Luminescence properties of M
2TiO
4:Eu
3+, Li
+(M:Mg, Ca)
and MgAl
2O
4:RE
3+(RE
3+:Ho
3+, Sm
3+, and Yb
3+)
Esra O¨ ztu¨rk•Erkul Karacaoglu
Received: 29 August 2014 / Accepted: 28 October 2014 / Published online: 4 December 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract In this study, we aimed to prepare perovskite-related and Eu3?-activated Mg2TiO4, Ca2TiO4 and (Mg, Ca)2TiO4 doped with Eu
3?
, Li?, and spinel-oxide-type MgAl2O4 doped with Ho3?, Sm3?, and Yb3? through a solid-state reaction method under open atmosphere. The thermal behaviors of the samples were characterized by DTA/TG. The phase properties were characterized by X-ray diffraction and the effects of rare-earth ions (Eu3?, Ho3?, Sm3?, and Yb3?) on the luminescence properties of the hosts were investigated using a photoluminescence spectrometer. The morphology and elemental analysis of each sample were determined by SEM/EDX.
Keywords Spinel structure Solid-state reaction method Open atmosphere Luminescence Rare-earth ions
Introduction
Phosphor materials are substances that generally exhibit the phenomenon of luminescence. Therefore, these mate-rials include both phosphorescent matemate-rials, which show a slow decay in brightness ([1 ms) and longer luminescence property, and fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphor materials are widely applied in lighting, displays, lasers, and scintillators, and additionally it is necessary that the phosphor host structure should exhibit good optical, mechanical, and thermal properties [1,2].
Recently, there has been a rise of interest in the study of the spinel oxide AB2O4due to its variety of physical and chem-ical properties and advanced technologchem-ical applications. Spinel oxides are one of the large families that include a number of compounds and variety of known physical prop-erties such as ferromagnetism, metal–insulator transition, charge ordering, superconductivity, and in luminescence [3]. Magnesium aluminate (MgAl2O4), corresponding to the mineral spinel, is known to be an optically inert host and because of its chemical and thermal stability, it is attractive as a host for such rare-earth ions. Rare-earth ions, especially in the trivalent charge state where luminescence is generally due to f–f electron transitions, are often used as activators or luminescence centers for phosphor materials [4].
This paper presents the luminescent properties of the perovskite-related Mg2TiO4, Ca2TiO4, and (Mg,Ca)2TiO4 structures and spinel-oxide-type MgAl2O4 activated with different types of rare earths prepared by solid-state reac-tion under open atmosphere.
Materials and methods
In the present study, the high temperature solid-state reaction method, also known as the ceramic method, was used for the production of samples. According to the nominal compositions of [Mg2TiO4, Ca2TiO4, and (Mg,Ca)2TiO4]:Eu2O3, Li2CO3 (amount of %1 mol) and MgAl2O4:RE (amount of % 0.5 mol) (RE = Ho2O3, Sm2O3or Yb2O3), appropriate amounts of the high purity starting reagents, such as 4MgCO3Mg(OH)25H2O (A.R.), CaCO3(99.9 %), Li2CO3(99.9 %), Al2O3(99.0 %), Eu2O3 (99.99 %), Ho2O3(99.99 %), Sm2O3(99.99 %), and Yb2O3 (99.99 %) were thoroughly mixed and ground in a ball mill (Retsch PM 100) at 250 rpm for 1 h in propanol-2 which E. O¨ ztu¨rk (&) E. Karacaoglu
Department of Materials Science and Engineering, Engineering Faculty, Karamanog˘lu Mehmetbey University, Karaman, Turkey e-mail: [email protected]
was added to facilitate the grinding process and to enhance the particle mixing. Subsequently, the mixed batch forms were dried via magnetic stirrer. Then the powders were sintered in pure alumina crucibles using a tubular furnace (Protherm PTF 16/50/450) between 1,100 and 1,350°C for 1–4 h under open atmosphere for each sample; the samples were then cooled down slowly to the room temperature. The synthesized phosphors were ground to powder form prior to the characterization.
After the weighing and milling processes, simultaneous differential thermal analysis (DTA) and thermogravimetric (TG) analysis (Seiko Instruments Inc./Exstar TG/DTA 6200) at a heating rate of 10°C/min in an inert argon atmosphere from room temperature to 1,000°C were employed to analyze the decomposition and the oxidation process of the precursor. Then the heat treatments were applied, and a BRUKER AXS D8 ADVANCE model X-ray diffractometer, which was run at 40 kV and 30 mA (Cu–Ka radiation) in a step-scan mode (0.02°/2h), was used to determine the phases after sintering. The excitation and emission spectra of the synthesized phosphors were obtained by a fluorometer (Photon Technology Interna-tional (PTI), QuantaMasterTM30). Finally, the morphology and particle size distributions of the sintered, dry-milled, and sieved powders were designated by a LEO 440 scan-ning electron microscope (SEM).
Results and discussion Thermal analysis
In order to examine the thermal behavior of Mg2TiO4, Ca2TiO4, and (Mg,Ca)2TiO4 and MgAl2O4 composed of
TG analysis were carried out between 50 and 1,000°C (Figs. 1–3).
As shown by Fig.1a, the former precursor has the dehydration of the 4MgCO3Mg(OH)25H2O ingredient. The following dramatic mass losses starting from 200 to 325 °C (A0) and a much bigger decrease from 325 to 550 °C (B0) are related to the decomposition of 4MgCO
3-Mg(OH)25H2O and Li2CO3 in the crystal system. The thermal behavior of the starting raw materials reactions under heating can be summarized as follows:
4MgCO3 Mg OHð Þ25H2O ! 4MgCO3
Mg OHð Þ2þ5H2O; ð1Þ 4MgCO3 Mg OHð Þ2! 4MgCO3þ MgO þ H2O; ð2Þ
4MgCO3! 4MgO þ 4CO2; ð3Þ
200 60.0 65.0 70.0 75.0 80.0 85.0 90.0 95.0 100.0 70.0 75.0 80.0 85.0 90.0 95.0 100.0 TG/% DTA/μV TG/% DTA/μV 400 600 Temperature/°C 800 1000 0.00 –20.00 –15.00 –10.00 –5.00 0.00 5.00 Endo Exo 10.00 200 400 600 Temperature/°C 800 1000 Endo Exo A A B B C A' A' B' (a) (b)
Fig. 1 DTA/TG curves of a Mg2TiO4:Eu3?, Li?and b Ca2TiO4:Eu3?, Li?
200 65.0 70.0 75.0 80.0 85.0 90.0 95.0 100.0TG/% A' B' C' B A C D 400 600 Temperature/°C 800 1000 Endo Exo –4.00 –2.00 0.00 2.00 4.00 6.00 8.00 10.00 DTA/μV
The first endothermic peak (at 261°C) is attributed to the deviation of the hydroxyl group from Mg(OH)2. The second endothermic peak (at 437°C) shows the decom-position of MgCO3which changes into MgO. Li2CO3was used as charge compensator in the system, so the amount of Li2CO3 in the system is relatively small and its decom-position rate is about 0.16 %.
The endothermic peak at 811°C is believed to be the formation temperature for the Mg2TiO4 for the solid-state-assisted process. The TG curve exhibits a total mass loss equal to 35 % which is almost similar to the calcu-lated mass loss (*35.6 %) attributed to the complete decomposition process of 4MgCO3Mg(OH)25H2O and Li2CO3.
Figure1 (b) shows the DTA/TG curves for Ca2TiO4 :-Eu3?, Li?. The curve between 610 and 830°C includes the decomposition of CaCO3 and Li2CO3. The endothermic peak (at 795°C, point A’) is attributed to the decomposi-tion of CO2 from CaCO3 and Li2CO3 to CaO and Li2O. Li2CO3was used as charge compensator in the system, so
the amount of Li2CO3in the system is relatively small and its decomposition is about 0.16 %.
The small endothermic peak at 865°C is believed to be the crystallization temperature for the formation of Ca 2-TiO4for the solid-state-assisted process, but XRD studies show that it is not a phase formation peak.
Figure2 shows that the dehydration of 4MgCO 3-Mg(OH)25H2O and decomposition of CaCO3 in the sys-tem, which are similar to the DTA/TG curves of Mg2TiO4:Eu3?, Li?and Ca2TiO4:Eu3?, Li?naturally.
The DTA/TG curves of the MgAl2O4system phosphors which were activated by Ho3?, Sm3?, and Yb3?, respectively, are almost similar to themselves because the main crystal and raw materials are the same. Additionally, when the MgAl2O4 system phosphors include 4MgCO3Mg(OH)25H2O, see Fig.3a, b, and c, it is seen that the dehydration of 4MgCO 3-Mg(OH)25H2O in the system is similar to the DTA/TG curves of Mg2TiO4:Eu3?, Li?. Also the endothermic peak is at 811°C which is believed to be the crystallization temperature for the formation of MgAl2O4for the solid-state-assisted process. 70.0 75.0 80.0 85.0 90.0 95.0 100.0 70.0 75.0 80.0 85.0 90.0 95.0 100.0 75.0 80.0 85.0 90.0 95.0 100.0 200 400 600 Temperature/°C 800 1000 200 400 600 Temperature/°C 800 1000 200 400 600 Temperature/°C 800 1000 0.00 5.00 10.00 15.00 20.00 0.00 5.00 10.00 15.00 20.00 0.00 5.00 10.00 15.00 25.00 20.00 TG/% TG/% TG/% DTA/μV DTA/μV DTA/μV Endo Exo Endo Exo Endo Exo A' A' A' B' B' B' A B C D A B A B (a) (b) (c)
X-ray diffraction (XRD) analysis
After the thermal analysis, the heat treatment temperatures for each sample were determined according to the DTA/TG results. The first crystal formation temperatures were more than 800°C, so the sintering temperatures were applied as a pre-sintering stage at 800°C for 2 h and the main
sin-1,200°C for 1.5 h, and 1,350 °C for 1.5 h for Mg2TiO4, Ca2TiO4, and (Ca,Mg)2TiO4type phosphors, respectively. The sintering process 1,100°C for 4 h was applied for MgAl2O4system phosphors. After the sintering processes, XRD analysis was conducted.
Figure4a shows the XRD pattern of the Mg2TiO4:Eu3?, Li?sample which resembles Ca2TiO4:Eu3?,Li? (Fig.4b) 10 15 20 25 30 35 40 45 2θ/° 50 55 60 65 70 75 80 85 90 10 15 20 25 30 35 40 45 2θ/° 50 55 60 65 70 75 80 85 90 10 15 20 25 30 35 40 45 2θ/° 50 55 60 65 70 75 80 85 90 10 15 20 25 30 35 40 45 2θ/° 50 55 60 65 70 75 80 85 90 10 0 0 25 50 75 100 125 150 175 0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 10 20 30 40 50 60 Intensity/cps Intensity/cps Intensity/cps Intensity/cps Intensity/cps Intensity/cps 70 80 90 100 110 0 5 10 15 20 25 30 35 40 45 50 55 60 65 0 25 50 75 100 125 150 175 15 20 25 30 35 40 45 2θ/° 50 55 60 65 70 75 80 85 90 10 15 20 25 30 35 40 45 2θ/° 50 55 60 65 70 75 80 85 90 (a) (b) (c) (d) (e) (f)
Fig. 4 XRD patterns of a Mg2TiO4:Eu3?,Li?, b Ca2TiO4:Eu3?, Li?, c (Mg,Ca)2TiO4:Eu3?, Li?, d MgAl2O4:Ho3?, e MgAl2O4:Sm3?, and
XRD pattern of MgAl2O4:Ho3?is given, which is similar to the MgAl2O4:Sm3?(Fig.4e) and MgAl2O4:Yb3? (Fig.4f) XRD results.
In the literature, Mg2TiO4must have an inverse cubic spinel structures which is obtained by very long and higher sintering temperatures [5]. For the Ca2TiO4system phos-phor, the main crystal could not be synthesized even by raising the sintering temperature and lengthening the sin-tering time. Additionally, the Ca2TiO4structure does not exist in literature studies [6]. XRD results indicated that the (Mg, Ca)2TiO4:Eu3?, Li? phosphor did not have any crystal system. Furthermore, this system is unique and has not even been studied in the literature.
The last group of phosphors which were studied have a spinel structure (in other words garnet structure) that is named after the mineral spinel (MgAl2O4) and the general composition is AB2O4[7]. The trivalent charge state rare earths of Ho3?, Sm3?, and Yb3?were added as activator to the MgAl2O4 host in this study, respectively. Despite thermal treatment being applied for this type of phosphor, according to DTA/TG results, this crystal system could not be indexed in XRD analysis.
All of the samples XRD results proved that none of the samples crystal systems could be indexed by XRD because of single phase formation was not achieved by heat applied. Photoluminescence properties
All of the photoluminescence studies gave surprising results in terms of excitations and emissions, although none of the crystal systems in this study could be indexed by XRD. The excitation and emission bands are due to the activated rare earths which are luminescence centers.
Figure 5 shows similar photoluminescence spectra to Mg2TiO4:Eu3?, Li? and Ca2TiO4:Eu3?, Li? phosphor powders upon excitation with 398 nm in UV region (as indicated in Fig.5) and obtained emission with 617 nm wavelength in the red region.
The photoluminescence analysis of these two systems indicates that the max emission band at 617 nm, and the other emission bands are at 593, 653, 698, and 536 nm attributed to 5D
0?7F2, 5D0?7F1, 5D0?7F3, 5D0?7F4, and 5
D0?7F0transitions of the Eu3?ion, respectively [8, 9]. Furthermore, the maximum excitation band at 398 nm, and the other excitation bands at 383 and 362 nm are attributed to 7
F0?5L6,7F0?5L7, and5D0?7F0,1,2transitions of the Eu3?ion, respectively [9,10]. 350 0 4 8 12 16 20 24 Excitation 400 450 500 Wavelength/nm Intensity/a.u. 0 1 2 3 4 Intensity/a.u. 550 600 650 700 350 400 450 500 Wavelength/nm 550 600 650 Excitation Emission Emission 617 nm 593 nm 536 nm 398 nm 383 nm 419 nm 362 nm 653 nm698 nm 398 nm 383 nm 362 nm 521 nm 617 nm 593 nm (a) (b)
Fig. 5 Photoluminescence spectra of a Mg2TiO4:Eu3?, Li?and b Ca2TiO4:Eu3?, Li?
300 0 40 80 120 160 200 240 280 344 nm 422 nm 658 nm 400 500 Wavelength/nm Intensity/a.u. 600 700 800 721 nm
Fig. 6 Photoluminescence spectra of (Ca,Mg)2TiO4:Eu3?, Li?
Figure6 shows the photoluminescence spectra of (Ca,Mg)2TiO4:Eu3?, Li? phosphor powders upon excita-tion with 344 nm wavelength in the UV region and the maximum emission was obtained with 658 nm wavelength in the red region.
The photoluminescence analysis results indicate that the maximum emission band at 658 nm is attributed to the 5D
0?7F4transition of the Eu3?ion [8, 9] which is also one of the emissions of Mg2TiO4:Eu3?, Li?and Ca2TiO4:Eu3?, Li?. The other emission band, at 721 nm is attributed to the 5D
0?7F5 transition of the Eu3? ions. In addition, the excitation bands at 344 and 422 nm are attributed to the 7F
0,1,2 ?5D0transitions of the Eu3?ion [10].
The last three photoluminescence analysis results are related to the same host (MgAl2O4) doped with Ho3?, Sm3?, and Yb3?, respectively (Figs.7–9).
The photoluminescence spectra of MgAl2O4:Ho3? (Fig.7) show that the yellow-green emission observed between 500 and 600 nm is assigned to the transition from 5F
4and5S2states to the5I8ground state under both 360 and 425 nm excitations. Also the red and NIR emissions at 695 and 710 nm resulted from the5F5?5I8(630–680 nm) and (5F5, 5S2) ?5I7 (700–800 nm) transitions, respectively [11], under 425 nm excitation. The 360 and 425 nm exci-tation bands originate from 5I8?3G5, 3H5,6 and 5I
8? (5G,3G)5transitions [12].
It is well known that an incompletely filled 4f shell qualifies the electron configurations of rare-earth ions. The 4f orbital is situated within the ion and is guarded from the surroundings by the filled 5s2and 5p6orbitals. Therefore, the effect of the host lattice on the optical transitions in 4fnconfiguration is very little, but this effect is necessary and then emission transition efficiency is indicated by sharp lines in the spectra. Figure8presents the excitation and emission spectra of the MgAl2O4:Sm3?. The emission of Sm3?(4f5) is positioned in the orange-red spectral region and includes transitions from the4G5/2level to the ground state6H5/2and higher levels6HJ (J [ 5/2) [13]. The 4G5/2–6H5/2 (560 nm), 4G5/2–6H7/2 (617 nm),4G5/2–6H10/2(695 nm),4G5/2–6H5/2(515 nm), and 4
G5/2–6H9/2(650 nm) transitions are typical emission bands of Sm3?[14,15]. Also the excitation bands can be listed as: 270, 353, 365, 422, 485, and 401 nm; therefore, these excitations are assigned to 7F0?5FJ,6H5/2?4H7/2, 6H5/2?4L17/2, 6H
5/2?6P5/2?4P5/2, 6H5/2?4I9/2, and 6H5/2?4L13/2 ?6P3/2?4F7/2transitions, respectively [13–16].
Figure9 shows the excitation and emission bands of MgAl2O4:Yb3?phosphor. According to the PL results, the characteristic emission peaks of Yb3?, which are given in detail in the literature [17] for the ion, could not be determined. Concerning the Yb-activated materials, 200 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Excitation Emission 203 nm 278 nm 360 nm 395 nm 425 nm 544 nm 695 nm 578 nm 619 nm 548 nm 250 300 350 400 450 Wavelength/nm Intensity/a.u. 500 550 600 650 700 750 710 nm
Fig. 7 Photoluminescence spectra of MgAl2O4:Ho3?phosphor
200 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 270 nm 365 nm 422 nm 401 nm 353 nm 311 nm 485 nm 541 nm 560 nm 617 nm 695 nm 515 nm 4.0 4.5 5.0 5.5 6.0 Excitation Emission 250 300 350 400 450 Wavelength/nm Intensity/a.u. 500 550 600 650 700
Fig. 8 Photoluminescence spectra of MgAl2O4:Sm3?phosphor
200 0 1 2 3 4 5 6 280 nm 254 nm 345 nm 544 nm 529 nm 425 nm 420 nm 617 nm 685 nm 694 nm 760 nm 565 nm 542 nm 260 nm 7 8 9 Excitation Emission 250 300 350 400 450 500 550 Wavelength/nm Intensity/a.u. 600 650 700 750 800
various investigations have been performed on the tech-nological potential of these materials because they may generate tunable lasers in the IR region from 900 to 1,060 nm and also in the visible region at about 500 nm emission [17].
Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis
It can be seen from the SEM that the grains of Mg2TiO4 :-Eu3?, Li?, Ca2TiO4:Eu3?, Li?, and (Ca,Mg)2TiO4:Eu3?, 0 0 5 10 2000 4000 6000 8000 Counts Energy/keV Mg Ti O Ca TiEu T C
Fig. 10 SEM/EDX results of Mg2TiO4:Eu3?, Li?phosphor
0 0 2000 4000 6000 8000 10000 Counts Ca Ti Ti O Ca C Ti Eu Ca 5 Energy/keV 10 Fig. 11 SEM/EDX results of
Ca2TiO4:Eu3?, Li?phosphor
0 0 5 10 2000 Mg O Ti Ca Ca C C Ti Ti Eu 4000 6000 Counts Energy/keV Fig. 12 SEM/EDX results of
(Mg,Ca)2TiO4:Eu3?, Li?
phosphor 0 0 5 10 5000 10000 15000 Counts Energy/keV Mg Ho O C
Fig. 13 SEM/EDX results of MgAl2O4:Ho3?phosphor
Li?are more well grained in terms of the shape of particles than MgAl2O4:Ho3?, and MgAl2O4:Sm3?, MgAl2O4:Yb3? system phosphors (Figs.10–15). These results clearly indi-cated that the morphology of particles could differ according to crystal system although all of them were prepared by same thermal treatment process. In addition, although the particle size distributions of Mg2TiO4:Eu3?, Li?, Ca2TiO4:Eu3?, Li?, and (Ca,Mg)2TiO4:Eu3?, Li?are homogenous that of the MgAl2O4system is not. With EDX analysis, it was found that all the phosphor systems comprised major elements and small amounts of rare-earth elements according to their crystal system.
Conclusions
The novel phosphors, Mg2TiO4:Eu 3?
, Li?, Ca2TiO4:Eu 3?
, Li?, (Ca,Mg)2TiO4:Eu3?, Li?, MgAl2O4:Ho3?, MgAl 2-O4:Sm3?, and MgAl2O4:Yb3? were prepared by conven-tional solid-state reaction method under open atmosphere and their photoluminescence properties were investigated. The most important point is that, although the phosphors did not have any crystal system, the photoluminescence bands occurred concerning the activator ions. Briefly, photoluminescence analysis exhibited that all of the dif-ferent activated hosts exhibit emissions due to emission centers which are in the trivalent charge state. Therefore,
3?
Additionally, Ho3?- and Sm3?-activated MgAl2O4 gave green and red region emissions as a result of these rare-earth ions’ transitions. Finally, it is noted that the activator ion emissions in these phosphor systems are independent of the phase forming process.
Acknowledgements The authors would like to thank Karamanoglu Mehmetbey University, Scientific Research Projects Commission (BAP), project number: 48-M-12.
References
1. C¸ ırc¸ır E, Ozpozan Kalaycioglu N. Host-sensitized phosphores-cence of Mn4?, Pr3?, 4?and Nd3?in MgAl2Si2O8. Mat Res Bul.
2012;47(5):1138–41.
2. Karacaoglu E, Karasu B. The effects of re-firing process under oxidizing atmosphere and temperatures on the properties of strontium aluminate phosphors. Mat Res Bul. 2013;48:3702–6. 3. Skvortsova V, Mironova-Ulmane N, Ulmanis U. Neutron
irradi-ation influence on magnesium aluminium spinel inversion. Nucl Instr Meth Phy Res B. 2002;191:256–60.
4. Tan TTY. Rare Earth Nanotechnology. Singapore: Pan Stanford Publishing Pte. Ltd; 2012. p. 208–9.
5. Kumar S, Kumar R, Koo BH, Choi H, Kim DU, Lee CG. Structural and electrical properties of Mg2TiO4. J Ceram Soc Jpn.
2009;117(1365):689–92.
6. Lu Z, Le Z, Xu NC, Wang LX, Zhang QT. Luminescent prop-erties of Eu3? doped layered perovskite structure M2TiO4
(M = Ca, Sr, Ba) red-emitting phosphors. spectrosc Spectr An. 2012;32(10):2632–6. 0 0 5 Sm O Mg C 10 5000 10000 15000 Counts Energy/keV Fig. 14 SEM/EDX results of
MgAl2O4:Sm3?phosphor 0 0 5 Yb O Mg C 10 5000 10000 15000 20000 Counts Energy/keV Fig. 15 SEM/EDX results of
8. Zhou L, Zhang L, Xu NC, Wang LX, Zhang QT. Luminescent Properties of Eu3? doped layered perovskite structure M2TiO4
(M = Ca, Sr, Ba) red-emitting phosphors. Spectrosc and Spectr An. 2012;32(10):2632–6.
9. Lee EY, Kim YJ. Optical excitation and emission spectra of YNb4O:Eu3?. J Korean Electrochem Soc. 2009;12(3):234–8.
10. Loan TT, Ha LH, Long NN. Optical transition of Eu3? in
Mg(Al1-xEux)2O4. VNU J Sci Math-Phys. 2007;23:84–91.
11. Boyer JC, Vetrone F, Capobianco JA, Speghini A, Bettinelli M. Optical transitions and upconversion properties of Ho3? doped
ZnO–TeO2glass. J App Phys. 2003;93:9460.
12. Wang Z, Yin Y, Yuan D. Optical transitions in Ho3?-doped La3Ga5SiO14crystals. J Alloys Compd. 2007;436:364–8.
13. Ðordevic V, Nikolic MG, Antic Zˇ , Mitric M, Dramicanin MD. LaYO3:Sm3?nanocrystalline phosphor: preparation and emission
properties. Acta Phys Pol A. 2011;120:2.
14. Patra A, Reisfeld R, Minti H. Influence of aluminium oxide on intensities of Sm3? and Pr3? spectral transitions in sol–gel glasses. Mater Lett. 1998;37:325–9.
15. Baur F, Katelnikovas A, Petry R, Po¨ttgen R, Ju¨stel T. A Novel Sm3 ? Activated Orange Emitting Molybdate. Germany: Rare Earths Elem Conf; 2013.
16. Rudramadevi BH, Buddhudu S. Spectral and thermal analysis of Sm3? and Dy3?:B
2O3-BaO-LiF/AlF3Glasses. Indian Pure App
Phys. 2008;46:825–32.
17. Del Cacho VD, Kassab LRP, de Oliveira SL, Morimoto NI. Blue cooperative emission in Yb3?-doped GeO2–PbO glasses. Mater
