Long wavelength Ce3+ emission in Y–Si–O–N materials

Journal of Alloys and Compounds 268 (1998) 272–277

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Long wavelength Ce 31 emission in Y–Si–O–N materials a, a a b J.W.H. van Krevel *, H.T. Hintzen , R. Metselaar , A. Meijerink a

Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Debye Institute, University Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Received 21 October 1997

Abstract An investigation of new luminescent materials, Y 5 (SiO 4 ) 3 N:Ce, Y 4 Si 2 O 7 N 2 :Ce, YSiO 2 N:Ce and Y 2 Si 3 O 3 N 4 :Ce is presented. In a series of these oxynitride materials, long wavelength emission of Ce 31 is observed, the exact position determined by the nephelauxetic effect, the crystal-field splitting and the Stokes shift. It was found that the crystal-field splitting becomes larger when more N 32 versus O 22 coordinates to Ce 31 . This is ascribed to the higher formal charge of N 32 compared to O 22 . Furthermore, several parameters, like increased nitrogen versus oxygen coordination, more non-bridging versus bridging nitrogen coordinated to Ce 31 , as well as its coordination by free oxygen ions, contribute to the increase of the covalency, which shifts the centre of gravity of the 5d state to lower energy (nephelauxetic effect). The Stokes shift is observed to become smaller for lattices with a more extended silicon network formation when more N 32 is incorporated. The smaller Stokes shift is ascribed to the increasing rigidity of the lattice. The energy difference between the lowest 5d excitation band of Tb 31 and that of Ce 31 amounts to 12–15*10 3 cm 21 , which is in agreement with literature.  1998 Elsevier Science S.A. Keywords: Oxynitride; Luminescence; Cerium

1. Introduction When Ce 31 is incorporated in an oxidic host lattice, 5d→4f emission in the near-ultraviolet or blue region of the spectrum is normally found [1,2]. The emission will shift to the red (e.g. up to 699 nm for Y 2 O 2 S:Ce [3]) when: the crystal-field splitting is strong, the covalency increases, or the Stokes shift is large [1,2]. A strong crystal-field splitting is found for, e.g. Y 3 Al 5 O 12 :Ce [4], yielding emission maxima at about 500 nm. The covalency is, for example, increasing in the series YF 3 :Ce, YOCl:Ce and Y 2 O 2 S:Ce and, as a result of this, the barycentre of the 5d level decreases from 44 200 [5] or 33 700 [2] to 21 600 21 cm [3] respectively. The large Stokes shift in both SrY 2 O 4 :Ce [2] and Y 2 O 2 S:Ce [3] results in emission at lower energy (575 nm for SrY 2 O 4 :Ce and 699 nm for Y 2 O 2 S:Ce). In this paper a study concerning a series of new luminescent materials is presented, viz. Ce 31 -doped silicon oxynitride materials. The lattices investigated are: Y 5 (SiO 4 ) 3 N (apatite-type structure), Y 4 Si 2 O 7 N 2 (cuspidine-type structure), YSiO 2 N (pseudowollastonite-type *Corresponding author. Tel.: 131 40 2473132; fax: 131 40 2445619; e-mail: [email protected] 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00550-1

structure) and Y 2 Si 3 O 3 N 4 (melilite-type structure) [6]. In 31 31 these lattices Ce is substituted on the Y sites. The 32 22 N /O ratio increases in the series Y 5 (SiO 4 ) 3 N, Y 4 Si 2 O 7 N 2 , YSiO 2 N to Y 2 Si 3 O 3 N 4 and also the N 32 / O 22 31 coordination to Y [6,12,13,15]. Silicon is tetrahedrally coordinated with oxygen and nitrogen ions. The SiO 42x N x tetrahedra are interconnected differently in these series of compounds: there are isolated Si(O,N) 4 tetrahedra in Y 5 (SiO 4 ) 3 N [6–10], dimeric Si 2 O 5 N 2 units in Y 4 Si 2 O 7 N 2 [8,9,11,12], trimeric Si 3 O 6 N 3 rings in YSiO 2 N [7–9,13] and sheets of (Si 3 O 3 N 4 ) n in Y 2 Si 3 O 3 N 4 [8,12,14–16]. For these Y–Si–O–N materials, the influence of nitrogen incorporation and the interconnection of SiO 42x N x tetrahedra on the Ce 31 luminescence is described. The structure of the four compounds will be further discussed in the results, where the Ce 31 luminescence in every compound is described separately.

2. Experimental 31

All samples were doped with 5 at.% Ce . Starting ˆ materials were CeO 2 (Rhone-Poulenc, purity 99.95%), ˆ Y 2 O 3 (Rhone-Poulenc, purity 99.999%), SiO 2 (Degussa Aerosil ox50) and a-Si 3 N 4 (Starck LC-12). The starting

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powders were suspended in acetone, and mixed in a planetary ball-mill for 10 h. The resulting mixture was dried and fired in Mo crucibles under a 5%H 2 / 95%N 2 atmosphere. Y 5 (SiO 4 ) 3 N:Ce and Y 4 Si 2 O 7 N 2 :Ce were fired for 2 h at 16008C, YSiO 2 N:Ce for 2 h at 15008C, and Y 2 Si 3 O 3 N 4 :Ce for 2 h at 17008C. After firing the samples were ground to obtain a fine powder. X-ray powder diffraction with Cu Ka radiation (Philips PW 1120) showed that Y 5 (SiO 4 ) 3 N:Ce and Y 4 Si 2 O 7 N 2 :Ce are single phase. YSiO 2 N:Ce contained some Y 4 Si 2 O 7 N 2 :Ce and a small amount of Y 4 Si 2 O 7 N 2 :Ce was also present in Y 2 Si 3 O 3 N 4 :Ce. A Perkin-Elmer LS 50B spectrophotometer equipped with a Xe flash light, was used to perform luminescence measurements at room temperature. For correction of the excitation spectra, part of the excitation beam was led to a second photomultiplier by a beam-splitter. The excitation intensity was determined as a ratio of the two photomultiplier signals. Correction of the emission spectra was performed between 340 and 900 nm using the observed versus the calibrated signal of a W-lamp. Excitation spectra were also recorded in the range 160– 350 nm using a Spex Fluorolog spectrofluorimeter equipped with a deuterium lamp. The light path was flushed with nitrogen to prevent oxygen absorption in the

273

region 160–200 nm. The spectra were corrected by taking the ratio of the observed excitation signal versus that of sodium salicylate.

3. Results

3.1. General Emission and excitation spectra of the four Y–Si–O– N:Ce compounds show broad bands Fig. 1(a,d), characteristic for Ce 31 [1,2]. The positions of the emission maxima vary significantly for the different Y–Si–O–N:Ce compounds: emission at about 400–450 nm is observed for YSiO 2 N:Ce, whereas emission near 500 nm is found for Y 2 Si 3 O 3 N 4 :Ce. In Y 4 Si 2 O 7 N 2 :Ce, the emission maximum is even beyond 500 nm, which is quite exceptional [1,2]. The low energetic position of the 5d band in Y 4 Si 2 O 7 N 2 :Ce results in absorption of visible light. In the excitation spectra, the 5d excitation band extends to 460 nm (Fig. 1(b) for Y 4 Si 2 O 7 N 2 :Ce) and also covers the blue part of the spectrum (420–480 nm). As a result, Y 4 Si 2 O 7 N 2 :Ce is a yellow powder. The excitation spectra of Y 5 (SiO 4 ) 3 N:Ce, Y 2 Si 3 O 3 N 4 :Ce and YSiO 2 N:Ce cover the UV part of the spectrum and the powders of these

Fig. 1. Emission and excitation spectra of Y 5 (SiO 4 ) 3 N:Ce (a); Y 4 Si 2 O 7 N 2 :Ce (b); YSiO 2 N:Ce (c); and Y 2 Si 3 O 3 N 4 :Ce (d). The excitation ( lexc ) and monitoring wavelengths ( lmon ) are given in the figure (nm), except for Y 5 (SiO 4 ) 3 N:Ce. For this last compound, excitation spectra are recorded for emission 490 nm (thick line) and 380 nm (thin line). The excitation wavelengths of the emission spectra are 285 (thin line) and 380 nm (thick line).

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compounds are white: the slightly yellow colour of YSiO 2 N:Ce and also in Y 2 Si 3 O 3 N 4 :Ce (shoulder at 425 nm) is probably due to the presence of some Y 4 Si 2 O 7 N 2 :Ce. Table 1 shows luminescence data measured for the several compounds. The data collected for the Ce 31 luminescence are obtained from emission and excitation measurements between 280 and 650 nm (Fig. 1) and, moreover, from additional excitation spectra measured between 160 and 350 nm. The excitation bands below 250 nm are generally weak for all compounds, probably as a result of photoionisation [17]. The splitting of the 5d band depends on the symmetry of the coordination and the crystal-field strengths (due to the ligands). In a well-known site symmetry, often the measured splitting into, for example, five crystal-field components can be related to a crystal-field for cubic symmetry. Examples for this procedure can be found in, e.g. Refs. [2,5]. For a more correct comparison we have, in our case, tried to derive a cubic crystal-field splitting, but in view of the uncertainties in the site symmetries of the different sites and in the assignment of the different 5d bands observed, it seemed better to take the difference between the energy of the lowest and of the highest energy 5d band as a measure of the crystal-field splitting. Although this estimate introduces an error in the crystal-field splitting due to the effect of the ligands alone, it gives a qualitative idea of the influence of the ligands on the crystal-field strength.

The value for the centre of gravity for the 5d state is estimated by averaging the observed excitation maxima. The difference between the lowest energy maximum for the excitation band and the highest energy emission maximum is taken as the value for the Stokes shift of the Ce-doped samples (Table 1). This procedure can introduce an error due to the high Ce 31 concentrations used (5 at.% Ce 31 versus Y 31 ). Reabsorption of the highest energy emission can occur in concentrated samples, due to a spectral overlap between the lowest energy excitation band and the highest energy emission. This results in a shift of the emission maximum to longer wavelengths.

3.2. Y5 ( SiO4 )3 N: Ce The compound first mentioned in Table 1 is Y 5 (SiO 4 ) 3 N:Ce and contains the lowest nitrogen amount. Y 5 (SiO 4 ) 3 N shows great similarity with existing hexagonal apatite structures, such as for example Ca 5 (PO 4 ) 3 OH [6]. The Y(1) site has a coordination number of 7 [6,10] in which Y 31 is bonded to a free oxygen ion [6,9,10], as well as oxygen and nitrogen ions which belong to the silicon tetrahedra [7,9]. Y(2) is bonded to nine oxygen and nitrogen ions, which are all attached to Si [7–9]. Emission spectra, recorded at varying excitation wavelengths, revealed two separate emission maxima at about 423 and 475 nm (Fig. 1(a)). These results indicate the presence of two types of luminescent centres.

Table 1 Emission maxima (EM) and halfwidth (EHW), excitation maxima (ExM), the crystal-field splitting (CFS), the centre of gravity (CG), the Stokes shift (SS), and the relative emission intensity (Irel ) at an optimum excitation wavelength for different Y–Si–O–N:Ce materials Compound

EM (nm / *10 3 cm 21 )

EHW (*10 3 cm 21 )

ExM (nm / *10 3

Y 5 (SiO 4 ) 3 N:Ce

(1) 475 / 21.1

4.5

(2) 423 / 23.6

6

Y 4 Si 2 O 7 N 2 :Ce

504 / 19.8

YSiO 2 N:Ce

Y 2 Si 3 O 3 N 4 :Ce

21

CFS (*10 3 cm 21 )

CG (*10 3 cm 21 )

SS (*10 3 cm 21 )

Irel ( lexc )

(1) 240 / 41.7 290 / 34.5 325 / 30.8 355 / 28.2 (2) 225 / 44.4 280 / 35.7 310 / 32.3 325 / 30.8

(1) 13.5

(1) 33–35

(1) 7.161

40 (325)

(2) 13.6

(2) 34–37

(2) 7.261

4.8

240 / 41.7 6290 / 34.5 355 / 28.2 390 / 25.6

16.1

30–35

5.861

30 (390)

405 / 24.7 442 / 22.6

4.2

6195 / 51.3 (23) 265 / 37.7 6340 / 29.4 6370 / 27.0

24.3

38–40

3.361

100 (370)

493 / 20.3

4.3

6195 / 51.3 260 / 38.5 310 / 32.3 390 / 25.6

25.7

35–39

3.261

60 (390)

)

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Excitation spectra recorded with emissions at 423 and 475 nm yielded two different excitation spectra (Fig. 1(a)). The maxima of these bands for the two sites are given in Table 1. Based on the excitation spectra, we estimated the barycentre at 33–35*10 3 cm 21 for Ce 31 on the Y-site with emission at 475 nm, and for the other it was estimated at about 34–37*10 3 cm 21 . According to Lammers and Blasse and others, the excitation and emission bands at lower energy of Ceactivated Gd 4.67 h 0.33 (SiO 4 ) 3 O with the apatite structure, 31 originate from Ce on the Y(1) site [18,19]. This assumption is based on the fact that the free oxygen ion coordinates the Y(1) site, which is believed to increase the covalency and results in a shift of the Ce 31 5d band to lower energy (increase in nephelauxetic effect). Following this interpretation, we denote the luminescent centre having an emission maximum at 475 nm and a centre of gravity of the 5d level at 33–35*10 3 cm 21 (Table 1) to Ce 31 on the Y(1) site, and the other with an emission maximum at 423 nm and a barycentre at 34–37*10 3 cm 21 to Ce 31 on the Y(2) site.

3.3. Y4 Si2 O7 N2 : Ce The emission maximum of Y 4 Si 2 O 7 N 2 :Ce is at about 504 nm (Fig. 1(b)). This is rather exceptional and only few compounds show emission at longer wavelengths, which corresponds with a Ce 31 5d→4f transition. Compared to the three other compounds dealt with in this paper, this maximum is at the longest wavelength, i.e. at lowest energy (Table 1). Four different Y sites are present in Y 4 Si 2 O 7 N 2 , three having seven-fold and one six-fold coordination [12]. Every Y site in Y 4 Si 2 O 7 N 2 has low point symmetry (C1 ) and two sites are surrounded by one and the other two by three free oxygen ions [12]. Fig. 1(b) shows that there is a broad excitation band extending from about 460 to the UV part of the spectrum. Four local maxima can be discerned in this excitation band, i.e. at about 240, 290, 355 and 390 nm (Table 1). Despite the presence of four different types of Y sites in Y 4 Si 2 O 7 N 2 , excitation spectra measured for different emission wavelengths and emission spectra recorded at different excitation wavelengths revealed only small differences as compared to the spectra in Fig. 1(b). Furthermore, only a slightly larger halfwidth was observed (Table 1), compared to YSiO 2 N:Ce and Y 2 Si 3 O 3 N 4 :Ce, where the Ce 31 luminescence originates from a single type of luminescent centre. Obviously, strong overlap between the luminescence spectra belonging to different Ce 31 ions takes place, preventing disentanglement of the individual spectra. It is remarkable that, in spite of the presence of four different sites, the difference between the excitation band at highest and lowest energy is only 16 000 cm 21 . This indicates that the crystal-field splitting for all Ce 31 ions on various Y sites cannot be extremely large.

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3.4. YSiO2 N: Ce YSiO 2 N:Ce shows an emission spectrum with a maximum at about 442 nm and a shoulder at about 405 nm (Fig. 1(c)). The tail at the long-wavelength side of the emission spectrum (.490 nm) originates from contamination with some Y 4 Si 2 O 7 N 2 :Ce, as also determined with powder X-ray diffraction. This is confirmed by different excitation spectra, obtained at monitoring wavelengths varying between 500 and 650 nm, which are similar to the excitation spectrum of Y 4 Si 2 O 7 N 2 :Ce. The emission band at 442 nm and the shoulder at 405 nm (Fig. 1(c)) have a similar excitation spectrum. Moreover, excitation in different levels belonging to the 5d state of YSiO 2 N:Ce revealed (apart from the influence due to the Y 4 Si 2 O 7 N 2 :Ce contamination) a similar emission spectrum with a maximum at 442 nm and a shoulder at 405 nm, indicating a single type of Ce 31 centre. The presence of only one type of Ce 31 luminescent centre corresponds with the fact that one crystallographic Y site is reported to be present in YSiO 2 N [13]. The energy difference between these two emissions at 405 and 442 nm is about 2.1*10 3 cm 21 , corresponding with the ground-state splitting of the Ce 31 ion (free ion: 2 | 2250 cm 21 [20]). Emission spectra of earlier F7 / 2 2 2 F5 / 2 5 reported 1% Ce 31 -doped samples showed that the highenergy emission band of the split 4f ground state dominates [2]. Here, however, the intensity of the low-energy emission band at 442 nm is larger. Obviously the emission at 405 nm is partially quenched, which is ascribed to reabsorption present for high Ce 31 concentrations due to overlap of excitation and emission bands. Y 31 in YSiO 2 N is surrounded by six O 22 and two N 32 ions, and has a low symmetry (C1 ) [13]. The symmetry of the site can be deduced from (ideal) S6 symmetry, in which two N 32 ions are axially coordinated and triangles of oxygen ions are at both sides of the Y ion [13]. For this coordination, the excitation band is expected to split into three levels [21], which are observed around 195, 265 and approximately 350 nm. Due to further distortion of the S6 symmetry, resulting in C1 symmetry, the Ce 31 5d excitation band is expected to split up into five excitation levels [21]. In agreement with this expectation, the bands near 195 and 350 nm are relatively broad, the crystal-field splitting of the latter resulting in two maxima at approximately 340 and 370 nm (Fig. 1(c)). The centre of gravity was estimated at about 38–40*10 3 cm 21 .

3.5. Y2 Si3 O3 N4 : Ce Irrespective of the excitation wavelength, one band with a maximum at about 493 nm is present in the emission spectrum of Y 2 Si 3 O 3 N 4 :Ce (Fig. 1(d)). For different monitoring wavelengths, the shape of the excitation spectrum does not change, except for a shoulder at 400–450 nm, whose intensity varies. This is ascribed to the lumines-

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cence of Y 4 Si 2 O 7 N 2 :Ce, whose presence is also demonstrated by powder X-ray diffraction. The presence of one type of excitation and emission spectrum is expected as only one crystallographic Y 31 site is present in Y 2 Si 3 O 3 N 4 :Ce [12,15]. The maxima of this excitation spectrum are at 195, 260, 310 and 390 nm. The centre of gravity of the 5d level was calculated at 35–39*10 3 cm 21 .

4. Discussion

4.1. Crystal-field splitting The crystal-field splitting, observed in the different Ce 31 -doped Y–Si–O–N compounds, increases going from about 13–14*10 3 cm 21 for both sites in Y 5 (SiO 4 ) 3 N:Ce, to about 16.1*10 3 cm 21 for Y 4 Si 2 O 7 N 2 :Ce, about 24.3*10 3 cm 21 for YSiO 2 N:Ce and about 25.7*10 3 cm 21 for Y 2 Si 3 O 3 N 4 :Ce (Table 1). The results indicate that the crystal-field splitting shows an upwards trend for higher nitrogen contents which we ascribe to the higher formal charge of N 32 versus O 22 .

4.2. Centre of gravity The value of the centre of gravity decreases to lower energy in the series: YSiO 2 N:Ce (38–40*10 3 cm 21 ) . Y 2 Si 3 O 3 N 4 :Ce (35–39*10 3 cm 21 ) . Y(2) site in Y 5 (SiO 4 ) 3 N:Ce (34–37*10 3 cm 21 ) . Y(1) site in Y 5 (SiO 4 ) 3 N:Ce (33–35*10 3 cm 21 ) . Y 4 Si 2 O 7 N 2 :Ce (30–35*10 3 cm 21 ). It can be expected that the centre of gravity of the 5d level comes down in energy when more nitrogen versus oxygen coordinates to Ce 31 as the Ce–N bond is more covalent than the Ce–O bond (nephelauxetic effect). However, more effects play a role as this effect cannot explain the observed trend. When structural features of the lattices are considered, it must be noted that in YSiO 2 N [13] and also in Y 2 Si 3 O 3 N 4 [15], the nitrogen ions are only present as bridging between two silicon tetrahedrons. In Y 5 (SiO 4 ) 3 N only terminal nitrogen ions, i.e. nitrogen bonded to a single silicon tetrahedron, coordinates to Ce 31 , whereas in Y 4 Si 2 O 7 N 2 both types of N 32 ions coordinate to Ce 31 . We suggest that, when nitrogen bridges two silicon tetrahedra, the bonding with Ce 31 is reduced compared to a terminal nitrogen ion. This reduction in bonding with Ce 31 is assumed to decrease the nephelauxetic effect. Further, it is known that the covalency increases when free (oxygen) ions coordinate to Ce 31 and this appears to be the case for the Y(1) site in Y 5 (SiO 4 ) 3 N:Ce and all sites in Y 4 Si 2 O 7 N 2 :Ce. In YSiO 2 N:Ce and Y 2 Si 3 O 3 N 4 :Ce, however, only oxygen and nitrogen anions, which are linked to silicon, are present. Therefore, we feel that the combined influence of three

effects (the coordination of nitrogen to Ce 31 , presence of non-bridging versus bridging nitrogen, and the presence of free oxygen ions) give the observed range in barycentres of these Y–Si–O–N:Ce compounds.

4.3. Stokes shift The Stokes shift varies significantly for the different Y–Si–O–N:Ce compounds (Table 1). It decreases in the series: Y 5 (SiO 4 ) 3 N:Ce (7100–7200 cm 21 ) . 21 Y 4 Si 2 O 7 N 2 :Ce (5800 cm ) . YSiO 2 N:Ce (3300 cm 21 ) ¯ Y 2 Si 3 O 3 N 4 :Ce (3200*10 3 cm 21 ). The observed value are in the range of Stokes shifts reported by Blasse and Bril for various Ce 31 -activated materials (from 1200 cm 21 for ScBO 3 :Ce to 8000 cm 21 for SrY 2 O 4 :Ce [2]). A large Stokes shift, observed for both luminescent centres in Y 5 (SiO 4 ) 3 N:Ce, was also observed by Lammers and Blasse for its oxidic counterpart Ln 4.67 (SiO 4 ) 3 O:Ce with Ln5La, Gd (8000–9000 cm 21 ) [18]. From the above-mentioned values for the different Y– Si–O–N:Ce compounds, it can be concluded that the Stokes shift decreases with extended interconnection of the silicon tetrahedra, as induced by the incorporation of nitrogen, and resulting in a three-dimensional network. Obviously, a higher degree of network formation leads to an increase of the rigidity of the lattice, which causes a lower Stokes shift. Finally, it is noted that the trend in the Stokes shift is reflected by the room temperature emission intensity, measured for the Y–Si–O–N:Ce compounds. It is high for YSiO 2 N:Ce and Y 2 Si 3 O 3 N 4 :Ce (with small Stokes shifts) and lower for the other two compounds (with large Stokes shifts). A large Stokes shift causes quenching at relatively low temperatures.

4.4. Relation between energy of Tb 31 and Ce 31 5 d excitation bands The energy difference between the lowest 5d-component of Tb 31 and Ce 31 in the same Y-Si-O-N compound is observed to be in the range of 12–15*10 3 cm 21 [22]. This agrees with results in literature where a value of about 14*10 3 cm 21 was found for a couple of compounds [18].

5. Conclusions Luminescence of Ce 31 is reported in four different Y–Si–O–N materials. A variation of the Ce 31 emission is observed with maxima up to 504 nm for Y 4 Si 2 O 7 N 2 :Ce. The position of the emission can be explained by three effects. (1) Crystal-field splitting which becomes larger when more N 32 versus O 22 coordinates to Ce 31 . This is

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ascribed to the higher formal charge of N 32 compared to O 22 . (2) Shift of Ce 31 5d centre of gravity to lower energy with increase in covalency (nephelauxetic effect). Several parameters, such as increased nitrogen versus oxygen coordination, more non-bridging versus bridging nitrogen coordinated to Ce 31 , as well as the presence of free oxygen ions contribute to this effect. Obviously, these parameters partially compensate each other, as none dominates in our series of materials. (3) Stokes shift which becomes smaller for lattices with an extended silicon network formation when more nitrogen is incorporated. The smaller Stokes shift is ascribed to the increasing rigidity of the lattice. The energy difference between the energy of the lowest 5d excitation band of Tb 31 and that of Ce 31 is about 12–15*10 3 cm 21 , which is in accordance with literature.

[6] [7]

[8]

[9]

[10]

[11]

[12]

Acknowledgements [13]

The authors would like to thank M. v.d. Berg and M. v. Dorst for the preparation of the samples. Furthermore, we would like to thank Dr. D. van der Voort (Philips Lighting, Eindhoven, The Netherlands) and Dr. C. Ronda (Philips Research Laboratories, Aachen, Germany) for reading the manuscript.

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