LETTER TO THE EDITOR

ELSEVIER Journal of Non-Crystalline Solids 204 (1996) 196-201

J O U R N A L OF

NON-C SOI

Letter to the Editor

The influence of oxide doping on some optical properties of fluoroaluminate glasses

V.D. Khalilev *, P. Ebeling, N.U. Vinogradova St. Petersburg's State Institute of Technology, St. Petersburg, Russia

Received 15 November 1995; revised 17 May 1996

Abstract

The optical constants and transmission spectra of fluoroaluminate glasses from the system A1F3-YF3-~ZRF2 (R = Ba, Ca, Sr, or Mg) doped with small (0.25-1.00 tool%) amounts of oxides into the batch and with oxygen from the melting atmosphere were measured. The displacement of UV-transmission limit toward the longwave region, as compared to undoped glass, was observed to be slight (about 20 nm) in the case of oxide doped glasses and significant (about 100 nm) in the case of glass doped with oxygen from the melting atmosphere. It is concluded that fluoroaluminate glasses accept only limited quantity of oxides. The glasses concerned are transparent (up to 91%) in the range of wavelengths of 220 to 6300 r i m .

1. Introduct ion

Fluoride glasses are known for their high trans- parency in the range of wavelengths of 200-5500 nm and for their low linear and non-linear refractive indices, which makes them attractive materials for fibre and laser optics [1]. The first fluoride glasses, f luoroberyllate glasses (FBG), were obtained. The influence of oxygen content in FBG on their optical properties, particularly on transparency, was studied in Ref. [2]. FBG materials were melted in an atmo- sphere containing oxygen. At first, transparency in- creased, caused by oxidative effect of oxygen on refractory nitrides forming in the glass in the pres-

* Corresponding author.

ence of ammonia residue after fluorination by am- monia bifluoride. With an increase of melting time in the same atmosphere, an absorption band of 270 nm appeared and bands at 350 and 430 nm were ob- served in the spectra of glasses of B e F 2 - K F - C a F 2 system containing no AIF 3. Research on FBG is restricted because of the high toxicity of beryll ium fluoride.

There are many publications on fluorozirconate glasses and there are significantly fewer publications on fluoroaluminate glasses (FAG), though the letter have a higher transformation temperature (about 450°C) and better chemical durability [3].

To produce transparent non-crystallizing FAG (of high quality), it is necessary to use additional fluori- nation. NH4HF 2 is one of the best fluorinating agents. Ammonia residue reacts with batch fluorides,

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particles of which decrease the transparency and crystallization resistance of glass. In this case, the doping of glass with small amounts of oxygen leads to oxidation of refractory nitrides and to an increase in transparency.

A better method consists in melting a batch of monocrystalline fluorides (produced by vacuum high temperature technology with lead bifluoride being used) followed by remelting of crushed glass with addition of fluoride acid in a reactive dry mixed atmosphere of oxygen and argon of high purity. In this case, ammonia bifluoride is not used. This method increased the transparency of FAG and de- creased the concentration of dispersing defects to the level of the best fluorozirconate glasses [4]. The question of the effects of oxygen is not solved. As shown in Ref. [4], oxygen content in FAG can reach 5 at.%.

This paper presents the results of an investigation into the influence of oxygen doping into the batch

and from the melting atmosphere on some optical properties of FAG.

2. Experimental

The basic glass for this investigation has the composition A1F3-YF3-ERF 2 (R = Ba, Ca, Sr, or Mg) system containing 36 mol% A1F 3 and will be called FAG-36. The starting composition for this glass was the mineral usovite Ba2CaMgA12F~4. The batches were prepared from monocrystalline raw ma- terials, except A1F 3. FAG-36 was doped with oxygen in two ways: (1) from the melting atmosphere (reac- tive mixture of argon of high purity and oxygen of superior purity in the ratio 2:1). These samples will be indicated by index (02); (2) as oxide compounds in small additions to the batch.

The composition of synthesised glasses is given in Table 1. The doping of every oxide started from 0.10

T a b l e 1

C o m p o s i t i o n o f s a m p l e s s y n t h e s i s e d

N o . o f s a m p l e M g O a ( m o l % ) C a O a ( m o l % ) A 1 2 0 3 a ( m o l % ) S r O " ( m o l % ) Y 2 0 3 ~ ( m o l % ) B a O a ( m o l % ) P b O a ( m o l % )

1 0 . 2 5 . . . . . .

2 0 . 5 0 . . . . .

3 0 . 7 5 . . . . . .

4 - 0 . 1 0 . . . . .

5 - 0 . 2 5 . . . . .

6 - 0 . 5 0 . . . . .

7 - 0 . 7 5 . . . . .

8 - 1 . 0 0 . . . . .

9 - 0 . 2 5 0 . 2 5 . . . .

1 0 - - 0 . 2 5 - - -

1 1 - - 0 . 5 0 - - -

1 2 - - - 0 . 2 5 - - -

1 3 - - - 0 . 5 0 - - -

1 4 - - - 0 . 7 5 - - -

1 5 - - - 1 . 0 0 - - -

1 6 - - - 0 . 2 5 - -

1 7 - - - 0 . 5 0 - -

1 8 . . . . . 0 . 2 5

1 9 . . . . . 0 . 5 0

2 0 . . . . . 0 . 7 5

2 1 . . . . . 1 . 0 0

2 2 . . . . . . 0 . 2 5

2 3 . . . . . 0 . 5 0

2 4 ( 0 2 ) - - - 0 . 5 0 - -

a T o 1 0 0 m o l % B a C a M g A 1 2 F t 4 , t h e s e m o l % o f o x i d e w e r e a d d e d .

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198 V.D. Khalilet, et al. / Journal of Non-C~stalline Solids 204 (1996) 196-201

or 0.25 mol% and finished when the next glass of the series was opalescent or milky. The glasses with additions of rare earth element oxides Ga, Ge, Hf, Zr were prepared, but the colour of the glasses was grey and opaque at 0.25 tool% oxide addition, so they were not investigated.

Melting of glasses was carried out in vitreous carbon crucibles at 900-1000°C for 1 h and 1 h 20 min, in the atmosphere of high purity argon (except samples with index 02). The rate of cooling was about 2°C/s. The weight loss of the glasses did not exceeded 3%.

For a comparison, some fluoroberyllate glasses (FBG) doped with oxides were synthesised. The composition of basic glass FBG-54 was as follows (tool%): BeF 2 54, A1F 3 10, KF 20, CaF 2 12. The batch containing BeO, A1203, KHF 2, CaF 2 was melted in platinum crucibles, NH4HF 2 was used as a fluorinating agent in 50 mass% excess. FBG-54 doped either with BeO or with AI203, K20 or CaO were melted in argon atmosphere, other conditions being equal.

Optical properties were determined by a refrac- tometer (Hpqb-23). Transmission spectra were mea- t~'/! (a)#

50

~00 300 ~bO ,~ , n m

TT, dO0

50

(b) . r

900 3do ~o0 ,~, nm

50

(c)

P "Z

f ~

(d) 400"

5O

/

z -Z

~6o ~tbo ,l, nm Jbo #bo .~, rvn

Fig. 1. UV transmission spectra of samples of FAG-36 (a) and of FAG-36 doped with following oxides: BaO (b) where the curve I corresponds to 0.25, II to 0.50, III to 0.75, IV to 1.00 mol%; A1203, Y203 (c) where curve I corresponds to 0.25 mol% of Y203, II to 0.25 mol% of AI203, III to 0.50 tool% of Y203 (A1203); mixture of CaO + A1203 (curve I) and PbO (II) (d).

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Table 2

Optical constants of glasses synthesised

199

No. of sample n,. (q-4 X 10 -4) n d ( ± 4 X 10 -4) nf (-t-4 X 10 4) v = (nd - l ) / ( n f - n c)

FAG-36(O 2) 1.4253 1.4267 1.4297 98.5 l 1.4255 1.4269 1.4299 97.3 2 1.4249 1.4257 1.4285 99.4

3 opalescent sample 4 1.4267 1.4280 1.4311 97.6

5 1.4252 1.4264 1.4295 98.7 6 1.4251 1.4264 1.4295 98.1 7 1.4254 1.4268 1.4297 99.4 8 opalescent sample 9 1.4252 1.4266 1.4296 96.6

10 1.4252 1.4265 1.4296 97.6 11 opalescent sample 12 1.4250 1.4262 1.4293 97.6 13 1.4253 1.4267 1.4297 98.3 14 1.4258 1.4271 1.4301 99.3 15 1.4267 1.4280 1.4310 98.6 16 1.4263 1.4276 1.4306 98.7

17 opalescent sample 18 1.4251 1.4263 1.4294 98.9 19 1.4245 1.4258 1.4288 99.3 20 1.4268 1.4282 1.4312 96.9 21 opalescent sample 22 1.4263 1.4276 1.4306 98.2

23 1.4264 1.4277 1.4308 96.0 24(02 ) 1.4256 1.4269 1.4299 98.8

sured on samples of FAG, 7 and 0.5 mm thick, and on samples of FBG, l0 mm thick, with spectrome- ters (Hitachi and Shimadzu).

3. Results

Optical constants of the glasses are given in Table 2. It should be mentioned that the ground glass, FAG-36, has stable optical constants, refractive in- dex particularly n d = 1.4267 _ (4 × 10 4). The dop- ing of FAG with small amounts of oxides has a small influence on the refractive index of FAG-36, for instance n a varies from 1.4257 to 1.4280. The density of oxide doped FAG lies between 3.81 and 3.88 g / c m 3. The highest density samples are those containing PbO.

Ultraviolet transmission spectra of FAG-36 and oxide doped FAG are shown in Fig. 1. The samples are transparent (I, II, III, ~ 91%) at wavelengths

< 250 nm. In Fig. l(b) and (c), the spectra of opalescent samples (III, IV) are also shown (trans- parency < 40% at A > 400 nm).

~o0

50

J 20o ~ 0~0 ~, r~n

Fig. 2. UV transmission spectra of the samples of FAG-36(O2). Thicknesses of samples are (a) 7 and (b) 0.5 ram.

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200 V.D. Khalileu et al. / Journal of Non-Crystalline Solids 204 (1996) 196-201

T,7. too

50

40' ' • ~ .a ;o 2ooo 42oo cm ~6oo

Fig. 3. Typical IR transmission spectra of the samples of oxide doped FAG-36, 7 mm thick (dashed line) and of the samples FAG-36(O 2) and glass no. 24(02) (a) 7 and (b) 0.5 mm thick.

UV transmission spectra of samples FAG-36(O 2) with thickness 7 and 0.5 mm are shown in Fig. 2(a). The limit of UV transmission is shifted into the longwave region. In the spectrum of the thin sample of FAG-36, three absorption bands were observed: 215, 235, and 255 nm.

Typical infrared transmission spectra of FAG with

~Z t00

d - 2~0 zb0 ~k0 0/~0 a],n,,

Fig. 4. UV transmission spectra of the samples of FBG-54 (a) and FBG-54 doped with following oxides: 0.4 mol% of AI203 (CaO, K20) (b); BeO (c) 0.2; and (d) 0.4 tool%.

samples 7 and 0.5 mm thick are shown in Fig. 3. Spectra of thick samples (a) have absorption band at 3600 cm -1 (2.8 p~m). For the samples with index (O2), another absorption band at 2170 cm -1 (4.6 ~m) is observed.

UV transmission spectra of FBG samples are shown in Fig. 4. In the case of undoped FBG-54, an absorption band at 270 nm is resolved. In the case of FBG-54 (a) doped with K20, AI203 or CaO, the absorption at 270 nm is greater. Other bands at 350 and 430 nm occur only in samples with addition of BeO ((c), (d)), the absorption being greater when concentration of BeO increases from 0.2 to 0.4 tool%.

4. Discuss ion

Upon analysis of 'oxide-fluoride' phase equilib- rium diagrams [5], for instance of AIzO3-CaF 2 sys- tem, when even the smallest addition of AI203 to CaF 2 decreases the melting temperature of the sys- tem, we assume that addition of a small amount of oxides to FAG would decrease their tendency to crystallization, because the reduction of liquidus temperature of composition leads to an increase in the possibility of glass formation. Experimental data had confirmed this hypothesis only partly.

As mentioned in Ref. [4], oxygen content in FAG- 36(02) is about 5 at.% (according to XPS data). The calculated content of 'batch' oxygen in our FAG is not above 0.45 at.%. It should be noted that the glasses were milky or opalescent when the calculated oxygen content was about 0.29 at.%. We believe that in FAG the solubility of oxide is limited quantity, and the oxide over this amount increases crystalliza- tion.

We do not think that slight displacement of the UV transmission limit toward the longwave region for oxide doped FAG, as compared to undoped FAG-36, as we observed, can be a result of the doped cation. Logically, UV transmission limit should shift into the longwave region with addition of heavy cations, such as PbO and Y203, and to the shortwave region with addition of light ones. These shifts are not observed in the spectra. So we attribute the limit displacement observed to the effects of oxygen. Based on the data obtained, it seems diffi-

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V.D. Khalilec et al. / Journal o f Non-Crystalline Solids 204 (1996) 196-201 201

cult to determine the form of this relation between either UV transmission limit shifts, or the variations of optical constants, and calculated oxygen content; we note that there is a small uncontrolled pick-up of air oxygen during some of the operations, and the content of A I 2 0 3 residue in commercial A1F 3 of high purity fluctuates.

The combined state of oxygen in FAG-36(O 2) (so-called 'a tmospher ic ' oxygen) and the combined state of 'ba tch ' oxygen might differ. The displace- ment of UV transmission limit in the case of 'a tmo- spheric ' oxygen is significant and three absorption bands (215, 235 and 255 nm) are resolved in the spectra of thin samples. We assign the bands con- cerned to the influence of 'a tmospher ic ' oxygen.

The maximum transparency was observed for the glasses no. 23 and 9 doped with 0.5 mol% PbO and 0.25% CaO + 0.25% AI20 3, respectively. These two additions have the lowest melting temperature, as compared to the other ones.

Absorption bands in IR transmission spectra seem to be caused by the presence in the glass of struc- turally bound water (3600 cm E) and oxygen linked to carbon as CO (2170 cm - l ) [6], because of the interaction between 'a tmospher ic ' oxygen and the vitreous carbon crucible.

In the case of FBG, absorption bands at 350 and 430 nm are caused by the presence of oxygen linked to beryll ium in the glass. In the case of FBG contain- ing no A1F 3, and its increased hydroscopicity, the same absorption bands (350 and 430 nm) are caused by the formation of B e - O bond as a result of pyrohydrolysis of glass melt.

5. Conclusions

Oxide solubility in fluoroaluminate glasses (FAG) is limited (up to oxygen content in glass, ~ 0.29 at.%). The optical quality of FAG-36 remains after doping the batch with Y203, A1203, MgO up to 0.25 mol%; PbO, CaO + A120 3 up to 0.50 tool%; BaO, CaO up to 0.75 mol%; SrO up to 1.00 mol%. The fundamental absorption limit (the wavelength corre- sponding to 50% of transmission with the sample thickness 7 mm) is ~ 6.4 Ixm in the IR-region, and ~ 245 nm in the UV-region for oxide doped FAG and 315 nm for FAG-36(O 2) and glass no. 24(02). 'Atmospher ic ' and 'ba tch ' oxygen exist in FAG in different states and influence the optical properties of FAG differently. In fluoroberyllate glasses, the ab- sorption bands at 350 and 430 nm are caused by presence of oxygen linked to beryll ium in the glass.

References

[1] M. Weber, D. Milan and W. Smith, Engineering 5 (17) (1978) 468.

[2] V.D. Khalilev, Steklo, Trudy instituta stecla (Russia) 1 (1966) 101.

[3] V.D. Khalilev and L.V. Bogdanov, Vysokochistye veshestva (Russia) 4 (1992) 50.

[4] L.D. Bogomolova, N.A. Krasil'nikova, O.A. Trul. V.L. Bog- danov, V.D. Khalilev, K.V. Panfilov and F. Caccavale, J. Non-Cryst. Solids 175 (1994) 84.

[5] N.A. Toropov and V.P. Brakovski, Diagrammy sostojanija silikatny sistem, Spravochnik 'Dvojnye sistemy' (Nauka, Moscow, 1965) p. 468.

[6] C. Nakamoto, Infrakrasnye spectry neorganicheski i coordina- cionny soedineni (Mir, Moscow, 1966) p. 106.