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Determination of oxygen diffusivity in tetragonal YBa2Cu3Ox using a solid state electrochemical method
s
SOLID STATE
mm
l!iB .
__
ELSEWIER
Determination
Solid State Ionics 86-88
(1996)
IONICS
1385-1389
of oxygen diffusivity in tetragonal YBa,Cu,O, a solid state electrochemical method 0. Porat”, Z. Rosenstock,
I. Shtreichman,
using
I. Riess
Physics Department and the Crown Center for Superconductivity, Technion IIT, 32600 Ha@, Israel
Abstract A solid state electrochemical technique is used to determine the oxygen chemical and component diffusion coefficients in the tetragonal phase of YBa,Cu,OX. The measurements are performed at 85O”C, as a function of oxygen partial pressure and oxygen nonstoichiometry. The sample is held in a calcia-stabilized zirconia titration cell. An oxygen ionic current pulse is applied to the cell, and the EMF signal is monitored as a function of time. The diffusion coefficients are evaluated by analyzing the time dependence of the EMF, during and after the current pulse. It is found that the oxygen chemical diffusion coefficient increases weakly with x, with an average value of D - 5. lo-’ cm*/s at T = 850°C. The oxygen component diffusion coefficient is found to be independent of nonstoichiometry, with a value of Do - 5 . 10m5 cm*/s. Keywords: Oxygen diffusion;
Diffusion
coefficient;
Titration
cell
1. Introduction The sensitivity of the superconductor YBa,Cu,O, (YBCO) to oxygen nonstoichiometry, x, is well documented. This includes superconducting and normal electrical transport, magnetic and structural properties [l-7]. Classical methods for obtaining the oxygen chemical diffusion coefficient, 6, and the oxygen component diffusion coefficient, Do, are thermogravimetry, in which one follows the mass change of a sample during an abrupt change of the oxygen partial pressure (PO,) [8,9], and electrical conduc*Corresponding author. Present address: Department of Materials Science and Technology, Massachusetts gy, Cambridge, MA 02139, USA. 0167.2738/96/$15.00 Copyright PII SO167-2738(96)00317-7
01996
Institute of Technolo-
tivity measurements, where sample resistance is detected as a function of time after the PO, has been changed [lO,ll]. Other useful techniques are based on electrochemical methods, e.g. permeation measurements [12] and EMF and current relaxation measurements [ 13,141. In most of the methods either b or Do is measured directly, while the other is determined indirectly using thermodynamic properties of the material. Despite the numerous published works on both 6 and D, in YBCO, there are still disagreement and lack of knowledge regarding the dependence of the diffusion coefficients upon x and P 02’ In this report we used a current pulse titration technique, using a stabilized zirconia electrochemical cell, to determine both D and Do in the YBCO tetragonal phase [ 15,161. The advantage of this
Elsevier Science B.V. All rights reserved
1386
0. Porat et al. I Solid State Ionics 86-88
method is that it allows measuring the PO*-x relation (i.e. the titration curve) and the chemical diffusion coefficient simultaneously, therefore allowing determination of both D and Do as a function of PO, and X.
2. Experimental
and theoretical
considerations
Fig. 1 presents the coulometric titration cell used in this work. The electrochemical component is a calcia-stabilized zirconia (CSZ) tube, closed at one end. The sample is spring loaded against the closed end, inside the tube. This side is heated by a furnace. Sample temperature is controlled by a Pt/Pt+ lO%Rh thermocouple (not shown in Fig. 1). The cold end of the CSZ tube is closed by a metal fitting, which allows sealing the cell, when needed. An alumina rod is inserted to the CSZ tube in order to minimize the cell “dead volume”. Two Pt electrodes are painted on either side of the tube hot tip. The EMF developed between the electrodes, under open circuit conditions, is given by the Nemst equation:
(1) where R is the gas constant, F is the Faraday constant and T is the temperature. POg+ is the oxygen partial pressure over the sample. The outer Pt electrode is exposed to air. The EMF is measured using two Pt leads. The sample serves as part of the electrical circuit. Thermal EMF which might be generated on the sample is eliminated taking care to have only small temperature gradient on the sample. Coulometric titration is performed by passing a current through the electrodes and the CSZ electrolyte, thereby pumping oxygen into/out of the cell.
(1996) 1385-1389
The change in the stoichiometry of the material, x, due to a titration step of current I and time t, is:
(2)
where m and M are the sample weight and the molecular weight, respectively. Using titration combined with open circuit EMF measurement, one can get the PO, --x relation of the material [17-191. For the measurement of the diffusion coefficients we used a galvanostatic intermittent method, developed by Weppner and Huggins [ 151. The measurement is performed by applying a constant current for a short time, and measuring the EMF vs. time during and after titration. The EMF is a measure of the PO, near the sample-electrolyte interface at any time (or the activity of oxygen at the sample surface). Starting from a steady state EMF, E,, (reflecting an initial equilibrium PO surrounding the sample, at composition x, ) and p;mping out oxygen, the EMF increases during titration, due to the reduction of the sample mainly at the surface. After a time r, the titration is stopped. The EMF reaches a maximum value of E, +AE,, where AE, is the transient voltage during titration. The EMF now relaxes slowly to a new steady state EMF value, E2, which corresponds to a new equilibrium PO, at the new composition x2. This decay reflects the redistribution of oxygen in the sample. For determining the chemical diffusion coefficient, one solves Fick’s second law, with initial condition of homogeneous composition of the sample, and boundary conditions of constant ionic flux at one interface and no flux at the opposite sample interface. Using the measured values of E,, E2, AE,, Z, 7, one gets an expression for the chemical diffusion coefficient of oxygen [15]:
leads
Pt Electrodes
YBa,Cu,O, Sample
Alumina Rod
Fig. 1. Schematic
Spring/’
of the titration cell.
0. Porat et al. I Solid State Ionics 86-88
where s is the sample surface area and V, the molar volume. Few assumptions have to be made in order to obtain Eq. (3) [15,16]: (a) The titration time is sufficiently small so that (1) 7
(1996) 1385-1389
1387
of titration steps of the order of 10 mA for 10 min were carried out, in order to alter the sample composition. Open circuit EMF was measured after relaxation takes place. At certain compositions shorter titration steps (0.5 mA for 2 min) were also performed.
3. Results and discussion Fig. 2 presents the deviation from stoichiometry x in YBa,Cu,O, vs. PO, at the temperature of 850°C. The measured data points extend between 6.05 IX 5 6.35 and 4. 10-3sPo2 0.21 atm respectively, and are in good agreement with previous titration experiment [ 181. Fig. 3 presents both Lj and Do as a function of X. The values of fi were determined using the ZEMF characteristic as explained beforehand. Do is determined using D, and the thermodynamic factor W which is obtained from the PO,-x relation (Fig. 2). It is found that W is close to unity throughout the measured PO, range, in agreement with Ref. [17]. Therefore Do is close to fi. One notes that Do is independent of x (and PO,), with average value of -5.10p5 cm*/s. b, on the other hand, seems to increase weakly with x (and therefore also increases with PO -see Fig. 2), with values of 4. 1O-5sfi 57. 10m5 cm*/s at 6.081x16.25, respectively. A com-
where a(O) is the activity of oxygen in the sample and c(0) is the oxygen concentration. Assuming that diffusion takes place mainly through oxygen sites in the Cu-0 chains of YBCO structure, and taking [17,18], Eq. (4) a(0) = K(T)P;** and c(O)=x-6 becomes:
&AD 2
alo&* O
>
a log& - 6) .
(5)
One can therefore calculate Do using the predetermined value of fi and the measured titration curve, i.e. PO, -x relation. YBCO samples were prepared by standard ceramic procedure. A YBCO sample was placed in the CSZ tube and heated in air to 85O”C, where it was kept for 24 h to reach equilibrium. The sample composition is fixed at 6.35 at these PO and T values [20-221. The cell was then sealed and2 a series
Fig. 2. Nonstoichiometry T= 850°C.
in YBa,Cu,O,
as a function
of PO2 at
0. Porat et al. I Solid State Ionics 86-88
(1996) 1385-1389
This is supported also by the value of W which is closed to unity, i.e. a(O)-C(0).
4. Summary
6
61
6.2
x
63
in YBa,Cu,O,
Fig. 3. The chemical diffusion coefficient and the component diffusion coefficient of oxygen in YBa,Cu,O,, as a function of X, at T= 850°C.
parison with previous data is not straightforward, since most previous works were performed at lower temperatures and higher x. We find our results in fair agreement with the data of Maier et al. [14] and Patrakeev et al. [12], when the later are extrapolated to higher T and lower n. D is usually reported to increase with increasing x [8,12], although it is not clear whether the dependence is due to the thermodynamic factor (i.e. the PO,-x relation) or due to a Do-x dependence. We find that D, remains constant at the measured x range, and the dependence of Lj on x is due to W. This is in disagreement with Kishio et al. [8] and Patrakeev et al. [12], who find that D, increases with x. They suggested an interstitial diffusion mechanism, in which the activation energy for ion jump decreases with x, due to ion-ion interaction, therefore gives rise to increasing D,. On the other hand, McManus et al. [ 131 report on an opposite trend, in which D, decrease with x. They suggested vacancy mechanism. However these results seem somewhat inaccurate, given also the fact that the values of b are much higher than in any other report. We believe the explanation of refs. [8] and [12] to be reasonable, and that the independent D, -x in the present work is due to the composition range for which measurements were performed, which is close to x=6. In this composition the mobile oxygen interstitial species are not interacting.
The oxygen chemical and component diffusion coefficients of tetragonal YBa,Cu,O, were measured as a function of PO, and x at T= 850°C using an intermittent titration method. The measured range is 6.085x56.25 and 10~‘5Poz110-2 atm. The values of D and D, are found to be relatively close, of the order of 5.10m5 cm2/s, due to the thermodynamic factor which is close to unity. Our data are comparable with data of others, when the later are extrapolated to the measured temperature and x ranges. It is found that D increases weakly with x (and PO,). This increase is due to the dependence of the thermodynamic factor W on x. D, is found to be constant in the measured x range.
Acknowledgments This research was supported by the Fund Promotion of Research in the Technion.
for
References 111R.J. Cava, B. Batlogg,
C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Phys. Rev. B36 (1987) 5719. 121W.E. Farneth, R.K. Bordia, E.M. McCarron, M.K. Crawford and R.B. Flippen, Solid State Commun. 66 (1988) 953. [31 J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, I.K. Schuller C.U. Segre, K. Zhang and MS. Kleefisch, Phys. Rev. B36 (1987) 3608. 1.0. Lelong, A. Kessel and J. r41 B. Fisher, J. Genossar, Ashkenazi, Physica C153-155 (1988) 1348. [51 I.A. Leonidov, Ya.N. Blinovskov, E.E. Flyatau, P. Ya. Novak and V.L. Kozhevnikov, Physica Cl58 (1989) 287. [61 G.M. Choi, H.L. Tuller and M-J. Tsai, in: Nonstoichiometric Compounds, eds. J. Nowotny and W. Weppner (Kluwer, Dordrecht, 1989) p.451. [71 J.H. Brewer et al, Phys. Rev. Lett. 60 (1988) 1073. PI K. Kishio, K. Suzuki, T. Hasegawa, T. Yamamoto and K. Fueki, J. Solid State Chem. 82 (1989) 192. [91 T. Umemura, K. Egawa, M. Wakata and K. Yoshizaki, Jpn. J. Appl. Phys. 28 (1989) L1945. [lOI K.N. Tu, N.C. Yeh, S.I. Park and C.C. Tsuei, Phys. Rev. B39, (1989) 304.
0. Porat et al. I Solid State Ionics 86-88 [ll] J.R. LaGraff and D.A. Payne, Phys. Rev. B47 (1993) 3380. [12] M.V Patrakeev, LA. Leonidov, V.L. Kozhevnikov, V.I. Tsidilkovskii and A.K. Demin, Physica C210 (1993) 213. [I 31 J. McManus, D, Fray and J. Evetts, Physica Cl69 (1990) 193. [14] J. Maier, P. Murugaraj, G. Pfundtner and W. Sitte, Ber. Bunsenges. Phys. Chem. 93 (1989) 1350. [15] W. Weppner and R.A. Huggins, J. Electrochem. Sot. 124 (1977) 1569. [16] G. Schmidt-Kongeter and W. Weppner, in: Reactivity of Solids, eds. P. Barret and L.-C. Dufour (Elsevier, Amsterdam, 1985) p. 393.
(1996) 1385-1389
1389
[17] 0. Porat, I. Riess and H.L. Tuller, Physica Cl92 (1992) 60. [18] I. Riess, 0. Porat and H.L. Tuller, J. Supercond. 6 (1993) 313. [ 191 0. Porat and I. Riess, Materials Science and Engineering B22 (1994) 310. [20] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) Ll228. [21] E.D. Specht, C.J. Sparks, A.G. Dhere, I. Brynestad, O.B. Cavin, M.D. Kroeger and H.A. Oye, Phys. Rev. B37 (1988) 7426. [22] P. Strobel, J.J. Capponi, M. Marezio and P. Monod, Solid State Commun. 64 (1987) 513.
SOLID STATE
mm
l!iB .
__
ELSEWIER
Determination
Solid State Ionics 86-88
(1996)
IONICS
1385-1389
of oxygen diffusivity in tetragonal YBa,Cu,O, a solid state electrochemical method 0. Porat”, Z. Rosenstock,
I. Shtreichman,
using
I. Riess
Physics Department and the Crown Center for Superconductivity, Technion IIT, 32600 Ha@, Israel
Abstract A solid state electrochemical technique is used to determine the oxygen chemical and component diffusion coefficients in the tetragonal phase of YBa,Cu,OX. The measurements are performed at 85O”C, as a function of oxygen partial pressure and oxygen nonstoichiometry. The sample is held in a calcia-stabilized zirconia titration cell. An oxygen ionic current pulse is applied to the cell, and the EMF signal is monitored as a function of time. The diffusion coefficients are evaluated by analyzing the time dependence of the EMF, during and after the current pulse. It is found that the oxygen chemical diffusion coefficient increases weakly with x, with an average value of D - 5. lo-’ cm*/s at T = 850°C. The oxygen component diffusion coefficient is found to be independent of nonstoichiometry, with a value of Do - 5 . 10m5 cm*/s. Keywords: Oxygen diffusion;
Diffusion
coefficient;
Titration
cell
1. Introduction The sensitivity of the superconductor YBa,Cu,O, (YBCO) to oxygen nonstoichiometry, x, is well documented. This includes superconducting and normal electrical transport, magnetic and structural properties [l-7]. Classical methods for obtaining the oxygen chemical diffusion coefficient, 6, and the oxygen component diffusion coefficient, Do, are thermogravimetry, in which one follows the mass change of a sample during an abrupt change of the oxygen partial pressure (PO,) [8,9], and electrical conduc*Corresponding author. Present address: Department of Materials Science and Technology, Massachusetts gy, Cambridge, MA 02139, USA. 0167.2738/96/$15.00 Copyright PII SO167-2738(96)00317-7
01996
Institute of Technolo-
tivity measurements, where sample resistance is detected as a function of time after the PO, has been changed [lO,ll]. Other useful techniques are based on electrochemical methods, e.g. permeation measurements [12] and EMF and current relaxation measurements [ 13,141. In most of the methods either b or Do is measured directly, while the other is determined indirectly using thermodynamic properties of the material. Despite the numerous published works on both 6 and D, in YBCO, there are still disagreement and lack of knowledge regarding the dependence of the diffusion coefficients upon x and P 02’ In this report we used a current pulse titration technique, using a stabilized zirconia electrochemical cell, to determine both D and Do in the YBCO tetragonal phase [ 15,161. The advantage of this
Elsevier Science B.V. All rights reserved
1386
0. Porat et al. I Solid State Ionics 86-88
method is that it allows measuring the PO*-x relation (i.e. the titration curve) and the chemical diffusion coefficient simultaneously, therefore allowing determination of both D and Do as a function of PO, and X.
2. Experimental
and theoretical
considerations
Fig. 1 presents the coulometric titration cell used in this work. The electrochemical component is a calcia-stabilized zirconia (CSZ) tube, closed at one end. The sample is spring loaded against the closed end, inside the tube. This side is heated by a furnace. Sample temperature is controlled by a Pt/Pt+ lO%Rh thermocouple (not shown in Fig. 1). The cold end of the CSZ tube is closed by a metal fitting, which allows sealing the cell, when needed. An alumina rod is inserted to the CSZ tube in order to minimize the cell “dead volume”. Two Pt electrodes are painted on either side of the tube hot tip. The EMF developed between the electrodes, under open circuit conditions, is given by the Nemst equation:
(1) where R is the gas constant, F is the Faraday constant and T is the temperature. POg+ is the oxygen partial pressure over the sample. The outer Pt electrode is exposed to air. The EMF is measured using two Pt leads. The sample serves as part of the electrical circuit. Thermal EMF which might be generated on the sample is eliminated taking care to have only small temperature gradient on the sample. Coulometric titration is performed by passing a current through the electrodes and the CSZ electrolyte, thereby pumping oxygen into/out of the cell.
(1996) 1385-1389
The change in the stoichiometry of the material, x, due to a titration step of current I and time t, is:
(2)
where m and M are the sample weight and the molecular weight, respectively. Using titration combined with open circuit EMF measurement, one can get the PO, --x relation of the material [17-191. For the measurement of the diffusion coefficients we used a galvanostatic intermittent method, developed by Weppner and Huggins [ 151. The measurement is performed by applying a constant current for a short time, and measuring the EMF vs. time during and after titration. The EMF is a measure of the PO, near the sample-electrolyte interface at any time (or the activity of oxygen at the sample surface). Starting from a steady state EMF, E,, (reflecting an initial equilibrium PO surrounding the sample, at composition x, ) and p;mping out oxygen, the EMF increases during titration, due to the reduction of the sample mainly at the surface. After a time r, the titration is stopped. The EMF reaches a maximum value of E, +AE,, where AE, is the transient voltage during titration. The EMF now relaxes slowly to a new steady state EMF value, E2, which corresponds to a new equilibrium PO, at the new composition x2. This decay reflects the redistribution of oxygen in the sample. For determining the chemical diffusion coefficient, one solves Fick’s second law, with initial condition of homogeneous composition of the sample, and boundary conditions of constant ionic flux at one interface and no flux at the opposite sample interface. Using the measured values of E,, E2, AE,, Z, 7, one gets an expression for the chemical diffusion coefficient of oxygen [15]:
leads
Pt Electrodes
YBa,Cu,O, Sample
Alumina Rod
Fig. 1. Schematic
Spring/’
of the titration cell.
0. Porat et al. I Solid State Ionics 86-88
where s is the sample surface area and V, the molar volume. Few assumptions have to be made in order to obtain Eq. (3) [15,16]: (a) The titration time is sufficiently small so that (1) 7
(1996) 1385-1389
1387
of titration steps of the order of 10 mA for 10 min were carried out, in order to alter the sample composition. Open circuit EMF was measured after relaxation takes place. At certain compositions shorter titration steps (0.5 mA for 2 min) were also performed.
3. Results and discussion Fig. 2 presents the deviation from stoichiometry x in YBa,Cu,O, vs. PO, at the temperature of 850°C. The measured data points extend between 6.05 IX 5 6.35 and 4. 10-3sPo2 0.21 atm respectively, and are in good agreement with previous titration experiment [ 181. Fig. 3 presents both Lj and Do as a function of X. The values of fi were determined using the ZEMF characteristic as explained beforehand. Do is determined using D, and the thermodynamic factor W which is obtained from the PO,-x relation (Fig. 2). It is found that W is close to unity throughout the measured PO, range, in agreement with Ref. [17]. Therefore Do is close to fi. One notes that Do is independent of x (and PO,), with average value of -5.10p5 cm*/s. b, on the other hand, seems to increase weakly with x (and therefore also increases with PO -see Fig. 2), with values of 4. 1O-5sfi 57. 10m5 cm*/s at 6.081x16.25, respectively. A com-
where a(O) is the activity of oxygen in the sample and c(0) is the oxygen concentration. Assuming that diffusion takes place mainly through oxygen sites in the Cu-0 chains of YBCO structure, and taking [17,18], Eq. (4) a(0) = K(T)P;** and c(O)=x-6 becomes:
&AD 2
alo&* O
>
a log& - 6) .
(5)
One can therefore calculate Do using the predetermined value of fi and the measured titration curve, i.e. PO, -x relation. YBCO samples were prepared by standard ceramic procedure. A YBCO sample was placed in the CSZ tube and heated in air to 85O”C, where it was kept for 24 h to reach equilibrium. The sample composition is fixed at 6.35 at these PO and T values [20-221. The cell was then sealed and2 a series
Fig. 2. Nonstoichiometry T= 850°C.
in YBa,Cu,O,
as a function
of PO2 at
0. Porat et al. I Solid State Ionics 86-88
(1996) 1385-1389
This is supported also by the value of W which is closed to unity, i.e. a(O)-C(0).
4. Summary
6
61
6.2
x
63
in YBa,Cu,O,
Fig. 3. The chemical diffusion coefficient and the component diffusion coefficient of oxygen in YBa,Cu,O,, as a function of X, at T= 850°C.
parison with previous data is not straightforward, since most previous works were performed at lower temperatures and higher x. We find our results in fair agreement with the data of Maier et al. [14] and Patrakeev et al. [12], when the later are extrapolated to higher T and lower n. D is usually reported to increase with increasing x [8,12], although it is not clear whether the dependence is due to the thermodynamic factor (i.e. the PO,-x relation) or due to a Do-x dependence. We find that D, remains constant at the measured x range, and the dependence of Lj on x is due to W. This is in disagreement with Kishio et al. [8] and Patrakeev et al. [12], who find that D, increases with x. They suggested an interstitial diffusion mechanism, in which the activation energy for ion jump decreases with x, due to ion-ion interaction, therefore gives rise to increasing D,. On the other hand, McManus et al. [ 131 report on an opposite trend, in which D, decrease with x. They suggested vacancy mechanism. However these results seem somewhat inaccurate, given also the fact that the values of b are much higher than in any other report. We believe the explanation of refs. [8] and [12] to be reasonable, and that the independent D, -x in the present work is due to the composition range for which measurements were performed, which is close to x=6. In this composition the mobile oxygen interstitial species are not interacting.
The oxygen chemical and component diffusion coefficients of tetragonal YBa,Cu,O, were measured as a function of PO, and x at T= 850°C using an intermittent titration method. The measured range is 6.085x56.25 and 10~‘5Poz110-2 atm. The values of D and D, are found to be relatively close, of the order of 5.10m5 cm2/s, due to the thermodynamic factor which is close to unity. Our data are comparable with data of others, when the later are extrapolated to the measured temperature and x ranges. It is found that D increases weakly with x (and PO,). This increase is due to the dependence of the thermodynamic factor W on x. D, is found to be constant in the measured x range.
Acknowledgments This research was supported by the Fund Promotion of Research in the Technion.
for
References 111R.J. Cava, B. Batlogg,
C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Phys. Rev. B36 (1987) 5719. 121W.E. Farneth, R.K. Bordia, E.M. McCarron, M.K. Crawford and R.B. Flippen, Solid State Commun. 66 (1988) 953. [31 J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, I.K. Schuller C.U. Segre, K. Zhang and MS. Kleefisch, Phys. Rev. B36 (1987) 3608. 1.0. Lelong, A. Kessel and J. r41 B. Fisher, J. Genossar, Ashkenazi, Physica C153-155 (1988) 1348. [51 I.A. Leonidov, Ya.N. Blinovskov, E.E. Flyatau, P. Ya. Novak and V.L. Kozhevnikov, Physica Cl58 (1989) 287. [61 G.M. Choi, H.L. Tuller and M-J. Tsai, in: Nonstoichiometric Compounds, eds. J. Nowotny and W. Weppner (Kluwer, Dordrecht, 1989) p.451. [71 J.H. Brewer et al, Phys. Rev. Lett. 60 (1988) 1073. PI K. Kishio, K. Suzuki, T. Hasegawa, T. Yamamoto and K. Fueki, J. Solid State Chem. 82 (1989) 192. [91 T. Umemura, K. Egawa, M. Wakata and K. Yoshizaki, Jpn. J. Appl. Phys. 28 (1989) L1945. [lOI K.N. Tu, N.C. Yeh, S.I. Park and C.C. Tsuei, Phys. Rev. B39, (1989) 304.
0. Porat et al. I Solid State Ionics 86-88 [ll] J.R. LaGraff and D.A. Payne, Phys. Rev. B47 (1993) 3380. [12] M.V Patrakeev, LA. Leonidov, V.L. Kozhevnikov, V.I. Tsidilkovskii and A.K. Demin, Physica C210 (1993) 213. [I 31 J. McManus, D, Fray and J. Evetts, Physica Cl69 (1990) 193. [14] J. Maier, P. Murugaraj, G. Pfundtner and W. Sitte, Ber. Bunsenges. Phys. Chem. 93 (1989) 1350. [15] W. Weppner and R.A. Huggins, J. Electrochem. Sot. 124 (1977) 1569. [16] G. Schmidt-Kongeter and W. Weppner, in: Reactivity of Solids, eds. P. Barret and L.-C. Dufour (Elsevier, Amsterdam, 1985) p. 393.
(1996) 1385-1389
1389
[17] 0. Porat, I. Riess and H.L. Tuller, Physica Cl92 (1992) 60. [18] I. Riess, 0. Porat and H.L. Tuller, J. Supercond. 6 (1993) 313. [ 191 0. Porat and I. Riess, Materials Science and Engineering B22 (1994) 310. [20] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) Ll228. [21] E.D. Specht, C.J. Sparks, A.G. Dhere, I. Brynestad, O.B. Cavin, M.D. Kroeger and H.A. Oye, Phys. Rev. B37 (1988) 7426. [22] P. Strobel, J.J. Capponi, M. Marezio and P. Monod, Solid State Commun. 64 (1987) 513.