- Email: [email protected]
Color-tunable self-activated oxyfluoride phosphors induced by anion-defect with cation-disorder
Journal of Alloys and Compounds 756 (2018) 212e216
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Color-tunable self-activated oxyfluoride phosphors induced by aniondefect with cation-disorder Sungjun Yang, Sung-Hoon Kim, Sangmoon Park* Center for Green Fusion Technology and Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 46958, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2018 Received in revised form 2 May 2018 Accepted 4 May 2018 Available online 6 May 2018
Cation-disordered oxyfluoride materials, Sr3-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7), were prepared via solid-state reactions and subsequently induced with defects under appropriate reduction conditions. The anion-deficient self-activating phosphors can be identified using broadband-shaped emissions ranging from 300 to 700 nm when excited with ultraviolet (UV) light. The luminescence properties of the defect-induced self-activating phosphors were examined using cation-disordered [(Sr,Ba) (Al,Sn)O4]3- and [(Sr,Na)2F]3þ clusters in the oxyfluoride lattice. The luminescent emissions of Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors were significantly enhanced by the combination of the cation disorder with anion defect. Color-tunable emissions in the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphor were obtained. The desired Commission Internationale de I'Eclairage (CIE) values corresponding to the spectral emission from blue to white and yellow regions of the spectra were attained by exciting the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors with UV light. © 2018 Elsevier B.V. All rights reserved.
Keywords: Self-activating phosphors Defects Cation-disorder Oxyfluorides
1. Introduction Rare-earth and transition ions - Ce3þ, Eu2þ, Eu3þ, Tb3þ, or Mn2þ ions - in various host lattices can be generally used as activators for an appropriate luminescence process, which is defined as the excitation and emission progression through the electronic transitions of the activators [1,2]. Line-shaped or broadband-shaped emission could arise from an activator, and is assigned to f-f, f-d, or d-d electronic transitions; especially, the activators showing broadband-shaped transitions can be affected by their coordinated environments in host lattices [1e5]. Notably, unusual emission without using the activators can be observed via an electronic transition between an anion electron and empty cation orbital; for example, an electron transfer from the 2p orbital of O2 to the vacant 3d orbital of V5þ in the (VO4)3- complex results in broadband-shaped emission [1,6]. Such phosphors are called selfactivated phosphors because the activators of rare-earth or transition ions are not involved in the process. Similar luminescence phenomena were widely observed in (WO4)2-, (MoO4)2-, and (MnO4)- complexes during the investigation of self-activated
* Corresponding author. E-mail address: [email protected] (S. Park). https://doi.org/10.1016/j.jallcom.2018.05.041 0925-8388/© 2018 Elsevier B.V. All rights reserved.
CaWO4 phosphors, which were used as a scintillator for the detection of X-rays in 1896 [7e10]. Furthermore, the defectinduced self-activating phosphors obtained owing to cation or anion vacancies in host lattices, which generate self-trapped electron or hole states, emit broadband-shaped luminescence lights without the activators [11e13]. These defects can be produced with cation-disordered substitutions in the host lattice or by altering material preparation conditions using X-ray radiation, Arþ irradiation, or appropriate reduction environments [13e17]. In a previous study, defect-induced self-activating (Sr,Ca,Ba)3AlO4-aF1d phosphors controlled by anion defects were investigated [16]. In this study, cation-disordered Sr2.5-qBa0.5NaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) oxyfluorides are prepared. The luminescence properties of defectinduced self-activating oxyfluoride phosphors are examined by controlling the anion defect structure generated under postsynthesis reduction conditions when excited with ultraviolet (UV) light. A significant emission enhancement is expected to arise from a combination of the cation disorder with anionic defects. Colortunable emissions of defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors are observed. The desired CIE values corresponding to the spectral emission regions from blue to white and yellow are attained using the defect-induced, self-activating oxyfluoride phosphors.
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
2. Experimental details Samples of Sr3-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) were prepared by heating the appropriate stoichiometric amounts of SrCO3 (Alfa 99%), BaCO3 (Alfa 99.8%), SrF2 (Alfa 99%), Na2CO3 (Alfa 99.5%), Al2O3 (Alfa 99.95%), and SnO2 (Alfa 99.9%) at temperature up to 1200 C for 3 h in air. The as-made samples were subsequently annealed at 800 C in reducing atmosphere (8% H2/92% Ar) for 1 h to compose self-activating luminescent materials [15e18]. Phase identification of the phosphors was done using a Shimadzu XRD-6000 powder diffractometer using CoKa radiation and the unit cell parameters were determined by using the least squares refinement program Rietica. Ultravioletevisible spectroscopy to measure the excitation and emission spectra of the fluoride phosphors was done with a spectrofluorometer (Sinco Fluromate FS-2). 3. Results and discussion The phase identification of Sr2.5-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) samples was performed by studying the powder X-ray diffraction (XRD) patterns after the Ba2þ, Naþ, and/or Sn4þ ions were replaced with Sr2þ or/and Al3þ ions in the Sr3AlO4F host lattice. The XRD patterns of the single-phase host lattice features without any evident impurities were observed. Fig. 1 (a)e(e) indicate the calculated Sr3AlO4F (ICSD 50736, a ¼ 6.7822(1) and c ¼ 11.1437(2) Å) and the obtained Sr3-pBapAlO4F (p ¼ 0, 0.5), Sr2.475-qBa0.5NaqAlO4F (q ¼ 0.05), and Sr2.5-qBa0.5NaqAl1-zSnzO4F (q, z ¼ 0.05) XRD patterns. The larger Ba2þ (r ¼ 1.52 Å, CN ¼ 10) and smaller Naþ (r ¼ 1.18 Å, CN ¼ 8) ions can be accommodated in the 10-coordinated Sr(1) (r ¼ 1.36 Å, CN ¼ 10) and 8-coordinated Sr(2) (r ¼ 1.26 Å, CN ¼ 8) sites in the structure, respectively. Sr2þ in the obtained Sr3AlO4F lattice (a ¼ 6.7735(2) and c ¼ 11.12840(5) Å) was partially substituted by Ba2þ (p ¼ 0.5) in Sr3-pBapAlO4F (a ¼ 6.8378(2) and c ¼ 11.1615(6) Å) and this resulted in a slight shift in the positions of the Bragg reflections between 2q ¼ 35 and 37 to lower angles as the cell size increased as shown in Fig. 1 (c). Furthermore, when Sr2þ ions were replaced by smaller Naþ ions in Sr2.475Ba0.5Na0.05AlO4F (a ¼ 6.8373(3) and c ¼ 11.1563(8) Å), a slight peak shift from the XRD pattern of the Sr2.5Ba0.5AlO4F lattice was observed in Fig. 1 (d). It is conceivable that an Sr2þ ion was replaced by two Naþ ions in the host lattice. The substitution of Al3þ (r ¼ 0.395 Å, CN ¼ 4) ions by Sn4þ (r ¼ 0.55 Å, CN ¼ 4) ions in the Sr2.45Ba0.5Na0.05AlO4F structure resulted in a clear shift to lower angles as the cell size increased to a ¼ 6.8484(3) and c ¼ 11.1765(5) Å for the
213
Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4F lattice as shown in Fig. 1 (e). In Fig. 2 (a), the broadband emission spectra and photos in the blue, green, white, yellow, and orange regions of the visible light with UV excitation light ranging from 254 to 313 nm are shown when Sr3-pBapAlO4-aF1-d (p ¼ 0e0.7) powder was exposed to a reducing atmosphere (8% H2/92% Ar) at 800 C. The structure of Sr3AlO4F lattice contains two types of 10-fold and 8-fold coordinated Sr2þ ions, Sr(1)O8F2 and Sr(2)O6F2, with 4-fold coordinated 3þ Al3þ cation (AlO4) in the stacking Sr(1)AlO3 layers 4 and Sr(2)2F [16,19]. As reported before, the anion-deficient self-activating photoluminescence of Sr3AlO4-aF1-d was suggested to be induced by two defect clustersd[SrAlO4]3- and [Sr2F]3þ blocks along the c-axis [16]. When the content of Ba2þ ions was increased, the emission spectra exhibited a red shift from 467 to 582 nm. The energy gap steadily increased with the substitution with larger Ba2þ ions, which occupy the 10-fold Sr(1) site in the Sr3AlO4F host lattice with the local electronic structure of [(Sr,Ba)AlO4]3- clusters. The maximum intensity of the photoluminescence emission under UV excitation light was observed for the solid-state series Sr3-pBapAlO4aF1-d with p ¼ 0.5. When the defect-induced Sr3-pBapAlO4-aF1þ d (p ¼ 0e0.7) phosphors were doped with Na ions, the emission bands showed a clear shift to the yellow region. The blue emission of the Sr3-pBapAlO4-aF1-d (p ¼ 0) phosphor evidently shifted to the green region when substituted by Naþ ions; furthermore, the green emission was observed in Sr2.975-pBapNa0.05AlO4-aF1-d (p ¼ 0.3) phosphors under the excitation of 254 and 312 nm with a hand lamp as shown in Fig. 2 (b). The maximum intensity of the yellow photoluminescence emission of Sr2.5Ba0.5AlO4-aF1-d with p ¼ 0.5 under UV excitation light was enhanced by Naþ substitution in the structure; in addition, reddish emission was observed in Sr2.975pBapNa0.05AlO4-aF1-d (p ¼ 0.5, 0.7) under 365 nm excitation with a hand lamp. The Sr2þ ion was replaced by two Naþ ions in the Sr2.975þ pBapNa0.05AlO4-aF1-d lattice. The Na ions can play the role of a charge compensator; moreover, the over-doped Naþ ions will possibly occupy the interstitial positions in the defect-induced oxyfluoride lattice. Naþ ions promote the emission of the selfactivating phosphors, when the anion defect process occurs, thereby reducing the number of cation vacancies. The defectinduced Sr2.95-pBapNa0.05Al0.95Sn0.05O4-aF1-d (p ¼ 0e0.7) phosphors show two excitation and emission centers as shown in Fig. 2 (c). In the Sr2.95-pBapNa0.05Al0.95Sn0.05O4F host lattices, Naþ and Sn4þ ions occupy Sr2þ and Al3þ sites, respectively. The cation disorders in the host structures exhibited defect-trap centers and unusual emission spectra according to the previous reports [11,13]. When the Sr2.95-pBapNa0.05Al0.95Sn0.05O4F lattice was induced with defects by substituting Sr2þ and Al3þ with Naþ and Sn4þ,
Fig. 1. XRD patterns of the calculated Sr3AlO4F (ICSD 50736) and observed (b) Sr3AlO4F, (c) Sr2.5Ba0.5AlO4F, (d) Sr2.425Ba0.5Na0.05AlO4F, and (e) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4F.
214
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
Fig. 2. The excitation and emission spectra and photographs of (a) Sr3-pBapAlO4-aF1-d, (b) Sr2.975-pBapNa0.05AlO4-aF1-d, (c) Sr2.95-pBapNa0.05Al0.95Sn0.05O4-aF1-d (p ¼ 0e0.7) phosphors.
respectively, new excitation from 250 to 400 nm and emission from 300 to 700 nm were obtained; furthermore, the yellow emission of self-activating Sr2.95-pBapNa0.05Al0.95Sn0.05O4-aF1-d phosphors was enhanced compared with that of Sr2.975-pBapNa0.05AlO4-aF1-d phosphors. In addition, the excitation band of the defect-induced phosphors according to the substitution of Sn4þ ions in the Sr2.95xpBapAlO4F host structure shifted to longer wavelengths (~400 nm) owing to apparent changes of the local electronic structure and band gap. Fig. 3 shows the emission spectra and the schematic diagrams of (A) Sr2.5Ba0.5AlO4-aF1-d, (B) Sr2.45Ba0.5Na0.05AlO3.975-aF1-d, (C) Sr2.475Ba0.5Na0.05AlO4-aF1-d, (D) Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1-d, and
(E) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors. With the substitution by Ba2þ, Naþ, and Sn4þ ions in the Sr3AlO4F lattice, the local electronic structures of [SrAlO4]3- and [Sr2F]3þ blocks are changed into (A) [Sr0.5Ba0.5AlO4]3and [Sr2F]3þ, (B) 2.95 [Sr0.5Ba0.5AlO3.975] and [Sr1.95Na0.05F]2.95þ, (C) [Sr0.5Ba0.5AlO4]3and [Sr1.975Na0.05F]3þ, (D) [Sr0.5Ba0.5Al0.95Sn0.05O4.025]3and [Sr2F]3þ, and (E) [Sr0.5Ba0.5Al0.95Sn0.05O4]2.95 and [Sr1.95Na0.05F]2.95þ blocks. As shown in Fig. 2 (a), the self-activating yellow emission of Sr2.5Ba0.5AlO4-aF1-d reached a maximum intensity. Furthermore, when two Naþ ions in the Sr2.475Ba0.5Na0.05AlO4-aF1-d lattice were substituted, the defect-induced yellow emission was improved by
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
215
demonstrated in the emission spectra when excited approximately at 281 and 312 nm and 282 and 316 nm, respectively, as shown in Fig. 3. When the Zr4þ ions occupied the Y3þ site of the Na5YZrSi6O18 lattice, the usual defect-induced emission was observed owing to their cation disorder [13]. As Sn4þ ions occupied the Al3þ site in both [Sr0.5Ba0.5Al0.95Sn0.05O4.025]3and [Sr0.5Ba0.5Al0.95Sn0.05O4]2.95 clusters of the oxyfluoride lattice, strong blue emission spectra could be observed in the 300e500 nm region owing to the positive defects in the Al3þ site. The unusual excitation band centered at ~281 nm could be only observed in Sn4þ-substituted oxyfluoride phosphors, which showed a strong blue emission band centered at ~366 nm. The defect-induced selfactivating Naþ, Sn4þ-substituted Sr2.5Ba0.5AlO4-aF1-d phosphors (D, E) produced clear excitation and emission bands centered at ~310 and ~555 nm, respectively. When excited with lights of wavelengths ~281 or ~310 nm, a major blue or yellow emission spectrum was observed. Fig. 4 shows the emission spectra of the Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors by varying the excitation wavelength from 283 to 319 nm. The self-activating blue or yellow emission band reached the maximum intensity when the excitation wavelength was 283 or 319 nm. By controlling the excitation wavelength, color-tunable self-activated Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors were obtained. The inset in Fig. 4 shows photographs of blue to white and yellow emission in the defect-induced self-activating phosphors under illumination from 283 to 319 nm (aek). The CIE coordinates of the self-activating phosphors were (a) x ¼ 0.2870, y ¼ 0.2620, (f) x ¼ 0.3430, y ¼ 0.3533, and (k) x ¼ 0.3982, y ¼ 0.4356 in Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d (EX ¼ 283, 297, and 319 nm) phosphors, respectively. A white-emitting self-activated phosphor induced by anion defects with cation disorders was obtained when excited at the wavelength of 297 nm.
4. Conclusions
Fig. 3. The emission spectra with the schematic diagrams of (A) Sr2.5Ba0.5AlO4-aF1-d, (B) Sr2.45Ba0.5Na0.05AlO3.975-aF1-d, (C) Sr2.475Ba0.5Na0.05AlO4-aF1-d, (D) Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1-d, and (E) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors.
approximately 24% owing to the reduction of cation vacancies. However, when an Sr2þ ion was replaced by an Naþ ion in the Sr2.5Ba0.5AlO4F lattice, the defect-induced emission of the Sr2.45Ba0.5Na0.05AlO3.975-aF1-d phosphors abruptly decreased because the excessive anion vacancy in the [Sr0.5Ba0.5AlO3.975]2.95 cluster ultimately impeded the anion defect process of the selfactivating emission. The emission of cation-disordered self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors showed an improvement of approximately 172% compared with the original emission of the Sr2.5Ba0.5AlO4-aF1-d phosphors. The improved covalency between [Sr0.5Ba0.5Al0.95Sn0.05O4]2.95 and [Sr1.95Na0.05F]2.95þ clusters from [Sr0.5Ba0.5AlO4]3- and [Sr2F]3þ clusters through cationic disorder (Al3þ-Sn4þ and Sr2þ-Naþ) resulted in the effective red shift of the luminescence with the enhancement of emission. However, when an Al3þ ion was replaced by an Sn4þ ion in the Sr2.5Ba0.5AlO4F lattice, the defect-induced emission of the Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1-d phosphors was weakened because the oxygen-enriched [Sr0.5Ba0.5Al0.95Sn0.05O4.025]3- cluster inhibited the anion defect process. When the Sn4þ ions were substituted in the oxyfluoride lattice, two clear bands centered at 367 and 555 nm and 366 and 574 nm for the defect-induced (D) Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1d and (E) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors were
Cation-disordered Sr3-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) compounds were prepared via solid-state reactions at 1200 C. The aniondeficient self-activating Sr3-p-qBapNaqAl1-zSnzO4-aF1-d and Sr2.975pBapNa0.05AlO4-aF1-d phosphors were prepared via defect induction under a reducing atmosphere (8% H2/92% Ar) at 800 C for 1 h. The anion-deficient self-activating phosphors were identified using
Fig. 4. The emission spectra with photographs and CIE coordinates of the colortunable Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors under the excitation wavelength from 283 to 319 nm (aek). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
216
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
broadband-shaped emissions ranging from 300 to 700 nm when excited with UV light. Broadband-shaped emissions centered approximately at 360 and 570 nm were observed by substituting Naþ and/or Sn4þ ions in the Sr2.5Ba0.5AlO4-aF1-d lattice when excited with UV light of wavelength approximately 256e320 nm. The defect-induced yellow emission of the Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors was enhanced by approximately 172% compared with the emission of the Sr2.45Ba0.5AlO4-aF1d phosphors. Color-tunable emissions from blue to yellow regions were obtained with the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphor; moreover, the white emitting light with the desired CIE value (x ¼ 0.3430, y ¼ 0.3533) was attained with the self-activated phosphor excited under UV light of wavelength 297 nm. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (NRF2015R1D1A1A01059655). References [1] W.M. Yen, S. Shionoya, Phosphor Handbook, CRC Press, Boca Raton, 1999. [2] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R 71 (2010) 1e34. [3] J.L. Wu, G. Gundiah, A.K. Cheetham, Structure-property correlations in Cedoped garnet phosphors for use in solid state lighting, Chem. Phys. Lett. 441 (2007) 250e254. [4] M. Nazarov, C. Yoon, Controlled peak wavelength shift of Ca1-xSrx(SySe1-y): Eu2þ phosphor for LED application, J. Solid State Chem. 179 (2006) 2529e2533.
[5] W. Yang, S.H. Kim, S. Park, Multi-color tunable Ce3þ-Mn2þ cooperative Y7O6F9 vernier phosphors, J. Alloys Compd. 673 (2016) 1e7. [6] J. Zhou, F. Huang, J. Xu, H. Chen, Y. Wang, Luminescence study of a selfactivated and rare-earth activated Sr3La(VO4)3 phosphor potentially applicable in W-LED, J. Mater. Chem. C 3 (2015) 3023e3028. [7] M. Nikl, Scintillation detectors for x-rays, Meas. Sci. Technol. 17 (2006) R37eR54. [8] B. Ding, H. Qian, C. Han, J. Zhang, S.-E. Lindquist, B. Wei, Z. Tang, Oxygen vacancy effect on photoluminescence properties of self-activated yttrium tungstate, J. Phys. Chem. C 118 (2014) 25633e25642. [9] Y. Zhai, M. Wang, Q. Zhao, J. Yu, X. Li, Fabrication and Luminescent properties of ZnWO4:Eu3þ, Dy3þ white light-emitting phosphors, J. Lumin. 172 (2016) 161e167. [10] X. Chen, Z. Xia, M. Yi, X. Wu, H. Xin, Rare-earth free self-activated and rareearth activated Ca2NaZn2V3O12 vanadate phosphors and their color-tunable luminescence properties, J. Phys. Chem. Solid. 74 (2013) 1439e1443. [11] X. Xu, Y. Wang, Y. Gong, W. Zeng, Y. Li, Effect of oxygen vacancies on the red phosphorescence of Sr2SnO4:Sm3þ phosphor, Optic Express 18 (2010) 16989e16994. [12] C.C. Kang, R.S. Liu, J.C. Chang, B.J. Lee, Synthesis and luminescent properties of a new yellowish-orange afterglow phosphor Y2O2S: Ti,Mg, Chem. Mater. 15 (2003) 3966e3968. [13] Y. Guan, L. Qin, Y. Huang, C. Qin, D. Wei, H.J. Seo, A self-activated silicate phosphor of Na5YZrSi6O18, Mater. Res. Bull. 54 (2014) 24e27. [14] D. Kan, T. Terashima, R. Kanda, A. Masuno, K. Tanaka, S. Chu, H. Kan, A. Ishizumi, Y. Kanemitsu, Y. Shimakawa, M. Takano, Blue-light emission at room temperature from Arþ-irradiated SrTiO3, Nat. Mater. 4 (2005) 816e819. [15] S. Park, T. Vogt, Near UV excited line and broad band photoluminescence of an anion-ordered oxyfluoride, J. Am. Chem. Soc. 132 (2010) 4516e4517. [16] S. Park, T. Vogt, Defect monitoring and substitutions in Sr3-xAxAlO4F (A ¼ Ca, Ba) oxyfluoride host lattices and phosphors, J. Phys. Chem. C 114 (2010) 11576e11583. [17] S. Yang, S. Park, Defect-induced self-activating Sr2-p-3q/2ApSmqLiSiO4-aF1-d (A ¼ Ba, Ca, p ¼ 0-0.1, q ¼ 0-0.002) phosphors, J. Alloys Compd. 695 (2017) 1802e1806. [18] S. Park, New sensitizing roles of defect-induced self-activator in Sr2.53x/ 2Ca0.5PrxAl1yInyO4F (0.001 x 0.05 y ¼ 0, 0.1) Phosphors, Opt. Mater. 35 (2013) 516e519. [19] S. Park, Structureecompositioneluminescence correlations in Sr2.53x/ 2Ba0.5SmxAl1yInyO4F (0.001x, y0.1) oxyfluorides, J. Solid State Chem. 186 (2012) 201e207.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Color-tunable self-activated oxyfluoride phosphors induced by aniondefect with cation-disorder Sungjun Yang, Sung-Hoon Kim, Sangmoon Park* Center for Green Fusion Technology and Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 46958, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2018 Received in revised form 2 May 2018 Accepted 4 May 2018 Available online 6 May 2018
Cation-disordered oxyfluoride materials, Sr3-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7), were prepared via solid-state reactions and subsequently induced with defects under appropriate reduction conditions. The anion-deficient self-activating phosphors can be identified using broadband-shaped emissions ranging from 300 to 700 nm when excited with ultraviolet (UV) light. The luminescence properties of the defect-induced self-activating phosphors were examined using cation-disordered [(Sr,Ba) (Al,Sn)O4]3- and [(Sr,Na)2F]3þ clusters in the oxyfluoride lattice. The luminescent emissions of Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors were significantly enhanced by the combination of the cation disorder with anion defect. Color-tunable emissions in the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphor were obtained. The desired Commission Internationale de I'Eclairage (CIE) values corresponding to the spectral emission from blue to white and yellow regions of the spectra were attained by exciting the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors with UV light. © 2018 Elsevier B.V. All rights reserved.
Keywords: Self-activating phosphors Defects Cation-disorder Oxyfluorides
1. Introduction Rare-earth and transition ions - Ce3þ, Eu2þ, Eu3þ, Tb3þ, or Mn2þ ions - in various host lattices can be generally used as activators for an appropriate luminescence process, which is defined as the excitation and emission progression through the electronic transitions of the activators [1,2]. Line-shaped or broadband-shaped emission could arise from an activator, and is assigned to f-f, f-d, or d-d electronic transitions; especially, the activators showing broadband-shaped transitions can be affected by their coordinated environments in host lattices [1e5]. Notably, unusual emission without using the activators can be observed via an electronic transition between an anion electron and empty cation orbital; for example, an electron transfer from the 2p orbital of O2 to the vacant 3d orbital of V5þ in the (VO4)3- complex results in broadband-shaped emission [1,6]. Such phosphors are called selfactivated phosphors because the activators of rare-earth or transition ions are not involved in the process. Similar luminescence phenomena were widely observed in (WO4)2-, (MoO4)2-, and (MnO4)- complexes during the investigation of self-activated
* Corresponding author. E-mail address: [email protected] (S. Park). https://doi.org/10.1016/j.jallcom.2018.05.041 0925-8388/© 2018 Elsevier B.V. All rights reserved.
CaWO4 phosphors, which were used as a scintillator for the detection of X-rays in 1896 [7e10]. Furthermore, the defectinduced self-activating phosphors obtained owing to cation or anion vacancies in host lattices, which generate self-trapped electron or hole states, emit broadband-shaped luminescence lights without the activators [11e13]. These defects can be produced with cation-disordered substitutions in the host lattice or by altering material preparation conditions using X-ray radiation, Arþ irradiation, or appropriate reduction environments [13e17]. In a previous study, defect-induced self-activating (Sr,Ca,Ba)3AlO4-aF1d phosphors controlled by anion defects were investigated [16]. In this study, cation-disordered Sr2.5-qBa0.5NaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) oxyfluorides are prepared. The luminescence properties of defectinduced self-activating oxyfluoride phosphors are examined by controlling the anion defect structure generated under postsynthesis reduction conditions when excited with ultraviolet (UV) light. A significant emission enhancement is expected to arise from a combination of the cation disorder with anionic defects. Colortunable emissions of defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors are observed. The desired CIE values corresponding to the spectral emission regions from blue to white and yellow are attained using the defect-induced, self-activating oxyfluoride phosphors.
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
2. Experimental details Samples of Sr3-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) were prepared by heating the appropriate stoichiometric amounts of SrCO3 (Alfa 99%), BaCO3 (Alfa 99.8%), SrF2 (Alfa 99%), Na2CO3 (Alfa 99.5%), Al2O3 (Alfa 99.95%), and SnO2 (Alfa 99.9%) at temperature up to 1200 C for 3 h in air. The as-made samples were subsequently annealed at 800 C in reducing atmosphere (8% H2/92% Ar) for 1 h to compose self-activating luminescent materials [15e18]. Phase identification of the phosphors was done using a Shimadzu XRD-6000 powder diffractometer using CoKa radiation and the unit cell parameters were determined by using the least squares refinement program Rietica. Ultravioletevisible spectroscopy to measure the excitation and emission spectra of the fluoride phosphors was done with a spectrofluorometer (Sinco Fluromate FS-2). 3. Results and discussion The phase identification of Sr2.5-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) samples was performed by studying the powder X-ray diffraction (XRD) patterns after the Ba2þ, Naþ, and/or Sn4þ ions were replaced with Sr2þ or/and Al3þ ions in the Sr3AlO4F host lattice. The XRD patterns of the single-phase host lattice features without any evident impurities were observed. Fig. 1 (a)e(e) indicate the calculated Sr3AlO4F (ICSD 50736, a ¼ 6.7822(1) and c ¼ 11.1437(2) Å) and the obtained Sr3-pBapAlO4F (p ¼ 0, 0.5), Sr2.475-qBa0.5NaqAlO4F (q ¼ 0.05), and Sr2.5-qBa0.5NaqAl1-zSnzO4F (q, z ¼ 0.05) XRD patterns. The larger Ba2þ (r ¼ 1.52 Å, CN ¼ 10) and smaller Naþ (r ¼ 1.18 Å, CN ¼ 8) ions can be accommodated in the 10-coordinated Sr(1) (r ¼ 1.36 Å, CN ¼ 10) and 8-coordinated Sr(2) (r ¼ 1.26 Å, CN ¼ 8) sites in the structure, respectively. Sr2þ in the obtained Sr3AlO4F lattice (a ¼ 6.7735(2) and c ¼ 11.12840(5) Å) was partially substituted by Ba2þ (p ¼ 0.5) in Sr3-pBapAlO4F (a ¼ 6.8378(2) and c ¼ 11.1615(6) Å) and this resulted in a slight shift in the positions of the Bragg reflections between 2q ¼ 35 and 37 to lower angles as the cell size increased as shown in Fig. 1 (c). Furthermore, when Sr2þ ions were replaced by smaller Naþ ions in Sr2.475Ba0.5Na0.05AlO4F (a ¼ 6.8373(3) and c ¼ 11.1563(8) Å), a slight peak shift from the XRD pattern of the Sr2.5Ba0.5AlO4F lattice was observed in Fig. 1 (d). It is conceivable that an Sr2þ ion was replaced by two Naþ ions in the host lattice. The substitution of Al3þ (r ¼ 0.395 Å, CN ¼ 4) ions by Sn4þ (r ¼ 0.55 Å, CN ¼ 4) ions in the Sr2.45Ba0.5Na0.05AlO4F structure resulted in a clear shift to lower angles as the cell size increased to a ¼ 6.8484(3) and c ¼ 11.1765(5) Å for the
213
Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4F lattice as shown in Fig. 1 (e). In Fig. 2 (a), the broadband emission spectra and photos in the blue, green, white, yellow, and orange regions of the visible light with UV excitation light ranging from 254 to 313 nm are shown when Sr3-pBapAlO4-aF1-d (p ¼ 0e0.7) powder was exposed to a reducing atmosphere (8% H2/92% Ar) at 800 C. The structure of Sr3AlO4F lattice contains two types of 10-fold and 8-fold coordinated Sr2þ ions, Sr(1)O8F2 and Sr(2)O6F2, with 4-fold coordinated 3þ Al3þ cation (AlO4) in the stacking Sr(1)AlO3 layers 4 and Sr(2)2F [16,19]. As reported before, the anion-deficient self-activating photoluminescence of Sr3AlO4-aF1-d was suggested to be induced by two defect clustersd[SrAlO4]3- and [Sr2F]3þ blocks along the c-axis [16]. When the content of Ba2þ ions was increased, the emission spectra exhibited a red shift from 467 to 582 nm. The energy gap steadily increased with the substitution with larger Ba2þ ions, which occupy the 10-fold Sr(1) site in the Sr3AlO4F host lattice with the local electronic structure of [(Sr,Ba)AlO4]3- clusters. The maximum intensity of the photoluminescence emission under UV excitation light was observed for the solid-state series Sr3-pBapAlO4aF1-d with p ¼ 0.5. When the defect-induced Sr3-pBapAlO4-aF1þ d (p ¼ 0e0.7) phosphors were doped with Na ions, the emission bands showed a clear shift to the yellow region. The blue emission of the Sr3-pBapAlO4-aF1-d (p ¼ 0) phosphor evidently shifted to the green region when substituted by Naþ ions; furthermore, the green emission was observed in Sr2.975-pBapNa0.05AlO4-aF1-d (p ¼ 0.3) phosphors under the excitation of 254 and 312 nm with a hand lamp as shown in Fig. 2 (b). The maximum intensity of the yellow photoluminescence emission of Sr2.5Ba0.5AlO4-aF1-d with p ¼ 0.5 under UV excitation light was enhanced by Naþ substitution in the structure; in addition, reddish emission was observed in Sr2.975pBapNa0.05AlO4-aF1-d (p ¼ 0.5, 0.7) under 365 nm excitation with a hand lamp. The Sr2þ ion was replaced by two Naþ ions in the Sr2.975þ pBapNa0.05AlO4-aF1-d lattice. The Na ions can play the role of a charge compensator; moreover, the over-doped Naþ ions will possibly occupy the interstitial positions in the defect-induced oxyfluoride lattice. Naþ ions promote the emission of the selfactivating phosphors, when the anion defect process occurs, thereby reducing the number of cation vacancies. The defectinduced Sr2.95-pBapNa0.05Al0.95Sn0.05O4-aF1-d (p ¼ 0e0.7) phosphors show two excitation and emission centers as shown in Fig. 2 (c). In the Sr2.95-pBapNa0.05Al0.95Sn0.05O4F host lattices, Naþ and Sn4þ ions occupy Sr2þ and Al3þ sites, respectively. The cation disorders in the host structures exhibited defect-trap centers and unusual emission spectra according to the previous reports [11,13]. When the Sr2.95-pBapNa0.05Al0.95Sn0.05O4F lattice was induced with defects by substituting Sr2þ and Al3þ with Naþ and Sn4þ,
Fig. 1. XRD patterns of the calculated Sr3AlO4F (ICSD 50736) and observed (b) Sr3AlO4F, (c) Sr2.5Ba0.5AlO4F, (d) Sr2.425Ba0.5Na0.05AlO4F, and (e) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4F.
214
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
Fig. 2. The excitation and emission spectra and photographs of (a) Sr3-pBapAlO4-aF1-d, (b) Sr2.975-pBapNa0.05AlO4-aF1-d, (c) Sr2.95-pBapNa0.05Al0.95Sn0.05O4-aF1-d (p ¼ 0e0.7) phosphors.
respectively, new excitation from 250 to 400 nm and emission from 300 to 700 nm were obtained; furthermore, the yellow emission of self-activating Sr2.95-pBapNa0.05Al0.95Sn0.05O4-aF1-d phosphors was enhanced compared with that of Sr2.975-pBapNa0.05AlO4-aF1-d phosphors. In addition, the excitation band of the defect-induced phosphors according to the substitution of Sn4þ ions in the Sr2.95xpBapAlO4F host structure shifted to longer wavelengths (~400 nm) owing to apparent changes of the local electronic structure and band gap. Fig. 3 shows the emission spectra and the schematic diagrams of (A) Sr2.5Ba0.5AlO4-aF1-d, (B) Sr2.45Ba0.5Na0.05AlO3.975-aF1-d, (C) Sr2.475Ba0.5Na0.05AlO4-aF1-d, (D) Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1-d, and
(E) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors. With the substitution by Ba2þ, Naþ, and Sn4þ ions in the Sr3AlO4F lattice, the local electronic structures of [SrAlO4]3- and [Sr2F]3þ blocks are changed into (A) [Sr0.5Ba0.5AlO4]3and [Sr2F]3þ, (B) 2.95 [Sr0.5Ba0.5AlO3.975] and [Sr1.95Na0.05F]2.95þ, (C) [Sr0.5Ba0.5AlO4]3and [Sr1.975Na0.05F]3þ, (D) [Sr0.5Ba0.5Al0.95Sn0.05O4.025]3and [Sr2F]3þ, and (E) [Sr0.5Ba0.5Al0.95Sn0.05O4]2.95 and [Sr1.95Na0.05F]2.95þ blocks. As shown in Fig. 2 (a), the self-activating yellow emission of Sr2.5Ba0.5AlO4-aF1-d reached a maximum intensity. Furthermore, when two Naþ ions in the Sr2.475Ba0.5Na0.05AlO4-aF1-d lattice were substituted, the defect-induced yellow emission was improved by
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
215
demonstrated in the emission spectra when excited approximately at 281 and 312 nm and 282 and 316 nm, respectively, as shown in Fig. 3. When the Zr4þ ions occupied the Y3þ site of the Na5YZrSi6O18 lattice, the usual defect-induced emission was observed owing to their cation disorder [13]. As Sn4þ ions occupied the Al3þ site in both [Sr0.5Ba0.5Al0.95Sn0.05O4.025]3and [Sr0.5Ba0.5Al0.95Sn0.05O4]2.95 clusters of the oxyfluoride lattice, strong blue emission spectra could be observed in the 300e500 nm region owing to the positive defects in the Al3þ site. The unusual excitation band centered at ~281 nm could be only observed in Sn4þ-substituted oxyfluoride phosphors, which showed a strong blue emission band centered at ~366 nm. The defect-induced selfactivating Naþ, Sn4þ-substituted Sr2.5Ba0.5AlO4-aF1-d phosphors (D, E) produced clear excitation and emission bands centered at ~310 and ~555 nm, respectively. When excited with lights of wavelengths ~281 or ~310 nm, a major blue or yellow emission spectrum was observed. Fig. 4 shows the emission spectra of the Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors by varying the excitation wavelength from 283 to 319 nm. The self-activating blue or yellow emission band reached the maximum intensity when the excitation wavelength was 283 or 319 nm. By controlling the excitation wavelength, color-tunable self-activated Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors were obtained. The inset in Fig. 4 shows photographs of blue to white and yellow emission in the defect-induced self-activating phosphors under illumination from 283 to 319 nm (aek). The CIE coordinates of the self-activating phosphors were (a) x ¼ 0.2870, y ¼ 0.2620, (f) x ¼ 0.3430, y ¼ 0.3533, and (k) x ¼ 0.3982, y ¼ 0.4356 in Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d (EX ¼ 283, 297, and 319 nm) phosphors, respectively. A white-emitting self-activated phosphor induced by anion defects with cation disorders was obtained when excited at the wavelength of 297 nm.
4. Conclusions
Fig. 3. The emission spectra with the schematic diagrams of (A) Sr2.5Ba0.5AlO4-aF1-d, (B) Sr2.45Ba0.5Na0.05AlO3.975-aF1-d, (C) Sr2.475Ba0.5Na0.05AlO4-aF1-d, (D) Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1-d, and (E) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors.
approximately 24% owing to the reduction of cation vacancies. However, when an Sr2þ ion was replaced by an Naþ ion in the Sr2.5Ba0.5AlO4F lattice, the defect-induced emission of the Sr2.45Ba0.5Na0.05AlO3.975-aF1-d phosphors abruptly decreased because the excessive anion vacancy in the [Sr0.5Ba0.5AlO3.975]2.95 cluster ultimately impeded the anion defect process of the selfactivating emission. The emission of cation-disordered self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors showed an improvement of approximately 172% compared with the original emission of the Sr2.5Ba0.5AlO4-aF1-d phosphors. The improved covalency between [Sr0.5Ba0.5Al0.95Sn0.05O4]2.95 and [Sr1.95Na0.05F]2.95þ clusters from [Sr0.5Ba0.5AlO4]3- and [Sr2F]3þ clusters through cationic disorder (Al3þ-Sn4þ and Sr2þ-Naþ) resulted in the effective red shift of the luminescence with the enhancement of emission. However, when an Al3þ ion was replaced by an Sn4þ ion in the Sr2.5Ba0.5AlO4F lattice, the defect-induced emission of the Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1-d phosphors was weakened because the oxygen-enriched [Sr0.5Ba0.5Al0.95Sn0.05O4.025]3- cluster inhibited the anion defect process. When the Sn4þ ions were substituted in the oxyfluoride lattice, two clear bands centered at 367 and 555 nm and 366 and 574 nm for the defect-induced (D) Sr2.5Ba0.5Al0.95Sn0.05O4.025-aF1d and (E) Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors were
Cation-disordered Sr3-p-qBapNaqAl1-zSnzO4F (p ¼ 0e0.7 and q, z ¼ 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p ¼ 0e0.7) compounds were prepared via solid-state reactions at 1200 C. The aniondeficient self-activating Sr3-p-qBapNaqAl1-zSnzO4-aF1-d and Sr2.975pBapNa0.05AlO4-aF1-d phosphors were prepared via defect induction under a reducing atmosphere (8% H2/92% Ar) at 800 C for 1 h. The anion-deficient self-activating phosphors were identified using
Fig. 4. The emission spectra with photographs and CIE coordinates of the colortunable Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors under the excitation wavelength from 283 to 319 nm (aek). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
216
S. Yang et al. / Journal of Alloys and Compounds 756 (2018) 212e216
broadband-shaped emissions ranging from 300 to 700 nm when excited with UV light. Broadband-shaped emissions centered approximately at 360 and 570 nm were observed by substituting Naþ and/or Sn4þ ions in the Sr2.5Ba0.5AlO4-aF1-d lattice when excited with UV light of wavelength approximately 256e320 nm. The defect-induced yellow emission of the Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphors was enhanced by approximately 172% compared with the emission of the Sr2.45Ba0.5AlO4-aF1d phosphors. Color-tunable emissions from blue to yellow regions were obtained with the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-aF1-d phosphor; moreover, the white emitting light with the desired CIE value (x ¼ 0.3430, y ¼ 0.3533) was attained with the self-activated phosphor excited under UV light of wavelength 297 nm. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (NRF2015R1D1A1A01059655). References [1] W.M. Yen, S. Shionoya, Phosphor Handbook, CRC Press, Boca Raton, 1999. [2] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R 71 (2010) 1e34. [3] J.L. Wu, G. Gundiah, A.K. Cheetham, Structure-property correlations in Cedoped garnet phosphors for use in solid state lighting, Chem. Phys. Lett. 441 (2007) 250e254. [4] M. Nazarov, C. Yoon, Controlled peak wavelength shift of Ca1-xSrx(SySe1-y): Eu2þ phosphor for LED application, J. Solid State Chem. 179 (2006) 2529e2533.
[5] W. Yang, S.H. Kim, S. Park, Multi-color tunable Ce3þ-Mn2þ cooperative Y7O6F9 vernier phosphors, J. Alloys Compd. 673 (2016) 1e7. [6] J. Zhou, F. Huang, J. Xu, H. Chen, Y. Wang, Luminescence study of a selfactivated and rare-earth activated Sr3La(VO4)3 phosphor potentially applicable in W-LED, J. Mater. Chem. C 3 (2015) 3023e3028. [7] M. Nikl, Scintillation detectors for x-rays, Meas. Sci. Technol. 17 (2006) R37eR54. [8] B. Ding, H. Qian, C. Han, J. Zhang, S.-E. Lindquist, B. Wei, Z. Tang, Oxygen vacancy effect on photoluminescence properties of self-activated yttrium tungstate, J. Phys. Chem. C 118 (2014) 25633e25642. [9] Y. Zhai, M. Wang, Q. Zhao, J. Yu, X. Li, Fabrication and Luminescent properties of ZnWO4:Eu3þ, Dy3þ white light-emitting phosphors, J. Lumin. 172 (2016) 161e167. [10] X. Chen, Z. Xia, M. Yi, X. Wu, H. Xin, Rare-earth free self-activated and rareearth activated Ca2NaZn2V3O12 vanadate phosphors and their color-tunable luminescence properties, J. Phys. Chem. Solid. 74 (2013) 1439e1443. [11] X. Xu, Y. Wang, Y. Gong, W. Zeng, Y. Li, Effect of oxygen vacancies on the red phosphorescence of Sr2SnO4:Sm3þ phosphor, Optic Express 18 (2010) 16989e16994. [12] C.C. Kang, R.S. Liu, J.C. Chang, B.J. Lee, Synthesis and luminescent properties of a new yellowish-orange afterglow phosphor Y2O2S: Ti,Mg, Chem. Mater. 15 (2003) 3966e3968. [13] Y. Guan, L. Qin, Y. Huang, C. Qin, D. Wei, H.J. Seo, A self-activated silicate phosphor of Na5YZrSi6O18, Mater. Res. Bull. 54 (2014) 24e27. [14] D. Kan, T. Terashima, R. Kanda, A. Masuno, K. Tanaka, S. Chu, H. Kan, A. Ishizumi, Y. Kanemitsu, Y. Shimakawa, M. Takano, Blue-light emission at room temperature from Arþ-irradiated SrTiO3, Nat. Mater. 4 (2005) 816e819. [15] S. Park, T. Vogt, Near UV excited line and broad band photoluminescence of an anion-ordered oxyfluoride, J. Am. Chem. Soc. 132 (2010) 4516e4517. [16] S. Park, T. Vogt, Defect monitoring and substitutions in Sr3-xAxAlO4F (A ¼ Ca, Ba) oxyfluoride host lattices and phosphors, J. Phys. Chem. C 114 (2010) 11576e11583. [17] S. Yang, S. Park, Defect-induced self-activating Sr2-p-3q/2ApSmqLiSiO4-aF1-d (A ¼ Ba, Ca, p ¼ 0-0.1, q ¼ 0-0.002) phosphors, J. Alloys Compd. 695 (2017) 1802e1806. [18] S. Park, New sensitizing roles of defect-induced self-activator in Sr2.53x/ 2Ca0.5PrxAl1yInyO4F (0.001 x 0.05 y ¼ 0, 0.1) Phosphors, Opt. Mater. 35 (2013) 516e519. [19] S. Park, Structureecompositioneluminescence correlations in Sr2.53x/ 2Ba0.5SmxAl1yInyO4F (0.001x, y0.1) oxyfluorides, J. Solid State Chem. 186 (2012) 201e207.