Photoluminescence properties of rare-earth activated BaSi7N10

Journal of Alloys and Compounds 487 (2009) 28–33

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Photoluminescence properties of rare-earth activated BaSi7 N10 Y.Q. Li a,b,∗ , A.C.A. Delsing a,c , R. Metslaar a , G. de With a , H.T. Hintzen a,c a

Laboratory of Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b College of Materials Science and Engineering, Nanjing University of Technology, New Model Road 5, Nanjing, Jiangsu 210009, China c Material and Devices for Sustainable Energy Technologies, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

a r t i c l e

i n f o

Article history: Received 19 June 2009 Received in revised form 31 July 2009 Accepted 1 August 2009 Available online 15 August 2009 Keywords: Luminescence Barium silicon nitride Europium Cerium Terbium X-ray powder diffraction Structure

a b s t r a c t The photoluminescence properties of Eu2+ -, Ce3+ - and Tb3+ -activated BaSi7 N10 have been studied. The transitions of f ↔ d of the Eu2+ and Ce3+ ions occur at relatively high energies for nitrides which are principally attributed to weak bonding strength of Ba–N and small crystal field splitting of 5d excitation levels of Eu2+ and Ce3+ due to a large Ba crystallographic site in BaSi7 N10 . As a result, it was found that BaSi7 N10 :Eu2+ showed blue emission at about 475 nm originating from the 4f6 5d1 → 4f7 transition of Eu2+ upon UV excitation. While BaSi7 N10 :Ce3+ , Li+ exhibited UV-blue emission with a maximum at about 400 nm caused by the 5d → 4f transition of Ce3+ . In BaSi7 N10 :Tb3+ , Li+ , the luminescence spectrum of Tb3+ was dominated by green line emission arising from the 5 D4 → 5 FJ (J = 3, 4, 5, 6) transitions. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The photoluminescence properties of novel rare-earth doped alkaline-earth silicon or aluminosilicon nitride/oxynitride have been extensively investigated in recent years. Particularly, Eu2+ and Ce3+ -activated silicon/aluminosilicon nitride-based luminescent materials have been rapidly developed in order to meet urgent requirements of white LED lighting [1–17]. Unlike alkaline-earth silicates/aluminosilicates [18–20], silicon oxynitride/nitride-based compounds have unique structural features due to the presence of nitrogen in the lattices [21–24]. Therefore, significant differences on the luminescence properties are expected, particularly for the Eu2+ and Ce3+ activator ions [25] whose excitation and emission bands can be shifted to lower energies due to high degree of covalent bonding and large crystal field splitting of the 5d excited levels. With a medium ratio of M/Si or M/Si(Al) around 0.5, M2 Si5 N8 :Eu2+ (M = Ca, Sr, Ba) shows unusual Eu2+ orange to red emission in 580–660 nm offering high quantum efficiency (>70%) [7,10,11,26,27]. MAlSiN3 :Eu2+ (M = Ca, Sr) emits red light in 620–680 nm depending on the composition with the external

∗ Corresponding author at: Laboratory of Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail address: [email protected] (Y.Q. Li). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.08.019

quantum efficiency >70% [13–17]. While M2 Si5 N8 :Ce3+ exhibits broad emissions ranging from 450 to 550 nm and peaking at about 470, 550, 450 nm for M = Ca, Sr, Ba, respectively [28]. CaAlSiN3 :Ce3+ [12] also gives highly efficient orange emission centers at about 580–600 nm depending on the Ce3+ concentration when excited by blue light. With a high ratio of M/Si = 1, MSiN2 :Eu2+ (M = Ca, Sr, Ba) shows red emission in 610–630 nm [29–32]. Differentially, in the cases of Ce3+ , MSiN2 :Ce3+ shows yellow to red emission from 550 to 640 nm for M = Ca [32,33] depending on the Ce3+ concentration and the type of the crystal structures. However, it just shows blue emission at about 485 nm for M = Sr, Ba [31]. Regarding to the ratio of M/Si(Al) in 0.2–0.3, Eu2+ doped SrAlSi4 N7 [34] and Ba2 AlSi5 N9 [35] were found to give red emission at about 632 nm and yellow emission at about 584 nm, respectively. Surprisingly, with a further lower ratio of Sr/Si ∼0.17, SrSi6 N8 :Eu2+ can only provide blue emission at about 450 nm which could be mainly attributed to the Si–Si bond [36]. For even a lower ratio of Ba/Si ∼0.14 in BaSi7 N10 , which has higher degree of cross-linking among ternary silicon nitrides and both edge- and corner-sharing of SiN4 tetrahedra exist in BaSi7 N10 structure [37]. Originally, BaSi7 N10 :Eu2+ (10 mol%) was reported to emit red light at about 650 nm upon excitation at 400 nm [26]. In a subsequent study, however, BaSi7 N10 :Eu2+ (1 mol%) has been found to be blue emitting (∼484 nm) phosphor under UV excitation [27]. These contradictory results may be caused by different purity of the prepared materials arising from oxygen contamination in the raw materials and different Eu2+ con-

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tents. In addition, up to date the luminescence properties of Ce3+ or Tb3+ doped BaSi7 N10 have not been reported. On the basis of our previous investigations, Eu2+ , Ce3+ and Tb3+ doped nitride-based materials can yield very efficient luminescence not only at long but also at short excitation wavelength region. With the same purpose it is also interesting to know the photoluminescence properties of Eu2+ , Ce3+ and Tb3+ -activated BaSi7 N10 for use as the potential LED conversion phosphors as well as the traditional lamp phosphors. Therefore, in this work we firstly synthesized high purity undoped and rare-earth (RE = Eu2+ , Ce3+ and Tb3+ ) doped BaSi7 N10 and then we focus on the investigation of the photoluminescence properties of Eu2+ , Ce3+ and Tb3+ -doped BaSi7 N10 at low rare-earth ion concentrations correlated with the local structure parameters of BaSi7 N10 . 2. Experimental 2.1. Synthesis The binary nitride compounds with approximate formulations of Ba3 N2 , EuN, and TbN were pre-synthesized by nitridation of small piece of barium (Aldrich, 99.9%), europium (Csre, 99%) and terbium (Csre, 99%) metals under flowing dried nitrogen atmosphere at 550, 780 and 800 ◦ C, respectively, for about 12 h with twice firing processes in a horizontal tube furnace. Powder samples of undoped and Eu2+ -, Ce3+ -, Tb3+ -doped BaSi7 N10 were prepared by a solid state reaction approach. Then Ba3 N2 , EuN, TbN, Ce and Li metals as well as ␤-Si3 N4 (Cerac S-1177, ␤ content: 90.9%, oxygen ∼0.75%) powders were thoroughly mixed in the appropriate molar ratio and ground in an agate mortar. Based on our initial experiments, high concentration of rare-earth ions, particularly for Eu2+ , could easily lead to form the secondary phase (Ba, RE)2 Si5 N8 (see Sections 3.1 and 3.2 for details). In order to synthesize singlephase BaSi7 N10 :RE (RE = Eu2+ , Ce3+ , Tb3+ ) for better understanding of their natural luminescence properties of rare-earth ions in the BaSi7 N10 host, the rare-earth ion concentrations were set at 0.2 mol% for Eu2+ , 0.5–1 mol% for Ce3+ and 1 mol% for Tb3+ with respect to Ba2+ in BaSi7 N10 in the present work. The powder mixture was then transferred into a molybdenum crucible. All the processes were carried out in a purified nitrogen-filled glove-box to prevent oxidation or hydrolysis of the metal nitrides. Subsequently, the powder mixture was fired twice in a horizontal tube furnace at 1300–1400 ◦ C for 12 and 16 h, respectively, under flowing N2 –10%H2 atmosphere with an intermediate grinding in between. In the case of BaSi7 N10 :Tb3+ , Li+ , an additional heat processing was carried out at 1550 ◦ C for 12 h in a N2 –10%H2 atmosphere in order to obtain a chemical stability phosphor in air.

Fig. 1. X-ray powder diffraction patterns of BaSi7 N10 and Eu2+ -, Ce3+ - and Tb3+ doped BaSi7 N10 . The vertical bars on the bottom represent the positions of Bragg reflections.

examples, the second phase was BaSi6 N8 O [39] for lower oxygen content, and BaSi6 N8 O and/or BaSi2 O2 N2 [40] for high oxygen content. As mentioned above, a nearly single-phase BaSi7 N10 :RE could only be obtained by using ␤-Si3 N4 as a starting material. Therefore, in this work the ␤-Si3 N4 powder with very low oxygen content (∼0.75 wt%) was used for the subsequent experiments.

2.2. Characterization The crystalline phase formation was checked by X-ray powder diffraction (XRD) analysis with CuK␣ radiation and graphite monochromator at 40 kV–30 mA (Rigaku D/MAX-B) at room temperature. Diffraction data were collected by a step scanning from 10◦ to 120◦ in 2 with a step size of 0.02◦ and a counting time of 12 s per step. The structural parameters of Eu2+ -, Ce3+ -, and Tb3+ -doped BaSi7 N10 were analyzed by powder X-ray Rietveld refinement using the GSAS packages [38]. The obtained phosphors were also observed by scanning electron microscope (SEM, JEOL 840A). Diffuse reflectance and luminescence spectra were measured by a Perkin Elmer LS 50B spectrophotometer (Xe flashlight, R952 photomultiplier) at room temperature. For the diffuse reflectance measurement, BaSO4 powder and black felt were used as the standard references.

3. Results and discussion 3.1. Phase formation and structural characteristics Fig. 1 plots the X-ray powder diffraction patterns of undoped and Eu2+ -, Ce3+ - and Tb3+ -doped BaSi7 N10 . It can be seen that nearly single-phase BaSi7 N10 and BaSi7 N10 :RE (RE = Eu2+ , Ce3+ , Tb3+ ) have been prepared by using ␤-Si3 N4 as a starting material due to its low impurity oxygen. It should be noted that the purity of BaSi7 N10 is significantly sensitive to oxygen impurities from the starting materials and the subsequent processing. Since the capacity of oxygen in the BaSi7 N10 lattice is much lower than that in M2 Si5 N8 (M = Ca, Sr, Ba), it has been found to be very difficult to obtain single-phase BaSi7 N10 :RE using ␣-Si3 N4 powder due to its high oxygen content (e.g., >1.5 wt%). Accordingly, small amounts of the secondary phases were always found in the final phosphors apart from the main phase of BaSi7 N10 if ␣-Si3 N4 powder was used as a raw material. As typical

Fig. 2. Scanning electron micrograph of (a) BaSi7 N10 :Eu2+ and (b) as-received ␤Si3 N4 powder.

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Fig. 3. Projection of the crystal structure and the coordination polyhedron of Ba with nitrogen atoms of BaSi7 N10 :RE (RE = Eu2+ , Ce3+ , Tb3+ ). The large sphere (red) represents the (Ba, RE) atoms. The small white and blue spheres represent the Si and N atoms, respectively. Fig. 4. Diffuse reflection spectra of undoped BaSi7 N10 and BaSi7 N10 :Eu2+ .

Fig. 2a shows a scanning electron micrograph of the obtained BaSi7 N10 :Eu2+ powder. As seen, it mainly consists of short tabularlike particles of about 2–3 ␮m and large amount of long rod-like particles of 4–10 ␮m in length and 2 ␮m in width which may originate from its mother-particles of ␤-Si3 N4 , evidently showing in Fig. 2b. The lattice parameters and some selected structural parameters of BaSi7 N10 :RE (RE = Eu2+ , Ce3+ , Tb3+ ) were summarized in Table 1. The unit cell volume of BaSi7 N10 :RE slightly shrinks (∼0.1%) with a decrease of the ionic size of RE going from Eu2+ via Ce3+ to Tb3+ . The BaRE (RE = Eu2+ , Ce3+ , Tb3+ ) ion is located in a void of the network structure of [Si7 N10 ]2− and directly coordinated with 12 N atoms (<0.35 nm) having similar average bond length of BaRE –N around 0.326 nm. Correspondingly, the calculated polyhedron coordination volume (Vp ) [41] of BaRE N12 ranges from 0.077 to 0.079 nm3 (Fig. 3 and Table 1). It is worth noting that as the RE ions have different low doping concentrations the effect of the ionic size on the average and local structure of BaSi7 N10 actually can be neglectable. By contrast, these obtained Vp are about 4.4 and 3.7 times larger than the Ba(1) and Ba(2) polyhedron coordination volume, respectively, in Ba2 Si5 N8 (∼0.0176 nm3 and 0.0212 nm3 for Ba(1)N6 (C.N. = 6) and Ba(2)N7 (C.N. = 7), respectively) [42]. In other words, the Vp of BaNx takes about 36.5 vol.% (x = 12) and 21 vol.% (x = 6 and 7) space within the unit cell of BaSi7 N10 and Ba2 Si5 N8 , respectively. Thus, the lower solubility of Eu2+ , Ce3+ and Tb3+ can be expected in the BaSi7 N10 lattice due to large metal cation size mismatching between the activator and the Ba ion in agreement with our experimental results that the higher concentrations of rare-earth ions would result in forming the second phases, like (Ba, RE)2 Si5 N8 confirmed by the X-ray powder diffraction analysis. In this way, even the second phase is in small amounts, for example for (Ba, Eu)2 Si5 N8 , the luminescence intensity of Ba2 Si5 N8 :Eu2+ is expected to be very strong due to its higher efficiency, which would show intense red emission when excited above 400 nm (see next section).

in Fig. 5. The excitation spectrum of BaSi7 N10 :Eu2+ (0.2 mol%) only consists of an asymmetric broad band peaking at about 297 nm in significantly narrow spectral range of 240–400 nm. Furthermore, the excitation band of Eu2+ can be roughly decomposed into three largely overlapping Gaussians subbands centered at about 296, 332, 363 nm (see inset in Fig. 5). The emission spectrum of BaSi7 N10 :Eu2+ showed a typical broad band emission peaking at about 475 nm (Fig. 5) corresponding to the 4f6 5d → 4f7 transition of Eu2+ . This result is in consistent with Ref. [27] that BaSi7 N10 :Eu2+ emits blue light with the emission band at about 484 nm. The wrong observation of red emission was made for BaSi7 N10 :Eu2+ (10 mol%) [26] with a high Eu2+ concentration, evidently surpassing the solubility limit resulting in the segregation of (Ba, Eu)2 Si5 N8 phase as detected by X-ray powder diffraction. With varying excitation wavelengths (Fig. 5), only a single emission band was observed, in agreement with one crystallographic Ba site in the BaSi7 N10 lattice [37]. Both the excitation and emission bands of Eu2+ are located at higher energies in a more rigid BaSi7 N10 lattice when compared with Ba2 Si5 N8 :Eu2+ [7,10] (e.g., the emission maximum at ∼570 nm for Ba2 Si5 N8 :Eu2+ (0.5 mol%)). In Ba2 Si5 N8 , there are two individual crystallographic Ba sites with 6 and 7 folder coordinations with the nitrogen atoms [42]. However, the coordination number of the Ba2+ ion (C.N. = 12 within 0.355 nm) in BaSi7 N10 is nearly twice as much as in Ba2 Si5 N8 (Fig. 3). More specifically, the coordination polyhedron volume (Vp ) of BaEu N12 (76.922 × 10−3 nm3 ) and the average bond length of BaEu –N (0.3248 nm) in BaSi7 N10 :Eu2+

3.2. Photoluminescence of rare-earth doped BaSi7 N10 3.2.1. BaSi7 N10 :Eu2+ Fig. 4 shows the diffuse reflection spectra of BaSi7 N10 and BaSi7 N10 :Eu2+ (0.2 mol%). The optical band gap of the obtained BaSi7 N10 powder was estimated to be 4.8 eV by the diffuse reflection spectrum of BaSi7 N10 . This value is in fair agreement with the first-principles calculated one (∼3.8–4.0 eV) if the underestimation is considered in this calculation method [43]. The absorption band of Eu2+ was distributed from 250 to 420 nm in the reflection spectrum, which fairly accorded with its excitation spectrum, as shown

Fig. 5. Excitation and emission spectra of Ba0.998 Eu0.002 Si7 N10 . Inset shows the decomposed excitation spectrum.

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Table 1 Selected structural parameters of Eu2+ -, Ce3+ -, Tb3+ -doped BaSi7 N10 (e.s.d.s. in parentheses). Compound

Ba0.998 Eu0.002 Si7 N10

Ba0.99 Ce0.005 Li0.005 Si7 N10

Ba0.98 Tb0.01 Li0.01 Si7 N10

Space group a (nm) b (nm) c (nm) ˇ (◦ ) V (×10−3 nm3 ) C.N. (BaRE )* Vp (BaRE N12 )** (×10−3 nm3 ) Mean BaRE –N (nm)

P c (No. 7) 0.68659(1) 0.67042(1) 0.96260(1) 106.256(1) 425.38(1) 12 76.92 ± 1.58 0.3248

0.68659(1) 0.67037(1) 0.96258(1) 106.254(1) 425.34(1) 12 78.21 ± 1.59 0.3260

0.68643(1) 0.67029(1) 0.96238(2) 106.257(1) 425.09(1) 12 79.11 ± 1.60 0.3270

* **

C.N. (BaRE ): coordination number of Ba with the N atoms within the range of 0.35 nm. Vp : polyhedron coordination volume around BaRE .

(see Table 1) is much larger than these in Ba2 Si5 N8 (Vp (Ba(1)N6 ) = 17.6 × 10−3 nm3 and Vp (Ba(2)N7 ) = 21.2 × 10−3 nm3 ; Ba(1)–N = 0.2832 nm and Ba(2)–N = 0.2885 nm). Consequently, the crystal field splitting (CFS ∼ 6200 cm−1 in BaSi7 N10 vs.18300 cm−1 in Ba2 Si5 N8 ) of the 5d excitation levels of Eu2+ is small, because the crystal field strength around BaEu is very low due to longer BaEu –N distances and larger Vp , in consistent with the fact of largely overlapping excitation subbands of Eu2+ in its excitation spectrum. Furthermore, the above arguments are also supported by the firstprinciples calculation results which evidently indicate that there are weak coupling between the Ba 5p and N 2s and 2p states with almost empty Ba 6s states in the valence band of BaSi7 N10 [43]. Moreover, it is worth pointing out that the estimated Stokes shift of BaSi7 N10 :Eu2+ is about 6500 cm−1 which is smaller than that in Ba2 Si5 N8 :Eu2+ (∼7500 cm−1 ) as expected in a more rigid lattice due to the high degree of cross-linking of SiN4 tetrahedra. Finally, it is worth noting that oxygen impurity has relatively small effect on the luminescence properties of BaSi7 N10 :Eu2+ since the efficiency of Eu-doped second phase is very low. It was found that when ␣-Si3 N4 was used as a starting material, besides the major emission band of BaSi7 N10 :Eu2+ (Fig. 5), a weak emission shoulder at about 400 nm was present in the spectrum due to the formation of (Ba, Eu)Si6 N8 O second phase in BaSi7 N10 :Eu2+ . 3.2.2. BaSi7 N10 :Ce3+ , Li+ Fig. 6 shows the diffuse reflection spectrum of Ba1−2x Cex Lix Si7 N10 (x = 0, 0.005). It was clear that the absorption edge of BaSi7 N10 was slightly shifted to long wavelength by the incorporation of Ce3+ and Li+ . The absorption band of Ce3+ is centered at about 325 nm starting from ∼420 nm and is well in

Fig. 6. Diffuse reflection spectra of undoped BaSi7 N10 and BaSi7 N10 :Ce3+ , Li+ (0.5 mol%).

consistent with the observed excitation spectra of BaSi7 N10 :Ce3+ , Li+ (Fig. 7). Similar to BaSi7 N10 :Eu2+ , the excitation and emission bands of Ce3+ were also found at higher energies compared to Ba2 Si5 N8 :Ce3+ , Li+ caused by the same reasons for Eu2+ . The excitation bands of Ce3+ are also accumulated in narrow range of 240–360 nm with a maximum at about 327 nm. In the excitation spectrum approximate four subbands of Ce3+ at about 255, 285, 306 and 328 nm could be fitted (see inset in Fig. 7). The emission spectrum exhibited a broad band peaking at about 400 nm with a weak shoulder at about 370 nm, which could be related to the transition of Ce3+ from the lowest 5d level to the doublet levels, viz. 2 F5/2 and 2 F7/2 , of the ground state of Ce3+ separated by about 2027 cm−1 on the energy scale [19]. The luminescence intensity of BaSi7 N10 :Ce3+ , Li+ was significantly decreased with increasing the Ce3+ concentration due to its significant concentration quenching behavior of Ce3+ , which may be attributed to the increased number of defect levels due to an aliovalent substitution of Ce3+ → Ba2+ as well as larger size mismatching (between Ba2+ and Ce3+ or Li+ ) by which the excited 5d electrons of Ce3+ are easily trapped by the defects in BaSi7 N10 lattice. On the other hand, photoionization could be also responsible for quenching luminescence at room temperature due to the hybridization of the upper 5d excited levels of Ce3+ with the host excitation bands (Fig. 6). Combined the above-mentioned factors with a small Stokes shift of about 5580 cm−1 in BaSi7 N10 :Ce3+ , Li+ , the integrated emission intensity of Ce3+ was reduced about 30% from x = 0.005 to x = 0.01. As usual, higher Ce3+ content also resulted in a red shift of the emission band, and the emission band of Ce3+ was positioned at shorter

Fig. 7. Excitation and emission spectra of BaSi7 N10 :Ce3+ , Li+ (0.5, 1 mol%). Inset shows the decomposed excitation spectrum with the Gaussian subbands of BaSi7 N10 :Ce3+ , Li+ (0.5 mol%).

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integrated emission intensity from blue to green. In contrast, exci1 band at about 270 nm tation into the spin-forbidden 4f8 → 4f7 5d  resulted in a relative high value of 8.8 for I5 D4 / I5 D3 . The different ratios for excitation in spin-forbidden versus spin-allowed suggest different feeding ratios from 5d levels to 5 D3 and 5 D4 emitting levels, respectively. On the other hand, low Tb3+ luminescence intensity at room temperature in BaSi7 N10 could be also related to the photoionization processes. 4. Conclusions

Fig. 8. Diffuse reflection, excitation and emission spectra of BaSi7 N10 :Tb3+ , Li+ (1 mol%) (exc = 270 nm and em = 546 nm).

wavelength than that of Eu2+ in BaSi7 N10 in full agreement with the published literature [19,44]. On the basis of the structural and luminescence properties of Eu2+ or Ce3+ doped BaSi7 N10 , it is clearly seen that the appropriate doping sites (i.e., suitable coordination number and bond length) are really necessary for rare-earth ions in order to obtain long wavelength excitation/absorption besides a rigid lattice within a nitrogen-rich environment. 3.2.3. BaSi7 N10 :Tb3+ , Li+ The diffuse reflection, excitation and emission spectra of BaSi7 N10 :Tb3+ , Li+ (1 mol%) are illustrated in Fig. 8. From the diffuse reflection spectra it can be seen that the absorption edge of BaSi7 N10 :Tb3+ , Li+ slightly shifts to longer wavelengths due to the introduction of Tb3+ as found in the case of Ce3+ . The absorption of f–d of Tb3+ was very weak and no absorption peaks from the transitions of f–f of Tb3+ could be found in its reflection spectrum due to low Tb3+ concentration. From the excitation spectrum, the strongest broad band covering 230–260 nm may be ascribed to the hybrid bands of the host lattice excitation band and the spin-allowed 4f8 → 4f7 5d1 transition band. The second broad band centered at about 271 nm with relative low intensity in the range of 260–300 nm is probably originated from the spin-forbidden 4f8 → 4f7 5d1 transition. Meanwhile a number of sharp transitions at lower energy (340–500 nm, not all of them are shown in Fig. 8) can be assigned to the intra-4f8 transitions of Tb3+ [19]. Normally, the energy difference between the lowest 5d excitation band for Ce3+ and Tb3+ in the same host is about 11,000–15,000 cm−1 for the spin-allowed f–d transitions, while 5000–9000 cm−1 for spin-forbidden f–d transitions [19,45]. In regarding to Ce3+ - and Tb3+ -doped BaSi7 N10 , the estimated energy differences between Ce3+ and Tb3+ are about 14,800 and 9110 cm−1 for the spin-allowed and spin-forbidden f–d transitions of Tb3+ , respectively, which further supports the assignment of the excitation bands. Under UV excitation, BaSi7 N10 :Tb3+ , Li+ (1 mol%) exhibited green-yellowish emission which composed of two sets of emission lines originating from the transitions of the excited states of the 5 D3 and 5 D4 levels to the ground state of 7 FJ of Tb3+ [19,20,46]. In BaSi7 N10 :Tb3+ , Li+ (1 mol%) both blue emissions from the 5 D3 levels (5 D3 → 7 FJ (J = 2–6)) and green emissions from 5 D4 levels (5 D4 → 7 FJ (J = 3–6)) have been observed. The dominating emission peak was located at about 546 nm corresponding to the 5 D4 → 7 F5 transition, which demonstrated that stronger cross-relaxation of Tb3+ [19] have already occurred to some extent. Excitation into the spin-allowed 4f8 → 4f7 5d1 band at about  235 nm  yielded a value I5 D4 / I5 D3 , viz. the of 3.5 for the emission intensity ratio of

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