Indium and gallium fluorochloride glasses

JOURNAL OF

ELSEVIER

Journal of Non-Crystalline Solids 221 (1997) 78-83

Indium and gallium fluorochloride glasses G. Zhang, J. Jiang, M. Poulain * Laboratoire des Mat~riaux Photoniques, CEMA, Universit~ de Rennes 1, 35042 Rennes, France

Received 12 March 1997; revised 17 June 1997

Abstract Systematic attempts of chlorine incorporation were implemented in fluorogallate and fluoroindate glasses. High chloride content was achieved in the InF3-BaFz-PbC12-NaC1 system. Up to 15 mol% NaC1 or 7.5 mol% PbCI 2 could be incorporated in glasses based on GaF3 or GaF3-InF 3. The effect of chlorine content on glass formation and properties has been investigated. IR cut-off is shifted toward longer wavelength and 1G4 fluorescence lifetime of Pr 3+ doping ions is increased. Such compositions have been used for the core of experimental optical fibers. © 1997 Elsevier Science B.V.

I. Introduction Fluorochloride glasses make an interesting group of halide glasses as they extend the usual range of physical properties of fluoride glasses. A typical example is cadmium fluorochloride glasses which contain high chlorine content and are more stable than the fluoride glasses based on CdF 2 [1,2]. Early work [3] carried out on fluorozirconate glasses showed that chlorine incorporation increases refractive index and IR transmission range while it decreases Tg, Synthesis appears more difficult as starting materials may be hygroscopic. In addition chlorine containing glasses are usually more sensitive to water attack. Only limited experiments have been implemented in indium and gallium fluorochloride glasses. The main reason lies in glass forming ability which is often limited in fluoroindate glasses. Sample prepara-

* Corresponding author. Tel.: +33-2 99 28 62 63; fax: +33-2 99 28 69 72; e-mail: [email protected].

tion is still more difficult when chlorides enter the glass composition. Despite these problems, chlorine incorporation has been investigated because it may help to understand the glass formation mechanism, extension of the IR transmission range will be very helpful for some applications and spectroscopic properties of rare earth ions may be modified, and quantum efficiency of selected transitions could be increased.

2. Experimental By comparison with standard fluoride glasses synthesis of fluorochloride glasses requires special care. In order to ensure CI incorporation in glass, preparation is carried out in two steps. First, the batch containing all fluorides is heated to melting, and cooled to room temperature. Then, the chlorides which have been dried are added into the crucible which is heated until a clear melt is obtained. Fining is implemented at a temperature well below that used for fluoride glasses. In this way, chlorine loss is

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G. Zhang et al. / Journal of Non-Crystalline Solids 221 (1997) 78-83

BaF2

were implemented using the same methods as for gallium and indium fluoride glasses, described elsewhere [4]. The 1G4 --->3H 5 transition characteristics of the Pr 3+ doped samples were measured at the

40 mol% InFa

/

~

@ glass-ceramics 0

ORC, Southampton. The fluorescence spectra were recorded using a Ti:sapphire laser as the excitation source. Lifetimes were obtained from the emission decay curve by e 3 in fluorescence intensity. The q u a n t u m efficiency (QE) was calculated as

glass

QE = ( I G 4 l i f e t i m e ) / ( p u m p t i m e ) .

NaCI

PbClz

Fig. 1. The glass forming range in the 40InF3-BaF2 -PbC12 -NaCI system.

3. Results The vitreous area in a pseudo ternary system, 4 0 I n F 3 - B a F 2 - P b C 1 2 - N a C 1 , is shown in Fig. 1. The characteristic temperatures and the stability factor, A T = ( T ~ - Tg), of some compositions are listed in Table 1, Other non-systematic investigations have been made in this quaternary system. The content of PbC12 and NaC1 can reach 50 and 30 m o l % respectively. The C 1 / F ratio m a y be higher than 1.0. Tg is lower as chlorine concentration increases. It is also sensitive to the relative a m o u n t s of Pb and Ba. Glass forming ability seems to be e n h a n c e d by comparison

greatly reduced. This loss originates from exchanges with atmosphere. Chemical action of gaseous o x y g e n is limited but the formation of volatile GaC13 and InCl 3 m a y result in a large difference b e t w e e n nominal and actual compositions. Glass properties studied here include characteristic temperatures (Tg for glass transition temperature, T~ and Tp for the onset and peak temperatures of crystallization respectively), refractive index (riD), density and IR transmission spectrum. Measurements

Table 1 Characteristic temperatures of some InF3-based glasses. Tg, Tx and Tp are temperatures for glass transition, onset of crystallization and exotherm maximum, respectively, AT = Tx -- Tg is a stability index Composition (mol%)

Tg (o)

TX(°)

Tp (°C)

AT (o)

40InF3-20BaF2-4OPbC12 401nE~-10BaF2-30PbCI2-20NaCI 40InF3-20BaF2-20PbCI2-20NaCI 40InF3-20BaF2-20PbF2-20NaF

184 + 2 189 __+2 214 ___2 224 _+2

229 _+2 251 ± 2 266 + 2 266 + 2

237 + 268 + 272 ± 270 +

45 62 52 42

2 2 2 2

Table 2 Glass formation in GaF3-MF2-PbF2-PbCI2 (M = Zn, Mn, Cd) systems Composition (mol%)

Glass formation

35GaF3-24ZnF2-36PbF2-5PbC12 35GaF3-24ZnF2-31PbF2- 10PbCl2 35GaF3-24ZnF2-21PbF2-20PbCl 2 35GaF3-24MnF2-36PbF2-5PbCI 2 35GaF3-24MnF2-31PbF2-10PbCI2 35GaF3-24CdF2-41PbF2-0PbC12 35GaF3-24CdF2-36PbF2-5PbC12 35GaF3-24CdF2-30PbF2- 11PbC12

glass glass-ceramic ceramic glass-ceramic glass < 1 mm, Tg = 212°C, Tx = 272°C glass-ceramic glass > 1 ram, Tg= 225°C, Tx = 282°C glass < 1 mm

G. Zhang et al. / Journal of Non-Crystalline Solids 221 (1997) 78-83

80

1 0 0

,

,

,

,

,

,

,

,

100

,

80 u

80

V

60

60

t--

m

a ~

b

o

'~ 40

,~_

'i

c

l: 40

i

m r-

i

~

20

20

0 ' 4000

~

I

3100

I

I

0

I

2200

130(

Wavenumber

(crn q )

4000

400

Fig. 2. IR spectra of indium fluoride and fluorochloride glasses. Sample thickness: 1 mm. Glass compositions: (a) 40InF 3 2 0 B a F 2 - 2 0 P b F 2 - 2 0 N a F and (b) 40InF3-20BaF2-20PbC12 20NaC1.

with the corresponding fluoride glasses. However the maximum glass thickness in this system is still less than 2 mm. Chlorine was also introduced in 35GaF3-24MF 241PbF 2, ( M = M n , Zn, Cd) glasses using the PbC12/PbF 2 substitution. The results are reported in Table 2. The incorporation of 5-10 mol% PbC1 z enhances glass formation in the three groups of glasses. The 35GaF3-24CdF2-36PbF2-5PbCI 2 glass is the most stable one, although AT is only 57°C which corresponds to a limited sample thickness. Other experimental work has been implemented in 4 0 G a F 3 - 4 5 C d F 2 - ( 1 5 - x ) P b F z - x N a C 1 and 40GaF3-10PbF2-(50- x)CdF2-xNaC1 systems (see Table 3). In both systems, NaC1 content can reach 15 tool%. Glass stability is improved in the 5-10 tool%

I

I

I

I

I

I

t

I

I

31 O0 2200 1300 W a v e n u m b e r ( c m -1 )

,~1~

400

Fig. 3. IR spectra of gallium fluoride glasses, pure fluorides (a) and Cl-containing (b). Sample thickness: 1 mm. Glass compositions: (a) 4 0 G a F 3 - 1 0 P b F 2 - 5 0 C d F 2 and (b) 40GaF3-10PbF 2 40CdF 2 -10NaC1. Sample (b) exhibits an important extrinsic absorption band around 1000 cm - I .

range by comparison with the all-fluoride glasses. AT value is large (90°C) and glass thickness is increased to 4-5 mm. IR transmission spectra of fluoride and fluorochloride glasses based on InF 3 and GaF 3 are shown in Figs. 2 and 3. IR cut-off is shifted toward longer wavelength in indium fluorochloride glass. But there are absorption peaks at 3450, 3120 and 1600 cm-1 in the indium fluorochloride glass which originate from water. A first contribution arises from bulk OH as in fluorozirconate glasses. A second source lies in the water molecules adsorbed at glass surface, exemplifying the increased sensitivity of chloride compounds to moisture. No evident shift in infrared transmission is observed in gallium fluorochloride

Table 3 Effect of substitutions of NaC1 for CdF 2 and PbF 2 in GaF3-based system: vitreous compositions and characteristic temperatures. Tg, TX and Tp are temperatures for glass transition, onset of crystallization and exotherm maximum, respectively, z~T = Tx - Tg is a stability index Composition (mol%)

Tg (°C)

Tx (°C)

40GaF3-45CdF 2- 15PbF 2 40GaF 3- 4 5 C d F 2- 10PbF2-5NaC1 40GaF 3-45CDF 2- 5 P b F 2- 10NaCI 40GaF 3- l 0PbF 2-50CdF 2 40GaF 3- 10PbF2-45CdF2-5NaCI 40GaF 3-10PbFz-42.5CdF2-7.5NaCI 40GaF 3- 10PbF 2- 4 0 C d F 2-10NaCI 40GaF 3- 10PbF2-35CdF 2- 15NaC1

291 263 257 284 263 258 253 243

365 353 344 366 353 347 345 307

+ 2 + 2 + 2 +_ 2 + 2 + 2 + 2 + 2

+ 2 + 2 + 2 + 2 + 2 + 2 5:2 + 2

Tp (°C)

AT (°C)

371 356 356 354 404

74 90 87 82 90 89 92 64

+ + + + +

2 2 2 2 2

G. Zhang et al./ Journal of Non-C~stalline Solids 221 (1997) 78-83

glasses. However, the observation of the transmission spectrum shows an important extrinsic absorption band around 1000 cm -~ which is related to sulfate impurities in glass precursors. This screens the real multiphonon absorption edge, and further (a) 400

81

Table 4 ~G4~3H5 fluorescence lifetime and bandwidth (FWHM) of praseodymium-doped 20GaF3-151nF3-30PbFz-20CdF2 - 15ZnF~ (PGICZ) glasses Dopants IG4 lifetime (Ixs) FWHM(nm) 600 ppm PrF3 600 ppm PrC13 5NaC1+ 600 ppm PrF 3 5NaC1+ 600 ppm PrC13

146___5 1554-5 169 + 5 1804-5

81 +2 91 4-2 85 4-2 924-2

Tp

.30C

E 250 ~

20C

' 0

i 3

i 6

i 9

12

15

x (tool%)

(b) 5.8

E 5.6

r5.4

5.2 0

i

i

i

i

3

6

9

12

15

x Imol%)

(c) 1.580 1.576

1.572 a t,-. 1.568

1.564

1,561

J 3

I 6

I 9

= 12

15

x (mol%)

Fig. 4. Evolutionof (a) characteristic temperatures, (b) density and (c) refractive index versus NaCI content in 30GaF3-5InF325CdF2-20PbF 2-(20- x)ZnF2-xNaC1 glasses.

studies should be carried out on samples of higher purity. The influence of chlorine incorporation on glass properties has been studied in these glasses and the results are exemplified in Fig. 4. As expected, C1 increases n o value and decreases Tg. Chlorine containing glasses are less dense because C 1 / F substitution reduces the number of heavy cations per unit volume. As observed in previous GaF3-based systems, glass forming ability appears maximum at 10 mol% of NaC1. The 1G4 ---~3H5 transition of Pr 3+ ion is dominated by multiphonon relaxation, and the quantum efficiency (QE) of PDFA is low in current glass hosts. For example, it is only 3% in ZBLAN. In order to improve QE, research efforts focus on new materials with lower phonon energy [5,6]. While the predominant factor lies in the vibrational characteristics of the glass matrix, it is possible that the local chemical environment of the Pr 3÷ ions may influence the 1G4 ~ 3 H5 transition. In practice praseodymium may be incorporated as PrC13 or PrF 3 and there is some probability that some chlorine anions remain close to Pr 3÷. This effect may be combined with the decrease of the glass phonon energy expected from chlorine incorporation. Results of experiments implemented on this basis are reported in Table 4. While the accuracy of the measured lifetime does not exceed 5 p~s, these data suggest that NaC1 introduction may increase lifetime. This has been confirmed by other measurements 1. However this effect is more important when Pr is introduced as PrCI 3. In addition F W H M is also increased by roughly 10 nm.

J Unpublished data from Le Verre Fluor6 SA.

G. Zhang et al. / Journal of Non-Crystalline Solids 221 (1997) 78-83

82

Table 5 Physical properties of a PGICZ preform and fiber. Tg is glass transition temperature, ot is thermal expansion coefficient between 50 and 200°C, n D is refractive index and NA is the numerical aperture of the fiber. Core glass: 20GaF3-15InF3-29PbF2-20CdF 2-13ZnF2-3PbC12. Cladding glass: 22GaF3-13InF3-30PbF2-18CdFz-13ZnF2-2NaF-2GdF 3 pr3+-doped preform

Tg (°C)

ot (10- 7 K -

PGICZFCI core PGICZF cladding

221 + 2 238 + 2

160 + 3 162 + 3

1) (50_200oC)

This result suggests that the Pr-C1 bonds remained in the glass after the melting process. So this approach could define a way toward more efficient PDFA. Considering that t h e 1G4 lifetime is very sensitive to OH content [7,8] and that water impurities are usually associated with chlorides, samples synthesized in more rigorous conditions should give more attractive results than those of Table 4. Indeed we have observed a lifetime of 210 ~s for dry processed PGICZ samples [8]. The calculated quantum efficiency reaches 6-7% which is two times higher than in ZBLAN. Preforms and fibers with a Cl-containing PGICZ core have been obtained. Some characteristics of a pair of core/clad glasses are reported in Table 5.

4. Discussion

Difficulty in obtaining stable glasses in fluorochloride systems may reflect differences in the vitrifying mechanism. We have already discussed the influence of C1/F mixing effect on glass formation [9]. The introduction of heavy halogen ions increases the confusion of glass structure, but also this creates weaker bonds. Glass scientists often try to correlate chemical bonding to glass formation. The underlying idea is that a glass cannot exist without vitreous network and such a network implies strong bonds between anions and vitrifying cations, while these bonds are weaker for modifying cations. In this respect, what is the influence of chlorine incorporation in fluoride glasses? As C1-M bonds are weaker than F - M bonds, one may expect changes in physical properties, especially the decrease of the glass transition temperature. But the result as to glass forming ability is more contrasted: while it seems beneficial in CdF z- and MnF2-based glasses, conclusions are less clear in fluorozirconate and fluoroalu-

nD

NA

Fiber loss (dB/m)

1.624 + 0.0005 1.595 + 0.0005

0.30

12

minate glasses. Various chlorofluorozirconate glasses have been reported [10,11] but it seems that most attempts for synthesizing stable chlorofluoroaluminate have failed. From current criteria for assessing the vitrifying ability in fluoride systems, either single bond strength or cationic field strength, both gallium and indium appear in an intermediate situation. But their respective behavior is different. Results indicate that chlorine incorporation is more difficult in fluorogallate glasses than in fluoroindates. Crystal chemistry of halide complexes provides a starting point for the explanation of this observation. Glass formation is correlated to the low nucleation and growth rate of the crystalline phases. In a general way, the substitution of fluorine by chlorine anions enlarges the size of the coordination polyhedra, insofar as coordination number is not changed, and also reduces the strength of the links between close coordination polyhedra. This may lead to some depolymerization of the melt, which in turn influences glass transition. Depending on vitreous systems, difference between M - F and M-C1 bonds may result in separate MFn and MC1 n polyhedra, leading to fluorine- and chlorine-rich clusters in the melt, enhancing nucleation. When this separation occurs, one may expect chloride phases to crystallize first as they exhibit lower liquidus temperatures. In other cases, the vitreous network is constructed from stable MFnC1m mixed polyhedra. Then devitrification rate is reduced as important diffusion processes are required for the growth of pure chloride or pure fluoride crystalline phases. The larger ionic radius of In 3+ by comparison with Ga 3+ (0.80 vs. 0.62 A) results in lower field and bond strength and may be more favorable to the formation of mixed polyhedra. This study provides prospects for praseodymiumdoped fiber amplifiers and also raises questions. Lifetime measurements are the basis for QE assessments, but there are numerous sources of uncer-

G. Zhang et al. / Journal of Non-Crystalline Solids 221 (1997) 78-83

tainty. The first lies in the calculation method, as it is based on the fitting of the experimental decay curve with the theoretical relation. But there are also the intrinsic features of the material, such as impurity content and glass homogeneity. It is expected that impurities with high phonon energy will act as quenching centers of the active ions. Also other rare earths cations may lead to de-excitation. The control of the impurity level is difficult and may result in discrepancies between measurements implemented on similar samples. For this reason, the significance of the lifetime measurements is greater when making comparisons between samples processed in similar conditions using the same starting materials and equipment. Results strongly suggest that the local environment of the cations is dependent on the nature of the starting materials. This means that physical properties may be different for two glasses of the same chemical composition. This provides a new way for increasing the QE of active elements in a given vitreous host. In the case of fluoride fibers for optical amplifiers, background losses are usually high, by comparison with silica or Z B L A N fibers. It would be attractive to increase lifetime and quantum efficiency of Pr 3-- ions in a Z B L A N fiber while keeping losses at a low level. This could be achieved if the local structure around the active ions could be controlled.

5. Conclusion New indium and gallium fluorochloride glasses have been synthesized in multicomponent systems InF3-BaF2-PbC12-NaC1 and G a F 3 - M F 2 - P b F z PbCI 2. Evolution of physical properties versus chlorine concentration is reported. As expected, chlorine incorporation results in higher refractive index, lower Tg increases fluorescence lifetime of the IG 4 ---->3H 5

83

transition in the Pr 3+ ions. Higher quantum efficiency is expected from samples doped with PrCI 3 instead of PrF 3. Experimental fibers were drawn from these glasses, but the level of background losses is important. Further work on glass composition and processing should allow low loss singlemode optical fibers to be obtained.

Acknowledgements This work was supported by the European Research program RACE Fluor lI. Authors are indebted to ORC Southampton, Brunel and Barcelona Universities for their assistance and encouragement.

References [1] M. Matecki, M. Poulain, J. Non-Cryst. Solids 140 (1992) 82. [2] G. Zhang, M. Poulain, J. Jha, J. Non-Cryst. Solids 184 (1995) 72. [3] M. Poulain, J.L. Adam, unpublished results. See also J.L. Adam, thbse de doctorat de 3~ cycle, University of Rennes, 1983, pp. 3-29. [4] G. Zhang, B. Friot, M. Poulain, J. Non-Cryst. Solids 213&214 (1976) 6. [5] D.W. Hewak, R.S. Deol, J. Wang, G. Wylangowski, J.A. Medeiros Neto, B.N. Samson, W.S. Brocklesby, R.I. Laming, A. Jha, G. Zhang, C. Le Deit, M. Poulain, Proc. SPIE 2073 (1993) 127. [6] R.F. Bartholomew, B.G. Aitken, M.A. Newhouse, J. NonCryst. Solids 184 (1995) 229. [7] S. Jordery, M. Naftaly, A. Jha, Proc. 9th Int. Symp. on Non-oxide Glasses, Hangzhou, China, May 1994, p. 412. [8] G. Zhang, thbse de doctorat, Universit6 de Rennes, 1996. [9] G. Zhang, C. Zhang, J. Non-Cryst. Solids 140 (1992) 345. [10] M. Poulain, A. Elyamani, Mater. Sci. Forum 32&33 (1988) 73-85. [11] M. Poulain, A. Elyamani, Mater. Sci. Forum 67&68 (1991) 119.