Conductivity relaxation in zirconium fluoride glasses: effect of substitution of Zr4+ by Y3+ ions

Solid State Ionics 120 (1999) 27–32

Conductivity relaxation in zirconium fluoride glasses: effect of substitution of Zr 41 by Y 31 ions M. Sural, A. Ghosh* Solid State Physics Department, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700032, India Received 24 June 1998; accepted 4 December 1998

Abstract The electrical conductivity and the conductivity relaxation of (55 2 x)ZrF 4 –15BaF 2 –xYF 3 –30LiF glasses were studied in the temperature range from 300 K to just below the glass transition temperature and in the frequency range from 10 Hz to 2 MHz. No large changes in the conductivity were observed with the substitution of Zr 41 by the Y 31 ions. The activation energy remained almost constant up to 20 mol.% YF 3 content and increased for higher YF 3 content in the glass compositions. The frequency dependent conductivity was analyzed in terms of modulus formalism. The distribution parameter for the conductivity relaxation times remained almost unchanged with the substitution of YF 3 with an increase for 40 mol.% YF 3 content. The distribution of relaxation times of the present glasses was much broader than that for the YF 3 -free zirconium fluoride glasses. The glass decoupling index decreased and the modulus relaxation rate increased with the increase of YF 3 content in the glass compositions with an anomaly for the composition having 20 mol.% YF 3 content.  1999 Elsevier Science B.V. All rights reserved. Keywords: Ionic conductivity; Fluoride; Yttrium fluoride; Glass

1. Introduction Zirconium fluoride glasses have attracted considerable attention because of their potential use for making infrared optical components and ultra low loss optical fibers [1–3]. These glasses also show a relatively high electrical conductivity, which makes them a likely candidate for use as solid electrolytes [4]. Zirconium fluoride glasses are multicomponent materials, which contain a network former ZrF 4 , glass modifiers such as BaF 2 , LaF 3 , alkali metal fluorides, etc. and network stabilizers such as AlF 3 , *Corresponding author. E-mail address: [email protected] (A. Ghosh)

YF 3 , etc. [5]. The alkali metal free ZrF 4 -based glasses are fluorine ion conductors, while glasses containing alkali metal fluorides are either fluorine ion conductors or mixed fluorine and alkali ion conductors or alkali ion conductors depending on the nature and content of alkali ions in the glass compositions [6–10]. The effect of the substitution of AlF 3 for ZrF 4 on the electrical properties of the zirconium fluoride glasses has been studied [5,11]. A shallow minimum in the DC conductivity was observed and was attributed to arising from a change in the high frequency dielectric constant. A decrease in the probability of the ion jump in the structure was observed with the progressive replacement of Zr 41 ions by Al 31 ions. The objective of the present work

0167-2738 / 98 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00551-7

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is to study the electrical conductivity and the modulus spectra of ZrF 4 -based glasses by the progressive substitution of ZrF 4 by YF 3 in the frequency range 10 Hz–2 MHz and in the temperature range from 300 K to just below glass transition temperature.

2. Experiment Fluoride glass samples of compositions (55 2 x)ZrF 4 –15BaF 2 –xYF 3 –30LiF, where x 5 5–40 mol.%, were prepared using the chemicals ZrF 4 , YF 3 , BaF 2 and LiF (Aldrich). The appropriate proportions of these chemicals were mixed with an excess of ammonium bifluoride. The mixtures were melted in a platinum crucible in an electrical furnace at 8508C for 10 min. The melts were then poured into a preheated aluminium mould. Residual mechanical stresses were removed by annealing the samples at a temperature 508C below the glass transition temperatures (T g ) determined by differential thermal analysis using a heating rate of 208C / min. The amorphous nature of the samples was confirmed from X-ray diffraction. For electrical measurements, gold electrodes were deposited on both surfaces of the polished samples of diameter 1.5 cm and thickness 0.1 cm by vacuum evaporation. The gold coated samples were then heat treated at 1508C for the stabilization of the electrodes. The electrical measurements such as capacitance and conductance of the samples were carried out in the frequency range 10 Hz–2 MHz using a QuadTech RLC meter (model 7600) interfaced with a computer. Measurements were made over a temperature range from 300 K to just below T g . The DC conductivity was obtained either from the extrapolation of the frequency-dependent AC conductivity or from the complex impedance plots.

3. Results and discussion The DC conductivity of all glass compositions is shown in Fig. 1 as a function of 10 3 /T. It is clear from the figure that the variation of the conductivity with temperature can be represented by an Arrhenius equation s 5 s o exp(2Ws /kT ). The values of the

Fig. 1. Temperature dependence of the DC conductivity of (55 2 x)ZrF 4 –15BaF 2 –xYF 3 –30LiF glasses for different values of x (shown).

preexponential factor s o and the activation energy Ws were obtained from the least square straight line fits to the data and are shown in Table 1 for all glass compositions. Table 1 also includes the values of the conductivity at 2008C (s 200 ) for all compositions. These values of Ws and s 200 are close to the values obtained for other zirconium fluoride glasses [9,10]. No large changes in the conductivity are observed in Fig. 1 by the substitution of Zr 41 by Y 31 ion. Table 1 shows that Ws is almost constant up to 20 mol.% YF 3 content and increases beyond this composition. The NMR studies of similar glasses showed that the charge carriers in them are mainly Li 1 ions [10]. The AC conductivity data have been analyzed in the framework of modulus formalism M* 5 M9 1 jM0 5 1 / ´*, where ´* is the complex dielectric constant [12,13]. The frequency dependence of M9 and M0 for the glass composition with 20 mol.% YF 3 is shown in Fig. 2 at several temperatures. It is observed that M9 tends to a constant value M ` 5 1 / ´ ` at higher frequencies. It is also observed in Fig. 2 99 ) that M0 shows an asymmetric maximum (M max centered at the dispersion region of M9. The maxi-

M. Sural, A. Ghosh / Solid State Ionics 120 (1999) 27 – 32

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Table 1 Electrical parameters for the (55 2 x)ZrF 4 –15BaF 2 –xYF 3 –30LiF glasses x (mol.%)

Log s 0 (S cm 21 ) (60.01)

Ws (eV) (60.02)

Log s 200 (S cm 21 ) (60.01)

Tg (8C)

5 10 20 30 40

2.49 2.86 2.43 2.81 2.61

0.75 0.78 0.77 0.87 0.89

2 5.49 2 5.44 2 5.58 2 5.50 2 5.87

235 248 267 260 300

mum shifts to higher frequencies with the increase in temperature. The region to the left of the peak is where the charge carriers are mobile over long distances, while the region to the right is where they are spatially confined to their potential wells. The frequency v c where the maximum in M0 occurs is indicative of the transition from a short range to a long range mobility at decreasing frequency and is defined by the condition v c t c 5 1, where t c is the conductivity relaxation time. The temperature and frequency dependence of M9 and M0 for other glass compositions are similar. We have shown in Fig. 3 a 99 versus log 10 (v / master plot of M9 / M ` and M0 / M max v c ) for all temperatures for a glass composition. It

Fig. 2. The frequency dependence of M9 and M0 at different temperatures (shown) for the glass composition with x 5 20.

99 versus log 10 (v / v c ) for the Fig. 3. Plot of M9 / M ` and M0 / M max same temperatures and compositions as in Fig. 3. The solid lines are the best fits to the modulus formalism.

M. Sural, A. Ghosh / Solid State Ionics 120 (1999) 27 – 32

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may be observed that plots for all temperatures superpose perfectly, which shows that the dynamical processes occurring at different frequencies exhibit the same thermal activation energy. The data for M9 and M0 for all glass compositions were fitted at all temperatures simultaneously to the theoretical values given by the modulus formalism [12,13], using the Kohlrausch–Williams–Watts (KWW) function f(t) 5 exp[2(t / t c )b ], where the exponent b tends to unity for Debye relaxation. A best fit for one glass composition is shown in Fig. 3. Other glass compositions also showed similar fits. The values of ´ ` and b obtained from the fits are shown in Table 2. The temperature dependence of the inverse relaxation times (t 21 c ) obtained from the peak frequency v c is shown in Fig. 4 as a function of 10 3 /T for all glass compositions. It is clear in the figure that the data 21 can be fitted well to an Arrhenius equation t 21 c 5 to exp(2Wc /kT ). The values of t o , and Wc obtained from the fits are shown in Table 2. It is observed from Table 1 and Table 2 that the activation energy Ws for the DC conductivity is very close to Wc for the conductivity relaxation times, which suggests a hopping mechanism for charge carriers. It may be noted in Table 2 that the conductivity relaxation rate (t 21 o ) increases with the increase of YF 3 content in the glasses with an anomaly for the glass with 20 mol.% YF 3 content. This result is in sharp contrast with the result of the AlF 3 substituted zirconium fluoride glasses in which, when ZrF 4 was substituted with AlF 3 , the conductivity relaxation rate decreased [5]. It may be observed in Table 2 that the values of b are almost independent of YF 3 content in the compositions but increases for the glass with 40 mol.% YF 3 content. A comparison of the values of b for the present glass compositions with those of other ZrF 4 -based glasses [9,10] shows that the values of b ( | 0.46) for the present glass compositions are substantially smaller than those ( . 0.50) for the

Fig. 4. Temperature dependence of the inverse conductivity relaxation time for the same glass compositions as shown in Fig. 1.

YF 3 -free ZrF 4 glasses. Therefore, the substitution of ZrF 4 by the YF 3 leads to a broader distribution of the relaxation times in the present glasses compared to the YF 3 -free ZrF 4 -based glasses. The small values of b also suggest that the cooperation between charge carriers in the conductivity relaxation [14,15] is much higher than that of YF 3 -free ZrF 4 glasses [9,10]. The glass decoupling index R t (T g ), defined [16,17] as the ratio R t (T g ) 5 kt s (T g )l / kt c (T g )l, where kt s (T g )l and kt c (T g )l are the average structural and the conductivity relaxation times at the glass transition temperature T g , can be calculated from the

Table 2 Different parameters of the (55 2 x)ZrF 4 –15BaF 2 –xYF 3 –30LiF glasses obtained from conductivity relaxation model x (mol.%)

t 0 (s)

Wc (eV) (60.02)

b

´` 5 1 / M`

R t (T g )

5 10 20 30 40

1.96 3 10 215 1.20 3 10 215 3.39 3 10 215 9.75 3 10 216 5.07 3 10 216

0.75 0.77 0.76 0.87 0.87

0.46 0.47 0.47 0.47 0.54

13.08 12.48 13.34 12.88 14.14

3.22 3 10 8 8.73 3 10 7 9.52 3 10 6 1.63 3 10 7 8.33 3 10 6

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conductivity relaxation times. R t (T g ) describes the extent to which the conducting ion motion in a given glass can be decoupled from the viscous motion of the glassy matrix and is consequently related to the ability of mobile ions to migrate in the glass electrolytes at T g . R t (T g ) has been calculated by 21 3 extrapolating the log 10 t c versus 10 /T plots (Fig. 4) at T g and assuming the structural relaxation time equal to 200 s [17,18], and are shown in Table 2. It may be noted that the values of R t (T g ) decrease with the increase of YF 3 content in the composition with an anomaly for 20 mol.% YF 3 content. This observation is in agreement with the variation of the conductivity with composition. A remarkable feature of the conductivity relaxation observed in the present glass compositions is that the temperatures of the maxima of the M0[T max (M0)] measured at 1 kHz (Fig. 5) bear a constant ratio, to the glass transition temperature T g for all compositions [T max (M0) /T g ¯ 0.65]. Since the values of activation energy Wc for the conductivity relaxation times for various compositions do not vary significantly, it implies that the calorimetric relaxation rate which is used to designate the glass transition temperature is lower than the conductivity relaxation rate by a factor that remains unchanged

31

with composition. Due to the substitution of ZrF 4 by YF 3 in the glass compositions the heights of the maxima of M0 do not change significantly for the different glass compositions which gives almost composition independent values of ´ ` (Table 2). The variation of the conductivity relaxation rate with the substitution of YF 3 can be explained on the basis of structural considerations. Based on infrared and Raman spectroscopy, X-ray diffraction, molecular dynamics and computer simulation studies, a number of possible structures of barium-metaflurozirconate glasses has been proposed [17–20]. Although such structures are relevant to ZrF 4 –BaF 2 glasses containing different amounts of ZrF 4 , a feature common to these structures is that Zr atoms are coordinated with F which acts as a bridging atom to form zigzag chains and that Ba atoms are ionically bonded to F and occupy interstitial sites within the neighbouring chains. It has been suggested [21] that F 2 ions are not only vibrating thermally at their lattice sites, but also migrating from one site to another, while Zr 41 and Ba 21 ions vibrate thermally only in their respective sites. Substitution of the ZrF 4 content in the glasses by YF 3 causes both the coordination number and the number of bridging F atoms to change. Although the details of structures of the present glasses are not known, it is reasonable to assume that the essential structure remains that of zigzag chains of alternating Zr and F atoms containing cross links between the chains. It has been suggested that [22] YF 3 are good stabilizers of ZrF 4 based glasses similar to AlF 3 and that they occupy network forming positions, rather than network modifying positions. Raman spectroscopy studies [23] of the fluoride glasses containing YF 3 suggest that these glasses have the broadest distribution of local structural configurations. We suggest that this increases the probability of F 2 ion forming a bridge with the neighbouring chains as a result of thermal excitation and that this increases the conductivity relaxation rate.

4. Conclusions

Fig. 5. Temperature dependence of M0 at a fixed frequency of 1 kHz for three glass compositions (shown).

The substitution of ZrF 4 by YF 3 in the zirconium fluoride glasses does not lead to large changes in the electrical conductivity of the glass compositions. The

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activation energy remains almost constant up to 20 mol.% YF 3 content and increases for higher YF 3 content in the compositions. The distribution parameter for the conductivity relaxation times also remains constant with a slight increase for the glasses with 40 mol.% YF 3 content. However, the distribution is much broader than that for the YF 3 -free ZrF 4 -based glasses. The glass decoupling index decreases and the relaxation rate increases with an anomaly for 20 mol.% YF 3 content, despite the increase of nonbridging fluorine atoms with the increase of YF 3 content in the glass compositions.

Acknowledgements The financial support for the work by the Department of Science and Technology, Govt. of India (via Grant No. SP/ S2 / M26 / 93) is gratefully acknowledged.

References [1] M. Poulain, M. Chanthanasinh, J. Lucas, Mater. Res. Bull. 12 (1977) 151. [2] J. Qiu, J. Mater. Sci. 31 (1996) 3597. [3] J. Lucas, J. Mater. Sci. 24 (1984) 1. [4] J.M. Bobe, J. Senegas, J.M. Reau, M. Poulain, J. Non-Cryst. Solids 162 (1993) 169.

[5] C. Mai, G.P. Johari, Phys. Chem. Glasses 30 (1989) 116. [6] J.M. Reau, J. Senegas, H. Aomi, P. Hagenmuller, M. Poulain, J. Solid State Chem. 60 (1985) 159. [7] J.M. Reau, J. Senegas, J.M. Rojo, M. Poulain, J. Non-Cryst. Solids 116 (1990) 175. [8] J.M. Reau, H. Kahnt, M. Poulain, J. Non-Cryst. Solids 119 (1990) 347. [9] J.M. Bobe, J.M. Reau, J. Senegas, M. Poulain, Solid State Ionics 82 (1995) 39. [10] J.M. Bobe, J.M. Reau, J. Senegas, M. Poulain, J. Non-Cryst. Solids 209 (1997) 122. [11] G.V. Chandrasekhar, M.W. Shafer, Mater. Res. Bull. 15 (1980) 221. [12] F.S. Howell, R.A. Bose, P.B. Macedo, C.T. Moynihan, J. Phys. Chem. 78 (1974) 639. [13] P.B. Macedo, C.T. Moynihan, R. Bose, Phys. Chem. Glasses 13 (1972) 171. [14] C.A. Angel, Solid State Ionics 9–10 (1983) 3. [15] C.A. Angel, Solid State Ionics 18–19 (1986) 72. [16] K.L. Ngai, J. Phys. C2 (1992) 61. [17] R.M. Almedia, F. Lau, J.D. Mackenzie, J. Non-Cryst. Solids 69 (1984) 945. [18] J. Lucas, C.A. Angel, S. Tamaddon, Mater. Res. Bull. 19 (1984) 945. [19] C.A. Angel, P.A. Cheeseman, S. Tamaddon, J. Phys. C9 (1982) 381. [20] Y. Kawamoto, Mater. Sci. Forum 6 (1985) 417. [21] Y. Kawamoto, T. Horisaka, K. Kiran, N. Soga, J. Chem. Phys. 83 (1985) 2398. [22] I.T. Hamt, J.M. Parkar, Mater. Sci. Forum 6 (1985) 437. [23] B. Bednow, P.K. Banerjee, J. Lucas, A. Fontneau, M.G. Drexhage, Mater. Sci. Forum 6 (1985) 471.