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Effect of Ag2O on the conductive behaviour of silver vanadium tellurite glasses: Part II
Solid State Ionics 146 (2002) 323 – 327 www.elsevier.com/locate/ssi
Effect of Ag2O on the conductive behaviour of silver vanadium tellurite glasses: Part II R.A. Montani *, A. Lorente, M.A. Frechero Laboratorio de Fisicoquı´mica, Departamento de Quı´mica e Ingenieria Quı´mica, Universidad Nacional del Sur, Avenida Alem 1250, 8000 Bahı´a Blanca, Argentina Received 28 February 2001; received in revised form 16 October 2001; accepted 29 October 2001
Abstract In a previous work, the effect of the Ag2O on the electrical conductivity of vanadium tellurite glasses of the form XAg2O(1 X)V2O52TeO2 has been studied by using the impedance spectroscopy in a wide range of temperature and composition. The obtained results confirm the existence of a transition from a typically electronic (polaronic) conductive regime when the molar fraction (X) of Ag2O is equal to 0, to an ionic conductive regime when X tends to be 1. This transition is characterised by a deep minimum in the electrical conductivity of about three orders of magnitude. In the present paper, a complementary study of the system of the form 0.27Ag2O0.73[ YV2O5(1 Y)TeO2] is presented. In this system, there also exists such a transition, but now, from an ionic to an electronic conductive regime. The correlated behaviour between conductivity and the mean silver-silver and vanadium-vanadium distances indicates that a concentration-based explanation is appropriate. D 2002 Published by Elsevier Science B.V. Keywords: Glasses; Ionic conductivity; Polarons
1. Introduction In a previous work (referred in the future as Paper 1 and System 1), we studied the effect of silver on the electronic (polaronic) conductivity of vanadium tellurite glasses of the form XAg2O(1 X)V2O52TeO2, for X = 0, 0.2, 0.4, 0.5, 0.6, and 0.8 [1]. For this system, the existence of a deep minimum in the isotherms of conductivity was verified and the explan-
*
Corresponding author. Tel.: +54-91-28034/35; fax: +54-91551447. E-mail address: [email protected] (R.A. Montani).
ation for this minimum was given assuming the existence of two kinds of independent migrating paths: one kind of path consisting of an electronic transfer in the chain V(IV) – V(V) and the other kind of path made by the regular position of non-bridging oxygen along the network-former chains allowing the ion displacement. This explanation is an alternative to the existence of an ion – polaron effect as proposed by Baza´n et al. [2]. The aim of the present work is to study the complementary system of the form 0.27Ag 2 O0.73[ YV2O5(1 Y)TeO2]: this system starts on what we define as Zone II in Paper 1, where the ionic regime for conductivity prevails [1]. In the present work, new extra data for System 1 is also included.
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 1 0 2 3 - 2
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R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
2. Experimental The experimental procedure for sample preparation has been given in the previous work [1]. The obtained samples correspond to the following concentrations: Y = 0.09 (equivalent to X = 0.8 in System 1), 0.2, 0.27, 0.3, 0.35, and 0.4. For higher values of Y, samples could not be obtained because it showed a strong tendency to form a second crystalline phase. The colour of the obtained glass were brownish in the whole range of Y. The amorphous character of the resulting solid was tested by X-ray diffraction analysis and confirmed by the existence of only one semicircle on the complex impedance plots at all compositions and temperatures. Glass disks of thickness between 0.6 and 1.3 mm were cut from the obtained cylinder and polished with very fine quality lapping papers. The electrodes for electrical measurements were made
Fig. 2. Isotherms of conductivity at several temperatures as a function of Y.
using silver-conducting paint to which metallic leads were attached. The conductivities of the samples were determined in a temperature domain below the transition temperature by standard AC impedance methods in a frequency range from 5 Hz to 2 MHz.
3. Results
Fig. 1. Logarithmic plot of electrical conductivity using Eq. (1) as a function of 1000/T at several compositions.
In Fig. 1, the results of the conductivity are plotted as a function of the temperature for all studied glass compositions. The results fit an Arrhenius-type equation relating conductivity and temperature Eq. (1). From this figure, it is clear that in the whole investigated range of temperatures, a single activation energy is verified suggesting, for each composition, that only a single conduction process takes place. Fig. 2 shows isothermal variations of conductivity as a
R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
Fig. 3. Activation energy, Ea, calculated from slopes of Fig. 1, as a function of Y.
function of Y and the conductivity passing through a minimum at approximately Y = 0.3. This figure suggests a kind of transition in the conductive regime, but in the present case, this minimum seems to be not so deep as for System 1. In Fig. 3, the activation energy, Ea, obtained from the slopes in Fig. 1, is plotted as a function of Y. Again as for System 1, the minimum in conductivity observed in Fig. 2 is accompanied by an inverse behaviour of Ea. Then, from Figs. 1 – 3, the existence of two zones defined by Y becomes evident: Zone I for Y values lower than 0.27 and Zone II for Y values greater than 0.3.
325
Fig. 4. The glass transition temperature, Tg, as a function of Y.
diminution of the total amount of glass former oxides. On the other hand, on System 2, the relative amounts of alkaline modifier oxide and of former oxides, remains constant, with no or less sensible changes on the mechanical properties of the glassy matrix. Figs. 5 and 6 show the mean distance between silver and between vanadium ions estimated using the corresponding density values. For System 1, the
4. Discussion Fig. 4 shows the glass transition temperature, Tg, for System 2 clearly; whereas for System 1, there is a strong decay on Tg as a function of X (Fig. 4 on Paper 1), in System 2, these values remain almost constant for the whole range of Y. This behaviour is ascribed to the nature of the systems. In fact, both Systems 1 and 2 are composed of an alkaline modifier oxide (Ag2O), a former-modifier oxide (V2O5), and a definitive former oxide (TeO2). Then, when in System 1, one of the former oxides (V2O5) is replaced by the alkaline modifier oxide, the glassy matrix is strongly modified in its mechanical properties by the relative
Fig. 5. The mean distance between ions as a function of X for System 1.
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Fig. 7. The Meyer – Neldel dependence for System 1.
Fig. 6. The mean distance between ions as a function of Y for System 2.
glass compositions. The results fit the general formula for electrical conductivity of the form: crossover between the values of these distances occurs at the same value of X corresponding to the minimum in the isotherms of conductivity (Fig. 2, Paper 1), supporting the explanation given there. Also from the same figure, it is clear that for Zone II, i.e., where ionic conductivity prevails, the distance between silver ions is under 0.55 nm and the distance between vanadium ions is also under that value in Zone I, where electronic conductivity prevails. For System 2, Fig. 6 shows that the distance between silver ions remains under 0.55 nm in the whole interval of Y, and this value for the distance between vanadium ions is reached at Y = 0.27, i.e., at the minimum in the isotherms in Fig. 2. From this concentration, the distance between vanadium ions diminishes with a corresponding increase of the conductivity in the isotherms in Fig. 2. Note the correspondence between the slopes of the isotherms in both zones of Fig. 2 and the slopes in Fig. 6: a slow increase of the distance between silver ions is accompanied with a strong decrease in conductivity, whereas a slow decrease in the distance between vanadium ions is also accompanied by a similar strong increase of conductivity in Zone II. In Fig. 1, the results of the conductivity were plotted as a function of the temperature for all studied
r¼
r0 expðEa =kT Þ; T
ð1Þ
where r0 is the pre-exponential term, Ea the activation energy for the involved process, T the absolute tem-
Fig. 8. The Meyer – Neldel dependence for System 2.
R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
perature, and k the Boltzmann constant. Remarkably, this kind of dependence fits the data for both electronic [3] and ionic conductivity [4]. In Figs. 7 and 8, the pre-exponential term as a function of the activation energy Ea is plotted for the Systems 1 and 2, respectively. In both systems, the results fit a linear dependence as predicted by the Meyer – Neldel rule [4] (or compensation law) as: logr0 ¼ aEa þ b;
ð2Þ
where a and b are constant. For System 1 in the two zones previously defined, two different behaviours for this rule are found: in the electronic domain (Zone I), the parameter a is negative (also known as the antiMeyer – Neldel rule), whereas in the ionic domain (Zone II), a is positive (the Meyer – Neldel rule). In Zone II, the systems appear to have the same behaviour for AgI-Ag oxisalt glasses and polycrystalline Lisicons solid solutions as was discussed by Almond and West [4].
327
transfer in the chain V(IV) – V(V) and the other kind of path made by the regular position of non-bridging oxygen along the network-former chains allowing the ion displacement. This seems to be the most likely explanation for the present system, but now going from an ionic regime to an electronic one. In fact, considering again (Figs. 2, 3, and 6), it is clear that a strong decrease on the ionic conductivity of about 1.5 orders of magnitude appears when the concentration is slightly reduced (or equivalently, the Ag – Ag distance slightly increased to about 0.05 nm). This strong effect of the concentration on conductivity is also known in the physics of ionic-conducting glasses as the ‘‘anomalous’’ dependence between conductivity and concentration and its explanation is, at present, one of the most challenging questions in the field [6– 8]. On the other hand, the simultaneous diminution of the V –V distance promotes the electronic transport and, as a consequence of these two phenomena, the relative minimum in the isotherms of Fig. 2 appears.
Acknowledgements 5. Conclusions Two well-defined regimes for conductivity are found for the system 0.27Ag2O0.73[ YV2O5(1 Y) TeO2]: Zone I ( Y < 0.27) and Zone II ( Y > 0.27). At present, two explanations have been proposed for this kind of phenomenon. Firstly, the one proposed by Baza´n et al. [2] for sodium-molybdenum phosphate glasses, where a kind of interaction between ion and polaron is assumed. The argument is as follows: mobile electrons or polarons formed by the capture of the moving electron by a V(IV) atom are attracted to the Ag + ions with an opposite charge. This soformed cation –polaron pair tends to move together as a neutral entity. Then, the migration of these pairs does not involve any net displacement of charge so this process does not contribute to the electrical conductivity [2]. The other explanation proposed has been given by Jayanasinghe et al. [5] for sodium – vanadium tellurite glasses, who have suggested the existence of two kinds of independent migrating paths: one kind of path consisting of an electronic
Financial support by CONICET, CIC, and FONCYT of Argentina is gratefully acknowledged. RAM is a Research Fellow of the CIC of Argentina. We wish to thank Drs. R.S. Pettigrosso and V. Pedroni for DTA and X-ray analysis, respectively.
References [1] R.A. Montani, A. Lorente, M.A. Vincenzo, Solid State Ionics 130 (2000) 91 – 95. [2] J.C. Baza´n, J.A. Duffy, M.D. Ingram, M.R. Mallace, Solid State Ionics 86 – 88 (1996) 497. [3] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41. [4] D.P. Almond, A.R. West, Solid State Ionics 9 – 10 (1983) 277. [5] G.D.L.K. Jayasinghe, M.A.K.L. Dissanayake, M.A. Careem, J.L. Souquet, Solid State Ionics 93 (1997) 291. [6] M.D. Ingram, Phys. Chem. Glasses 28 (1987) 215. [7] M.D. Ingram, in: J. Zarzycki (Ed.), Electrical Properties of Glasses, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology: A Comprehensive Treatment, vol. 9, VCH, Weinheim, 1991, p. 715. [8] R.A. Montani, Phys. Chem. Glasses 42 (2001) 112.
Effect of Ag2O on the conductive behaviour of silver vanadium tellurite glasses: Part II R.A. Montani *, A. Lorente, M.A. Frechero Laboratorio de Fisicoquı´mica, Departamento de Quı´mica e Ingenieria Quı´mica, Universidad Nacional del Sur, Avenida Alem 1250, 8000 Bahı´a Blanca, Argentina Received 28 February 2001; received in revised form 16 October 2001; accepted 29 October 2001
Abstract In a previous work, the effect of the Ag2O on the electrical conductivity of vanadium tellurite glasses of the form XAg2O(1 X)V2O52TeO2 has been studied by using the impedance spectroscopy in a wide range of temperature and composition. The obtained results confirm the existence of a transition from a typically electronic (polaronic) conductive regime when the molar fraction (X) of Ag2O is equal to 0, to an ionic conductive regime when X tends to be 1. This transition is characterised by a deep minimum in the electrical conductivity of about three orders of magnitude. In the present paper, a complementary study of the system of the form 0.27Ag2O0.73[ YV2O5(1 Y)TeO2] is presented. In this system, there also exists such a transition, but now, from an ionic to an electronic conductive regime. The correlated behaviour between conductivity and the mean silver-silver and vanadium-vanadium distances indicates that a concentration-based explanation is appropriate. D 2002 Published by Elsevier Science B.V. Keywords: Glasses; Ionic conductivity; Polarons
1. Introduction In a previous work (referred in the future as Paper 1 and System 1), we studied the effect of silver on the electronic (polaronic) conductivity of vanadium tellurite glasses of the form XAg2O(1 X)V2O52TeO2, for X = 0, 0.2, 0.4, 0.5, 0.6, and 0.8 [1]. For this system, the existence of a deep minimum in the isotherms of conductivity was verified and the explan-
*
Corresponding author. Tel.: +54-91-28034/35; fax: +54-91551447. E-mail address: [email protected] (R.A. Montani).
ation for this minimum was given assuming the existence of two kinds of independent migrating paths: one kind of path consisting of an electronic transfer in the chain V(IV) – V(V) and the other kind of path made by the regular position of non-bridging oxygen along the network-former chains allowing the ion displacement. This explanation is an alternative to the existence of an ion – polaron effect as proposed by Baza´n et al. [2]. The aim of the present work is to study the complementary system of the form 0.27Ag 2 O0.73[ YV2O5(1 Y)TeO2]: this system starts on what we define as Zone II in Paper 1, where the ionic regime for conductivity prevails [1]. In the present work, new extra data for System 1 is also included.
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 1 0 2 3 - 2
324
R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
2. Experimental The experimental procedure for sample preparation has been given in the previous work [1]. The obtained samples correspond to the following concentrations: Y = 0.09 (equivalent to X = 0.8 in System 1), 0.2, 0.27, 0.3, 0.35, and 0.4. For higher values of Y, samples could not be obtained because it showed a strong tendency to form a second crystalline phase. The colour of the obtained glass were brownish in the whole range of Y. The amorphous character of the resulting solid was tested by X-ray diffraction analysis and confirmed by the existence of only one semicircle on the complex impedance plots at all compositions and temperatures. Glass disks of thickness between 0.6 and 1.3 mm were cut from the obtained cylinder and polished with very fine quality lapping papers. The electrodes for electrical measurements were made
Fig. 2. Isotherms of conductivity at several temperatures as a function of Y.
using silver-conducting paint to which metallic leads were attached. The conductivities of the samples were determined in a temperature domain below the transition temperature by standard AC impedance methods in a frequency range from 5 Hz to 2 MHz.
3. Results
Fig. 1. Logarithmic plot of electrical conductivity using Eq. (1) as a function of 1000/T at several compositions.
In Fig. 1, the results of the conductivity are plotted as a function of the temperature for all studied glass compositions. The results fit an Arrhenius-type equation relating conductivity and temperature Eq. (1). From this figure, it is clear that in the whole investigated range of temperatures, a single activation energy is verified suggesting, for each composition, that only a single conduction process takes place. Fig. 2 shows isothermal variations of conductivity as a
R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
Fig. 3. Activation energy, Ea, calculated from slopes of Fig. 1, as a function of Y.
function of Y and the conductivity passing through a minimum at approximately Y = 0.3. This figure suggests a kind of transition in the conductive regime, but in the present case, this minimum seems to be not so deep as for System 1. In Fig. 3, the activation energy, Ea, obtained from the slopes in Fig. 1, is plotted as a function of Y. Again as for System 1, the minimum in conductivity observed in Fig. 2 is accompanied by an inverse behaviour of Ea. Then, from Figs. 1 – 3, the existence of two zones defined by Y becomes evident: Zone I for Y values lower than 0.27 and Zone II for Y values greater than 0.3.
325
Fig. 4. The glass transition temperature, Tg, as a function of Y.
diminution of the total amount of glass former oxides. On the other hand, on System 2, the relative amounts of alkaline modifier oxide and of former oxides, remains constant, with no or less sensible changes on the mechanical properties of the glassy matrix. Figs. 5 and 6 show the mean distance between silver and between vanadium ions estimated using the corresponding density values. For System 1, the
4. Discussion Fig. 4 shows the glass transition temperature, Tg, for System 2 clearly; whereas for System 1, there is a strong decay on Tg as a function of X (Fig. 4 on Paper 1), in System 2, these values remain almost constant for the whole range of Y. This behaviour is ascribed to the nature of the systems. In fact, both Systems 1 and 2 are composed of an alkaline modifier oxide (Ag2O), a former-modifier oxide (V2O5), and a definitive former oxide (TeO2). Then, when in System 1, one of the former oxides (V2O5) is replaced by the alkaline modifier oxide, the glassy matrix is strongly modified in its mechanical properties by the relative
Fig. 5. The mean distance between ions as a function of X for System 1.
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R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
Fig. 7. The Meyer – Neldel dependence for System 1.
Fig. 6. The mean distance between ions as a function of Y for System 2.
glass compositions. The results fit the general formula for electrical conductivity of the form: crossover between the values of these distances occurs at the same value of X corresponding to the minimum in the isotherms of conductivity (Fig. 2, Paper 1), supporting the explanation given there. Also from the same figure, it is clear that for Zone II, i.e., where ionic conductivity prevails, the distance between silver ions is under 0.55 nm and the distance between vanadium ions is also under that value in Zone I, where electronic conductivity prevails. For System 2, Fig. 6 shows that the distance between silver ions remains under 0.55 nm in the whole interval of Y, and this value for the distance between vanadium ions is reached at Y = 0.27, i.e., at the minimum in the isotherms in Fig. 2. From this concentration, the distance between vanadium ions diminishes with a corresponding increase of the conductivity in the isotherms in Fig. 2. Note the correspondence between the slopes of the isotherms in both zones of Fig. 2 and the slopes in Fig. 6: a slow increase of the distance between silver ions is accompanied with a strong decrease in conductivity, whereas a slow decrease in the distance between vanadium ions is also accompanied by a similar strong increase of conductivity in Zone II. In Fig. 1, the results of the conductivity were plotted as a function of the temperature for all studied
r¼
r0 expðEa =kT Þ; T
ð1Þ
where r0 is the pre-exponential term, Ea the activation energy for the involved process, T the absolute tem-
Fig. 8. The Meyer – Neldel dependence for System 2.
R.A. Montani et al. / Solid State Ionics 146 (2002) 323–327
perature, and k the Boltzmann constant. Remarkably, this kind of dependence fits the data for both electronic [3] and ionic conductivity [4]. In Figs. 7 and 8, the pre-exponential term as a function of the activation energy Ea is plotted for the Systems 1 and 2, respectively. In both systems, the results fit a linear dependence as predicted by the Meyer – Neldel rule [4] (or compensation law) as: logr0 ¼ aEa þ b;
ð2Þ
where a and b are constant. For System 1 in the two zones previously defined, two different behaviours for this rule are found: in the electronic domain (Zone I), the parameter a is negative (also known as the antiMeyer – Neldel rule), whereas in the ionic domain (Zone II), a is positive (the Meyer – Neldel rule). In Zone II, the systems appear to have the same behaviour for AgI-Ag oxisalt glasses and polycrystalline Lisicons solid solutions as was discussed by Almond and West [4].
327
transfer in the chain V(IV) – V(V) and the other kind of path made by the regular position of non-bridging oxygen along the network-former chains allowing the ion displacement. This seems to be the most likely explanation for the present system, but now going from an ionic regime to an electronic one. In fact, considering again (Figs. 2, 3, and 6), it is clear that a strong decrease on the ionic conductivity of about 1.5 orders of magnitude appears when the concentration is slightly reduced (or equivalently, the Ag – Ag distance slightly increased to about 0.05 nm). This strong effect of the concentration on conductivity is also known in the physics of ionic-conducting glasses as the ‘‘anomalous’’ dependence between conductivity and concentration and its explanation is, at present, one of the most challenging questions in the field [6– 8]. On the other hand, the simultaneous diminution of the V –V distance promotes the electronic transport and, as a consequence of these two phenomena, the relative minimum in the isotherms of Fig. 2 appears.
Acknowledgements 5. Conclusions Two well-defined regimes for conductivity are found for the system 0.27Ag2O0.73[ YV2O5(1 Y) TeO2]: Zone I ( Y < 0.27) and Zone II ( Y > 0.27). At present, two explanations have been proposed for this kind of phenomenon. Firstly, the one proposed by Baza´n et al. [2] for sodium-molybdenum phosphate glasses, where a kind of interaction between ion and polaron is assumed. The argument is as follows: mobile electrons or polarons formed by the capture of the moving electron by a V(IV) atom are attracted to the Ag + ions with an opposite charge. This soformed cation –polaron pair tends to move together as a neutral entity. Then, the migration of these pairs does not involve any net displacement of charge so this process does not contribute to the electrical conductivity [2]. The other explanation proposed has been given by Jayanasinghe et al. [5] for sodium – vanadium tellurite glasses, who have suggested the existence of two kinds of independent migrating paths: one kind of path consisting of an electronic
Financial support by CONICET, CIC, and FONCYT of Argentina is gratefully acknowledged. RAM is a Research Fellow of the CIC of Argentina. We wish to thank Drs. R.S. Pettigrosso and V. Pedroni for DTA and X-ray analysis, respectively.
References [1] R.A. Montani, A. Lorente, M.A. Vincenzo, Solid State Ionics 130 (2000) 91 – 95. [2] J.C. Baza´n, J.A. Duffy, M.D. Ingram, M.R. Mallace, Solid State Ionics 86 – 88 (1996) 497. [3] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41. [4] D.P. Almond, A.R. West, Solid State Ionics 9 – 10 (1983) 277. [5] G.D.L.K. Jayasinghe, M.A.K.L. Dissanayake, M.A. Careem, J.L. Souquet, Solid State Ionics 93 (1997) 291. [6] M.D. Ingram, Phys. Chem. Glasses 28 (1987) 215. [7] M.D. Ingram, in: J. Zarzycki (Ed.), Electrical Properties of Glasses, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology: A Comprehensive Treatment, vol. 9, VCH, Weinheim, 1991, p. 715. [8] R.A. Montani, Phys. Chem. Glasses 42 (2001) 112.