted in the emission of white light (inset in Fig. 7). Aphosphor blend of (Ca0.98)La(PO4)7:0.005Eu2+,0.015Mn2+ andBaMgAl10O17:Eu2+ under a 365 nm LED chip shows a CRI of 91.5Ra and a CCT of 4496 K, as reported by Huang et al. [6]. Single-phaseBa8.8Eu0.1Mn0.1Y2Si6O24 orthosilicate phosphor shows a CRI of 92.9Ra and a CCT of 4705 K combined with 365 nm light. According tothis discussion, Eu2+ and Mn2+ co-activators in the Ba9Y2Si6O24

host structure evidently generate cool white light under NUV-executable LED light.

4. Conclusions

Single phase of Eu2+, Mn2+ co-doped Ba9Y2Si6O24 phosphorswas successfully prepared using a Li2CO3 flux below 1100 8C inreducing environment. By doping a Eu2+ emitter in the barium–yttrium–orthosilicate host lattice, efficient green emission wasachieved under NUV excitation. The maximum luminescentintensity of Ba9�mEumY2Si6O24 phosphors was reached when theEu2+ content corresponding to m = 0.1. The emission of Mn2+ d–dtransitions in the orthosilicate host lattices was raised by co-doping a Eu2+ sensitizer by transferring energy from Eu2+ to Mn2+

ions and their PL spectra were monitored as well. As the Eu2+

contents in Ba9�m�nEumMnnY2Si6O24 (m = 0.01–0.5, n = 0–0.7)phosphors increased, the noticeable quenching of the relativeintensity of the Mn2+ red emission occurred. The energy transfermechanism of the energy transfer from the Eu2+ to Mn2+ inBa9Y2Si6O24 phosphors was expressed by the linear plots of IS/ISO

versus CMna/3. The desired CIE values including emissions in

Ba9�m�nEumMnnY2Si6O24 phosphors throughout the green toorange regions of the spectra were achieved. The co-doping of

Eu2+ and Mn2+ in the single-phase orthosilicate structure resultedin the emission of white light under NUV LED light. These neworthosilicate phosphors are effective for applications requiringNUV executable white-light LEDs.

Acknowledgment

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology (2011-0010756).

References

[1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274.[2] M. Zachau, D. Becker, D. Berben, T. Fiedler, F. Jermann, F. Zwaschka, Proc. SPIE

6910 (2008) 691010.[3] G.Q. Yao, J.H. Lin, L. Zhang, G.X. Lu, M.L. Gong, M.Z. Su, J. Mater. Chem. 3 (1998)

585.[4] C.K. Chang, T.M. Chen, Appl. Phys. Lett. 90 (2007) 161901.[5] W.J. Yang, L. Luo, T.M. Chen, N.S. Wang, Chem. Mater. 1 (2005) 3883.[6] C.H. Huang, T.M. Chen, Opt. Express 18 (2010) 5089.[7] W.R. Liu, C.H. Huang, C.W. Yeh, J.C. Tsa, Y.C. Chiu, Y.T. Yeh, R.S. Liu, Inorg. Chem. 51

(2012) 9636.[8] Z. Hao, J. Zhang, X. Zhang, X. Sun, Y. Luo, S. Lu, Appl. Phys. Lett. 90 (2007) 26113.[9] Phosphor market expands as LEDs move into lighting applications, Solid State

Technology, May 30, 2012.[10] Y. Shimizu et al., US Patent 5,998,925.;

K. Ota et al., US Patent 6,943,380.[11] S. Lee, S. Park, J. Lumin. 143 (2013) 215.[12] L.H. Wang, L.F. Schneemeyer, R.J. Cava, T. Siegrist, J. Solid State Chem. 113 (1994)

211.[13] K. Toda, Y. Kawakami, S.I. Kousaka, Y. Ito, A. Komeno, K. Uematsu, M. Sato, IEICS

Trans. Electron. E89 (2006) 1406.[14] T. Nakano, Y. Kawakami, K. Uematsu, T. Ishigaki, K. Toda, M. Sato, J. Lumin. 126

(2009) 1654.[15] J.K. Park, M.A. Lim, C.H. Kim, H.D. Park, J.T. Park, S.Y. Choi, Appl. Phys. Lett. 82

(2003) 683.[16] M. Zachau, D. Becker, T. Fiedler, F. Jermann, F. Zwaschka, Proc. SPIE 6910 (2008)

691010.[17] S.H.M. Poort, J.H. van Krevel, R. Stomphorst, A.P. Vink, G. Blasse, J. Solid State

Chem. 122 (1996) 432.[18] Z.J. Zhang, A.C.A. Delsing, P.H.L. Notten, J.T. Zhao, H.T. Hintzen, Mater. Res. Bull. 47

(2012) 2040.[19] H.D. Nguyen, I.H. Yeo, S.I. Mho, ECS Trans. 28 (2010) 167.[20] D. Wang, Q. Yin, Y. Li, M. Wang, J. Lumin. 97 (2002) 1.[21] Y. Liu, X. Zhang, Z. Hao, X. Wang, J. Zhang, Chem. Commun. 47 (2011) 1067.