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Degenerated mixed cation effect in CuI–AgI–As2Se3 glasses: 64Cu and 110Ag tracer diffusion studies
Solid State Ionics 113–115 (1998) 697–701
Degenerated mixed cation effect in CuI–AgI–As 2 Se 3 glasses: 64 110 Cu and Ag tracer diffusion studies a ,c
A. Bolotov , E. Bychkov
a ,c ,
* ,1 , Yu. Gavrilov a , Yu. Grushko b , A. Pradel c , M. Ribes c , V. Tsegelnik a , Yu. Vlasov a
a St. Petersburg University, 199034 St. Petersburg, Russia St. Petersburg Institute for Nuclear Physics, 188350 Gatchina, Russia c LPMC, UMR 5617 CNRS, Universite´ Montpellier II, 34095 Montpellier, France b
Received 4 September 1998; accepted 12 September 1998
Abstract In contrast to the usual behaviour of mixed cation glasses (large deviations from additivity with a pronounced minimum at the conductivity isotherms and a diffusivity crossover when guest cations reduce the mobility of ions coming from the host material), the CuI–AgI–As 2 Se 3 glassy system exhibits remarkable differences in the transport properties. First, the ionic conductivity increases monotonically by four orders of magnitude from 3 3 10 28 to 5 3 10 24 S cm 21 when CuI is gradually replaced by AgI, without any minimum at intermediate concentrations. Secondly, no diffusivity crossover was observed. The 110 Ag tracer diffusion coefficient DAg follows the conductivity isotherm with nearly the same changes in DAg of four orders of magnitude, from ¯ 10 212 cm 2 s 21 to ¯ 10 28 cm 2 s 21 . The 64 Cu tracer diffusion coefficient DCu increases by a factor of 20 to 30 when copper is substituted by silver. Moreover, the values of DCu are always lower than DAg . The only exception is a 0.5CuI ? 0.5As 2 Se 3 glass, for which DAg and DCu are very similar. The results obtained can be described as a degenerated mixed cation effect with different trends for silver and copper mobile cations. 1998 Elsevier Science B.V. All rights reserved. Keywords: Mixed cation glasses; Copper iodide; Silver iodide; Ionic conductivity
1. Introduction The mixed alkali effect is one of the most remarkable features of ion transport in glasses. One observes a strongly non-linear change in the ionic *Corresponding author. Tel.: 133-328-658-250; fax: 133-328658-244; e-mail: [email protected] 1 Permanent address: MREID, Universite´ du Littoral, 59140 Dunkerque, France.
conductivity of mixed alkali or, more generally, mixed mobile cation glasses with a pronounced minimum of several orders of magnitude at intermediate concentrations, which is accompanied by a corresponding increase in the activation energy (see, for example, [1,2] and references therein). Tracer diffusion coefficients exhibit the diffusivity crossover when guest cations reduce significantly the mobility of ions coming from the host material. The mixed cation effect was also observed for chalcogenide
0167-2738 / 98 / $ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00397-X
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A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
glasses containing alkali and silver ions [3,4], however, no experiments have been reported concerning Ag 1 / Cu 1 mixed glasses. For our experiments, we have chosen CuI–AgI–As 2 Se 3 glasses, containing 50 mol.% of metal iodide. A 0.5CuI ? 0.5As 2 Se 3 glass is a mixed (Cu 1 , hole) conductor according to electrical and 64 Cu tracer diffusion measurements [5]. Its Ag-containing counterpart is nearly pure Ag 1 ion conductor [6]. In order to get additional and complementary information on ion transport in these mixed glasses, we have completed AC impedance measurements by tracer diffusion experiments using 110 Ag and 64 Cu isotopes.
2. Experimental details xCuI ? (0.5 2 x)AgI ? 0.5As 2 Se 3 glasses with r 5 AgI /(AgI 1 CuI) 5 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared from reagent grade CuI, AgI and As 2 Se 3 . The mixtures were sealed in a silica tube at 0.01 Pa and heated at 10008C for 3 days with repeated stirring of the melt. The glasses, annealed at 20–308C below the glass transition temperature, were characterised by X-ray diffraction. The homogeneity of the glasses was checked by optical and scanning electron microscopy. The density of each sample was determined using the Archimedes method in toluene. Electrical conductivity measurements were performed using a Hewlett-Packard 4192A impedance meter in the frequency range from 10 Hz to 10 MHz and the temperature range from 220 K to 400 K. Glass samples for conductivity measurements were discs of 5–8 mm in diameter and 1–3 mm in thickness with sputtered Au or Pt metallic contacts. For each composition, samples having the same conductivity parameters within experimental uncertainty were used in diffusion experiments. Both 110 Ag and 64 Cu tracer diffusion experiments were performed using a conventional sectioning after the diffusion anneal in case of the samples with relatively low diffusion coefficients (10 211 –10 29 cm 2 s 21 ). A non-destructive absorption method developed by Kryukov and Zhukhovitskii [7] was used for the samples with high diffusion coefficients (10 29 –10 27 cm 2 s 21 ). The applicability of this method for superionic chalcogenide glasses was
proved recently [8]. Both 110 Ag and short-living (t 1 / 2 5 12.8 h) 64 Cu isotopes were obtained in a research reactor of the St. Petersburg Nuclear Physics Institute. Other details concerning tracer diffusion measurements were published elsewhere [5,8,9].
3. Results
3.1. Glass homogeneity All of the synthesised glass compositions were found to be amorphous and homogeneous according to X-ray diffraction and optical microscopy measurements. Scanning electron microscopy (SEM) studies on selected compositions in the CuI–AgI–As 2 Se 3 system showed no evidence of phase separation, however, the repeated SEM measurements on the same samples after several months of storage in open air exhibit the presence of few microcrystallites of AgI at the surface of AgI-rich glasses. Presently, it is not clear whether the observed phenomenon is induced by interaction of the samples with the environment (contact with air and residual moisture, light illumination during 5 to 7 months) or it is an intrinsic property of these samples. Small-angle Xray scattering measurements on freshly prepared glasses as well as during their ageing are planned to verify the homogeneity.
3.2. Electrical conductivity All the glasses exhibit typical Arrhenius-like behaviour within the whole investigated temperature range. The resulting conductivity isotherm at 298 K for xCuI ? (0.5 2 x)AgI ? 0.5As 2 Se 3 glasses is shown in Fig. 1. One observes a monotonic increase of the total conductivity s t with increasing silver iodide fraction r 5 AgI /(AgI 1 CuI) from 2 3 10 27 S cm 21 to 5 3 10 24 S cm 21 without any indications of a minimum at intermediate compositions. Moreover, a slight downward curvature of the conductivity isotherm for Cu-rich glasses disappears when taking into account the copper ion transport number t Cu 1 ¯ 0.2 for a 0.5CuI?0.5As 2 Se 3 glass [5]. As a result, the ionic conductivity s i increases almost linearly by
A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
699
Fig. 1. Room-temperature conductivity of xAgI?(0.52x)CuI? 0.5As 2 Se 3 glasses vs. AgI fraction r5AgI /(AgI1CuI): (s) total conductivity s t measured using an impedance technique (s t coincides with ionic conductivity s i at r$0.4); (?) ionic conductivity s i for a 0.5CuI?0.5As 2 Se 3 glass calculated from s t using the copper ion transport number determined by the Tubandt method.
four orders of magnitude with r. As expected in this case, the conductivity activation energy Ea decreases monotonically from 0.56 eV (r50) to 0.28 eV (r51).
3.3. Silver and copper tracer diffusion Typical diffusion profiles for both 110 Ag and 64 Cu tracers are depicted in Fig. 2. The profiles, obtained by sectioning of the glass samples (Fig. 2a), are well described by the usual equations for the thin-layer geometry [10] A(x,t) 1 2 ]] 5 erf(q), A0
(1)
where x q 5 ]]] ]] , Π2 D 3t
(2)
A(x,t) is the corrected residual activity of the sample
Fig. 2. Typical 64 Cu and 110 Ag tracer diffusion profiles in xAgI? (0.52x)CuI?0.5As 2 Se 3 glasses (a) with relatively low diffusion coefficients (10 211 –10 29 cm 2 s 21 ), obtained by a conventional sectioning technique, and (b) with high diffusion coefficients (10 29 –10 27 cm 2 s 21 ) obtained by the absorption method [7].
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A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
after a thickness x was removed, t is the diffusion anneal time, A 0 is the initial residual activity and D* is the silver, DAg , or copper, DCu , tracer diffusion coefficient. Tracer absorption experiments for superionic conducting glasses (Fig. 2b) are well fitted within the Kryukov–Zhukhovitskii formalism [7] A1 2 A2 p 2 D* ln ]]] 5 K 2 ]] t, A1 1 A2 l2
(3)
where A 1 and A 2 are the sample activities measured at the diffusion and opposite faces, respectively, l is the sample thickness and K is a constant. Temperature dependencies of DAg and DCu obey the Arrhenius equation (Fig. 3). 110 Ag tracer diffusion measurements confirm the linearity of the ionic conductivity as a function of the AgI fraction r (Fig. 4). The value of DAg at 298 K increases also by four orders of magnitude, from ¯10 212 cm 2 s 21 to ¯10 28 cm 2 s 21 , with increasing r. More exciting, the 64 Cu tracer diffusion coefficient DCu increases by a factor of 20 to 30 when copper is substituted by silver (Fig. 4). Moreover, the values
Fig. 4. Room-temperature 110 Ag tracer diffusion coefficient DAg (s) and 64 Cu tracer diffusion coefficient DCu (?) for xAgI?(0.52 x)CuI?0.5As 2 Se 3 glasses vs. AgI fraction r5AgI /(AgI1CuI).
of DCu are always lower than DAg . The only exception is a 0.5CuI?0.5As 2 Se 3 glass, for which DAg and DCu are very similar.
4. Discussion
Fig. 3. 110 Ag tracer diffusion coefficients for xAgI?(0.52x)CuI? 0.5As 2 Se 3 glasses as a function of temperature.
The results obtained can be described as a degenerated mixed cation effect with different trends for silver and copper mobile cations. On the one hand, one observes a significant reduction in mobility of the Ag 1 ions when guest cations (Cu 1 ) are added to the host matrix (Fig. 4). In addition, DCu is lower by 2.5 orders of magnitude than DAg for a 0.5AgI? 0.5As 2 Se 3 glass. On the other, outside the region of AgI-rich glasses, where the values of DCu are nearly constant, (2–4)310 211 cm 2 s 21 , the mobility of the Cu 1 ions decreases and, in the limit of r→0, it becomes equal to that of the Ag 1 cations (Fig. 4). In other words, no diffusivity crossover is realised in this system. As a result, no minimum at the conductivity isotherm is observed (Fig. 1). A critical point in our results is, therefore, the
A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
observed decrease of DCu at r,0.6 instead of its increase with corresponding diffusivity crossover, as it would be expected in accordance with the usual findings for mixed cation glasses [1–3]. Nevertheless, the dramatic change in DCu seems to be reasonable taking into account general trends in the ion transport features of Cu 1 conducting chalcogenide and chalcohalide glasses [5]. Basically, the ionic conductivity and tracer diffusion coefficients are significantly higher (4–5 orders of magnitude) for Ag 1 conducting chalcogenide and chalcohalide glasses compared to their Cu 1 conducting counterparts, in contrast to Cu 1 and Ag 1 conducting oxide vitreous alloys [11,12]. It means that the microstructural organisation of Cu-containing chalcogenide glasses is much less favourable for ion migration than that of Ag-containing ones. Hence, an increased fraction of Cu-containing fragments in the mixed CuI–AgI–As 2 Se 3 glasses could reduce the ionic mobility of diffusing species whatever their nature (Ag 1 or Cu 1 ). However, the structural reason for the observed anomalies in the ion transport properties is not yet ¨ clear. Both 129 I-Mossbauer spectroscopy [5] and EXAFS [13] studies showed that CuI is dispersed on a molecular level in the glass network. On the contrary, preliminary EXAFS results indicated that interaction of silver iodide with the As 2 Se 3 glassy matrix is less pronounced [14]. However, it is not obvious that this particular difference in the microstructural organisation of the mobile ion component in the glass network is responsible for the observed degenerated mixed cation effect. Moreover, very similar molecular dispersion of silver iodide, as compared to the CuI–As 2 Se 3 system, was observed earlier for AgI–Sb 2 S 3 and AgI–Ag 2 S–As 2 S 3 glasses ¨ using 129 I-Mossbauer spectroscopy [15]. The ionic conductivity and silver tracer diffusion coefficients in these glasses do not differ significantly from those in the AgI–As 2 Se 3 glassy system.
5. Conclusions Degenerated mixed cation effect is observed for xCuI?(0.52x)AgI?0.5As 2 Se 3 glasses with r5AgI / (AgI1CuI)50.0, 0.2, 0.4, 0.6, 0.8 and 1.0. The ionic conductivity increases linearly by four orders of
701
magnitude with increasing r without any minimum at intermediate concentrations. The silver tracer diffusion coefficient DAg follows the ionic conductivity isotherm with the same change in DAg , from ¯10 212 cm 2 s 21 to ¯10 28 cm 2 s 21 . The 64 Cu tracer diffusion coefficient DCu increases by a factor of 20 to 30 when copper is substituted by silver. Moreover, the values of DCu are always lower than DAg . The only exception is a 0.5CuI?0.5As 2 Se 3 glass, for which DAg and DCu are very similar. The lack of the diffusivity crossover implies the absence of any minimum at the ionic conductivity isotherm. The structural reason for the observed degenerated mixed cation effect is not yet clear. It seems, however, that the microstructural organisation of Cu-containing fragments in the glass network is much less favourable for the ion migration and responsible for the absence of the diffusivity crossover.
References [1] D.E. Day, J. Non-Cryst. Solids 21 (1976) 343. [2] M.D. Ingram, Glasstech. Ber. Glass Sci. Technol. 67 (1994) 151. [3] M.P. Thomas, N.L. Peterson, E. Hutchinson, J. Am. Ceram. Soc. 68 (1985) 99. [4] G. Taillades, Ph.D. Thesis, University of Montpellier, France, 1996; C. Rau, Ph.D. work, University of Montpellier, private communication. [5] E. Bychkov, A. Bolotov, Yu. Grushko, Yu. Vlasov, G. Wortmann, Solid State Ionics 90 (1996) 289. [6] A. Bolotov, V. Tsegelnik, Yu. Gavrilov, E. Bychkov, unpublished results. [7] S.N. Kryukov, A.A. Zhukhovitskii, Doklady Akad. Nauk SSSR 90 (1953) 379. [8] Yu. Vlasov, E. Bychkov, V. Tsegelnik, Y.M. Ben-Shaban, Radiokhimiya 37 (1995) 281. [9] E. Bychkov, V. Tsegelnik, Y. Vlasov, A. Pradel, M. Ribes, J. Non-Cryst. Solids 208 (1996) 1. [10] G.H. Frischat, Ionic Diffusion in Oxide Glasses, Trans Tech, Aedermannsdorf, 1975. [11] C. Lui, C.A. Angell, Solid State Ionics 13 (1984) 105. [12] N. Machida, Y. Shinkuma, T. Minami, Solid State Ionics 45 (1991) 123. [13] E. Bychkov, A. Bolotov, P. Armand, A. Ibanez, J. NonCryst. Solids 232–234 (1998) 314. [14] P. Armand, A. Bolotov, E. Bychkov, A. Ibanez, unpublished results. [15] E. Bychkov, Y. Grushko, G. Wortmann, Hyperfine Interact. 69 (1991) 709.
Degenerated mixed cation effect in CuI–AgI–As 2 Se 3 glasses: 64 110 Cu and Ag tracer diffusion studies a ,c
A. Bolotov , E. Bychkov
a ,c ,
* ,1 , Yu. Gavrilov a , Yu. Grushko b , A. Pradel c , M. Ribes c , V. Tsegelnik a , Yu. Vlasov a
a St. Petersburg University, 199034 St. Petersburg, Russia St. Petersburg Institute for Nuclear Physics, 188350 Gatchina, Russia c LPMC, UMR 5617 CNRS, Universite´ Montpellier II, 34095 Montpellier, France b
Received 4 September 1998; accepted 12 September 1998
Abstract In contrast to the usual behaviour of mixed cation glasses (large deviations from additivity with a pronounced minimum at the conductivity isotherms and a diffusivity crossover when guest cations reduce the mobility of ions coming from the host material), the CuI–AgI–As 2 Se 3 glassy system exhibits remarkable differences in the transport properties. First, the ionic conductivity increases monotonically by four orders of magnitude from 3 3 10 28 to 5 3 10 24 S cm 21 when CuI is gradually replaced by AgI, without any minimum at intermediate concentrations. Secondly, no diffusivity crossover was observed. The 110 Ag tracer diffusion coefficient DAg follows the conductivity isotherm with nearly the same changes in DAg of four orders of magnitude, from ¯ 10 212 cm 2 s 21 to ¯ 10 28 cm 2 s 21 . The 64 Cu tracer diffusion coefficient DCu increases by a factor of 20 to 30 when copper is substituted by silver. Moreover, the values of DCu are always lower than DAg . The only exception is a 0.5CuI ? 0.5As 2 Se 3 glass, for which DAg and DCu are very similar. The results obtained can be described as a degenerated mixed cation effect with different trends for silver and copper mobile cations. 1998 Elsevier Science B.V. All rights reserved. Keywords: Mixed cation glasses; Copper iodide; Silver iodide; Ionic conductivity
1. Introduction The mixed alkali effect is one of the most remarkable features of ion transport in glasses. One observes a strongly non-linear change in the ionic *Corresponding author. Tel.: 133-328-658-250; fax: 133-328658-244; e-mail: [email protected] 1 Permanent address: MREID, Universite´ du Littoral, 59140 Dunkerque, France.
conductivity of mixed alkali or, more generally, mixed mobile cation glasses with a pronounced minimum of several orders of magnitude at intermediate concentrations, which is accompanied by a corresponding increase in the activation energy (see, for example, [1,2] and references therein). Tracer diffusion coefficients exhibit the diffusivity crossover when guest cations reduce significantly the mobility of ions coming from the host material. The mixed cation effect was also observed for chalcogenide
0167-2738 / 98 / $ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00397-X
698
A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
glasses containing alkali and silver ions [3,4], however, no experiments have been reported concerning Ag 1 / Cu 1 mixed glasses. For our experiments, we have chosen CuI–AgI–As 2 Se 3 glasses, containing 50 mol.% of metal iodide. A 0.5CuI ? 0.5As 2 Se 3 glass is a mixed (Cu 1 , hole) conductor according to electrical and 64 Cu tracer diffusion measurements [5]. Its Ag-containing counterpart is nearly pure Ag 1 ion conductor [6]. In order to get additional and complementary information on ion transport in these mixed glasses, we have completed AC impedance measurements by tracer diffusion experiments using 110 Ag and 64 Cu isotopes.
2. Experimental details xCuI ? (0.5 2 x)AgI ? 0.5As 2 Se 3 glasses with r 5 AgI /(AgI 1 CuI) 5 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared from reagent grade CuI, AgI and As 2 Se 3 . The mixtures were sealed in a silica tube at 0.01 Pa and heated at 10008C for 3 days with repeated stirring of the melt. The glasses, annealed at 20–308C below the glass transition temperature, were characterised by X-ray diffraction. The homogeneity of the glasses was checked by optical and scanning electron microscopy. The density of each sample was determined using the Archimedes method in toluene. Electrical conductivity measurements were performed using a Hewlett-Packard 4192A impedance meter in the frequency range from 10 Hz to 10 MHz and the temperature range from 220 K to 400 K. Glass samples for conductivity measurements were discs of 5–8 mm in diameter and 1–3 mm in thickness with sputtered Au or Pt metallic contacts. For each composition, samples having the same conductivity parameters within experimental uncertainty were used in diffusion experiments. Both 110 Ag and 64 Cu tracer diffusion experiments were performed using a conventional sectioning after the diffusion anneal in case of the samples with relatively low diffusion coefficients (10 211 –10 29 cm 2 s 21 ). A non-destructive absorption method developed by Kryukov and Zhukhovitskii [7] was used for the samples with high diffusion coefficients (10 29 –10 27 cm 2 s 21 ). The applicability of this method for superionic chalcogenide glasses was
proved recently [8]. Both 110 Ag and short-living (t 1 / 2 5 12.8 h) 64 Cu isotopes were obtained in a research reactor of the St. Petersburg Nuclear Physics Institute. Other details concerning tracer diffusion measurements were published elsewhere [5,8,9].
3. Results
3.1. Glass homogeneity All of the synthesised glass compositions were found to be amorphous and homogeneous according to X-ray diffraction and optical microscopy measurements. Scanning electron microscopy (SEM) studies on selected compositions in the CuI–AgI–As 2 Se 3 system showed no evidence of phase separation, however, the repeated SEM measurements on the same samples after several months of storage in open air exhibit the presence of few microcrystallites of AgI at the surface of AgI-rich glasses. Presently, it is not clear whether the observed phenomenon is induced by interaction of the samples with the environment (contact with air and residual moisture, light illumination during 5 to 7 months) or it is an intrinsic property of these samples. Small-angle Xray scattering measurements on freshly prepared glasses as well as during their ageing are planned to verify the homogeneity.
3.2. Electrical conductivity All the glasses exhibit typical Arrhenius-like behaviour within the whole investigated temperature range. The resulting conductivity isotherm at 298 K for xCuI ? (0.5 2 x)AgI ? 0.5As 2 Se 3 glasses is shown in Fig. 1. One observes a monotonic increase of the total conductivity s t with increasing silver iodide fraction r 5 AgI /(AgI 1 CuI) from 2 3 10 27 S cm 21 to 5 3 10 24 S cm 21 without any indications of a minimum at intermediate compositions. Moreover, a slight downward curvature of the conductivity isotherm for Cu-rich glasses disappears when taking into account the copper ion transport number t Cu 1 ¯ 0.2 for a 0.5CuI?0.5As 2 Se 3 glass [5]. As a result, the ionic conductivity s i increases almost linearly by
A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
699
Fig. 1. Room-temperature conductivity of xAgI?(0.52x)CuI? 0.5As 2 Se 3 glasses vs. AgI fraction r5AgI /(AgI1CuI): (s) total conductivity s t measured using an impedance technique (s t coincides with ionic conductivity s i at r$0.4); (?) ionic conductivity s i for a 0.5CuI?0.5As 2 Se 3 glass calculated from s t using the copper ion transport number determined by the Tubandt method.
four orders of magnitude with r. As expected in this case, the conductivity activation energy Ea decreases monotonically from 0.56 eV (r50) to 0.28 eV (r51).
3.3. Silver and copper tracer diffusion Typical diffusion profiles for both 110 Ag and 64 Cu tracers are depicted in Fig. 2. The profiles, obtained by sectioning of the glass samples (Fig. 2a), are well described by the usual equations for the thin-layer geometry [10] A(x,t) 1 2 ]] 5 erf(q), A0
(1)
where x q 5 ]]] ]] , Π2 D 3t
(2)
A(x,t) is the corrected residual activity of the sample
Fig. 2. Typical 64 Cu and 110 Ag tracer diffusion profiles in xAgI? (0.52x)CuI?0.5As 2 Se 3 glasses (a) with relatively low diffusion coefficients (10 211 –10 29 cm 2 s 21 ), obtained by a conventional sectioning technique, and (b) with high diffusion coefficients (10 29 –10 27 cm 2 s 21 ) obtained by the absorption method [7].
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A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
after a thickness x was removed, t is the diffusion anneal time, A 0 is the initial residual activity and D* is the silver, DAg , or copper, DCu , tracer diffusion coefficient. Tracer absorption experiments for superionic conducting glasses (Fig. 2b) are well fitted within the Kryukov–Zhukhovitskii formalism [7] A1 2 A2 p 2 D* ln ]]] 5 K 2 ]] t, A1 1 A2 l2
(3)
where A 1 and A 2 are the sample activities measured at the diffusion and opposite faces, respectively, l is the sample thickness and K is a constant. Temperature dependencies of DAg and DCu obey the Arrhenius equation (Fig. 3). 110 Ag tracer diffusion measurements confirm the linearity of the ionic conductivity as a function of the AgI fraction r (Fig. 4). The value of DAg at 298 K increases also by four orders of magnitude, from ¯10 212 cm 2 s 21 to ¯10 28 cm 2 s 21 , with increasing r. More exciting, the 64 Cu tracer diffusion coefficient DCu increases by a factor of 20 to 30 when copper is substituted by silver (Fig. 4). Moreover, the values
Fig. 4. Room-temperature 110 Ag tracer diffusion coefficient DAg (s) and 64 Cu tracer diffusion coefficient DCu (?) for xAgI?(0.52 x)CuI?0.5As 2 Se 3 glasses vs. AgI fraction r5AgI /(AgI1CuI).
of DCu are always lower than DAg . The only exception is a 0.5CuI?0.5As 2 Se 3 glass, for which DAg and DCu are very similar.
4. Discussion
Fig. 3. 110 Ag tracer diffusion coefficients for xAgI?(0.52x)CuI? 0.5As 2 Se 3 glasses as a function of temperature.
The results obtained can be described as a degenerated mixed cation effect with different trends for silver and copper mobile cations. On the one hand, one observes a significant reduction in mobility of the Ag 1 ions when guest cations (Cu 1 ) are added to the host matrix (Fig. 4). In addition, DCu is lower by 2.5 orders of magnitude than DAg for a 0.5AgI? 0.5As 2 Se 3 glass. On the other, outside the region of AgI-rich glasses, where the values of DCu are nearly constant, (2–4)310 211 cm 2 s 21 , the mobility of the Cu 1 ions decreases and, in the limit of r→0, it becomes equal to that of the Ag 1 cations (Fig. 4). In other words, no diffusivity crossover is realised in this system. As a result, no minimum at the conductivity isotherm is observed (Fig. 1). A critical point in our results is, therefore, the
A. Bolotov et al. / Solid State Ionics 113 – 115 (1998) 697 – 701
observed decrease of DCu at r,0.6 instead of its increase with corresponding diffusivity crossover, as it would be expected in accordance with the usual findings for mixed cation glasses [1–3]. Nevertheless, the dramatic change in DCu seems to be reasonable taking into account general trends in the ion transport features of Cu 1 conducting chalcogenide and chalcohalide glasses [5]. Basically, the ionic conductivity and tracer diffusion coefficients are significantly higher (4–5 orders of magnitude) for Ag 1 conducting chalcogenide and chalcohalide glasses compared to their Cu 1 conducting counterparts, in contrast to Cu 1 and Ag 1 conducting oxide vitreous alloys [11,12]. It means that the microstructural organisation of Cu-containing chalcogenide glasses is much less favourable for ion migration than that of Ag-containing ones. Hence, an increased fraction of Cu-containing fragments in the mixed CuI–AgI–As 2 Se 3 glasses could reduce the ionic mobility of diffusing species whatever their nature (Ag 1 or Cu 1 ). However, the structural reason for the observed anomalies in the ion transport properties is not yet ¨ clear. Both 129 I-Mossbauer spectroscopy [5] and EXAFS [13] studies showed that CuI is dispersed on a molecular level in the glass network. On the contrary, preliminary EXAFS results indicated that interaction of silver iodide with the As 2 Se 3 glassy matrix is less pronounced [14]. However, it is not obvious that this particular difference in the microstructural organisation of the mobile ion component in the glass network is responsible for the observed degenerated mixed cation effect. Moreover, very similar molecular dispersion of silver iodide, as compared to the CuI–As 2 Se 3 system, was observed earlier for AgI–Sb 2 S 3 and AgI–Ag 2 S–As 2 S 3 glasses ¨ using 129 I-Mossbauer spectroscopy [15]. The ionic conductivity and silver tracer diffusion coefficients in these glasses do not differ significantly from those in the AgI–As 2 Se 3 glassy system.
5. Conclusions Degenerated mixed cation effect is observed for xCuI?(0.52x)AgI?0.5As 2 Se 3 glasses with r5AgI / (AgI1CuI)50.0, 0.2, 0.4, 0.6, 0.8 and 1.0. The ionic conductivity increases linearly by four orders of
701
magnitude with increasing r without any minimum at intermediate concentrations. The silver tracer diffusion coefficient DAg follows the ionic conductivity isotherm with the same change in DAg , from ¯10 212 cm 2 s 21 to ¯10 28 cm 2 s 21 . The 64 Cu tracer diffusion coefficient DCu increases by a factor of 20 to 30 when copper is substituted by silver. Moreover, the values of DCu are always lower than DAg . The only exception is a 0.5CuI?0.5As 2 Se 3 glass, for which DAg and DCu are very similar. The lack of the diffusivity crossover implies the absence of any minimum at the ionic conductivity isotherm. The structural reason for the observed degenerated mixed cation effect is not yet clear. It seems, however, that the microstructural organisation of Cu-containing fragments in the glass network is much less favourable for the ion migration and responsible for the absence of the diffusivity crossover.
References [1] D.E. Day, J. Non-Cryst. Solids 21 (1976) 343. [2] M.D. Ingram, Glasstech. Ber. Glass Sci. Technol. 67 (1994) 151. [3] M.P. Thomas, N.L. Peterson, E. Hutchinson, J. Am. Ceram. Soc. 68 (1985) 99. [4] G. Taillades, Ph.D. Thesis, University of Montpellier, France, 1996; C. Rau, Ph.D. work, University of Montpellier, private communication. [5] E. Bychkov, A. Bolotov, Yu. Grushko, Yu. Vlasov, G. Wortmann, Solid State Ionics 90 (1996) 289. [6] A. Bolotov, V. Tsegelnik, Yu. Gavrilov, E. Bychkov, unpublished results. [7] S.N. Kryukov, A.A. Zhukhovitskii, Doklady Akad. Nauk SSSR 90 (1953) 379. [8] Yu. Vlasov, E. Bychkov, V. Tsegelnik, Y.M. Ben-Shaban, Radiokhimiya 37 (1995) 281. [9] E. Bychkov, V. Tsegelnik, Y. Vlasov, A. Pradel, M. Ribes, J. Non-Cryst. Solids 208 (1996) 1. [10] G.H. Frischat, Ionic Diffusion in Oxide Glasses, Trans Tech, Aedermannsdorf, 1975. [11] C. Lui, C.A. Angell, Solid State Ionics 13 (1984) 105. [12] N. Machida, Y. Shinkuma, T. Minami, Solid State Ionics 45 (1991) 123. [13] E. Bychkov, A. Bolotov, P. Armand, A. Ibanez, J. NonCryst. Solids 232–234 (1998) 314. [14] P. Armand, A. Bolotov, E. Bychkov, A. Ibanez, unpublished results. [15] E. Bychkov, Y. Grushko, G. Wortmann, Hyperfine Interact. 69 (1991) 709.