Disentangling Red Emission and Compensatory Defects in Sr[LiAl3N4

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Cite This: Chem. Mater. 2018, 30, 4493−4497

Disentangling Red Emission and Compensatory Defects in Sr[LiAl3N4]:Ce3+ Phosphor Julius L. Leaño, Jr.,†,‡,§ Agata Lazarowska,⊥ Sebastian Mahlik,⊥ Marek Grinberg,⊥ Hwo-Shuenn Sheu,¶ and Ru-Shi Liu*,†,∥ †

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Nanoscience and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Taiwan University, Taipei 106, Taiwan § Philippine Textile Research Institute, Department of Science and Technology, Taguig City 1631, Philippines ⊥ Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Gdańsk University, Wita Stwosza 57, 80-308 Gdańsk, Poland ¶ National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan ∥ Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan

Chem. Mater. 2018.30:4493-4497. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/05/18. For personal use only.

‡

S Supporting Information *

T

Nitridolithoaluminate SLA:Ce was prepared using all-nitride starting materials in a gas pressure sintering (GPS) furnace. The synthesized powder is cream-colored in appearance and looks yellow under the blue light (Figure S1, S2). The powder X-ray diffraction (XRD) patterns in Figure S3 reveal that the wide range of Ce3+ concentration remains to be in good correlation with CSD-427067. The Rietveld refinement (Figure S4) of the synchrotron powder XRD shows that the sample has a triclinic crystal structure that belongs to the P1̅ space group.1 The a = 5.85241(3) Å, b = 7.48525(4) Å, and c = 9.92537(5) Å parameters (Table S1) are comparable to the SLA host prepared using radiofrequency1 and solid-state2,3 approaches, with Eu as the dopant. The crystal structure of SLA possesses a rigid tetrahedral framework. It is characterized by edge-sharing and cornersharing Al and Li tetrahedra, with a degree of coordination, κ = 1.1 The ordered arrangement of Li and Al creates channels of two Sr crystallographic sites, Sr(1) and Sr(2) that are alternately stacked in pairs (Figure 1) forming a cuboid coordination sites with eight nitrogen atoms. The Sr(1) and Sr(2) sites are different with respect to the number and the arrangement of edge-sharing Li vs Al tetrahedra (Figure 1). Here, Sr(1) has more Li tetrahedra (six) in the pair of stacked cuboid sites, whereas in Sr(2), Li tetrahedra (four) is less. These Ce3+/Eu2+ ions consequently occupy the Sr sites and become the emitting centers, while the adjacent channels remain empty. The emission from two sites, considering their very similar chemical environments and characteristics, are almost superimposable, thereby having narrowband emission characteristic of SLA:Eu2+.1−3 These structural and electronic features of the SLA:Eu2+ exhibit superior emission intensity, quantum efficiency, and thermal stability.1−3

he cuboid-coordinated Sr[LiAl3N4] (SLA) has two Sr crystallographic sites. The structural similarities between these two sites lead to indistinguishable emission, or if occupancy is preferential, it has not been unequivocally established.1−3 The characteristic emission profile of Ce3+ consists of two emission bands attributed to the presence of spin−orbit coupled ground states 2F5/2 and 2F7/2 that are overlapped, thereby resulting in a broad composite emission band. In cases with more than one doping site, as in SLA, two Sr sites that preferentially accommodate the dopant in one or in both exist, and the two inherent relaxation energies of Ce3+ would redound to an even broader emission band.4−9 Should the longer wavelength emission peak reach approximately 620 nm, it would satisfy the red component for wLEDs. The red phosphor in white LED improves the CRI, but extremely redshifted emissions would result in serious IR spillover, thereby compromising the energy efficiency.10 Thus, two options may be undertaken to circumvent the problem of the excessively red-shifted emission of the SLA doped with Eu2+. It is possible to replace the Li and Al in the framework, and/or change the cation (activator) due to the substitutional variability of cuboid compounds. These approaches could still conserve the superior thermal stability, cuboid coordination, and ordered framework that redounds to higher rigidity. Thus, the substitution of the cation Sr(Eu) would be an appropriate starting point to obtain an LED emission with lesser IR spillover.11,12 However, the emission of SLA is shifted even more in the red region (∼670 nm) with Ca instead of Sr,13 and thus runs contrary to this goal. Thus, substitution of the activator ion, Eu2+ is an interesting possibility, and the interesting substitute is Ce3+. Here, we prepared and investigated the Ce3+-doped Sr[LiAl3N4], (SLA:Ce) prepared from all-nitride precursors via solid state reaction. The photoluminescence measurements shed light on the Ce occupancy in the host lattice while gaining further insight into the thermal and photoluminescence properties of this new phosphor. © 2018 American Chemical Society

Received: April 18, 2018 Revised: June 26, 2018 Published: June 27, 2018 4493

DOI: 10.1021/acs.chemmater.8b01561 Chem. Mater. 2018, 30, 4493−4497

Communication

Chemistry of Materials

However, these red-spanning emissions can be resolved and separately observed. Figure 2c presents the excitation spectra monitoring luminescence at 550 and 680 nm, which correspond to the Ce(1) and Ce(2) emission centers, respectively. When the spectrum excited at 442 nm increases significantly (615 nm emission band in comparison to 550 nm emission band), the same effect, although not as evident, is observed for the emission spectra excited at 570 nm. The preferred Ce3+ occupation between two sites changes with the concentration, and Sr(2) is more likely to be occupied with a higher loading of Ce. Thus, long wavelength part of the emission band increases with increased Ce3+ concentration. Diffuse reflectance spectra in Figure S6 reveal stronger absorption for sample x = 0.1 (blue curve) in comparison to the sample x = 0.01 (red curve) due to the increase in the concentration of Ce ions. Slight difference in the spectra especially in the ∼600 nm region of sample x = 0.1 supports our idea about increasing in the occupancy of Sr(2) site being stronger than Sr(1) site. Two-peaked emission from two crystallographic sites on Eu-doped systems has been possible but has not been done and successfully probed with SLA host.14−16 Across the different amounts of Ce3+(x) in the phosphors, the excitation spectra change with the luminescence wavelength at which it is monitored. The Ce3+ occupies two different emission centers, which is consistent with the presence of two Sr crystallographic sites. In Figure 2c, at 442 nm excitation, the broad emission band with two peaks that correspond to Ce3+: 5d → 4f (2F7/2) and 5d → 4f (2F5/2) are centered at 550 and 615 nm, respectively. A 570 nm excitation generates a broad emission with maxima at 620 and 680 nm. Ce(1) is more efficiently excited with 442 nm and is assigned to the more blue-shifted band, while Ce(2) is more efficiently excited with 570 nm and refers to the more red-shifted emission band (Figure 2c). The full excitation spectra show high-energy transitions in the UV range (290 nm, Figure S4). The quantum efficiency is ∼9%. The time-resolved luminescence spectra of x = 0.01 and x = 0.1 under a 460 nm pulse excitation (Figure 3a,c, respectively) were obtained in the 0−20 ns (red curves) and 40−80 ns (green curves) time scales. Different excitation wavelengths generate different emission spectra (Figure S7), where lowtemperature (10 K) PL data resolve the origin of multipleresolved emission peaks. The emission spectra change over time reckoned after the laser pulse, wherein the intensity of the short-wavelength part of the spectrum decreases in comparison to the long-wavelength region. This was the same strategy used to probe the two-peaked emission of Eu-doped phosphors.17,18 This behavior is more obvious in high Ce3+ concentration (x = 0.1, Figure 3c vs x = 0.01, Figure 3a), which means that the Ce(1) emission intensity decreases faster over time than the Ce(2) emission intensity. Thus, Ce(1) luminescence decays faster than the luminescence from the Ce(2). The decay curves of Sr1−x[LiAl3N4]:Cex luminescence with x = 0.01 observed at 545 and 700 nm, which correspond to Ce(1) and Ce(2) luminescence, are shown in Figure 3c,d, respectively. Reference to the emission of Ce(1) and Ce(2) as 545 and 700 nm avoids confusion of referring to the latter as 620 nm due to the overlap with the 615 nm emission of Ce(1). The decay curves are single exponential with decay times equal to τ1 = 39 ns and τ2 = 58 ns for Ce(1) and Ce(2), respectively. At a higher concentration of Ce3+ (x = 0.1), these are slightly shortened due to concentration quenching, and the obtained

Figure 1. Crystal structure showing the two Sr-sites, where Ce can be substituted into, thereby becoming Ce(1) and Ce(2) sites. Blue (Al) and fuchsia (Li) tetrahedra; green cuboid site (Sr1, left) and the green spheres (Sr2, right) as viewed along the a direction.

The photoluminescence excitation and emission spectra of Sr1−x[LiAl3N4]:Cex (x = 0.04) upon excitation at 470 nm has a broad two-peaked emission band at ∼560 and ∼620 nm (Figure 2a). The PL excitation profile observed at ∼615 nm

Figure 2. Spectroscopic data of Sr[LiAl3N4]:Ce3+. (a) Photoluminescence excitation (PLE) and photoluminescence (PL) of SLA:Ce; (b) emission from the two Ce-sites; (c) PLE and PL profiles of Sr1−x[LiAl3N4]:Cex3+ at various Ce concentrations (x = 0.01−0.1) under two different excitations and monitored at different wavelengths.

shows that the phosphor can be effectively excited by green light while a possible host excitation (band to band excitation) band at ∼280 nm is also noted (Figure S5). The excitation in the blue to green regions corresponds to the allowed transition from the 2F5/2 to the d states (5d1 electronic manifold). After vibrational relaxation, the emission from the lowest 5d1 state to the ground 4f1 electronic configuration split by the spin−orbit interaction into the 2F7/2 and 2F5/2 states, thus the two-peaked composite broad emission band (Figure 2b). The emission band from the Ce occupying the Sr(1) site, which is referred to as Ce(1), and the Ce occupying Sr(2), which is referred to as Ce(2), are strongly overlapping. 4494

DOI: 10.1021/acs.chemmater.8b01561 Chem. Mater. 2018, 30, 4493−4497

Communication

Chemistry of Materials

The equations do not depend on the lattice, and are almost equal to the difference of the free ions, whereas the difference between the energies of Ln2+ and Ln3+ does not depend on n, but can be different in different host lattices. ΔE(Ln 2 +, Ln3 +) = E(Ln 2 +, 4f n + 1 ) − E(Ln 3 +, 4f n )

(3)

2+

In Figure 4, the ground state of Eu in SLA is located approximately 17 400 cm−1 below the conduction band (CB),

Figure 3. Room temperature time-resolved luminescence spectra (a) x = 0.01, and (b) x = 0.1 obtained under 460 nm excitation. Luminescence decay curves of (c) x = 0.01, and (d) x = 0.1 of Ce(1) (obs. at 545 nm) and Ce(2) (obs. at 700 nm) centers, of Sr1−x[LiAl3N4]:Cex samples with different Ce3+ concentration. Figure 4. Energies of Ln3+ and Ln2+ with respect to the conduction band (CB) and valence band (VB) edges of SLA.

decay times are τ1 = 37 and τ2 = 54 ns for Ce(1) and Ce(2), respectively. At x = 0.1, the luminescence observed at 545 and 700 nm (corresponding to the Ce(1) and Ce(2) luminescence, respectively) are measured at different temperatures, ranging from 10 to 500 K, as shown in Figure S8, and summarized in Figure S9. For both Ce centers, the decay curves are single (or almost single exponential), and the temporal profiles do not change significantly as the temperature increases. In the case of Ce(2), the decay times do not change with temperature, while the decay times increase slightly for Ce(1). This increase can be attributed to the temperature-dependent broadening of the entire spectrum, resulting from the stronger overlapping of Ce(1) and Ce(2) emissions at high temperatures. The luminescence of the Ce(1) and Ce(2) centers are remarkably stable in the 10−500 K temperature range, and no significant luminescence quenching is observed. Figure S10 also shows the bulk temperature-dependent PL measurement. XANES Ce-L3 edge probed the valence states of Ce in the sample. Figure S11 shows the mixed valence states where Ce3+ and Ce4+ coexist but were not described earlier.19 The presence of Ce4+ can be stabilized by the disparity in the charges. Independently, charge compensation acceptor defects become indispensable, as Ce exists as Ce3+and Ce4+. In the same SLA host, Sr vacancy compensates the Eu3+ in the Sr2+ site.3 However, in a more general sense, Li vacancy and/or Li+ in the Sr2+ sites generate compensation defects (CD) in SLA. The same types of defects are necessary to compensate Ce3+ in Sr sites. To explain the existence of Ce4+ in the Sr2+ site, we consider the model proposed by Dorenbos. This presents the systematic position of the energy levels of Ln2+ and Ln3+ ions with respect to the band structure.20−23 This model plots the difference between the energies of the ground states of Ln2+ and Ln3+ in terms of the number of f electrons, n, as shown in eqs 1 and 2. ΔE(Ln 3 +, n , n + 1) = E(Ln 3 +, 4f n + 1 ) − E(Ln 3 +, 4f n )

(1)

ΔE(Ln 2 +, n , n + 1) = E(Ln 2 +, 4f n + 1 ) − E(Ln 2 +, 4f n )

(2)

and the ground state of Ce3+ should be located approximately 23 000 cm−1 below the conduction band. Thus, one can obtain ΔE(Ln2+, Ln3+) that is approximately equal to 42 700 cm−1. Here, the energy of the ground state of Ce3+ is lower than the energy of the ground state of Eu2+ by ∼5600 cm−1. To consider the influence of possible compensatory defects (CD) in SLA, we used the model proposed by Baran et al.24,25 In this model, Ln ions occupy the divalent metal sites and intrinsic acceptor states are created, that in turn captures the electrons from aliovalent Ln3+. Two different defects: a shallow compensatory defect, CD1 and a deep CD2 can considered and herein we propose the nature of the CD1 and CD2 states, where the former can be considered a Li vacancy or Li in a Sr site, and the latter can be Sr vacancies. The CD1 can compensate the Eu3+ or (Ce3+) in the Sr2+ site but they are not deep enough to compensate for Ce4+ in the Sr2+ site. The Sr vacancy creates the deeper CD2 site that can capture two electrons that can compensate two Eu3+ (Ce3+) in the Sr2+ site as well as a single Ce4+ in Sr2+ site (Figure 4). The large number of Ce4+ depends on the relative location of CD2 defect energy and Ce3+ energy with the band gap. It is the case when the energy of CD2 (Sr vacancy) is lower than the energy of the Ce3+ the CD2 can capture two electrons from Ce leaving the Ce3+ level empty. In such a case, the Ce4+ is the highest level occupied by an electron in Ce ion. Inductively coupled plasmaatomic emission spectroscopy (ICP-AES) proves the presence of this Sr and Li vacancy (Table S2). The use of SLA:Ce for white LED application was demonstrated using two fabrication strategies: sequential coating (Figure 5a) and direct phosphor mixture (Figure 5b). The green light-excitable property of SLA:Ce reasonably predisposes sequential coating, where the same green lightemitting, β-SiAlON was also used to generate white light in Sr[Mg2Al2N4]:Ce system.10 The introduction of SLA:Ce extends the spectral output of the blue LED (InGaN) + βSiAlON device toward the yellow and red regions (Figure 4495

DOI: 10.1021/acs.chemmater.8b01561 Chem. Mater. 2018, 30, 4493−4497

Communication

Chemistry of Materials

002-012-MY3, MOST 104-2119-M-002-027-MY3, MOST 104-2923-M-002-007-MY3) and National Center for Research and Development Poland Grant (No. PL-TW/V/1/2018), and financial support from the Epistar Corporation.

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(1) Pust, P.; Weiler, V.; Hecht, C.; Tucks, A.; Wochnik, A. S.; Henss, A. K.; Wiechert, D.; Scheu, C.; Schmidt, P. J.; Schnick, W. NarrowBand Red-Emitting Sr[LiAl3N4]:Eu2+ as a Next-Generation LedPhosphor Material. Nat. Mater. 2014, 13, 891−896. (2) Zhang, X.; Tsai, Y. T.; Wu, S.; Lin, Y.; Lee, J. F.; Sheu, H. S.; Cheng, B. M.; Liu, R. S. Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor for High Color Rendering Index White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2016, 8, 19612− 19617. (3) Tsai, Y. T.; Nguyen, H.; Lazarowska, A.; Mahlik, S.; Grinberg, M.; Liu, R. S. Improvement of the Water Resistance of a NarrowBand Red-Emitting SrLiAl3N4:Eu2+ Phosphor Synthesized under High Isostatic Pressure Through Coating with an Organosilica Layer. Angew. Chem. 2016, 128, 9804−9808. (4) Schmiechen, S.; Schneider, H.; Wagatha, P.; Hecht, C.; Schmidt, P. J.; Schnick, W. Toward New Phosphors for Application in Illumination-Grade White pc-LEDs: The Nitridomagnesosilicates Ca[Mg3SiN4]:Ce3+, Sr[Mg3SiN4]:Eu2+, and Eu[Mg3SiN4]. Chem. Mater. 2014, 26, 2712−2719. (5) Strobel, P.; Schmiechen, S.; Siegert, M.; Tücks, A.; Schmidt, P. J.; Schnick, W. Narrow-Band Green Emitting Nitridolithoalumosilicate Ba[Li2(Al2Si2)N6]:Eu2+ with Framework Topology whj for LED/ LCD-Backlighting Applications. Chem. Mater. 2015, 27, 6109−6115. (6) Li, Y. Q.; Hirosaki, N.; Xie, R. J.; Takeda, T.; Mitomo, M. Yellow-Orange-Emitting CaAlSiN3:Ce3+ Phosphor: Structure, Photoluminescence, and Application in White LEDs. Chem. Mater. 2008, 20, 6704−6714. (7) Chen, J.; Zhao, Y.; Li, G.; Mao, Z.; Wang, D.; Bie, L. Facile Synthesis of Yellow-Emitting CaAlSIN3:Ce3+ Phosphors and the Enhancement of Red-Component by Co-Doping Eu2+ Ions. Solid State Commun. 2017, 255 (Supplement C), 1−4. (8) Suehiro, T.; Hirosaki, N.; Xie, R. J.; Sato, T. Blue-Emitting LaSi3N5:Ce3+ Fine Powder Phosphor for UV-Converting White LightEmitting Diodes. Appl. Phys. Lett. 2009, 95, 051903. (9) Cai, L. Y.; Wei, X. D.; Li, H.; Liu, Q. L. Synthesis, Structure, and Luminescence of LaSi3N5:Ce3+ Phosphor. J. Lumin. 2009, 129, 165− 168. (10) Leaño, J. L.; Lin, S. Y.; Lazarowska, A.; Mahlik, S.; Grinberg, M.; Liang, C.; Zhou, W.; Molokeev, M. S.; Atuchin, V. V.; Tsai, Y. T.; Lin, C. C.; Sheu, H. S.; Liu, R. S. Green Light-Excitable Ce-Doped Nitridomagnesoaluminate Sr[Mg2Al2N4] Phosphor for White LightEmitting Diodes. Chem. Mater. 2016, 28, 6822−6825. (11) Narrow Red Phosphor; White Paper 20161202; Lumileds, 2016; pp 1−5. (12) Leaño, J. L.; Fang, M. H.; Liu, R. S. Critical ReviewNarrowBand Emission of Nitride Phosphors for Light-Emitting Diodes: Perspectives and Opportunities. ECS J. Solid State Sci. Technol. 2018, 7, R3111−R3133. (13) Pust, P.; Wochnik, A. S.; Baumann, E.; Schmidt, P. J.; Wiechert, D.; Scheu, C.; Schnick, W. Ca[LiAl3N4]:Eu2+A Narrow-Band RedEmitting Nitridolithoaluminate. Chem. Mater. 2014, 26, 3544−3549. (14) Sohn, K.-S.; Lee, S.; Xie, R.-J.; Hirosaki, N. Time-resolved Photoluminescence Analysis of Two-peaked Emission Behavior. Appl. Phys. Lett. 2009, 95, 121903. (15) Park, W. B.; Kim, H.; Park, H.; Yoon, C.; Sohn, K.-S. The Composite Structure and Two-peaked Emission Behavior of Ca1.5Ba0.5Si5O3N6:Eu2+. Inorg. Chem. 2016, 55, 2534−2543. (16) Sohn, K.-S.; Lee, B.; Xie, R. J.; Hirosaki, N. Rate-equation Model for Energy Transfer Between Activators at Different Crystallographic Sites in Sr2Si5N8:Eu2+. Opt. Lett. 2009, 34, 3427−3429.

Figure 5. White-LED. (a) Sequential coating; (b) mixed phosphor; (c) white light on the blackbody locus.

S12). Sequential coating and direct phosphor mixture assembly gave color rendering indices (CRI) of 61.1 and 66.1, and correlated color temperatures (CCT) of 5568 and 5077 K, respectively. Figure 5c shows the color coordinates in the CIE diagram and the actual images of the white lights generated via sequential coating (0.3309, 0.3518) and phosphor mixture approach (0.3445, 3705). In summary, a thermally stable SLA:Ce is successfully prepared by gas pressure sintering (1100 °C, 4 h, 0.9 MPa) and can be effectively excited by green light with a broadband emission that spans the yellow to red regions. At the optimum Ce-loading, the preferential occupancy of Ce3+ in the Sr(1) site reveals the resulting composite emission peaks at 560 and 615 nm. The peak in the red side increases with the amount of Ce, indicating that at higher Ce3+, it starts to occupy the Sr(2) site. The thermal-dependent decay of luminescence reveals single (and almost single) exponential decays even at high temperature (10−500 K). The mixed valences of Ce, as revealed by the Ce-L3 edge XANES in which the presence of some CDs stabilize the Ce species. The SLA:Ce can generate white light through a two-phosphor LED assembly via sequential phosphor coating and direct phosphor mixture coating approaches.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01561. Experimental details; crystallographic data; crystal structure; SXRD; photoluminescence and synthesis and analyses strategies and details (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*R. S. Liu. E-mail: [email protected]. ORCID

Ru-Shi Liu: 0000-0002-1291-9052 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (Contract Nos. MOST 104-2113-M4496

DOI: 10.1021/acs.chemmater.8b01561 Chem. Mater. 2018, 30, 4493−4497

Communication

Chemistry of Materials (17) Jung, Y. W.; Lee, B.; Singh, S. P.; Sohn, K.-S. Particleoptimi1zation-assisted Rate Equation Modelling of the Two-peaked Emission Behavior of Non-Stoichiometric CaAlxSi(7−3x)/4N3:Eu2+ Phosphors. Opt. Express 2010, 18, 17805−17818. (18) Park, W. B.; Song, Y.; Pyo, M.; Sohn, K.-S. Nonradiative Energy T r a n s f e r B e t w e e n T w o D i f f e r e n t A c t i v at o r S i t e s i n La4‑xCaxSi12O3+xN18‑x:Eu2+. Opt. Lett. 2013, 38, 1739−1741. (19) Cui, D.; Song, Z.; Xia, Z.; Liu, Q. Synthesis, Structure and Luminescence of SrLiAl3N4:Ce3+ Phosphor. J. Lumin. 2018, 199, 271−288. (20) Dorenbos, P. Systematic Behavior in Trivalent Lanthanide Charge Transfer Energies. J. Phys.: Condens. Matter 2003, 15, 8417− 8434. (21) Dorenbos, P.; Krumpel, A. H.; van der Kolk, E.; Boutinaud, P.; Bettinelli, M.; Cavalli, E. Lanthanide Level Location in Transition Metal Complex Compounds. Opt. Mater. 2010, 32, 1681−1685. (22) Dorenbos, P. Modeling the Chemical Shift of Lanthanide 4f Electron Binding Energies. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 165107−9. (23) Dorenbos, P. The Eu3+ Charge Transfer Energy and the Relation with the Band Gap of Compounds. J. Lumin. 2005, 111, 89− 104. (24) Baran, A.; Barzowska, J.; Grinberg, M.; Mahlik, B.; Szczodrowski, K.; Zorenko, J. Binding Energies of Eu2+ and Eu3+ Ions in β-Ca2SiO4 Doped with Europium. Opt. Mater. 2013, 35, 2107−2114. (25) Baran, A.; Mahlik, S.; Grinberg, M.; Cai, P.; Kim, S. I.; Seo, H. J. Luminescence Properties of Different Eu sites in LiMgPO4:Eu2+, Eu3+. J. Phys.: Condens. Matter 2014, 26, No. 385401.

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DOI: 10.1021/acs.chemmater.8b01561 Chem. Mater. 2018, 30, 4493−4497