SHORT RANGE STRUCTURES OF FLUORIDE GLASSES

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SHORT RANGE STRUCTURES OF FLUORIDE

GLASSES

R. Almeida, J. Mackenzie

To cite this version:

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JOURNAL

DE

PHYSIQUE

Colloque

C8,

supplCment au n012, T o m e 46, d6cembre 1985 page C8-75

SHORT RANGE STRUCTURES O F F L U O R I D E GLASSES R.M. Almeida and J.D. ~ackenzie*

Centro de Fisica MoZecuZar, Complexo I , I . S . T . , Av. Rovisco Pais

ZOO0 Lisboa, Portugal

* ~ a t e r i a Z s Science and Engineering Department, University of California, Los AngeZes, CA. 90024, U.S.A.

R6sum6

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La structure B courte distance des verres de fluorozirconate et de fluorohafnate a 6t6 6tudi6e par spectroscopie vibrationelle, diffraction des neutrons et des rayons X, et spectroscopie de photo6mission X. Les deux pre- mibres msthodes suggbrent que l'environnement en fluor des atomes Zr et Hf possbde une sym6trie Qlevse, avec des coordinations essentiellement 6 et 7. Le calcul donne un nombre de coordination F-F voisin de 5. La photo6mission Xdonnedes informations moins directes et ne permet pas de distinguer clairement les fluor pontants des non-pontants. Les proportions relatives de ces deux types d'atomes ont cependant une influence observable sur la structure de la bande de valence des fluorozirconates vitreux et cristal- lins.

Abstract - The short range structures of fluorozirconate and fluorohafnate glasses have been examined by vibrational spectroscopy, X-ray and neutron diffraction and X-ray photoemission spectroscopy. Results from the first two techniques were consistent with the occurrence of highly symmetrical fluorine atom environments about Zr and Hf atoms, with predominant coordinations between 6 and 7-fold. The F-P coordination number was calculated at -5. X-ray photoemission spectroscopy offered somewhat more indirect information, being unable at present to offer a clear distinction between bridging and non- -bridging fluorine atoms. The degree of bridging, however, had a significant effect on the structure observed in the valence band of both vitreous and crystalline fluorozirconates.

I

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INTRODUCTION

Due to their extended transmission in the infrared region, heavy metal halide glasses hold great technological potential, namely as optical waveguide materials. Their short range structures are still controversial, but the occurrence of net-

work-forming cation coordination numbers which are unusually high for nonmetallic glasses is now established in fluoride systems such as those based on ZrFq or HfF4 11-41. The ellucidation of the structure of these glasses requires the examination of several related systems by different complementary techniques.This paper discus- ses the short range structure of binary fluorozirconate and fluorohafnate glasses as derived from IR and Raman spectroscopy, X-ray and neutron diffraction and X-ray photoemission spectroscopy (XPS). The particular kinds of structural information which can be extracted with each method are considered in detail.

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JOURNAL DE PHYSIQUE

I1

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EXPERIMENTAL

A series of binary glasses were investigated, consisting of a network former (ZrFq or HfF4) and a network modifier (BaF2 or PbF2). Crystalline Li2ZrF6, K2ZrF6 and ZrF4 were also studied by XPS. Details of sample preparation and experimental meas- urements can be found elsewhere /2,4,5/. The binary glass compositions had the dizirconate or dihafnate (2:l) stoichiometry.

111

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RESULTS AND DISCUSSION

Pure amorphous ZrF4 or HfF4 have not yet been studied due to difficulties in preparation and handling. In the present case, by studying binary glasses, the struc- turally complex effects of a third fluoride (such as A1F3, LaF3 or ThF4) have been avoided.

Fig. 1 shows the IR-absorption and reduced 161 polarized Raman spectra of lead dihafnate glass. The two spectra were mutually exclusive, except for the strong 490 cm-I IR band which was weakly Raman active. Fig. 2 shows a similar set of spectra for a lead dizirconate glass. A comparison of the vibrational frequencies of the two glasses shows that the dominant Raman band does not involve Zr(Hf) cation motion. It can be concluded that (1) the IR response is dominated by stretching modes involving simultaneous motions of bridging fluorine atoms (Fb) and Zr(Hf) cations 1.21 and (2) the Raman spectrum is dominated by high frequency symmetric stretching vibrations of

non-bridging fluorine atoms (F ) about fixed Zr(Hf) cations 121. The low frequency "b

feature of the IR spectrum, whlch was found to increase from -85 cm-I to -110 cm-I on going from Pb to Ba-containing dihafnate glasses, can be attributed to localized vibrations of the modifying cations.

Fig. 1 (a) IR transmission and (b) reduced polarized Raman spectra of 2 HfF

.PbF2 glass. 4'

Fig. 2 (a) IR transmission and (b) reduced polarized Raman spectra of 2 ZrF

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The vibrational characteristics of these glasses, particularly the occurrence of a single dominant,

symmetric, completely polarized Raman line (depo- Zr or Hf larization ratio DR -0.08) near 580 em-', in con-

junction with a 2:l glass stoichiometry (F/Zr ratio equal to 5), are best interpreted in terms of highly symmetrical F atom environments about Zr or Hf atoms, predominantly six-fold coordinated. It is unlikely that vibrations of such high symmetry could be excited if the predominant coordination were, for example, seven-fold, as the diffraction results presented below appear to indicate. A possible structure compatible with the vibrational data is shown in Fig. 3, where every Zr(Hf) atom is surrounded by two bridging and four non-bridging F atoms in a chain-like fashion.

Fig. 4 shows the total correlation functions T(r) for barium dizirconate glass:

2

T(r)=<f(O)> {4drpO+ I(,)-K] sin(~r)dK] (1) 0 r 2ZrF4.BaF2- N . . ~ , O ~ r\ 4 -

25

N n n v

2 - Fig. 3 Chain-like structure

=:

L. of dizirconate (dihafnate)

"

+

glass.

o

-,

,

-

₄_.,I9

caIculated by Fourier analysis I _{of the structure factors I<K)}

6 - 2 Z r F 4 . BaF,-x-ray _{for the X-ray and neutron (time} of flight) diffraction experi-

-Exper. curve _{ments.,The first feature}₍_at

---

Zr-F _{-2.08 A)}_{can be assigned to}

n

...

F-F Zr-F correlations, with a Zr

coordination number of 6.720.5

N

171. The second feature (-2.70

x),

after Gaussian peak fitting,

CI was separated into two contribu-

;:Pi:

F-F- correlations at with a coordination

-

number

5 and Ba-F correlations at -2.8

,

with a coordination numberoof 14

.

The third feature

(-4.1 A) is likely to contain

I major contributions from Zr-Zr

0 2 4 6 correlations, from which the

r (

A)

average Zr-F-Zr bridging angle

was calculated at

-

170°. Fig. 4 Neutron and X-ray total correlation _{These results are in general} functions for barium dizirconate glass. (A- agreement with the structural dapted from ref. /7/ )

.

_{model of Fig. 3, except for a}

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JOURNAL DE PHYSIQUE

700 685 45 35 25 4 2 0 RELATIVE B I N D I N G ENERGY (eV)

F i g . 5 Core l e v e l and v a l e n c e band XPS s p e c t r a of c r y s t a l l i n e and v i t r e o u s f l u o r o z i r c o n a t e s .

h i g h e r t h a n s i x . This may be due t o a d d i t i o n a l i n t e r c h a i n Zr-F c o n t r i b u t i o n s involving Fnb atoms randomly l o c a t e d about t h e c e n t r a l Z r , which d i d n o t s i g n i f i c a n t l y af- f e c t t h e Raman response /2/. X-ray d i f f r a c t i o n d a t a were s i m i l a r f o r barium d i h a f n a t e g l a s s , a f t e r allowing f o r t h e d i f f e r e n c e s i n weight f a c t o r s , i n d i c a t i n g t h a t t h e two g l a s s e s a r e s t r u c t u r a l analogues.

I n o r d e r t o f u r t h e r c h a r a c t e r i z e t h e occurrence of b r i d g i n g and non-bridging f l u o - r i n e atoms i n t h e heavy metal f l u o r i d e g l a s s e s , some f l u o r o z i r c o n a t e compositions were a l s o s t u d i e d by XPS. F i g . 5 shows t h e F atom e l e c t r o n i c energy l e v e l peaks,

along w i t h one of t h e Z r c o r e l e v e l peaks. Although s m a l l v a r i a t i o n s i n t h e F c o r e l e v e l b i n d i n g e n e r g i e s could be 'detected among t h e d i f f e r e n t m a t e r i a l s s t u d i e d / 5 / , no s p l i t t i n g was observed f o r t h e F I s o r F 2s peaks, a s opposed t o t h e c a s e of modi- f i e d oxide g l a s s e s , where t h e 0 I s peak i s found t o s p l i t by -2 eV between b r i d g i n g and non-bridging oxygens / 8 / . The v a l e n c e band s p e c t r a , on t h e o t h e r hand, show con- s i d e r a b l e s t r u c t u r e i n t h e c a s e s of c r y s t a l l i n e K2ZrF6 and barium d i z i r c o n a t e g l a s s . Such s t r u c t u r e becomes even more pronounced when t h e BaF2 c o n t e n t of t h e barium f l u o r o z i r c o n a t e g l a s s e s i s i n c r e a s e d .

The d i f f e r e n c e s i n behaviour between oxide and t h e more i o n i c f l u o r i d e g l a s s e s can be a t t r i b u t e d t o t h e r e l a t i o n s h i p between t h e r e s o l v i n g power of t h e spectrometer (-1.0 eV) and t h e expected binding energy (BE) d i f f e r e n c e s between b r i d g i n g and non-bridg- i n g a n i o n i c s p e c i e s i n each c a s e . The p a r t i a l charge on t h e bonded a n i o n s , which i s

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The valence band spectra, also shown in Fig. 5, exhibited a considerable amount of structure, particularly for the more covalent materials

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crystalline K2ZrF6 and the dizirconate glass. Each sample showed two peaks, which could not be attributed to spin-orbit splittings. While the low BE peak could be assigned to the 2p non- -bonding levels of fluorine, the high BE peak was probably due to 0-like bonding levels with a predominant F 2p character. It can be concluded that the bonding schemes vary significantly from the more ionic halides, such as ZrFq, to the more covalent ones, such as K2ZrF6. Although it is not yet clear how this relates to the occurrence of bridging and non-bridging F atoms, the valence band structure of these glasses appears to be intermediate between those of ZrFq and K2ZrF6, with the extent of splitting being roughly proportional to the BaF2 content. Since K2ZrF6 contains both Fb and Fnb atoms, in this sense it is likely that the glasses contain both kinds of anions as well. It is possible that the use of a monochromatized X-ray source will allow a better characterization of the two types of fluorines.

IV - CONCLUSIONS

The short range structures of dizirconate and dihafnate fluoride glasses include Zr and Hf atoms with large coordination numbers, between 6 and 7. They also appear to contain both bridging and non-bridging F atoms. Vibrational spectroscopy shows fluorine atom environments of very high symmetry and it strongly suggests six-fold coordination, with the simultaneous occurrence of Fb and Fnb species. X-ray and neutron diffraction indicate a Zr-F coordination number near 7, an F-F coordination of -5, a Ba-F coordination near -14 and an average bridging angle of -1700, without providing direct evidence for the existence of Fb and Fnb anionic types. X-ray photoemission spectroscopy shows that the valence band structure of fluorozirconates is a strong function of the ZrFq content, probably as a result of the relative proportions of Fb and Fnb. AS a consequence of large network forming coordination numbers and a significant degree of ionicity, the structure of heavy metal fluoride glasses is more difficult to characterize than that of oxide or chalcogenide glasses.

We would like to thank NATO for supporting this work under NATO Research Grant No. 219.81 and also the AFOSR, Directorate of Chemical and Atmospheric Sciences for financial support.

REFERENCES

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2

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(1983) 6502.

/3/ Coupe, R., Louer, D., Lucas, J. and Leonard, A.J., J. Am. Ceram. Soc.

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(1983) 523.

141 Etherington, G. Keller, L., Lee, A., Wagner, C.N.J. and Almeida, R.M., 3. Non-Crystalline Solids 69 (1984) 69.

/5/ Almeida, R.M., Lau J. and xckenzie, J.D., J. Non-Crystalline Solids

2

(1984) 161.

161

Galeener, F.L. and Sen, P.N., Phys. Rev.

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(1978) 1928.

/7/ Etherington, G., Wagner, C.N.J., Almeida, R.M. and Faber Jr., J., Repts. Hahn-Meitner Institute (1984) 64.

I81 Bruckner, R., Chun, H. and Goretzki, H., Glastech. Ber. (1978) 1.