Some physical properties of new oxyfluoride glasses

J O U R N A L OF

ELSEVIER

Journal of Non-Crystalline Solids 184 (1995) 141-146

Some physical properties of new oxyfluoride glasses R. E1-Mallawany a, A.H. Khafagy a M.A. Ewaida a, I.Z. Hager M.J. Poulain b,,

b M.A. Poulain b

a Physics Department, Faculty of Science, Menoufia University, Shebin EI-Koom, Egypt b Laboratoire des Mat&iaux Photoniques, CEMA, UniversitO de Rennes I, Campus de Beaulieu, 35042 Rennes, France

Abstract New glasses have been synthesized in the quaternary system based on complex molybdenum oxyfluorides. Systematic investigations were implemented in the MoO3-BaF2-NaF-ZnF 2 quaternary system with a constant amount (30 mol%) of MoO 3. Samples of a few mm in thickness were obtained. The physical properties such as density, molar volume, glass transition temperature, activation energy of crystallization, stability criterion, thermal expansion coefficient, refractive index, molar refractivity and infrared measurements were performed and correlated with composition, especially with alkali fluoride content. By comparison with standard fluoride glasses, these glasses exhibit similar characteristic temperatures but higher refractive index and high ultraviolet absorption.

1. Introduction The influence of oxygen on glass-forming ability, devitrification and defects in fluoride systems appears as a topic of main importance because it is correlated with the problem of wavelength independent scattering in fluoride glass fibers [1]. Oxygen impurities are frequently cited as having an adverse effect on both optical transmission - through absorption and scattering - and stability of fluoride glasses [2,3]. These effects have currently been ascribed to trace amounts of oxide present in starting materials and to gaseous 0 2 or H 2 0 in the working atmosphere. However, experimental evidence was presented by Mitachi and Tick [4] who suggested that oxygen plays a positive role in the stabilization of

* Corresponding author. Tel: + 33 99 28 62 63. Telefax: + 33 99 28 69 72. E-mail: mpoulain@univ-rennes 1.fr.

some fluoride glasses such as the C L A P fluoroaluminate glass [5]. The addition of some oxides, especially P205, in fluoride melts, is effective for stabilizing the vitreous state. Yasui et al. [6] studied the influence of the addition of oxides upon glass-forming ability of fluoride glasses. The addition of A1PO 4 enhances vitrification, whereas the incorporation of V20 s or H3BO 3 rather stimulates crystallization. Tellurium oxide TeO 2 also showed some stabilizing effect, while the addition of CaMoO 4 appeared to have little influence. The study of oxyfluoride glasses provides basic information about glass-forming ability and may help the understanding of the vitrification mechanism and possibly the related structural aspects. Comparisons could be made between oxyfluoride glasses containing M O 4 polyhedra: until now main data refer only to fluorophosphate glasses, but the cation may be also Mo, W, Nb, Ta or even Si and Ge. The first aim

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00657-1

142

R. El-Mallawany et al. /Journal of Non-Crystalline Solids 184 (1995) 141-146

Table 1 Molybdenum oxyfluoride glass composition (mol%). Acronym

MoO3

BaF2

NaF

ZnF2

MOBNZnl MOBNZn2 MOBNZn3 MOBNZn4 MOBNZn5 MOBNZn6 MOBNZn7

30 30 30 30 30 30 30

40 35 30 25 20 15 10

10 15 20 25 30 35 40

20 20 20 20 20 20 20

of this work is the synthesis of fluoromolybdate glasses based on M o O 3. In addition, the main physical properties are reported and correlated with the composition, especially the alkali fluoride content.

2. Experimental 2.1. Glass preparation

Anhydrous fluorides are commercial products: BaF 2 from Prolabo, N a F and ZnF 2 are from Riedel de Haen. MoO 3 was supplied by Strem. A classical synthesis process was used [7]. Convenient amounts of starting materials are mixed and heated up to their melting temperature - typically 4 5 0 - 5 5 0 ° C - in a deep platinum crucible for 3 0 - 4 5 min. Then the melt is squeezed between metal plates (quenched glasses) for investigating the glass-forming area. This corresponds to quenching rates ranging from a few hundreds to several thousand K s -1. For more stable compositions, the melt is poured into a brass mould preheated around the glass transition temperature, Tg, which allows samples of several m m in thickness to be obtained. Samples were annealed near their glass transition temperature for 1 h. The furnace was then switched off and cooled slowly to room temperature.

sured using T M A SII Seiko instruments 5200. The density is obtained via the Archimedean method, using CCI 4 as an immersion liquid. The refractive index is measured using an A b b e refractometer. The uncertainty on the value of the physical characteristics is ___0.03 for density, + 5 × 10 4 for refractive index and ___3 × 10 7 for thermal expansion. The infrared (IR) absorption is recorded in a Bomem Michelson 100 instrument. The U V - v i s i b l e transmission is studied using a Cary 5 UV spectrophotometer.

3. Results The vitreous area in the 30% M o O 3 - B a F 2 - N a F ZnF 2 quaternary system is reported in Fig. 1. Glass samples are obtained as coloured parallellipipedic blocks. Some physical properties have been measured for typical compositions. A line was defined through the glass-forming region according to the general formula 30MOO3, 20 ZnF2, [50 - x ] BaF2, x N a F , with 10 < x < 40. A set of samples with NaF content ranging from 10 to 40 mol% were synthesized (see Table 1). As sodium fluoride content increases density, molar volume, refractive index and glass transition temperature decrease (see Table 2). This evolution is shown by Figs. 2 to 5.

BaFz MoO3

2.2. Measurements

The differential scanning calorimetry (DSC) measurements are implemented with a Seiko DSC 220 at a 10 K min -1 heating rate. Temperatures are given with an accuracy of + 2°C. Because systematic errors are the same in many cases, the relative evolution of these temperature correspond to a mean error of + I°C. The thermal expansion coefficient is mea-

E NaF

Zn~

20 40 60 Fig. l. Vitreousarea ~rMoO3-BaF2-NaF-ZnF2glasses.

R. El-Mallawany et al. /Journal of Non-Crystalline Solids 184 (1995) 141-146

143

1,6540

4,75 O

4,60

1,6520

¢.1 E u

4,45

x

1,6500

~

4,30

"-~

1,648O

~]

[]

[]

o

4,15

1,6460

4,00

1,6440 10

20

30

40

50 10

15

20

25

30

35

40

NaF %

Fig. 2. Density variation versus composition. The line is drawn as a guide for the eye.

NBF (mole%)

Fig. 4. Refractive index versus NaF content. The line is drawn as a guide for the eye.

28,0

27,5 ~:

27,0

:>

g

26,5

~> >

26,0

i

25,5

~n

n

~'

280

d. E P-

260

g m

~

25,0

240

m

24,5 24,0

10

20

30

40

220

'

10

50

20

30

NaF (mol %)

Fig. 5. Evolution of Tg versus composition. The line is drawn as a guide for the eye.

Table 2 The density, molar volume, Vm, glass transition temperature, Tg, activation energy of crystallization, refractive index, n D, and molar refractivity, Rm, of molybdenum oxyfluoride glasses Density (gm cm -3)

Vm (cm 3)

MOBNZnl MOBNZn2 MOBNZn3 MOBNZn4 MOBNZn5 MOBNZn6 MOBNZn7

4.71 4.65 4.48 4.22 4.19 4.09

27.87 26,82 26.32 26.39 24.98 24.00

50

NaF %

Fig. 3. Molar volume versus NaF content. The line is drawn as a guide for the eye.

Sample no.

40

Tg (°C)

E (kJ mol l)

S

(Tx-Tg) (oC)

298 285 275 265 252 239 226

113 114 112 111 111 101 113

0.82 1.20 1.75 2.22 3.11 5.64 2.05

42 57 59 66 75 52 54

E, stability factor, S, (Tx-Tg), nD

Rm (cm 3 mol l)

1.653 1.652 1.651 1.648 1.645

10.20 9.80 9.61 9.60 9.06

R. EI-Mallawany et al. /Journal of Non-CrystaUine Solids 184 (1995) 141-146

144 6,0

5,0 70

t.9

4,0

3,0 >, .Q ~.~

2,0

. _

50

1,0

0,0 10

20

I 40

30 NaF

i 50

40

%

10

Fig. 6. Stability factor versus composition. The line is drawn as a guide for the eye.

The DSC analysis of these glasses leads to values of characteristic temperatures. The glass transition occurs around 300°C for sodium free glasses. Glass stability may be evaluated using semi empirical factors such as the differences between the temperatures for onset of crystallization, Tx, and glass transition, Tg, or the S factor [8] defined as S = (Tx - Tg)(Tc It was also possible, from DSC scans, to calculate the activation energy for crystallization, using classical relations. The evolution of these parameters with respect to sodium concentration is displayed in Figs. 6-9. The resistance against devitrification is one of the major features of a glass as it determines the practical limits of manufacturing and use. It has been

Tx)/Tg.

I

I

I

20

30

40

50

NaF %

Fig. 8. (T×-Tg) versus composition. The line is drawn as a guide for the eye.

emphasized that more stable glasses correspond to low activation energies. A minimum value of E a is observed at 35% NaF. The correlation with the maximum of the stability criterion S is obvious from the comparison of Figs 6 and 7. The DSC scan of a corresponding glass is reported in Fig. 9. The refractive index is close to 1.65, which is a rather high value for a fluoride glass. This results from the high polarisability of the M o - O bonds. The plot of molar refractivity versus NaF content is linear. Optical transmission is limited in the UV spectrum by electronic transitions. While crystalline molybdate appears white, these glasses are usually brown, which indicates that some chemical reduction

120

339 °C

115

q

-2.

11o

255~

48O °C 330

IO0

i 10

20

30

40

50

150 NaF

%

Fig. 7. Activation energy, Ea, versus composition. The line is drawn as a guide for the eye.

218

308

394

482

570

Temperature *C Fig. 9. DSC trace of a fJuoromolybdateglass.

R. El-Mallawany et al. /Journal of Non-Crystalline Solids 184 (1995) 141-146 100

c m

0

5

10

Wavelength(micrometer)

Fig. 10. Infrared transmission specrum of a fluoromolybdateglass.

of Mo(VI) occurs. The corresponding UV transmission curve shows that the Urbach tail crosses the entire visible range. Further studies would be helpful in order to assess the influence of processing on UV absorption edge. In fluorozirconate glasses, the reduction of Zr(IV) results in a gray coloration which may be removed by oxidizing processing [9], e.g., RAP. In the IR spectrum which is shown in Fig. 10, the multiphonon absorption edge does not appear clearly on thick samples because it overlaps with a strong absorption band located around 5.7 txm. This band corresponds to the first overtone of the v 3 vibration mode of the M o O 4 tetrahedron [10] which behaves in this glass as an independent molecular species.

4. Discussion These fluoromolybdate glasses may be compared with fluorophosphate glasses. Their structure is likely to exhibit similar features. The exact coordination of molybdenum may be discussed as both tetrahedral and octahedral sites are observed in molybdenum complex oxides. The occurrence of the IR band at 5.7 Ixm rather suggests that oxygen anions are located around Mo a s M o O 4 tetrahedra. From the gross chemical formula - O / M o ratio is equal to 3 these tetrahedra cannot be isolated but should share two corners, resulting in chain-like arrangements. Then glass structure could be described as a random packing of fluorine anions, M o O 4 units and Ba ÷+ cations hosting the smaller Zn ÷+ and Na ÷ cations. Diffraction studies are required in order to determine if some molybdenum cations exhibit a sixfold coor-

145

dination with a random distribution of fluorine and oxygen in the anionic sites. The physical properties are closely related to composition. By comparison with fluorozirconate glasses, the glass transition temperature and density are similar, and the influence of sodium fluoride is the same as observed in fuoride glasses [11]. The decrease in molar volume is easily explained from the difference in the atomic weighs of BaF 2 and NaF, and also in the decrease of the total number of anions in the gross formula. The stability is still lower than that of standard fluorozirconate or fluoroaluminate glasses. In this pseudoquaternary system, a maximum stability is observed for 35% NaF and 15% BaF 2. This could be improved by composition adjustements. The practical applications of such glasses appear limited, but one may take advantage from their transmission range for special windows or fibres acting as optical filters. They could be helpful for manufacturing high numerical aperture optical fibres.

5. Conclusion New oxyfluoride glasses based on molybdenum have been identified and characterized. This study concentrates on glasses containing 30% molar MoO 3. Physical properties are correlated with composition: density, molar volume, Tg and n D decrease with NaF content increases. By comparison with standard fluoride glasses, optical transmission is limited both in the UV spectrum by electronic transitions arising from the partial reduction of molybdenum, and in the IR spectrum by the vibrations of the MoO 4 tetrahedra.

References [1] S. Mitachi, S. Sakaguchi, H. Yonezawa, K. Shikano, T. Shimgematsu and S. Takahashi, Jpn. J. Appl. Phys. 224 (1985) L827. [2] A.V. Longobardo and A.K. Varshneya, Mater. Sci. Forum 32&33 (1988) 203. [3] M. Poulain, Cryst. Latt. Def. Amorph. Mater 18 (1989) 337. [4] S. Mitachi and P.A. Tick, Mater. Sci. Forum 67&68 (1991) 169.

146

R. EI-Mallawany et aL /Journal of Non-Crystalline Solids 184 (1995) 141-146

[5] P.A. Tick, Proc. SPIE 843 (1987) 34. [6] I. Yasui, H. Hagihara and H. Inoue, J. Non-Cryst. Solids 140. (1992) 130. [7] M. Poulain, J. Non-Cryst. Solids 56 (1983) 1. [8] M. Saad and M. Poulain, Mater. Sci. Forum 19&20 (1987) 11.

[9] M. Poulain and G. Maz6, Chemtronics 3 (1988) 77. [10] K. Nakamoto, Infrared Spectra of Inorganic Compounds (Wiley, New York, 1970). [11] A. Lecoq and M. Poulain, J. Non-Cryst. Solids 34 (1979) 101.