Formation and UV absorption of cerium, europium and terbium ions in different valencies in glasses

Optical Materials 15 (2000) 7±25

www.elsevier.nl/locate/optmat

Formation and UV absorption of cerium, europium and terbium ions in di€erent valencies in glasses H. Ebendor€-Heidepriem *, D. Ehrt Otto-Schott-Institut, Friedrich Schiller University of Jena, Fraunhoferstr. 6, D-07743 Jena, Germany Received 14 December 1999; accepted 22 February 2000

Abstract Cerium, europium and terbium ions can exist in di€erent valencies in glasses. The formation and ultraviolet (UV) absorption features of the ions were studied in a ¯uoride phosphate (FP) and two phosphate glasses. Various melting conditions and X-ray irradiation were applied to change the redox states of the ions. Band separation of the UV absorption spectra was carried out to reveal the components and to determine their spectroscopic properties. The UV absorption spectra of the lower valent Eu2‡ , Tb3‡ and Ce3‡ ions are due to 4f±5d transitions, which are split in several bands by the local ®eld around the rare-earth (RE) ions. The crystal ®eld splitting of the trivalent ions di€ers from the one of the divalent ion. The UV absorption spectra of the higher valent Eu3‡ , Tb4‡ and Ce4‡ ions are caused by charge transfer (CT) transitions from oxygen and ¯uorine to the RE ions. The positions and oscillator strengths of the 4f±5d and CT transitions are studied in dependence on the RE and glass type. Furthermore, the ligand ®eld strength of Eu2‡ ions is investigated. Redox tendency, site symmetry and charge of the RE ions are important factors considering the in¯uence of the RE type. Di€erent polarizability and electron donor power of the ligands as well as di€erent RE site symmetry in the glasses cause the compositional dependence of the transition properties. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Cerium; Europium; Terbium; Absorption; Glasses; Fluorides; Phosphates

1. Introduction Because of their electronic structures, cerium, europium and terbium ions can exist in di€erent valencies. In the most glasses melted by conventional technique in air, the trivalent ions are stable. Eu3‡ serves as an electron acceptor, because it requires one electron to the half-®lled 4f shell. By contrast, Ce3‡ and Tb3‡ serve as electron donors, * Corresponding author. Tel.: +49-3641-948511; fax: +393641-948502. E-mail address: [email protected] (H. Ebendor€Heidepriem).

because Ce3‡ has one electron above the ®lled 5p shell and Tb3‡ has one electron above the half®lled 4f shell [1,2]. Therefore, reducing melting conditions lead to the formation of Eu2‡ ions [3± 12]. Oxidizing melting conditions as well as X-ray and ultraviolet (UV) radiation enable the oxidation to Ce4‡ and Tb4‡ [12±19]. The occurrence of two valencies in the case of cerium, europium and terbium ions a€ects the absorption properties of these ions. The electron donation ability of the lower valent ions facilitates the excitation of an electron from the 4f to the 5d shell. On the other side, the electron acceptance ability of the higher valent ions favours charge

0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 0 ) 0 0 0 1 8 - 5

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H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

transfer (CT) transitions from ligands to the rareearth (RE) ions. As a result, both 4f±5d transitions of the lower valent ions and CT transitions of the higher valent ions are shifted to lower energy into the UV range measurable by conventional spectrophotometers compared with RE ions which do not like to be reduced and oxidized, respectively [1,20]. Since 4f±5d and CT absorptions are allowed electronic transitions, they have high intensities and are sensitive to the host glass matrix [1,2,21]. The properties of the intense UV absorption of cerium, europium and terbium ions have been examined in oxide glasses by several authors [3± 7,13±15,20,22±30]. In the most publications, the pronounced 4f±5d band of Ce3‡ and Tb3‡ at lowest energy has been studied [3,15,22±24,27]. Detailed analysis of the 4f±5d absorption components of Ce3‡ , Tb3‡ and Eu2‡ are reported in [4,6,7,20,25,26,28]. The in¯uence of the ligands at the RE sites on the position of the pronounced 4f± 5d band of Ce3‡ and Tb3‡ has been studied by Reisfeld in glasses, crystals and complexes [20,30]. A nephelauxetic parameter has been de®ned which re¯ects the shift of the peak position in solids and complexes with respect to the free ion. The e€ect of the ligands on the nephelauxetic parameter has been described in terms of covalency between RE ions and surrounding ligands [20]. The nephelauxetic parameter de®ned by Reisfeld is similar to the one de®ned for transition metal ions from the Racah parameters [21]. Furthermore, it resembles the optical basicity de®ned by Du€y from the 6s± 6p transition of Pb2‡ [27]. Du€y [27] has studied the pronounced Ce3‡ 4f±5d band in dependence of the sodium content in a borate and a phosphate glass. He has compared the optical basicity determined from the Ce3‡ band with the optical basicity determined from the Pb2‡ band. The in¯uence of the RE environment on the position of the Eu3‡ CT band has been rationalized in terms of electronegativity by Jùrgensen [31] and has been studied in glasses by Reisfeld [20]. The optical electronegativity has been derived from the observed peak wave number of the CT absorption [31]. By means of CT absorption of transition metal ions, Du€y has studied the optical electronegativity of glasses in dependence on their optical basicity [27].

Investigations on the properties of the 4f±5d and CT transitions of RE ions are of fundamental and practical interest. The 4f±5d and CT transitions of RE ions are used in a wide range of applications. Because of their intense 4f±5d absorption at comparatively low energy, Ce3‡ , Eu2‡ and Tb3‡ are promising ions for Faraday rotators [8±10,33]. The luminescence properties of cerium, europium and terbium ions are of practical importance in phosphors and scintillators [1,34,35]. Recently, the long lasting phosphorescence of these ions after high-energy irradiation has received much attraction [14,36,37]. The photooxidation capability of these ions is responsible for their growing interest in Bragg grating and persistent spectral hole burning [38±40]. Both e€ects are the basis of new photonic elements [41,42]. The host sensitivity of the 4f±5d and CT transitions enables the use of cerium, europium and terbium ions as indicators for structure investigations [20,27,43]. Both their capability of redox state changes and their intense 4f±5d and CT transitions make these ions suitable indicators for examinations of defect center formation in glasses caused by high energy irradiation [3,5,13,15± 17,24,44,45]. In this paper, the absorption properties of the CT and 4f±5d transitions of cerium, europium and terbium ions are studied in di€erent glasses. Various melting conditions and X-ray irradiation were employed to change the valencies of these ions. The absorption spectra are resolved in their components by band separation method. The properties of the 4f±5d and CT transitions are discussed qualitatively in dependence on the RE type and glass composition, i.e. on the RE environment. This fundamental study is the basis for further investigations with regard to applications. Fluoride phosphate (FP) and phosphate glasses were chosen as host materials. They have a high UV transmission [46], which is required for observation of CT and 4f±5d transitions of RE ions. Furthermore, they are attractive candidates for high performance optics and for RE doped photonic elements [46±49]. Thus, investigations of RE ions in these glasses are of special interest.

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

2. Experimental A FP glass, a strontium-metaphosphate (MP) glass and a multicomponent ultraphosphate (UP) glass were used as host materials for the RE ions. The batch compositions of the host glasses are given in Table 1. The FP glass was doped with EuF3 and TbF3 , respectively. The two phosphate glasses were doped with Eu2 O3 and Tb4 O7 , respectively. The concentrations of the RE ions in the batches are reported in Table 1. Raw materials of high purity suitable for optical glasses were used. The FP and MP glass samples were melted from ¯uorides and Sr(PO3 )2 . The UP glass samples were prepared from oxides and carbonates. At ®rst, samples were melted from batches in air using platinum crucibles for the FP glasses and silica glass crucibles for the phosphate glasses. In the case of the FP glass, reducing melting conditions were employed by remelting the starting glasses in carbon crucibles in argon atmosphere at 1100°C. In the case of the MP glass, reducing melting conditions were applied by addition of sugar to the batches and subsequently melting the batches in silica glass crucibles in air. The UP glasses were melted under reducing conditions using several methods: remelting the starting glasses in carbon crucibles in argon atmosphere, addition of sugar or aluminium powder to the batches and then melting the batches in silica glass crucibles in air. In the case of all the three glass types, oxidizing melting conditions were applied by remelting the starting glasses using oxygen bubbling through the melts. The glasses melted in platinum and silica crucibles were poured into graphite moulds and then

9

slowly cooled to room temperature. The glasses melted in carbon crucibles were ®rst rapidly cooled in the crucibles to about 800 K and then slowly cooled to room temperature. After annealing, the glasses were cut and polished into samples for measurement of the optical properties. All glasses prepared are transparent and have an optical quality suitable for optical measurements. Absorption spectra in the range 195±1000 nm were recorded at room temperature using a double-beam spectrophotometer (Shimadzu UV3101PC). Band separations were carried out by least-squares ®tting using a commercial computer software (Jandel Scienti®c Peak®t). The absorptions caused by the RE ions are obtained from the di€erence between the spectra of the doped and undoped glass samples. The absorption spectra at wavelengths shorter than 200 nm have greater measuring error. Furthermore, possible di€erences in the impurity content and/or in the intrinsic transmission cut-o€ of doped and undoped samples would result especially in this spectral region to di€erences in the absorption behavior. Therefore, the separated bands in this wavelength region are not examined in detail, but they are necessary to enable a reliable band analysis of the intense absorption bands at longer wavelengths. 3. Results 3.1. Formation of the RE ions in di€erent valencies By using absorption, ¯uorescence and ESR spectroscopy as well as by comparing the spectra of glasses melted under di€erent redox conditions,

Table 1 Batch composition of the glasses studieda Glass

Batch composition (mol%)

RE ion density (1019 ions cmÿ3 ) Ce

Eu

Tb 1.0 100 1.0 100 1.0 100

FP

10 Sr(PO3 )2 ±10 MgF2 ±30 CaF2 ±15 SrF2 ±35 AlF3

1.5

1.0

MP

100 Sr(PO3 )2

2.0

1.0

UP

65 P2 O5 ±3 MgO±9 CaO±10 ZnO±9 BaO±4 Al2 O3

±

1.0

a

Note that in the case of Tb3‡ two series of doped glasses were prepared.

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H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

the valencies of the rare-earths in the glasses could be determined. In the case of terbium and europium, the trivalent ions are solely formed by melting the glasses from batches in air. Eu2‡ and Tb4‡ ions were not detected. Under reducing conditions by remelting the starting glasses in carbon crucibles in argon atmosphere, Eu2‡ ions were obtained in the FP glass, but were not found in the UP glass. The use of sugar and aluminium powder did not lead to the formation of Eu2‡ ions in the UP glass, too. In the MP glass, reducing melting conditions using sugar resulted in the formation of a small amount of Eu2‡ ions. Tb4‡ ions could not be formed in glasses by oxidizing melting conditions using oxygen bubbling through the melts. However, irradiation with X-rays and UV light is known to produce Tb4‡ in glasses [13,14,17,44]. In all the three glass types, Tb4‡ ions were obtained by irradiating samples with the un®ltered radiation of a Cu-cathode X-ray tube working at 50 kV and 160 mA. Often radiation-induced ions formed from Mn‡ are denoted as (Mn‡ )‡ and (Mn‡ )ÿ , respectively. In this paper, we denote radiation-induced terbium (IV) ions as Tb4‡ in order to emphasize the occurrence of tetravalent ions, which have di€erent properties with respect to trivalent ions. In the case of the FP glass, both Ce3‡ and Ce4‡ ions were observed by melting the glasses in air. Under reducing conditions by remelting the starting glasses in carbon crucibles in argon atmosphere, Ce3‡ ions were solely formed. In the MP glass, Ce4‡ ions were not found by melting in air. This is due to organic impurities in the Sr(PO3 )2 raw material commercial available, resulting in reducing conditions. The application of reducing conditions by using sugar did not lead to changes in the cerium absorption and Ce3‡ ¯uorescence spectra, indicating that reducing conditions were already obtained by melting the batches without sugar. Ce4‡ ions could be produced in the MP glass by oxygen bubbling through the melt. In summary, the trivalent state is the predominant redox state. Eu2‡ ions were formed in the FP glass in a larger amount than in the MP glass. They could not be obtained in the UP glass. Ce4‡ ions occur in the FP and MP glass melted in air

and melted using oxygen bubbling. Tb4‡ were produced in all the three glass types by X-ray irradiation. Because the results from glasses melted in air without reducing agents and the results from glasses melted using oxygen bubbling are similar, both melting conditions are labeled as ``ox''. 3.2. Band separation of the absorption spectra Both 4f±5d and CT transitions are allowed and thus have high extinction coecients [1,2,21]. The degeneracy of the 5d orbitals in free ions is lifted by the crystal ®eld in solids [6,7,20,28,30]. 4f±5d bands have moderate bandwidths (2000±5000 cmÿ1 ), whereas CT transitions are found to cause broad bands (8000±12 000 cmÿ1 ) [5±7,13±15,22± 24,28]. In FP glasses, several transition metal ions give rise to two broad CT bands in the UV and deep UV region (35 000±60 000 cmÿ1 ) [26]. According to these characteristics of 4f±5d and CT transitions as well as according to the electron donor and acceptor properties of the RE ions, several intense 4f±5d absorption bands of Ce3‡ , Eu2‡ and Tb3‡ ions as well as one or more intense and broad CT absorption bands of Ce4‡ , Eu3‡ and Tb4‡ ions are expected to appear in the UV spectra of the glasses investigated. On the basis of these assumptions, band separation was carried out as described below using Gaussian shape for all bands. In Figs. 1, 3±7, 9, the measured spectra are presented as thin lines and the spectra generated by the band analysis are presented as bold lines. The Ce3‡ absorption spectra of FP and MP glasses melted under reducing conditions (FP/red and MP/red) are shown in Fig. 1. At least six bands are necessary for a band separation. According to the interpretations of Ce3‡ spectra in glasses and water [20,30], the ®ve bands in the range 30 000±50 000 cmÿ1 are attributed to the 4f± 5d transition, indicating fully splitting of the 5d excited state (2 D). The band at highest energy (about 52 000 cmÿ1 ) is assigned to the 4f±6s transition. In the case of the FP/red glass, all ®ve 4f±5d bands have similar bandwidths. In order to obtain spectroscopic parameters being comparable with other RE ions and other glass types, band analysis using the same bandwidths for all 4f±5d bands

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

Fig. 1. Band analysis of the Ce3‡ absorption in FP and MP glasses melted under reducing conditions.

proved to be reasonable (Fig. 1). In the case of the MP/red glass, the 4f±5d band at the lowest energy and with the highest intensity has a clearly lower bandwidth that the other four bands of the 4f±5d transition. Thus, reasonable band analysis was obtained if the bandwidth of the intense band was adjustable without constraints and the bandwidths of the other four 4f±5d bands were set to have the same adjustable value (Fig. 1). In FP/ox and MP/ox glasses compared with FP/ red and MP/red glasses, the cerium absorption spectra have a higher intensity over the whole UV region (Fig. 2). At least two broad bands beside the six narrower bands of Ce3‡ are required for a reasonable band analysis (Fig. 3). These broad bands indicate CT transitions of Ce4‡ ions. Because the Ce4‡ bands are covered by the Ce3‡ bands, their parameters are not well determined and thus could only be estimated in a wider range (Table 2).

11

Fig. 2. Cerium absorption in FP and MP glasses melted under reducing and oxidizing conditions. The reduced glasses contain Ce3‡ ions, whereas the oxidized glasses contain both Ce3‡ and Ce4‡ ions.

In the case of the Tb3‡ 5d excited con®guration, the electrostatic interaction between 4f7 and 5d produces two excited states, the lower lying 9 D and the higher lying 7 D. The 7 F±7 D transition is spin allowed and thus has a high intensity. For this reason, the intense Tb3‡ absorption at >45 000 cmÿ1 in the glasses studied (Fig. 4) is attributed to the 7 F±7 D transition. The lower lying 7 F±9 D transition is spin forbidden and thus has a low intensity. In highly Tb3‡ doped glasses, an intense band in the range 35 000±45 000 cmÿ1 is observed, which is assigned to the 7 F±9 D transition (Fig. 5). The band separation of the Tb3‡ 7 F±7 D absorption was carried out similar to the Ce3‡ 2 F±2 D transition, i.e. we expected up to ®ve bands with similar bandwidths. At least three bands are necessary for a reasonable ®tting procedure of the

12

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

Fig. 3. Band analysis of the Ce3‡ and Ce4‡ absorption in FP and MP glasses melted under oxidizing conditions.

Tb3‡ 4f±5d absorption in the measured range (Fig. 4). The distances between the Tb3‡ 4f±5d bands are similar to the distances between the Ce3‡ 4f±5d bands. Furthermore, the bandwidths of the Tb3‡ and Ce3‡ bands have the same order of magnitude. The two Tb3‡ bands at higher energy are not clearly discernible by peaks in the absorption spectrum, and they are overlapped by the intense Tb3‡ band at lower energy. Moreover, they are located in the deep UV region (50 000 cmÿ1 ), where the absorption spectra have a greater measuring error as described in Section 2. For these reasons, the properties of the two bands were not examined in detail and band separation using di€erent bandwidths as in the case of the Ce3‡ absorption in the MP glass did not lead to reliable results. Absorption spectra with clearly separated Tb4‡ CT band were obtained in irradiated glass samples

as described in [44]. Under X-ray irradiation, not only Tb4‡ ions but also intrinsic defect centers absorbing in the visible (VIS) and UV region are obtained. In undoped samples, intrinsic defect pairs composed of hole centers (HC) and corresponding electron centers (EC) are formed due to the release and capture of electrons by precursors in the glass matrix as described in [50]. In Tb3‡ doped glasses, radiation-induced Tb4‡ ions representing HC and corresponding intrinsic EC are formed in addition to intrinsic defect pairs. Different absorption intensities of intrinsic HC were observed in doped and undoped samples. In order to eliminate this e€ect, the radiation-induced spectrum of the undoped sample was normalized to the radiation-induced spectrum of the doped sample in such a way that both spectra have the same intensities in the range of intrinsic HC absorption (6 18 000 cmÿ1 ) after the normalization. Under this condition, the di€erence between the spectrum of the doped sample and the normalized spectrum of the corresponding undoped sample is assumed to represent the absorption spectrum caused by Tb4‡ HC and corresponding intrinsic EC, whereas the absorption due to intrinsic defect pairs is removed. Such di€erence spectra of FP/ox, MP/ox and UP/ox glasses irradiated with X-rays are shown in Fig. 6. EC are known to absorb in the UV region at 6 33 000 cm ÿ1 [50,51]. Intrinsic HC absorbing in the VIS region is eliminated in the di€erence spectra as described above. Thus, the broad band at around 27 000 cmÿ1 is attributed to Tb4‡ CT transition, whereas the absorption at higher energy is due to the EC corresponding to the Tb4‡ HC. The position and width of the negative absorption band at about 47 000 cmÿ1 equals the Tb3‡ 4f±5d transition. The negative intensity indicates a decrease of the Tb3‡ concentration due to the irradiation. This con®rms the radiation-induced formation of Tb4‡ ions in the glasses. A weak negative absorption band in the VIS region has to be used for a reasonable ®t. The position and width of this band corresponds to one-absorption band of intrinsic HC [50]. The occurrence of this band indicates that the proportions between the various bands of the intrinsic HC are somewhat di€erent in doped and undoped samples. It is noticed that the intensity of this HC band is much

4f±5d

CT

CT

CT

Tb4‡

Ce4‡

Eu3‡

8

S±8 H

F±2 S

2

4f±6s

Eu2‡

F±2 D

2

4f±5d

F±7 D

7

Ce3‡

F±9 D

7

4f±5d

Tb3‡

Transition

41.0 ‹0.1 47.6 0.1 36.0 38.7 41.7 45.3 49.0 0.1 mb ˆ 41:1 52.5 0.1 34.1 0.1 40.9 0.1 mb ˆ 39:0 26.6 0.2 40 2 47.0 0.1 9.4 0.4 10.0 1.0 8.7 0.3

3.5 0.5 6.8 0.1 7.6 0.2

4.0 0.5 3.0 0.1 3.5 3.5 3.5 3.5 3.5 0.1

780 420 720 300 110 10

90 5 780 120 1760 300

0.6 0.1 250 20 300 370 320 220 160 15

mp FWHM e (103 cmÿ1 ) (103 cmÿ1 ) (l molÿ1 cmÿ1 )

FP

34 18 34 13 4.2 0.4

1.4 0.2 25 5 62 10

0.012 0.002 3.5 0.2 4.9 5.8 5.1 3.6 2.5 0.2

40.2 0.1 46.3 0.1 34.0 36.3 39.8 43.5 47.4 0.1 mb ˆ 38:9 51.2 0.2 31.7 0.3 40.7 0.1 mb ˆ 38:7 27.5 0.1 37 2 48.1 0.1 10.7 0.3 10.0 1.0 8.8 0.3

3.5 0.5 6.2 0.4 9.4 0.9

3.9 0.5 2.5 0.1 2.6 3.4 3.4 3.4 3.4 0.1

FWHM P (10ÿ3 ) mp (103 cmÿ1 ) (103 cmÿ1 )

MP

2230 300 2230 720 360 30

240 10 1300 600 2900 1200

2.5 0.2 550 40 640 260 260 250 200 15

e (l molÿ1 cmÿ1 )

109 16 100 40 14 1

3.9 0.6 37 15 127 50

0.045 0.008 6.5 0.5 7.6 4.1 4.0 4.0 3.1 0.2

P (10ÿ3 )

49.4 0.1

27.4 0.1

40.2 0.1 46.7 0.1

mp (103 cmÿ1 )

UP

8.1 0.2

10.9 0.3

3.5 0.4 2.2 0.1

FWHM (103 cmÿ1 )

480 60

3010 180

3.6 0.2 810 60

e (l molÿ1 cmÿ1 )

17 2

151 16

0.059 0.010 8.3 0.5

P (10ÿ3 )

Table 2 Peak wave number, mp , barycenter of the 5d orbitals, mb , full width at half maximum, FWHM, molar extinction coecient, e, oscillator strength, P, of the separated bands H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25 13

14

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

Fig. 4. Band analysis of the Tb3‡ 4f8 (7 F)±4f7 5d (7 D) absorption in FP, MP and UP glasses melted under oxidizing conditions.

lower in the di€erence spectra than in the spectra of the undoped samples. Furthermore, this band has only little in¯uence on the intense and broad Tb4‡ absorption, but it is necessary to enable a reasonable ®t. The positions and widths of the intrinsic defect center bands were ®xed according to [50]. The position and width of the Tb3‡ 4f±5d band were ®xed according to the results of the

Fig. 5. Band analysis of the Tb3‡ 4f8 (7 F)±4f7 5d (9 D) absorption in highly doped FP, MP and UP glasses melted under oxidizing conditions. The weak and narrow bands are due to 4f±4f transitions.

band separation of the unirradiated Tb3‡ doped glasses samples. The position and width of the Tb4‡ CT band and the intensities of all bands were determined by the ®t procedure. The Eu3‡ absorption spectra of the FP/ox, MP/ox and UP/ox glasses are shown in Fig. 7. First

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

Fig. 6. Band analysis of the radiation-induced absorption spectra of terbium doped FP, MP and UP glasses melted under oxidizing conditions.

in each case, the spectrum was only ®tted between about 35 000 cmÿ1 and the peak wave number using one broad band. Subsequently, the whole spectrum was ®tted using an additional band in the deep UV (DUV) region (>50 000 cmÿ1 ), whereas the position of the broad band in the UV region (<50 000 cmÿ1 ) was ®xed as determined beforehand. The band separation of the whole spectrum

15

Fig. 7. Band analysis of the Eu3‡ absorption in FP, MP and UP glasses melted under oxidizing conditions.

con®rmed that the intensity of the DUV band is negligible at the peak wavelength of the broad UV band as assumed in the ®rst ®t (Fig. 7). The broad band in the UV region is attributed to a CT transition from ligands to Eu3‡ ions. The absorption in the DUV region may be caused by a second CT transition as found for Ce4‡ (see above) and for several transition metal ions [26]. Besides, this absorption may be due to di€erent DUV absorption

16

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

intensity of doped and undoped glass samples as mentioned in Section 2. Because of these uncertainties, the properties of the second band at higher energy is not of interest. In the europium doped FP/red and MP/red glasses, both Eu2‡ and Eu3‡ ions are formed (Fig. 8). Since the absorption of the Eu3‡ ions is known from the oxidized glasses, the Eu3‡ absorption could be eliminated in the europium spectra of the reduced glasses. In the case of the FP/red glass, the amount of Eu3‡ ions could be estimated by ¯uorescence spectroscopy. By excitation at about 400 nm, intense Eu3‡ ¯uorescence at about 610 nm is observed. The ¯uoresence is not quenched up to 1 ´ 1021 Eu3‡ cmÿ3 . Eu2‡ absorption is neglibible at P 400 nm. Therefore, the Eu3‡ ¯uorescence intensity can be used to estimate the

Fig. 8. Europium absorption in FP and MP glasses melted under reducing and oxidizing conditions. The reduced glasses contain both Eu2‡ and Eu3‡ ions, whereas the oxidized glasses contain solely Eu3‡ ions.

Eu3‡ concentration in samples of the same glass composition. The observed decrease of the intensity in the FP/red samples compared with the FP/ ox sample indicates that about 20±40% of the Eu3‡ ions are reduced to Eu2‡ ions. Thus, 70% of the europium ions are assumed to be Eu3‡ ions in the FP/red glass. Accordingly, the Eu3‡ spectrum of the FP/ox glass was multiplied by 0.7 and subtracted from the europium spectrum of the FP/red glass. The resulting spectrum (Fig. 9) indicates Eu2‡ absorption. It is noted that the assumption of 50±100% Eu3‡ ions did not lead to large changes of the Eu2‡ absorption spectrum because of the low intensity of the Eu3‡ absorption compared with the Eu2‡ absorption (Fig. 8). In the case of the MP glass, the intensity at the peak wave number of the Eu3‡ CT band is almost equal in both the reduced and the oxidized glass. This result indicates that only a small fraction of the Eu3‡ ions are reduced to Eu2‡ ions in the MP/red

Fig. 9. Band analysis of the Eu2‡ absorption in FP and MP glasses melted under reducing conditions.

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

glass. Thus, the occurrence of 90±100% Eu3‡ ions is assumed in the determination of the Eu2‡ absorption by subtracting the appropriate Eu3‡ spectrum. The best-®t correlation coecient could be obtained by assuming 98% Eu3‡ ions in the MP/ red glass. The corresponding Eu2‡ spectrum is shown in Fig. 9. In both the FP/red and the MP/ red glass, the intense Eu2‡ absorption in the range 25 000±50 000 cmÿ1 could be separated with at least two bands (Fig. 9). In the FP/red glass, a band at about 50 000 cmÿ1 was required in addition to the two 4f±5d bands for a reasonable band analysis. The origin of this band is unknown. 3.3. Characterization and assignment of the separated bands 3‡

Five 4f±5d bands are found for Ce , indicating fully lifting of the degeneracy of the 5d orbitals. This result is consistent with Ce3‡ absorption in other glasses [20]. In the case of Tb3‡ , three bands are resolved in the measured spectral region. The similarity of the bandwidths and of the band distances for Ce3‡ and Tb3‡ suggests that the Tb3‡ 5d orbitals are fully split, too. The 4f±5d absorption of the divalent Eu2‡ ions consists of two bands with clearly higher bandwidths than the bands of the trivalent ions. The two bands are attributed to a splitting of 5d orbitals into eg and t2g orbitals. The fully splitting of the 5d orbitals in the case of the trivalent Ce3‡ and Tb3‡ ions indicates sites with symmetries lower than cubic, whereas the splitting of the 5d orbitals to two levels in the case of the divalent Eu2‡ ions demonstrates cubic sites of higher symmetry. Symmetry lower than cubic is con®rmed for trivalent RE ions by M ossbauer spectroscopy [52]. Fluorescence line narrowing spectroscopy and molecular dynamics simulations have shown that the RE3‡ site symmetry in oxide and ¯uoride glasses is very low, C2v or C1 [53±57]. Moreover, electron spin resonance spectroscopy has proven that axially symmetric RE3‡ sites do not exist in glasses [58]. The separation of the Eu2‡ 4f±5d transitions into two bands agrees with examinations of Eu2‡ absorption spectra in silicate glasses. 8- and 12-fold coordinated sites of cubic symmetry are proposed, resulting in a splitting to

17

eg orbitals at lower energy and t2g orbitals at higher energy [6,7]. The higher valent ions, Ce4‡ , Eu3‡ and Tb4‡ cause broad CT bands due to charge transfer from oxygen and ¯uorine to RE ions. In the case of Eu3‡ and Tb4‡ , one pronounced CT band is observed. In the case of Ce4‡ , two broad bands are found. The properties of the Ce4‡ bands could only be estimated in a wide range, since they are covered by the intense 4f±5d bands of the Ce3‡ ions. It was not possible to prepare glasses containing mainly Ce4‡ ions. 3.4. Spectroscopic parameters of the separated bands The separated bands are characterized by peak wave number, mp , full width at half maximum, FWHM, and intensity. The molar extinction coef®cients, e, are calculated from the peak intensities by e ˆ 10ÿ3 NA

E ; dN

…1†

where NA is the Avogadro number, d the sample thickness, N the ion density and E is the absorbance or optical density de®ned from the measured intensity, I, as E ˆ lg …I0 =I †:

…2†

The oscillator strengths, P, are determined from the integral band intensities by Carnall et al. [59] Z mc2 2:303  e…m† dm; …3† P ˆ 2 pe NA where m is the eletron mass, c the velocity of light, e the electron charge and m is the wave number. The characteristic parameters of the bands, mp , FWHM, e and P are reported in Table 2. The average position of the 4f±5d transition, which equals the average energy of the 5d orbitals, is characterized as barycenter, mb , by  R P Ei …m† dm i mp;i  R P ; …4† mb ˆ Ei …m† dm i where i designates the 4f±5d bands. The barycenters of Ce3‡ and Eu2‡ absorption are given in Table 2.

18

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

In the case of Eu2‡ absorption, the ligand ®eld strength is re¯ected by the energy di€erence 10 Dq between the eg and t2g orbitals of the 5d con®guration. In the case of Ce3‡ absorption, a ligand ®eld strength parameter comparable with Eu2‡ 10 Dq parameter and/or comparable for di€erent glass compositions cannot be extracted from the spectra because of di€erent splitting compared with Eu2‡ and di€erent shape in FP and MP glasses. The peak wave number of the ®rst 4f±5d bands and the one of the CT bands in the MP glass of this study agree with data of MP glasses from other authors (Table 3). 3.5. In¯uence of the RE type on the properties of the 4f±5d and CT transitions In FP and MP glasses, the absorption properties of all ions of the three corresponding RE ion pairs under consideration have been determined. Therefore, the in¯uence of the RE type on the barycenters, oscillator strengths and crystal ®eld strengths of the 4f±5d and CT transitions has been studied for these two glass types. Both the peak wave number of the ®rst 4f±5d band and the barycenter of all 4f±5d bands decreases from Ce3‡ to Eu2‡ , indicating similar dependence of both parameters on the RE type. The barycenter of the Tb3‡ 4f±5d absorption in the glasses investigated is unknown since a part

of the 4f±5d absorption is outside the measurable range. However, the similar behavior of the ®rst peak wave number and of the barycenter of Ce3‡ and Eu2‡ as well as the similar shape of the Tb3‡ and Ce3‡ absorption suggest that the peak wave number of the ®rst Tb3‡ band relative to the ®rst bands of Ce3‡ and Eu2‡ re¯ects the position of the barycenter of the Tb3‡ absorption with respect to the absorptions of the other two RE ions. The RE type has an opposite e€ect on the barycenters of the 4f±5d and CT transitions. The barycenter of the 4f±5d transitions decreases in the order Tb3‡ > Ce3‡ > Eu2‡ , whereas the barycenter of the ®rst CT transition at lowest energy increases in the order Tb4‡ < Ce4‡ < Eu3‡ (Fig. 10). As a result, the energy di€erence between the 4f±5d transition of the lower valent RE ion and the CT transition of the corresponding higher valent RE ion is di€erent for the three corresponding RE ion pairs studied. The Tb3‡ 4f±5d transition has a higher energy than the Tb4‡ CT transition, the Ce3‡ 4f±5d and Ce4‡ CT transition are observed in the same spectral region, and ®nally the Eu2‡ 4f± 5d transition has a lower energy than the Eu3‡ CT transition. The same dependence of the 4f±5d and CT energies on the RE type is found from data of phosphate and silicate glasses of other studies (Table 3). For both the ®rst 4f±5d band and the ®rst CT transition, the oscillator strengths of terbium and

Table 3 Wave number of the 4f±5d and CT transitions of europium, cerium and terbium ions in metaphosphate and sodium silicate glasses Glass composition (mol%)

Position (103 cmÿ1 )

Reference

First 4f±5d band Metaphosphate 50 CaO±50 P2 O5 50 SrO±50 P2 O5 50 BaO±50 P2 O5 50 Na2 O±50 P2 O5 Sodium-silicate 25 Na2 O±75 SiO2 25 Na2 O±75 SiO2 22 Na2 O±3 CaO±75 SiO2 22 Na2 O±3 CaO±75 SiO2

CT band

Eu2‡

Ce3‡

Tb3‡

Eu3‡

Ce4‡

Tb4‡

31.5 31.7

34.0

46.3 46.3 46.2

48.5 48.1 49.0

37

27.5

44.4

48.5

33.6 29.0 30.2

31.9 31.8

41.5 41.7

30.7

[5,28] This work [5,28] [27] [13] [24] [15] [3]

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

cerium are similar, but those of europium are di€erent (Fig. 11). In the case of the 4f±5d transitions, the higher valent Tb3‡ and Ce3‡ ions exhibit lower oscillator strengths than the lower valent Eu2‡ . By contrast, in the case of the CT transition, the higher valent Tb4‡ and Ce4‡ ions exhibit

Fig. 10. Peak wave number of the ®rst 4f±5d band (mp;1 ) and barycenter (mb ) of the 4f±5d and ®rst CT transition as a function of RE and glass type.

Fig. 11. Oscillator strength of the ®rst 4f±5d band (P1 ), of all 4f±5d bands (P) and of the ®rst CT band (P) as a function of RE and glass type.

19

higher oscillator strengths than the lower valent Eu3‡ ions. 3.6. In¯uence of the glass type on the properties of the 4f±5d and CT transitions Except for Ce4‡ , both the peak wave number of the ®rst 4f±5d band and the barycenter of the 4f± 5d transition are shifted to lower energy from the FP glass to the phosphate glasses (MP and UP), whereas the energy of the ®rst CT transition increases in this direction (Fig. 10). Because of the large uncertainty in the separation of the Ce4‡ CT bands due to superposition with the Ce3‡ 4f±5d bands, reliable results about the compositional dependence of the Ce4‡ CT energy were not obtained. The energy of the ®rst Tb3‡ 4f±5d band as well as of the Eu3‡ CT band increases from the MP to the UP glass (Fig. 10). The uncertainty in the Tb4‡ CT absorption due to superposition with defect center absorption prevents reliable results if the Tb4‡ CT energy is di€erent in MP and UP glasses. These results demonstrate that the compositional dependence of the 4f±5d and CT transition energies is similar for the two phosphate glasses studied. The energies increase from MP to UP glasses. By contrast, the transition energies of the FP glass compared with the phosphate glasses are di€erent for 4f±5d and CT transitions. The 4f±5d energy decreases but the CT energy increases from the FP to the phosphate glasses. The di€erent shape of the Ce3‡ 4f±5d absorption in the MP glass compared with the FP glass is conspicuous (Fig. 1). In the MP glass, the ®rst band has a clearly higher intensity than the other bands. In the FP glass, such a pronounced band is not observed. Similarly, the intensity of the ®rst Tb3‡ 4f±5d band increases with respect to the other 4f±5d bands in the order FP < MP < UP glasses (Fig. 4). It is noted that in silicate glasses Ce3‡ exhibits a pronounced 4f±5d bands like in phosphate glasses [3,15,24]. The oscillator strength of both 4f±5d and CT transitions increases with increasing phosphate content in the order FP < MP < UP (Fig. 11). The Eu2‡ 10 Dq parameter increases from the FP to the MP glass (Fig. 12).

20

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

Fig. 12. Ligand ®eld strength, 10 Dq, as a function of RE and glass type.

4. Discussion 4.1. In¯uence of the RE type The energy for the excitation of an electron from the 4f to the 5d shell decreases with increasing electron donor strength of the RE ion, i.e. with increasing capability of the ion to be oxidized [1]. Therefore, the shift of the 4f±5d transition to lower energy in the order Tb3‡ > Ce3‡ > Eu2‡ suggests an increase of the tendency for oxidation in the order Tb3‡ < Ce3‡ < Eu2‡ . The energy of a CT transition from ligands to metal ions decreases with increasing electron acceptor strength of the metal ion, i.e. with increasing capability of the ion to be reduced [1,2]. Therefore, the shift of the CT transition to lower energy in the order Eu3‡ > Ce4‡ > Tb4‡ suggests an increase of the tendency for reduction in the order Eu3‡ < Ce4‡ < Tb4‡ . This agrees with the oxidation tendency of the corresponding lower valent ions: Eu2‡ > Ce3‡ > Tb3‡ . The trends of the oxidation±reduction tendencies derived from the energy of the 4f±5d and CT transitions are con®rmed by the occurrence of the di€erent RE valencies in the glasses. By melting glasses in air, Eu3‡ occurs predominantly, which indicates high oxidation potential of Eu2‡ . Both Ce3‡ and Ce4‡ ions are present in glasses

melted in air and under oxidizing conditions. Tb4‡ ions could only be obtained by high-energy irradiation, which indicates low oxidation potential of Tb3‡ . The clear larger stability of Tb3‡ and Eu3‡ compared with Tb4‡ and Eu2‡ by melting glasses in air or under oxidizing conditions is consistent with the clearly higher energy of the Tb3‡ 4f±5d and the Eu3‡ CT absorption compared with the Tb4‡ CT and Eu2‡ 4f±5d absorption. The oscillator strengths of both 4f±5d and CT transitions depend in the same manner on the charge of the RE ions. The trivalent RE ions demonstrate lower oscillator strengths than the divalent and tetravalent ions. Di€erent RE site symmetry as re¯ected by the di€erent splitting of Eu2‡ and Ce3‡ 4f±5d transitions suggests an in¯uence of the RE ions charge on the RE site symmetry which is known to a€ect the oscillator strength. 4.2. In¯uence of the glass type In the glasses studied, ¯uorine and oxygen ions serve as ligands for the RE ions. The oxygens are bonded to phosphorous, whereby di€erent types of PO4 tetrahedra occur in the glasses. Usually, PO4 tetrahedra are characterized as Qn groups, where n is the number of bridging oxygens [60]. In the FP glass, mono- and di-phosphate (Q0 and Q1 ) groups are bonded into the chains of ¯uoroaluminates [18]. The MP glass contains mainly Q2 groups, which form long polyphosphate chains [60]. The UP glass has a cross-linked phosphate network composed of Q2 and Q3 groups [60,61]. According to these structures of the phosphate components in the glasses, the fraction of non-bridging oxygens in the PO4 groups increases in the order UP < MP < FP, which leads to an increase of the oxygen polarizability and electron donor power in the same order. It is noted that the bonding of the Q0 and Q1 groups in the FP glass into the ¯uoroaluminates decreases the oxygen polarizability and electron donor power compared with Q0 and Q1 groups, which are surrounded by network modi®er cations. Nevertheless, NMR and Raman spectroscopy indicate that the oxygen polarizability and electron donor power of the Q0 and Q1

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

groups in the FP glass is higher than the one of the Q2 and Q3 groups in phosphate glasses [18,62]. The di€erent shape of the 4f±5d absorption spectra in the FP glass on one side and the phosphate glasses on the other side indicates di€erent RE site symmetry in both glass types. The 4f±5d transition energy decreases the higher the covalency between RE ions and ligands due to lower interelectronic repulsion [20]. The covalency increases with increasing ligand polarizability. In the FP glass, ¯uorine and oxygen ions serve as ligands. In the phosphate glasses, the RE ions are solely surrounded by oxygen ions. The presence of ¯uoride ions having lower polarizability than oxide ions results in a lower degree of the entire covalency of all bonds of a RE site in the FP glass compared with the phosphate glasses. The higher oxygen polarizability in the MP glass compared with the UP glass is responsible for the higher covalency of the RE sites in the MP glass. Thus, the covalency of the RE sites increases in the order FP  UP < MP glasses. As a result, the 4f± 5d transition decreases in the same manner: FP  UP > MP (Table 2, Fig. 10). This behavior is consistent with results on the ®rst Tb3‡ 4f±5d band in various glasses. A decrease of the peak wave number with increasing ligand polarizability of the RE sites was found [43]. The lower ligand polarizability of the RE sites in the FP glass agrees with the low basicity found for this glass [32]. The energy of CT transitions from ligands to RE ions decreases with increasing electron donor power of the ligands. Among the phosphate glasses, the electron donor power of oxygens is higher in the MP than in the UP glass, leading to lower CT energy of the MP glass. This behavior coincides with the lower 4f±5d transition energy of the MP glass due to higher oxygen polarizability compared with the UP glass. By contrast, the CT energy of the FP glass is lower but its 4f±5d energy is higher compared with phosphate glasses. Although ¯uorine ions have a lower electron donor power due to higher electronegativity compared with oxygen ions, the RE ions in the FP glass exhibit a lower CT energy compared with the phosphate glasses. Two e€ects are proposed to explain this discrepancy. On one hand, the electron donor power of the oxygens is assumed to dominate the

21

CT energy, whereas the ¯uorine ions have only a minor in¯uence on the CT energy by a€ecting the electron donor power of the oxygens and/or by contributing to delocalized ligand orbitals which supply the electron of the CT process. Since the FP glass has a higher oxygen electron donor power due to Q0 and Q1 PO4 groups compared with Q2 and Q3 groups in the phosphate glasses, the CT energy is proposed to be lower in the FP glass than in the phosphate glasses. This e€ect is enhanced by the fact that RE ions in FP glasses favour oxygens as ligands [63,64]. On the other hand, the di€erent RE site symmetry in the FP glass as found from the di€erent shape of the 4f±5d absorption spectra is assumed to cause the lower CT energy in the FP glass. In this picture, the ¯uorine ions have an in¯uence on the CT energy by a€ecting the RE site symmetry. The predominance of the oxygen electron donor power and the minor in¯uence of the ¯uorine ions is substantiated by the results on the position of the ®rst CT transition of Cu2‡ ions in the FP glass and in sodium phosphate glasses [32]. The energy of this CT transition is in the 2Na2 O±3P2 O5 UP glass (44 800 cmÿ1 ) higher than in the FP glass (43 500 cmÿ1 ), which agrees with the higher CT transition energy of the UP glass in this work. By contrast, the 3Na2 O±2P2 O5 glass has a lower CT transition energy (42 400 cmÿ1 ) than the FP glass. Because of its large Na2 O content, this glass does not contain Q3 PO4 groups but many Q1 PO4 groups similar to the FP glass. Despite this similarity in the PO4 groups, the FP glass has a higher CT transition energy, which is due to the occurrence of ¯uorine ions decreasing the entire electron donor power at the metal ion sites in the FP glass. Comparing the compositional dependence of the 4f±5d and CT transition energies, an uniform behavior is found among the phosphate glasses, but an opposite behavior is observed for the FP glass compared with the phosphate glasses. Further examples about such phenomena are found for other hosts. An uniform compositional dependence is observed in RE complexes which di€er in their anions. The position of the ®rst 4f±5d band of Tb3‡ and Ce3‡ as well as the position of the Eu3‡ CT transition shifts to lower energy as the ligands in

22

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

RE complexes are changed in the order Fÿ > O2ÿ > Clÿ > Brÿ [20]. The electronegativity of the ligands decreases in the same manner, indicating increasing electron donor power. The polarizability of the ligands increases in the same order as the electron donor power. Due to this uniform behavior, the 4f±5d and CT transition energies depend in the same manner on the ligand type. The occurrence of an opposite compositional dependence of the 4f±5d and CT transition energies is substantiated by the properties of cerium and terbium ions in silicate glasses with respect to phosphate glasses (Table 3). In silicate glasses, the ®rst Ce3‡ and Tb3‡ 4f±5d band are shifted to lower energy, but the Ce4‡ and Tb4‡ CT transition are shifted to higher energy. The low network modi®er content of the silicate glasses under consideration suggests that they are composed of Q4 and Q3 SiO4 groups which have a lower number of non-bridging oxygens compared with the PO4 groups in the phosphate glasses. As a result, the oxygens in the silicate glasses exhibit a lower electron donor power, leading to a higher CT energy. Otherwise, the lower ®eld strength of Si4‡ compared with P5‡ increases the polarizability of the oxygens, leading to higher covalency between oxygen and RE ions. This decreases the 4f±5d transition energy from phosphate to silicate glasses. The opposite compositional dependence of the 4f±5d and CT transitions reveals di€erent dependence of the two types of transitions on the local structure in the RE environment. The 4f±5d transition energy is strongly a€ected by the average polarizability of all ligands at the RE sites. Thus, the presence of ¯uorine ions around RE ions in the FP glass and the higher polarizing e€ect of P5‡ on oxygen due to higher ®eld strength with respect to Si4‡ lead to a decrease of the 4f±5d transition energy in the order FP > phosphate > silicate glasses. Otherwise, the CT transition energy is more dependent on the electron donor power of the ligands, whereby the ligands with lower electronegativity such as oxygen have a dominant in¯uence. Therefore, the higher oxygen electron donor power in phosphate glasses compared with silicate glasses results in a lower CT energy of Ce4‡ and Tb4‡ in phosphate glasses. Similarly, the higher oxygen electron donor power in the FP glass

compared with the phosphate and silicate glasses is responsible for the lower CT energy of Eu3‡ and Tb4‡ in the FP glass. The higher electronegativity of the ¯uorine ions in the FP glass seems to have only a minor in¯uence on the average electron donor power of the RE environment. Furthermore, di€erent RE site symmetry in the FP glass compared with oxide glasses may be of importance. The increase of the oscillator strengths of both 4f±5d and CT transitions with increasing phosphate content is attributed to di€erent RE site symmetry in the three glass types studied as re¯ected by the di€erent 4f±5d absorption shapes of the glasses. The higher 10 Dq parameter of Eu2‡ 4f±5d absorption in the MP glass compared with the FP glass indicates stronger ligand ®eld. The absence of ¯uoride ions in the MP glass results in a higher electrostatic interaction and covalency between Eu2‡ and ligands. 5. Summary and conclusions Using di€erent melting conditions and X-ray irradiation, cerium, europium and terbium ions were produced in di€erent valence states in a FP, MP and UP glass. The UV absorption spectra of these ions were analyzed by band separation procedure. The lower valent ions give rise to several 4f±5d bands having moderate bandwidths. The ligand ®eld splitting of the 4fn±1 5d excited con®gurations is di€erent for trivalent and divalent ions. In the case of the trivalent Ce3‡ and Tb3‡ ions, fully splitting of the 5d orbitals were found, indicating RE sites of low symmetry. In the case of Eu2‡ ions, the splitting of the 5d orbitals into eg and t2g orbitals suggests cubic RE sites of higher symmetry. The higher valent ions cause broad bands, which are attributed to CT transitions from oxygen and ¯uorine to RE ions. The properties of the 4f±5d and CT transitions are a€ected by the RE type to a large extent as well as by the glass type to a lower extent. The in¯uence of the RE type on the transition energies re¯ects the oxidation±reduction tendency of the RE ions. The decrease of the 4f±5d energy in

H. Ebendor€-Heidepriem, D. Ehrt / Optical Materials 15 (2000) 7±25

the order Tb3‡ > Ce3‡ > Eu2‡ is consistent with the increasing tendency of the ions for oxidation. Otherwise, the decrease of the CT energy in the order Eu3‡ > Ce4‡ > Tb4‡ agrees with the increasing tendency of the ions for reduction. Furthermore, the higher Tb3‡ 4f±5d energy compared with the Tb4‡ CT energy as well as the higher Eu3‡ CT energy compared with Eu2‡ 4f±5d energy re¯ect the high redox stability of Tb3‡ and Eu3‡ in glasses melted under conventional conditions in air. In¯uence of the RE ions charge on the RE site symmetry is assumed to be responsible for the lower oscillator strength of the trivalent ions compared with the divalent and tetravalent ions. Considering the in¯uence of the glass matrix, the 4f±5d energy depends on the average polarizability of all ligands of the RE sites, whereas the CT energy is dominated by the electron donor power of oxygens and/or by symmetry e€ects. The low electron donor power of ¯uorine ions due to high electronegativity has a minor in¯uence on the CT energy. Comparing the FP glass with the phosphate glasses, the lower polarizability of ¯uorine ions compared with oxygen ions causes the higher 4f±5d energy in the FP glass. Otherwise, the higher electron donor power of the oxygen ions in the FP glass due to higher fraction of non-bridging oxygens in the PO4 groups and/or the di€erent RE site symmetry in the FP glass give rise to a lower CT energy. Comparing the two phosphate glasses studied, the higher polarizability and the higher electron donor power of the oxygens in the MP glass due to higher fraction of non-bridging oxygens in the PO4 groups lead to both lower 4f±5d and CT energy. The oscillator strengths of the 4f±5d and CT transitions increase in the order FP < MP < UP glasses. Di€erent RE site symmetry in the glasses may be a reason for this behavior. The crystal ®eld strength of the Eu2‡ sites increases from the FP to the MP glass due to higher charge and polarizability of the oxide ions compared with the ¯uoride ions, resulting in a higher electrostatic interaction and covalency between the ligands and the RE ions. The in¯uence of the RE and glass type on the positions of the 4f±5d and CT transitions is substantiated by data from other studies.

23

Acknowledgements We are grateful to P. Arnold, H. Storandt and R. Atzrodt from the University of Jena for the preparation of the cerium doped glasses. References [1] G. Blasse, B.C. Grabmeier, Luminescent Materials, Springer, Berlin, 1994. [2] J.C. Krupa, M. Que€elec, UV and VUV optical excitations in wide band gap materials doped with rare-earth ions, 4f±5d transitions, J. Alloys Comp. 250 (1997) 287. [3] V.I. Arbuzov, M.N. Tolstoi, M.A. Elerts, Ya.S. Trokshs, Photo-induced recharching of rare-earth metal ions in glasses and metastable valence state of the activator, Fizika i Khimiya Stekla (Glass Physics and Chemistry) 13 (1987) 581. [4] A.V. Dmitriuk, N.D. Soloveva, N.T. Timofeev, Spectroscopic photo-induced electron transfer in phosphate glasses doped with europium activator, Fizika i Khimiya Stekla (Glass Physics and Chemistry) 19 (1993) 33. [5] V.I. Arbuzov, N.S. Kovaleva, Radiation-induced reduction of Eu3‡ ions and its in¯uence on color centers formation in phosphate glass, Fizika i Khimiya Stekla (Glass Physics and Chemistry) 20 (1994) 492. [6] M. Nogami, Y. Abe, Enhanced emission from Eu2‡ ions in sol±gel derived Al2 O3 ±SiO2 glasses, Appl. Phys. Lett. 69 (1996) 3776. [7] M. Nogami, T. Yamazaki, Y. Abe, Fluorescence properties of Eu3‡ and Eu2‡ in Al2 O3 ±SiO2 glass, J. Lumin. 78 (1998) 63. [8] K. Tanaka, K. Fujita, N. Soga, J. Qiu, K. Hirao, Faraday e€ect of sodium borate glasses containing divalent europium ions, J. Appl. Phys. 82 (1997) 840. [9] J. Qiu, K. Tanaka, K. Hirao, Preparation and Faraday e€ect of ¯uoroaluminate glasses containing divalent europium ions, J. Am. Ceram. Soc. 80 (1997) 2696. [10] K. Tanaka, K. Fujita, N. Matsuoka, K. Hirao, N. Soga, Large Faraday e€ect and local structure of alkali silicate glasses containing divalent europium ions, J. Mater. Res. 13 (1998) 1989. [11] J. Qiu, K. Miura, N. Sugimoto, K. Hirao, Preparation and ¯uorescence properties of ¯uoroaluminate glasses containing Eu2‡ ions, J. Non-Cryst. Solids 213/214 (1997) 266. [12] H. Ebendor€-Heidepriem, D. Ehrt, Electron spin resonance spectra of Eu2‡ and Tb4‡ ions in glasses, J. Phys. Condens. Matter 11 (1999) 7627. [13] V.I. Arbuzov, M.A. Elerts, Photostimulated electron transfer between rare-earth coactivators in alkali silicate glasses, Sov. J. Glass Phys. Chem. 18 (1992) 216. [14] H. Hosono, T. Kinoshita, H. Kawazoe, M. Yamazaki, Y. Yamamoto, N. Sawanobori, Long lasting phosphorescence properties of Tb3‡ ± activated reduced calcium aluminate glasses, J. Phys. Condens. Matter 10 (1998) 9541.

24

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[36]

[37]

[38] [39]

[40]

[41] [42] [43] [44]

[45]

[46] [47]

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