Structure and optical properties of Eu3+-doped fluoroaluminate and fluorophosphate glasses

ELSEVIER Journal of Non-Crystalline Solids 222 (1997) 212-220

]OU~NAL OF

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Section 6. Optical properties

Structure and optical properties of Eu3+-doped fluoroaluminate and fluorophosphate glasses

Hiroyuki Inoue*, Kohei Soga, Akio Makishima

Department of Materials Science, School of Engineering, The University of Tokyo, Hong, Tokyo 113, Japan

Abstract

Emission spectra of Eu 3+ in fluoroaluminate and fluorophosphate glasses at 80 K were measured as a function of excitation energy by the fluorescence line narrowing (FLN) technique. The magnitudes of the splitting of the 7F 1 manifolds at lower excitation energies increased with increasing phosphate content. The relative intensity of 5D 0 ~ 7F 2 to 5D 0 ~ 7F 1 transitions increased by adding the phosphate compound. This increase decreased gradually with increasing excitation energy. The atomic structure around Eu 3+ in these glasses was simulated using molecular dynamics techniques. The Stark splitting and the transition probabilities between 5D 0 and 7Fj energy states at each simulated Eu 3+ ion were calculated on the basis of the point-charge approximation. The predicted energy differences between 5D 0 and (7F2, 7F 1 and 7F 0) energy levels agree within 100 cm -1 with the observed ones. The transition probabilities are discussed in terms of the crystal field potential and the configuration interaction parameters, -=(k,A). © 1997 Elsevier Science B.V.

1. Introduct ion

The opt ica l p r o p e r t i e s o f ra re e a r t h ions have p roved to be useful in the s tudy of local envi ron- m e n t for a n u m b e r of crys ta l l ine mat r ices [1]. The

*Corresponding author. Tel.: +81-3 3812 2111; fax: +81-3 3815 8363; e-mail: [email protected]

spectra of rare earth ions in disordered matrices exhibit inhomogeneous broadening and information about the local environment which is difficult to obta in . T h e d e v e l o p m e n t o f l a se r - induced flu-

o re scence l ine na r rowing (FLN) [2-4], however , has m a d e it poss ib le to o v e r c o m e a l imi ta t ion in the m e a s u r e m e n t of op t ica l spectra . S imple energy levels and the exci ta t ion f rom 7F 0 to 5D 0 levels o f an E u 3+ ion are sui table to descr ibe the

0022-9093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0022-3093(97)00362-1

H. lnoue et al. / Journal of Non-Crystalline Solids 222 (1997) 212-220 213

sites occupied by an Eu 3+ ion. Nevertheless, the difficulty still remains in the extraction of structural information from the FLN spectra. To solve this difficult problem, several approaches have been proposed and have reported structures around rare earth ions [2,4-10]. Brawer and We- ber [5,6] have proposed to use a computational technique that is capable of linking optical spectra of a doped glass and a simulated atomic structural model of the glass. They have demon- strated qualitative agreements in optical spectra of Eu 3+ ions in fluoroberyllate glasses. Recently Cormier and co-workers [7-9] have proposed an improvement of the technique and reported more accurate calculation of 4f-electron energies and transition probabilities between individual states of each J manifold.

Fluorophosphate glasses are attractive for laser application and for investigation of the local environment of a rare earth ion. Weber and co- workers [3,11] have concluded that FLN spectra indicate the existence of both fluoride and oxide sites in an Nd3+-doped fluorophosphate glass. Recently, Tanabe et al. [12] have reported the local environment of Eu 3+ ions in fluorophosphate glasses deduced from phonon sideband and Mtssbauer spectroscopy and the preferential coordination of oxide anions to the Eu 3 +.

In this study, the main interest is in the mixed- anion coordination states of Eu 3÷ ions and the effect of the asymmetry of the ligand field and covalency bonding on the optical properties. Flu- oroaluminate and fluorophosphate glasses doped with Eu 3+ ions were prepared. The emission spectra and lifetime of 5D 0 levels of the Eu 3+ ions were measured site selectively. We compared the experimental spectra with simulated ones from the structure models.

2. Experimental

2.1. Sample preparation and spectroscopy

Chemical compositions of four samples of Eu 3 +-doped fluoroaluminate and fluorophosphate glasses prepared are listed in Table 1. Ten grams of the batch were melted at ~ 1150°C for 30 min in a platinum crucible under an argon atmo- sphere. The melts were cast and annealed in a brass mold at 400°C.

The sample for the optical measurement was cooled at 80 K using a continuous flow cryostat (Oxford). The fluorescence line narrowing (FLN) spectra were excited using a dye laser (ND60 Continuum) operating with rhodamine 6G and B pumped by a pulsed frequency doubled Nd:YAG laser. The time-resolved fluorescence was de- tected by using a 1-m monochromator (SPEX) and a photomultiplier (Hamamatsu R1477).

2.Z Molecular dynamics simulation

Born-Mayer-type pairwise potentials were used. The potential parameters of the AIF3-CaF2-BaF 2 system have been reported [13]. Effective charges used for oxygen and phosphorous were - 1.5 and +3.0, respectively. The rest of the parameters were determined empirically such that they reproduced distances and coordination numbers reported by Shannon and Prewitt [14]. A cubic cell was used with periodic boundary conditions. The sizes and the compositions of the basic cells are given in Table 2. To obtain the variation of the Eu 3+ environments, more than 200 different sets of initial configurations for each composition were simulated, each containing one Eu 3+ ion. The temperature was decreased from 3000 to 273 K

Table 1 Chemical composit ions o f glasses prepared and their densities

Sample Molar ratio

MgF2 CaF 2 SrF 2 BaF 2 EuF 3 YF 3 A1F 3 Ca3(P04) 2

Density ( g / c m 3)

P00 8.7 26 8.7 8.7 1.0 12.0 35.0 - P05 8.7 18.5 8.7 8.7 1.0 12.0 35.0 2.5 P10 8.7 11 8.7 8.7 1.0 12.0 35.0 5.0 P15 8.7 3.5 8.7 8.7 1.0 12.0 35.0 7.5

3.70 3.70 3.69 3.68

214

Table 2 Compositions and sizes of the basic cells used in MD simulations

t4. Inoue et aL / Journal of Non-Crystalline Solids 222 (1997) 212-220

Sample Mg Ca Sr Ba Eu Y AI P O F Cell size (.~)

12 35 248 16.5388

12 35 5 20 233 16.6413

12 35 10 40 218 16.7553 12 35 15 60 203 16.8719

P00 9 26 8 9 1

P05 9 26 8 9 1

P10 9 26 8 9 1 P15 9 26 8 9 t

with 1 x 10 4 time steps (10 ps). PO43- units were introduced into the initial configuration. After 5 ps at 273 K the coordinates were used for the calculation of the structural parameters and crystal-field parameters.

to the nkq of all atoms inside a sphere of 20 nm radius with the E u 3+ lying at its center were summed. The experimental FLN spectra show the energy differences between 7F0, 7F1, 7F2, and SD 0 levels of Eu 3+. These energy levels were stimu- lated using a free ion and crystal-field Hamiltoni- ans, operating on a basis of 667 Stark levels including J mixing. Details of the treatment have been described [18-20]. The magnetic dipole and electric dipole transition probabilities were calculated for 5D 0 as an initial state and 7Fj as final state. In order to generate spectra from energy levels obtained, a Gaussian band shape, of which full-width at half-maximum (FWHM) was 50 cm- t, was assigned to each of the energies.

3. Results

2.3. Simulated energy levels o f Eu 3 + 3.1. Experimental F L N spectra

Parameters used in our calculation, elec- trostatic parameters E n (n =2,4,6), spin-orbit coupling constant ~'4e [15], 4f-electron radial integrals ( r k ) (k = 2,4,6) [16] and configuration interaction parameters ~(k,A) [17], of an Eu 3+ ion are listed in Table 3. The crystal-field parameters, Bkq, were calculated from the coordinates obtained from the MD simulations on the basis of the point-charge approximation. The contribution

Table 3 Parameters used in the calculation of spectra

Elec t ros t a t i c p a r a m e t e r (cm 1)

E 2 5573.0 E 4 26.71 E 6 557.39

Spin-orb i t coup l ing cons tan t (cm l )

~'4e 1326.0

4f -e lec t ron radia l in tegra l ( r 2 ) 2 . 3 4 × ( r 4 ) 1.34 × ( r 6 ) 1.65 ×

Conf igu ra t ion in te rac t ion p a r a m e t e r =-(1,2) - 1.08 ×

=-(3,2) 0.88 X =-(3,4) 0.90 × =-(5,4) - 1.27 x -=(5,6) - 2.70 x -=(7,6) 2.58 ×

10 -21 m 2 10 -41 m 4 10 -61 m 6

1 0 - 3 m 2 j I 10-23 m 4 j - 1 10 23 m 4 j - 1 10-43 m 6 j - l 10-43 m 6 j 1 10 63 m8 J - I

Time-resolved site-selective fluorescence spectra of the Eu 3+ ion were obtained at 80 K by using different resonant excitation wavelengths into the 7F0----~SD 0 inhomogeneous absorption band. A gate width of the time window was 8 ms at a delay of 2 ms after the excitation pulse. Fig. 1 shows emission spectra for samples P00 and P15, which were normalized to make the area of the 5D0--~7F 1 transition constant. The spectra for each sample exhibited gradual and systematic changes while scanning the excitation energies. As can be seen from the figure, the relative amplitude of the 5D 0 ~ 7F 2 transition to the SD 0

7F l transition increased with the introduction of phosphate compound. A similar increase for AIF3-AIPOa-EuF3-CaF 2 system has been reported, observed by means of broad band excitation [12]. This increase of the relative amplitude of SD 0 ~ 7F 2 transition decreased gradually with increasing excitation energy. For example, the ratios of the relative intensities were 1.9 and 1.3 at the excitation energies of 17 271.2 and 17 391.3 cm - t , respectively. The differences in the splitting of 7F 1 levels also were notable at a lower excitation energy range.

3.2. M D simulations

Several structural parameters obtained from

H. Inoue et aL / Journal of Non-Crystalline Solids 222 (1997) 212-220 215

• f • ~ ~ . _ (a)

0.2 ~ (b)

(c)

(e)

0.1 (fl

(g)

(h)

(i) 0 I I I

15500 16000 16500 17000 17500

Wavenumber (cm -l)

Fig. 1. The FLN spectra for P00 (thin line) and P15 (thick line) compositions. Excitation energy (cm-1): (a) 17 271.2, (b) 17286.1, (c) 17301.0, (d) 17316.0, (e) 17331.0, (f) 17346.1, (g) 17361.1, (h) 17376.2, (i) 17391.3.

MD simulations for P00 and P15 are summarized in Table 4. Total coordination numbers of anions around metal ions did not change with the addition of the phosphate compound. The oxygen coordination of cations increased monotonically with increasing oxygen concentration, O / ( F + O), in the basic cells. It was found that the number of oxygen ions in the first coordination sphere for

each cation was proportional to the oxygen concentration in the basic cells. For example, the ratios for an aluminum ion were 8.3%, 15.4% and 22.7%, in the P05, P10 and P15 compositions, respectively. The oxygen concentrations were 7.9%, 15.5% and 22.8% in the P05, and P10 and P15 basic cells, respectively. The oxygen concen- t rat ion in the coordination sphere of the aluminum ion was almost equal to the basic cell. The oxygen concentrations around other cations except a phosphorous ion were less than those in the basic cells. The selective oxygen coordination was due to the difference between potential energies of meta l -oxygen and meta l - f luor ine pairs. As the stretching vibration of a PO 3- unit is observed in infrared absorption spectra, the PO43- unit seemed to be stable in these glass compositions. It was found that oxygen ions preferentially coordinated to the phosphorous ions in the simulation which was started f rom random initial coordination. The oxygen coordination of phosphorous ions, however, was < 4. Thus phosphorous ions were introduced as PO 3- units into the initial configuration. Almost all phosphorous ions in the final coordinates could be coordinated with more than three oxygen ions.

The oxygen concentration in the first coordination sphere of an Eu 3+ ion was half that of the aluminum ion. However, our result of oxygen coordination around the Eu 3+ was inconsistent with that reported by Tanabe et al. [12]. They concluded the preferential coordination of oxygen ions to the Eu 3+ from the isomer shift of 151Eu M~ssbauer spectra and the coupling strength of

Table 4 Coordination number, CN, peak position of R-F and R-O pairs and full width at half maximum (FWHM)

Sample P00 P15

CN (RcN) R-F (FWHM) CN (F/O) R-F (FWHM)/R-O(FWHM)

Mg 6.2 (2.7) 2.03 (0.28) Ca 8.1 (3.1) 2.38 (0.35) Sr 8.8 (3.2) 2.44 (0.36) Ba 11.2 (3.6) 2.69 (0.45) Eu 8.4 (3.1) 2.29 (0.22) Y 7.8 (3.0) 2.15 (0.24) Al 6.1 (2.6) 1.81 (0.18) p - _

6.2 (5.4/0.8) 2.04 (0.29)/2.26 (0.30) 8.1 (6.9/1.2) 2.32 (0.35)/2.55 (0.48) 8.8 (7.4/1.4) 2.46 (0.36)/2.72 (0.50)

11.2 (9.1/2.1) 2.69 (0.41)/2.85 (-) 8.5 (7.5/1.0) 2.26 (0.23)/- (-) 7.8 (6.9/0.9) 2.12 (0.24)/2.29 (0.26) 6.1 (4.7/1.4) 1.84 (0.21)/1.87 (0.14) 4.1 (0.4/3.7) 1.55 (0.10)/1.58 (0.09)

216 H. lnoue et al. /Journal of Non-CrystaUine Solids 222 (1997) 212-220

P - O - stretching mode in the phonon sideband spectra. If the effect of the introduction of the first oxygen ion into the coordination sphere on these spectra is larger than that of the other oxygen ions, the changes of these spectra in the early stage of the addition of phosphate compound must be emphasized. However, the stabil- ity of an E u - O - P bond in our simulation could be evaluated only as the summation of the pairwise potentials, E u - O and P - O pairs. For a de- tailed discussion, we need the quantitative evalua- tion of the effect of the phosphate coordination to the Eu 3+ ion as well as a more accurate method to generate the atomic potentials.

3.3. Simulated energy levels and transition probabilities

For better agreement between observed and simulated spectra [8], there are several adjust- ments of the parameters, such as partial ionic charges and correction factor, 6k, for Bkq [18] and scaling factor, r, for the Ha r t r ee -Fock wave function of 4 f electrons [21]. In this study, no adjustment was performed in the first trial for fluorophosphate glass. We used the parameters listed in Table 3 only and the charges used were the same as in the MD simulations.

Fig. 2 shows the distribution of the simulated 7F0, 7F1, 7F 2 and 5D 0 energy levels as a function of 5Do-7F 0 transition energy. As shown in Fig. 2, the change of the 5D0-7F 0 transition energy was mainly due to a change of the energy of the 7F 0 level. The distribution of the lowest component of 7F l levels had a smaller width than the others in the 7F 1 manifolds and its width was independent of the 5D0-TF 0 transition energy. The distribution of other energies increased with increasing 5Do-7F 0 transition energy. The magnitude of the splitting of 7 F1 and 7F e manifolds increased cont- inuously with increasing the 5D0-7F 0 transition energy. These features of the splitting of 7F L and 7F 2 manifolds can be seen in the observed FLN spectra in Fig. 1.

Fig. 3 shows total and magnetic dipole transition probabilities of P00 from the 5D 0 level to the lower 7Fj levels as a function of the 5D0-TF 0 transition energy. The magnetic dipole transition

1 3 5 0 0 f 5D° ~ ®o .

13000[- i , I ~ I L

17100 17200 17300 17400 17500

:o ~ -3500 7F~ ~ , ~ ~

7F o -4000 ~- ® o

t Q O

I I I

17100 17200 17300 17400 17500

5D 0 - 7F 0 transition energy (cm -l)

Fig. 2. Energies of 5D 0, 7F 2, 7FI, and 7F 0 levels as a function of 5D0-7F 0 transition energy.

rate of a rare earth ion is little affected by its local environment. As can be seen from the figure, the constant magnetic dipole transition probabilities calculated were supported theoreti- cally, while the electric dipole transition probability is affected by a local environment produced by a large quenching rate. Thus, calculated total transition probabilities, which were obtained from the magnetic dipole transition probabilities, had a wide distribution from 65 to 320 s - l . The average transition rates calculated were 156, 158, 167 and 175 s-1 for the P00, P05, P10 and P15, respectively. A lifetime of the excited state is obtained as a reciprocal of total transition probability to the lower levels. Therefore, the calculated lifetimes were 6.4, 6.3, 6.0 and 5.7 ms, respectively. The observed average lifetimes for the P00, P05, P10 and P15 samples obtained were 6.7, 5.5, 4.9 and 4.4 ms, respectively. The random error in these data is +0.5 ms. The calculated lifetimes were of the same order as the values, though the effect of adding phosphate compound was less than that observed in the experimental data.

H. Inoue et aL / Journal of Non-Crystalline Solids 222 (1997) 212-220 217

300

• ~ 2oo t~

d ~

0 :=_

~. lOO

tI~ o o

0000 0

0 0 0 0

°°~ ° go, : , ° ° 0000_0 0 0

Cbo .9..i~:~ oOO 0 o O ~ ~000 0 0 0

o o

5o o o i~oo~.4m% g o o o%~ ~ o °

o OOo o

0 , I , I , I , 17100 17200 17300 17400 17500

5Do-7F 0 transition energy (cm -l)

Fig. 3. Total (open circle) and magnetic (closed circle) transition probabilities from 5D 0 level to the lower 7Fj for P00 sample.

4. Discussion

4.1. Observed optical properties

Fig. 4a shows the excitation spectra for samples P00 and P15 generated by the summation of each FLN emission spectra f rom 15 674 to 17 212 c m - and normalized to the intensity of the incident light. The full width at half maximum amplitudes (FWHMs) of the 7 F 0 ~ 5D 0 transition were ~ 65 cm-~. The peak positions shifted to the smaller energy side with the addition of phosphate and the difference between these excitation spectra were largest at ~ 17280 cm-1. The lifetimes of 5D 0, calculated using least-squares fitting to the decay curves of the emission intensities to 7F1, are shown in Fig. 4b. The effect of adding phosphate was observed in all excitation ranges. The maximum of the difference between samples P00 and P15 was at 17 286.1 c m - ~ excitation.

The effect of the introduction of phosphate was detectable and larger in the lower excitation energy range in FLN emission spectra, excitation intensities and lifetimes of 5D 0. The peak posi-

o.1

¢0

0.05 = 0 ¢o

• 0

17250 17300 17350 17400 8

(b) .~ 7 o-'°- ~)'-o.

6 "o. %)

"m 5 "o

.r.._o_e. ~ "o

4 l¢ "°'~.e. e "

3 I I I I 17250 17300 17350 17400

Excitation wavenumber (cml)

Fig. 4. Excitation spectra of 7F0-SD 0 transition (a) and lifetimes of 5D 0 levels of P00 (open circle) and P15 (closed circle) compositions. Triangles are the difference between PI 5 and P00 excitation spectra.

tion of the 7F 0 ~ S D 0 transition of Eu 3+ in Ca(PO3) 2 glass is at 17280 cm -1, reported by Nishimura et al. [22]. This position is almost equal to the peak of the difference curve in Fig. 4a. The relationship between the 7 F0 ~ 5D0 transition energy and the number of the oxygen ions in the coordination sphere around the Eu 3+ ion is not known. Tanabe et al. [12] reported that the isomer shift of 151Eu 3 + in f luorophosphate glass containing 20 mol% AIPO 4 was close to that of fluorides, such as 151EuF3, rather than those in phosphate glasses. The changes gradually occurred with increasing the phosphate content. Therefore, it is probable that the oxide anions coordinate to an Eu 3+ ion together with fluorine ions and the number of oxygen ions increase with increasing phosphate content in the chemical composition range of the f tuorophosphate glasses.

4.2. Simulated spectra

Fig. 5 shows the distribution of the 7 F0 ~ 5 D~ transition energy for the samples P00 and P15.

2 1 8 H. Inoue et aL /Journal of Non-Crystalline Solids 222 (1997) 212-220

2

0 )

o "

L ~

0 17100 17200 17300 17400 17500

5D 0 - 7F 0 transition energy (cm -l)

Fig. 5. The distributions of simulated 5Do-7F o energies for P00 (thin line) and P15 (thick line) samples.

The peak position of P00 was at 17 236 cm-1 and about 70 cm - l less than the experimental one. The shift of the distribution for P15 was larger than that in P00. This shift was opposite to the direction of the observed shift. Cormier et al. [8] have reported the effect of changes in oxygen and silicon charges on the simulated 7F 0 ~ 5D 0 transition energy in an N a 2 0 . 2 S i O 2 glass. They show that the transition energy decreased with decreas- ing partial ionic charges. Therefore, our simulation indicated that the negative charge of oxygen ions used have to decrease to reproduce the experimental spectra as well as introducing several adjustable parameters, such a s 6 k [18] and ~" [21].

Fig. 6 shows a comparison between experimental emission spectra for P00 and P15 broad band excitation of the simulated spectra. The experimental broad band spectra were generated by the summation of FLN emission spectra normalized with each intensity of the incident light. The simulated spectra were calculated from the summation of a Gaussian curve whose area corresponded to each transition probability. Both experimental and simulated spectra were plotted to make total areas of the 5 Do ~ 7 FI transition constant. The axes of abscissa of the simulated spectra were plotted with about a 100 cm -1 shift to larger energy. The shapes of the simulated spectra of the 5D 0 ---~7F 1 transitions reproduce the features of the experimental ones. Two large peaks in the spectra of the 5D 0 -~ 7F 2 transitions also could be reproduced. However, there were

--v. 0.03 (b)

.~-~ 0.02

"" 0.01 )

0 J 15500 16000 16500 17000

0.03 - 5D° - 7F2 5D° - 7FI

.~;~ 0.02

0.01

0 15500 16000 16500 17000 17500

Wavenumber (cm -l)

Fig. 6. The observed (a) and simulated (b) emission spectra of 5D0-7Fl and 7F 2 transitions for P00 (thin line) and P15 (thick line) samples.

several discrepancies between the experimental and simulated spectra. The simulated F W H M of 5D 0 ~ 7F 1 and 5D 0 ~ 7 F 2 transitions were 5 to 10 cm-1 wider and narrower than the experimental ones, respectively. The most remarkable discrep- ancy was the relative intensities of the 5D 0 ~ 7F 2 to 59 0 ~ 7F t transitions. The experimental relative amplitudes of the 5D 0 ~ 7F 2 transitions for sample P00 and P15 were 1.16 + 0.05 and 1.88 + 0.05, respectively. The simulated values for P00 and P15 were 1.60 and 1.76, respectively. The relative amplitude simulated for sample P00 was larger and the increase of the amplitude by the addition of the phosphate compound was less than the experimental values.

In our MD simulations the extremely large quenching rate and the primitive atomic potentials cause the discrepancies between the simulated and the observed spectra. There are other factors which we should consider. The configuration interaction parameters , ~(k,A), reported by

H. Inoue et aL /Journal of Non-Crystalline Solids 222 (1997) 212-220 219

Krupke [17] were obtained from the 4f-electron radial integrals, ( rk ) , whose magnitudes were more than twice larger than those reported by Freeman and Watson [16]. Therefore, it is probable that the transition intensity for P00 is overestimated when using the parameters reported by Krupke [17]. The effect of adding phosphate should be divided into two factors; one is the asymmetry of the ligand when an oxygen ion coordinates with an Eu 3÷ ion with fluorine ions and the other is the covalency of the rare earth- oxygen bonding. The asymmetry of the ligand was considered approximately on the basis of the point-charge approximation. Moreover the effect of covalency in the bonding can be divided into two parts; one is the decrease of effective charge and the other is the increase of ~(k,A), which are related to the radial integrals and energy differences between 4f -4f and 4f -5d orbitals of the rare earth ion. The spatial distribution of electrons on an ion with a covalent bond reduces the effective charge of the ion. Therefore, the charges of oxygen and phosphorous ions which were used

were - 1 . 5 and 3.0, respectively. If a rare earth ion has a covalent bond with its ligands, the electron density on the rare earth ion increases and the wave functions of 4f-electron are ex- tended spatially. The extension of 4f-electron causes an increase of the 4f-electron radial integrals and finally E(k,A) increases. In our calculation the same ~=(k,A)s listed in Table 3 were used even if oxygen ions coordinated the Eu 3+ ion. Therefore, this part of the effect of covalency was ignored. If we consider the effect, the relative intensity of 5D 0 ~ T F 2 transitions must increase by adding phosphate compound.

Fig. 7 shows the experimental and simulated FLN spectra for sample P00. The simulated spectra were calculated from the summation of the Gaussian curve whose area corresponded to each transition probability. They also indicated several of the discrepancies described above. However, the shape and feature of the experimental FLN spectra could be reproduced.

5. Conclusion

0.20 ~ ~ (a)

(b)

0.10 (c)

~ (d)

(e)

0 i 16000 16500 17000 17500

Wavenumber (cm -l)

Fig. 7. The observed (thin line) and simulated (thick line) FLN spectra for P00 compositions. The excitation energies were at (a) 17271, (b) 17301, (c) 17331, (d) 17361 and (e) 17 391 cm - 1.

The effect of the introduction of phosphate was detectable and in the lower excitation energy range of the FLN emission spectra, of the excitation intensities, and of the lifetimes of the 5D 0 level. The splitting of the 7F l manifolds at lower excitation energies increased with increasing phosphate content. The relative intensity of 5D 0 ---~7F 2 to 5Do----~7F 1 transitions increased by adding the phosphate compound. This increase decreased gradually with increasing excitation energy.

We conclude that the increase of the relative intensity of 5D o--->TF 2 to 5D 0 ---~7F l transitions observed is due to the increase of the covalency between the Eu 3+ ion and the ligands, such as oxygen coordination.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research of the Nippon Sheet Glass Foundation in 1996. The authors thank the Su- percomputer Center, Institute for Solid State

220 H. lnoue et al. / Journal of Non-Crystalline Solids 222 (1997) 212-220

Physics , U n i v e r s i t y o f T o k y o fo r t h e faci l i t ies a n d

t h e use o f t h e F A C O M V P P 5 0 0 .

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Documents

Structure and optical properties of Eu3+-doped fluoroaluminate and fluorophosphate glasses