Luminescence properties of Tb, SmCo-doped in alkali aluminoborate glasses

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Luminescence properties of Tb, SmCo-doped in alkali aluminoborate glasses. The mechanism of this phenomenon is attributed to the f-f intersaction for either Tb3+/Sm3. The ABL:Tb, Sm glasses can produce color emission from green to red by properly tuning the relative ratio between Tb3+ and Sm3+.
VNU Journal of Science: Mathematics Physics, Vol. 35, No. 1 (2019) 21-28
Original article
Luminescence Properties of Tb, SmCo-doped in Alkali
Aluminoborate Glasses
Tran Ngoc*, Phan Thi Hoai Thương, Hoang Sy Tai
Faculty of Natural Sciences, Quang Binh Unversity, 312 Ly Thuong Kiet, Quang Binh, Vietnam
Received 11December 2018
Revised 12March 2019; Accepted 17March 2019
Abstract: Spectroscopic properties of Tb3+ and Sm3+ ions co-doped in alkali aluminoborate
glasses (70-x-y)B2O3.28Li2O.2Al2O3.xSm2O3.yTb2O3 (ABL:Tb, Sm) fabricated by melting method
have been studied. The emission intensity of Tb3+ or Sm3+ in ABL:Tb or ABL:Sm glasses is
influenced by the Tb3+ or Sm3+doping concerning with the 0.75 mol% and 1.0 mol% optimum
concentrations, respectively. The concentration quenching effect of Tb3+ or Sm3+ in ABL:Tb, Sm
glasses is observed. The mechanism of this phenomenon is attributed to the f-f intersaction for
either Tb3+/Sm3. The ABL:Tb, Sm glasses can produce color emission from green to red by
properly tuning the relative ratio between Tb3+ and Sm3+. The emission intensity of Tb3+/Sm3+ in
ABL:Tb, Sm glasses can be enhanced by the energy transfer from Tb3+/Tb3+ and Sm3+/Sm3+. The
results indicate that ABL:Tb, Sm may be a promising double emission for white light emitting
diodes.
Keywords:Sm3+and Tb3+ ions; alkali aluminoborate glass, the concentration quenching mechanism.
1. Introduction
White light-emitting diodes (LEDs) offer many advantages such as long service lifetime, thermal
resistance, and high efficiency [1-3]. Therefore, white LEDs are expected to be a new light source in
the illumination field. Besides that, due to the unique structural and physicochemical properties, alkali
aluminoborate glasses doped with RE ions with the ingredients focus on the advantages: mechanical
stability,
chemical,
thermal,
low
melting
point,
solubility
of
rare
earth
ions
good
and
high
transparency[4-5]. The presence of structurally different borate units in alkali metal borate glasses is
favorable
for
spectroscopic
investigations
of
RE
ions.
These
structural
differences
are
usually
________
Corresponding author.
E-mail address: daotaoqb@gmail.com
https//doi.org/ 10.25073/2588-1124/vnumap.4301
21
22
T. Ngoc et al. / VNU Journal of Science: Mathematics Physics, Vol. 35, No. 1(2019) 21-28
correlated to chemical composition, type of modifiers and conditions during glass preparation. Broad
band emitters are usually used to sensitise the luminescence of rare earth and transition metal ions.
Particularly, the Tb3+and Sm3+ionsshow an efficient narrow band emission due to its ff electric dipole
allowed transitions. In most of the cases, this kind of energy transfer belongs to resonance type. In
recent years, the resonance energy transfer between impurity ions in a solid material has been the
subject of intense research, mainly because of its importance in the development of efficient phosphor
materials for solid state lighting. Among the rare-earth ions, Tb3+ and Sm3+ ions are two of the
important luminescent centers. Therefore, in the present paper, the luminescent characteristics Tb3+
and Sm3+ ions co-doped in ABL glasses are investigated. The glasses can create the color emission
from green to red. They can be effectively excited by near-UV, and are suitable for serving as white
LEDs.
2. Experiments
Alkali aluminoborate glass doped with Sm3+ or/and Tb3+ were prepared by conventional melt
quenching
technique.
The
stoichiometric
ratio
of
ABL
glasses
is
(70-x-y)B2O3-2Al2O3-28Li2O-
xSm2O3-yTb2O3. The metal oxides were weighed accurately in an electronic balance mixed thoroughly
and ground to a fine powder. The batches were then placed in quartz cup and melted in an electrical
furnace in air at 1323 K for 1.5 hours. The melt was quenched to room temperature in air. The glasses
were then annealed at 630 K for 2 hours. The glasses thus obtained were throughout, evenly, no
bubble. The samples were cut, grinded, polished to form cylinder block shape with product size:
thickness d = 0.1 mm, radius r = 6.0 mm (used for the measurement of refractive index n, density,
absorption and fluorescence); crushing and sorting grab particles range in size from 76 to 150 micron
powder products (used for X-ray diffraction). The glass formation was confirmed by powder X-ray
diffraction.
The measurement of the refractive index n is performed on the system Abbe refractometer at a
wavelength of Na lamp, 589 nm with C10H7Br (1 - bromonaphthalin ) used as the liquid in contact.
The measurement of density made by Archimede method, using xylene as immersion liquid form.
Optical absorption spectra were recorded in the wavelength regions 200 nm 2500 nm using Varian
spectrometer system Cary 5E UV-VIS-NIR, with a resolution of 1 nm. Fluorescence spectra were
obtained at room temperature using Flourolog - 3 Model FL3 - 22, resolution of 0.3 nm, excitation
light xenon (Vehicle).
3. Results and discussion
3.1. Spectral properties of ABL:Tb+3 glasses
Figure 1 depicts the excitation and emission spectra of ABL:Tb3+ glasses. The excitation spectrum
exhibits a series of sharp peaks between 250 nm and 400 nm. The sharp bands between 280 nm and
400 nm are assigned transitions within 4f8from 7F6 ground level to 5F5,4, 5H74, 5D1,0, 5L106, 5G62, and
5D24 excitation levels [1]. The emission spectrum of ABL:Tb3+ glasses
excited by 365 nm presents
four peaks at around 487 nm, 541 nm, 584 nm, and 621 nm which originate to the transitions between
the excited 5D4 level and 7FJ (J=6,5,4,3) level of Tb3+ ion, respectively [6]. The strongest emission
peak is located at 541 nm, corresponding to the typical transition 5D47F5of Tb3+ ion.
For ABL:Tb3+ glasses, the values of Tb3+optimum doping content are discussed (the results are
shown in Fig 2), the emission intensity first increases with the increasing ofTb3+doping content, and
4
0.6
T. Ngoc et al. / VNU Journal of Science: Mathematics Physics, Vol. 35, No. 1(2019) 21-28
23
reaches a maximum at 0.75 mol% Tb3+, then decreases. The phenomenon of decrease in emission
intensity after a certain concentration is called concentration quenching, the concentration quenching
mechanism due to concentration may be due to energy emitted from the excitatory state being lost
along the cross-relaxation. This is a restoration mechanism that occurs by transmitting the resonance
energy between two adjacent ions due to the unique energy level structure of these rare earth ions (in
Fig 3) or the non - radiative energy transfer from one Tb3+ to another Tb3+ ion may occur by exchange
interaction, radiation re - absorption or multiple - multiple interaction [7]. The transition of Tb3+ is
allowed while exchange interaction is responsible for the energy for forbidden transitions. It means that the
mechanism of exchange interaction plays main role in the energy transfer between Tb3+ ions in ABL:Tb3+
glasses [7].
1.0x106
160000
0.75 Tb(%mol)
80000
0.5 Tb(%mol)
1.0 Tb(%mol)
5.0x105
0
0.Tb3+ contens 0.8
1.0
0.25 Tb(%mol)
0.0
450
500
550
600
Wavelength (nm)
Fig 1. Emissionand excitation spectra
of Tb3+ doped in ABL glasses
Fig 2. Emission spectra of ABL:xTb3+ with
several different values of Tb3+ content.
Fig 3. The transmitting resonance energy
between two Tb3+ adjacent ions.
Intensity (a.u)
Intensity (a.u)
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Luminescence properties of Tb, SmCo-doped in alkali aluminoborate glasses. The mechanism of this phenomenon is attributed to the f-f intersaction for either Tb3+/Sm3. The ABL:Tb, Sm glasses can produce color emission from green to red by properly tuning the relative ratio between Tb3+ and Sm3+..

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VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1 (2019) 21-28 Original article Luminescence Properties of Tb, SmCo-doped in Alkali Aluminoborate Glasses Tran Ngoc*, Phan Thi Hoai Thương, Hoang Sy Tai Faculty of Natural Sciences, Quang Binh Unversity, 312 Ly Thuong Kiet, Quang Binh, Vietnam Received 11December 2018 Revised 12March 2019; Accepted 17March 2019 Abstract: Spectroscopic properties of Tb3+ and Sm3+ ions co-doped in alkali aluminoborate glasses (70-x-y)B2O3.28Li2O.2Al2O3.xSm2O3.yTb2O3 (ABL:Tb, Sm) fabricated by melting method have been studied. The emission intensity of Tb3+ or Sm3+ in ABL:Tb or ABL:Sm glasses is influenced by the Tb3+ or Sm3+doping concerning with the 0.75 mol% and 1.0 mol% optimum concentrations, respectively. The concentration quenching effect of Tb3+ or Sm3+ in ABL:Tb, Sm glasses is observed. The mechanism of this phenomenon is attributed to the f-f intersaction for either Tb3+/Sm3. The ABL:Tb, Sm glasses can produce color emission from green to red by properly tuning the relative ratio between Tb3+ and Sm3+. The emission intensity of Tb3+/Sm3+ in ABL:Tb, Sm glasses can be enhanced by the energy transfer from Tb3+/Tb3+ and Sm3+/Sm3+. The results indicate that ABL:Tb, Sm may be a promising double emission for white light emitting diodes. Keywords:Sm3+and Tb3+ ions; alkali aluminoborate glass, the concentration quenching mechanism. 1. Introduction∗ White light-emitting diodes (LEDs) offer many advantages such as long service lifetime, thermal resistance, and high efficiency [1-3]. Therefore, white LEDs are expected to be a new light source in the illumination field. Besides that, due to the unique structural and physicochemical properties, alkali aluminoborate glasses doped with RE ions with the ingredients focus on the advantages: mechanical stability, chemical, thermal, low melting point, solubility of rare earth ions good and high transparency[4-5]. The presence of structurally different borate units in alkali metal borate glasses is favorable for spectroscopic investigations of RE ions. These structural differences are usually ________ ∗Corresponding author. E-mail address: daotaoqb@gmail.com https//doi.org/ 10.25073/2588-1124/vnumap.4301 21 22 T. Ngoc et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1(2019) 21-28 correlated to chemical composition, type of modifiers and conditions during glass preparation. Broad band emitters are usually used to sensitise the luminescence of rare earth and transition metal ions. Particularly, the Tb3+and Sm3+ionsshow an efficient narrow band emission due to its f–f electric dipole allowed transitions. In most of the cases, this kind of energy transfer belongs to resonance type. In recent years, the resonance energy transfer between impurity ions in a solid material has been the subject of intense research, mainly because of its importance in the development of efficient phosphor materials for solid state lighting. Among the rare-earth ions, Tb3+ and Sm3+ ions are two of the important luminescent centers. Therefore, in the present paper, the luminescent characteristics Tb3+ and Sm3+ ions co-doped in ABL glasses are investigated. The glasses can create the color emission from green to red. They can be effectively excited by near-UV, and are suitable for serving as white LEDs. 2. Experiments Alkali aluminoborate glass doped with Sm3+ or/and Tb3+ were prepared by conventional melt quenching technique. The stoichiometric ratio of ABL glasses is (70-x-y)B2O3-2Al2O3-28Li2O-xSm2O3-yTb2O3. The metal oxides were weighed accurately in an electronic balance mixed thoroughly and ground to a fine powder. The batches were then placed in quartz cup and melted in an electrical furnace in air at 1323 K for 1.5 hours. The melt was quenched to room temperature in air. The glasses were then annealed at 630 K for 2 hours. The glasses thus obtained were throughout, evenly, no bubble. The samples were cut, grinded, polished to form cylinder block shape with product size: thickness d = 0.1 mm, radius r = 6.0 mm (used for the measurement of refractive index n, density, absorption and fluorescence); crushing and sorting grab particles range in size from 76 to 150 micron powder products (used for X-ray diffraction). The glass formation was confirmed by powder X-ray diffraction. The measurement of the refractive index n is performed on the system Abbe refractometer at a wavelength of Na lamp, 589 nm with C10H7Br (1 - bromonaphthalin ) used as the liquid in contact. The measurement of density made by Archimede method, using xylene as immersion liquid form. Optical absorption spectra were recorded in the wavelength regions 200 nm – 2500 nm using Varian spectrometer system Cary 5E UV-VIS-NIR, with a resolution of 1 nm. Fluorescence spectra were obtained at room temperature using Flourolog - 3 Model FL3 - 22, resolution of 0.3 nm, excitation light xenon (Vehicle). 3. Results and discussion 3.1. Spectral properties of ABL:Tb+3 glasses Figure 1 depicts the excitation and emission spectra of ABL:Tb3+ glasses. The excitation spectrum exhibits a series of sharp peaks between 250 nm and 400 nm. The sharp bands between 280 nm and 400 nm are assigned transitions within 4f8from 7F6 ground level to 5F5,4, 5H7−4, 5D1,0, 5L10−6, 5G6−2, and 5D2−4 excitation levels [1]. The emission spectrum of ABL:Tb3+ glasses excited by 365 nm presents four peaks at around 487 nm, 541 nm, 584 nm, and 621 nm which originate to the transitions between the excited 5D4 level and 7FJ (J=6,5,4,3) level of Tb3+ ion, respectively [6]. The strongest emission peak is located at 541 nm, corresponding to the typical transition 5D4–7F5of Tb3+ ion. For ABL:Tb3+ glasses, the values of Tb3+optimum doping content are discussed (the results are shown in Fig 2), the emission intensity first increases with the increasing ofTb3+doping content, and T. Ngoc et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1(2019) 21-28 23 reaches a maximum at 0.75 mol% Tb3+, then decreases. The phenomenon of decrease in emission intensity after a certain concentration is called concentration quenching, the concentration quenching mechanism due to concentration may be due to energy emitted from the excitatory state being lost along the cross-relaxation. This is a restoration mechanism that occurs by transmitting the resonance energy between two adjacent ions due to the unique energy level structure of these rare earth ions (in Fig 3) or the non - radiative energy transfer from one Tb3+ to another Tb3+ ion may occur by exchange interaction, radiation re - absorption or multiple - multiple interaction [7]. The transition of Tb3+ is allowed while exchange interaction is responsible for the energy for forbidden transitions. It means that the mechanism of exchange interaction plays main role in the energy transfer between Tb3+ ions in ABL:Tb3+ glasses [7]. 1.0x106 5.0x105 160000 80000 0 0.Tb3+ contens 0.8 1.0 0.75 Tb(%mol) 0.5 Tb(%mol) 1.0 Tb(%mol) 0.25 Tb(%mol) 0.0 450 500 550 600 Wavelength (nm) Fig 1. Emissionand excitation spectra of Tb3+ doped in ABL glasses Fig 2. Emission spectra of ABL:xTb3+ with several different values of Tb3+ content. Fig 3. The transmitting resonance energy between two Tb3+ adjacent ions. 24 T. Ngoc et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1(2019) 21-28 3.2.Spectralproperties of ABL: Sm3+ glasses Figure 4 depicts the excitation and emission spectra of ABL:Sm3+ glasses. The excitation spectrum exhibits a series of sharp peaks between 270 nm and 500 nm. The sharp bands between 300 nm and 500 nm are assigned to transitions between the 6H5/2 and 4I11/2, 6P3/2, and 4D3,5,7/2 levels [8].The emission spectrum of BLN: Sm3+ glasses excited by 400 nm presents four peaks at around 562nm, 600nm 646nm and 707nmwhich are originated from the transitions between the excited 4G5/2level and 6H5/2, 6H7/2, 6H9/2 and 6H11/2levels of Sm3+ ion, respectively [8, 9].The data analysed from the emission spectra has shown that three bands having strong intensity and narrow width appear at 562nm, 600nm and 646 nm. While, the wide range with weak strength is revealed at 700nm.On the other hand, the fluorescence spectra have seen that all 4 bands are overlapping each other a maximum of two (types are the same), as explained by the authors in [3,4,9], it is the splitting of the flow allowed by the electric dipole (which is allowed by the electric dipole). excitation of Sm3+ 1.0x107 8.0x106 6.0x106 4.0x106 emission of Sm3+ 4G5/2 6H7/2 6H5/2 6 9/2 3x105 2x105 1x105 BLN:Sm 1.0%mol 0.75%mol 0.5%mol 0.25%mol 2.0%mol 3x10 2x10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Smcontent(%mol) 2.0x106 0.0 300 6H11/2 400 500 600 700 Wavelength (nm) 0 550 Wavelength (nm) 700 750 Fig 4. Emissionand excition spectra of Sm3+doped ABL:Sm glasses Fig 5. Emission spectra of ABL: xSm with several different values of Sm3+ content. For ABL:Sm3+ glasses, the values of Sm3+optimum doping content were discussed (as shown in Fig.5). Firstly, The emission intensity increases with the in the increasing of Sm3+ doping content, and reaches a maximum at 1.0 mol% Sm3+ before decreasing.The decrease of luminescence intensity after a certain concentration is called concentration quenching or self-quenching (SQC). The SQC phenomenon is attributed to the nonradiative transfer processes, which consists multiphonon relaxation and energy transfer between the pairs of Sm3+ ions [9]. In the case of Sm3+ ions, the energy gap between fluorescent level 4G5/2and next lower level 6F11/2 is around 7300 cm-1. This energy gap is 5-6 times higher than the highest phonon energy in borate glass. Thus multiphonon relaxation rate is negligible and concentration quenching may be mainly due to energy transfer. The mainly interaction mechanism between the ions is usually dipole-dipole (DD), when the selected rule is not satisfied, it can happen by the interaction of higher order: dipole-quadrupole (DQ), quadrupole-quadrupole (QQ). The Inokuti and Hirayama model allows us to find the dominant interaction mechanism between the ions. T. Ngoc et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1(2019) 21-28 25 Fig. 6 presents the experimental decay curves obtained for different Sm3+ ion concentrations. By using the Inokuti-Hirayama model: when the migration process is negligible, decay curves can be expressed as: 3/S I = I exp − −Q   0  0   where t is the time after excitation, τ0 is the intrinsic decay time of donor in absence of acceptor. The value of S (=6, 8, 10) depends on whether the dominant mechanism of interaction is dipole– dipole, dipole–quadrupole or quadrupole–quadrupole, respectively. The energy transfer parameter (Q) is found in the fitting process and is calculate by: Q = 3 Γ1− S NR0 Г(x) is the gamma function, its value is equal to 1.77 for dipole–dipole (DD), 1.43 for dipole– quadrupole (DQ) and 1.30for quadrupole–quadrupole (QQ), respectively. N is the concentration of Sm3+ ions, R0 is the critical distance defined as donor–acceptor separation for which the rate of energy transfer to the acceptors is equal to the rate of intrinsic decay of the donor. The fluorescence decay curve of ABL:Sm3+ with 0.25 and 0.5 mol% of Sm3+ ions to have a best fitting with S = 6, where we used τ0 value (~2.62 ms) obtained for the ABL doped with 0.5 mol% of Sm3+ ions. The value of S = 6 indicated that the dominant interaction for energy transfer through cross-relaxation is of dipole-dipole type. The parameter (S) is good as agreement with other reports [8]. Energy transfer parameters Q, CDA and critical distance (R0) are calculated and were shown in table 1. Fig.6. Decay curves of Sm3+ ions in ABL glasses. Table 1. Variation of lifetime (τ, ms), quantum efficiency (η, %), cross-relaxation rate (WCR s-1), energy transfer parameter (Q), critical distance (R0, Å) and donor-acceptor interaction parameter (CDA,×10-42 cm6/s) with respect to concentration (mol%) of Sm3+ ions in BLN glasses. Concentrations τ (ms) η (%) (% mol) 0.25 2.95 89.3 0.5 2.62 73.7 WDA s-1 Q - -35.5 0.54 R0(Å) R (Å) 6.84 -6.84 9.56 CDA(,×10-40 cm6/s) -0.39 26 T. Ngoc et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1(2019) 21-28 From the emission spectra of ABL:Sm3+ glasses, the energy level diagram of Sm3+ ion is shown in Fig. 7. When Sm3+ ions are excited to the excited levels above the 4G5/2, there is a fast non-radiative relaxation to the 4G5/2 lowest excited level and emission takes place from 4G5/2level to its lower levels. The energy transfer process through cross – relaxation (CR) between the pair ofSm3+ ions (as shown in Fig.7) leads luminescence quenching. The cross – relaxation channels in ABL glasses may be estimated to be (4G5/2 → 6F5/2) → (6H5/2 → 6F11/2) and (4G5/2 → 6F11/2) → (6H5/2 → 6F5/2) as the energy difference between these transitions are negligible. The cross – relaxation is due to the energy transfer from the Sm3+ ion in an excited 4G5/2 state to a near Sm3+ ion in the ground state 6H5/2 state. This transfer leads the first ion into the intermediate level of 6F5/2 (or 6F11/2) at around 1373 nm (or 946 nm) and the second one in 6F11/2 (or 6F5/2) at around 946 nm (or 1373 nm), which occur in resonance with the 4G5/2 → 6F5/2 (or 4G5/2 → 6F11/2) transition. Then, from these states, the Sm3+ ions will relax to ground state by nonradiative relaxation. Thus, emission will be quenched [8, 9]. Fig 7. Energy level diagram and cross-relaxation channels for Sm3+ ions in ABL glasses. 3.3. Spectral properties of ABL:Sm3+,Tb3+ glasses Figure 8 exhibits the emission spectra of ABL:1.0Sm3+,xTb3+. With the increase of Tb3+ concentration, the phosphors can create color emission from blue to yellowish – green. Moreover, while the emission intensity of Sm3+ almost unchanged, the emission intensity of Tb3+ increases and has a maximum intensity at x= 0.75. The forbidden 4f – 4f electronic transitions within the 4fn configuration of Tb3+. Therefore, there is no effective energy transfer from the Sm3+ to the Tb3+ to enhance the emission of Tb3+[10-13]. The same results can also be seen from Fig 9whichexhibits the emission spectra of ABL:0.75Tb3+,xSm3+. It has shown that the color emission from blue to red can be observed with the increase of Sm3+ concentration. The emission intensity of Sm3+ increases until reaching a maximum intensity at x = 1.0%mol. T. Ngoc et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 1(2019) 21-28 27 1.5x106 1.0x106 O r i g i n P r o 8 E v a l u a t i o n 5D4-O r i g i n P r o 8 E v a l u a t i o n b3+ 0.5 mol% O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8 E v a l u a t i o n 1.25 mol% 4.0x105 O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8 E v a l u a t i o n mol% 1.25 mol% 0.75 mol% O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8/ E v a l u a t i o n mol% 1.75mol% O r i g i n P r o 8 E v a l u a t i o n 4 O r i g i n P r o 8 E v a l u a t i o n ap 5/2 5/2 O r i g i n P r o 8 E v a l u a t i o n 5.0x105 4 6 O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 87E v a l u a t i o n 4 4 D4- F3 O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8 E v a l u a t i o n 2.0x105 O r i g i n P r o 84E v a l u a t i o n 5/2 9/2 O r i g i n P r o 8 E v a l u a t i o n O r i g i n P r o 8 E v a l u a t i o n 4G5/2-6H11/2 0.0 450 480 510 540 570 600 630 660 Wavelength (nm) 0.0 500 600 700 Wavelength (nm) Fig.8. Emission spectra of ABL: xTb3+,1.0Sm3+ with several different values of Sm3+ content. Fig.9. Emission spectra of ABL: 0.75Tb3+,xSm3+ with several different values of Sm3+ content. As such, Tb3+ and Sm3+ ions are isolated emission centre. This can be explained by the fact that the structure of energy levels of these ions does not allow for the coherent energy transfer from Sm+3 ion to Tb3+ ion.

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