Some aspects of the existence of trivalent copper in YBa2Cu3O7−x superconductors and related corrosion phenomena

Physica C 171 (1990) 156-166 North-Holland

Some aspects of the existence of trivalent copper in YBa2Cu307_x superconductors and related corrosion phenomena F.P. Dousek The J. Heyrovskf~Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Dolej~kova 3, 182 23 Prague 8, Czechoslovakia Received 6 June 1990 Revised manuscript received 31 July 1990

Direct evidence was obtained for the presence of trivalent copper (3d 8) in the YBa2Cu3Ov_x (0~
1. Introduction The discovery of high-To, YBa2Cu307_x superconductors with a perovskite-like lattice was based on oxygen-containing superconductor compounds: LiTi204, BaPbo75Bio.2503 and Laz_xBaxCuO4_y. In all these compounds, one of the elements is present in two oxidation states: Ti 3+ and Ti 5+, Bi 3+ and Bi 5+, and Cu 2+ and Cu 3+, as a result of the overall stoichiometric content of oxygen [ 1-4 ]. The superconductivity of these substances is connected with this composition. Similarly, in YBa2Cu307_x (0~
Nonetheless, a n u m b e r of recent works almost unambiguously agree that the hole carriers in this substance, whose presence ensures the superconductivity, are not Cu 3+ in the superconductor lattice, but rather peroxide bonds c o m b i n e d with Cu 2+ [7,8]. The conclusion of ref. [9], where the products of dissolving of YBa2CuaO7_x in acidic electrolytes were studied, are not as u n a m b i g u o u s because of the high reactivity of this substance with such a high oxidation potential. No highly oxidizing species were found in solution several tens of milliseconds after dissolving the substance and therefore the definitive assignment of the oxidizing power to a location in the original material is not possible as charges can rearrange at an early stage. It is pointed out in ref. [ 10] that H2O 2 is not present in acid solutions of YBa2Cu307, indicating that peroxo c o m p o u n d s of divalent copper are not present [20]. This work describes the ability to dissolve the copper c o m p o n e n t s of the superconductor in alkaline media in the presence of a complexing agent and the use of this reaction for the specific detection o f C u m.

0921-4534/90/$03.50 © 1990 - Elsevier SciencePublishers B.V. (North-Holland)

F.P. Dousek / Cu HI in YBa2Cu307_ x superconductors

The stable C u III complex formed was studied in solution by spectrophotometry and polarography. The results were correlated with electrochemical measurements on electrodes of the superconductor (chronopotentiometry). Some aspects of the corrosion of superconductors are discussed from the point of view of electrochemistry.

2. Experimental The superconductor samples were prepared by E. Pollen of the Physics Institute of The Czechoslovak Academy of Sciences in Prague in the standard manner by calcination of a mixture of oxides, with subsequent grinding, pressing and annealing in an oxygen atmosphere. The samples were characterized by powder X-ray diffraction, measurement of the temperature dependence of the resistivity and chemical analysis of the Cu component. The details of these methods have already been published [ 11 ]. Prior to use, all the samples were ground in an agate dish so that the whole sample passed through a 40 ~tm sieve. CuO was prepared by heating Cu (OH)2 (300 ° C, 2 h ), obtained by precipitating a Cu (NO 3 ) 2 solution with ammonia [ 12 ]. The crystalline salt Na7[Cu(IO6)2].15H20 was provided by L. Jeniovsk~ of the Department of Inorganic Chemistry of the Charles University in Prague. The method of its preparation and chemical analysis have been described in detail previously [13,14]. The remaining reagents were of "for analysis" purity. Special care must be taken with KOH and its solutions, which must be completely free of substances that could he oxidized by trivalent copper. The Fluka product was completely satisfactory for our purposes; the local product (Lachema) had to be preboiled with a small amount of persulphate, oxidizing substances capable of being oxidized; excess persulphate was decomposed by boiling. Electrodes of the superconductor and of other materials (the working electrodes) with a diameter of 20 m m and a thickness of 0.5 m m were prepared by pressing powder substance (see above) with a force of 130 kN on a nickel screen (wire diameter 0.05 mm, hole size 0.1 m m ) acting as a support and current collector. The porosity of the pellets was about 26%.

157

The reference Cu/Cu20 electrode (which also acted as the auxiliary electrode), with diameter 20 m m and a thickness of 1.5 mm, was made of electrolytic copper powder in an analogous procedure (force 10 kN, porosity about 56%), and was then sintered in a hydrogen atmosphere (450°C, 30 min). After immersing in a 7.1N KOH solution in air, acting as the base electrolyte for the electrochemical measurements, it attained a potential determined by the system 2 C u + 2 O H - - , C u 2 0 + H 2 0 + 2 e , i.e. about - 3 6 0 mV vs. SHE (standard hydrogen electrode) [18]. The electrochemical cell employed to study the working electrodes consisted of a hermetically sealed compartment of plexiglass containing the working electrodes, distance ring (diameter 20/18 mm, thickness 1 m m ) , filled with a separator of unwoven polyamide fibres, and the reference electrode. The whole system was filled with base electrolyte under vacuum (500 Pa H20 vapour pressure above 7.1N K O H ) and electrical contact was established through a steel, chemically nickel-plated springs touching the nickel outlets of the cell. The details have already been described [ 15]. The cell was polarized at 25 °C by a constant direct current so that reduction processes occurred at the working electrode. The capacity of the reference electrode was selected so that no qualitative changes occurred at it during current passage, i.e. its potential practically did not change. Because of the high specific surface area of this porous electrode, its polarization at a current of _+ 1 mA is not greater than l0 mV. Measurement of the cell voltage with time at constant current thus yielded the dependence of the working electrode potential on time, i.e. the chronopoten!iometric curve, reflecting electrode reduction processes at the working electrode. A Minigor RE 501 (Goerz, Austria) pen recorder with an input resistance of 25 M ~ was employed. Polarographic measurements were carried out using a PA 4 instrument (Laboratorni piistroje, Prague) with a three-electrode system and a separated mercuric oxide reference electrode ( E = + 120 mV vs. SHE). The working electrode was either a dropping mercury electrode or a cylindrically-shaped vibrating platinum electrode with a diameter of 1 m m and length of 1 mm, sealed in glass (frequency of vibration 50/s, amplitude 1 m m ) . The auxiliary elec-

158

F.P. Dousek / CuTM in YBa2Cu307_x superconductors

trode consisted of a platinum foil, 0.5 × 1 cm in size. Spectrophotometric measurements were carried out on a Specord M 40 instrument (Carl Zeiss, Jena, G D R ) in 10 m m glass cuvettes. The reference solution always had the same composition as the solution employed for the extraction.

3. Results and discussion 3.1. Extraction o f the Cu-component in alkaline media

The compounds of trivalent copper are known to be more stable in alkaline than in acidic media [ 17,20,21 ]. This was also found to be true of the superconductor YBaCuO (sample 1), in agreement with the results of other authors [ 7-9,22 ]. It is also known that the compounds of Cu n dissolve in alkaline media to form blue hydroxo complexes of the type [ Cu n (OH)4 ] 2 - the formation of a red colour has been described for Cu nl, corresponding to very unstable [Cure(OH)4] ~- [20,21,26]. However, in alkaline media ( p H > 8 . 5 ) , Cum forms stable bisperiodato cuprate complexes with the [Cu m ( I 0 6 ) 2 ] 7- anion and bistellurato cuprate, with [ Cum (TeO6) 2 ] 9-, whose salts can also be prepared in crystalline form [ 13,14,17 ]. These properties were employed to demonstrate the presence of Cum in YBaCuO superconductor. The results of corrosion experiments are collected in table I, together with the chemical compositions of the tested samples. Sample 1, containing Cu In and Cu H, extracted by solutions of K O H alone (from 2 mol/1) yield blue solutions with evolution of oxygen with a characteristic absorption band on the spectrophotometric curves at about 640 nm. Sample 2, containing Cu n and Cu ~, and sample 3 (only Cu ~) yield analogous results, however without evolution of oxygen (see below). If the extraction is carried out using a solution of iodate in KOH (e.g. 2M K O H + 0 . 1 M KIO4), a basic change occurs in sample 1. Shortly after adding the finely powdered sample to the solution (25°C), a yellow colour appears around it and the evolution of oxygen is practically invisible. The rate of the extraction is dependent on the specific surface area of the sample and on stirring. After stirring for 70 h with

an electromagnetic stirrer and sedimentation for 48 h, the red-brown solution was found to contain 31.5% of the theoretical amount of Cu HI in sample 1. The Cum concentration did not increase further. This phenomenon is apparently a result of the presence of insoluble complexes of Ba and Y and also of the limited solubility of the Cu n complexes, blocking the surface of the sample grains and preventing further extraction. After a longer period of time, all these substances collect in a light-coloured suspension. The spectrophotometric transmittance curves of the extract of sample l, diluted 100-fold with pure solution E, and the curve for a 10-4M standard solution (sample 4) are depicted in fig. 1. The solutions are completely stable and can be stored for several weeks. It can be seen that the absorption spectra of the two solutions are identical and that extraction of the superconductor sample yielded the bisperiodato cuprate complex containing trivalent copper in the [ Cum (IO6) 2 ] 7- anion. The shapes of the curves, the measured extinction coefficients and the positions of the minima on the transmittance curves are completely in agreement with the literature data for this complex, which is one of the most intensely coloured inorganic substances [ 16,17 ]. The dependence of the absorbance ( E = - log I/Io) of the standard solution (sample 4) on the concentration is linear in the range l 0 - 6 t o l0 -4 mol/1 (fig. 2, full curve) and permits very sensitive quantitative determination of Cum in solution. Analogous results can be obtained by extraction of sample 1 with a saturated solution of K2TeO4 ( ~ 5 × 10-2M) in 2M KOH, where it was demonstrated that the [Cu m(TeO6)2 ]9- complex anion is formed, i.e. bistellurato cuprate. The solution obtained is once again yellow to brown in colour and the measured spectrophotometric characteristics are in agreement with the published values [16,17] :2mi,=405 nm, emi.=6400. The extinction coefficient (and thus also the sensitivity of the determination) is thus half that for the iodate complex. Both the iodate and tellurate complex of Cu m, obtained by extraction of sample I, were studied polarographically. The iodate complex yielded reduction waves corresponding to the reduction Cu 3+ + e - , C u 2+, using the procedure described in the literature [ 19 ] ; the concentration of CH Ill in solution could be determined quantitatively using the vibrating platinum electrode by the method of stan-

F.P. Dousek /

C u TM in

159

YBaeCu3Oz_x superconductors

Table I The chemical compositions of samples and spectrophotometric characteristics of solutions obtained in alkaline media. Sample

( 1 ) YBa2fu306.9o

Extracting Solution a) (20 ml)

Colour

A

414.4415.1 405

1.3X 104

traces02

6400

no

634.1 641.4

45 45

02 02

"~min b)

~min ¢)

[nm ]

Gas

evolution

7.87 wt.% Cu nl

B

21.27 wt.% Cu n (80.7 or 500 mg)

C D

yellow to red-brown yellow to brown light blue dark blue

(2) YBa2Cu30613 22.12 wt.% Cu n 7.1 wt.% Cu ~ (80.7 or 500 mg)

A B C D

dark blue blue light blue dark blue

621.5 622 630 641.4

45

no

(3) CuO 96.7 wt.% CuO 3.3 wt.% H20 ( 188 mg)

A B C D

dark blue colourless very light blue light blue

620.2 622 630 653.1

45

no

(4) NaT[Cu(IO6)2]- 15H20

E

yellow (10-4M)

414.7

1.3X 104

no

2M KOH + 0.1M KIO4, B: 2M KOH saturated with K2TeO4, C: 2M KOH, D: 7.1M KOH, E: 0.1M KOH + 0.01M b) 2m~,: wavelength of minimal transmittance. c) Cmi.:extinction coefficient at 2mi..

a~ A:

K I O 4.

%T 80

60

40

20

350

I

I

I

I

I

400

450

500

600

700

go0

~{nm) Fig. 1. Spectrophotometric transmittance curves (Cu m ). ( 1 ) Extract of 80.7 mg of sample 1 with solution A, diluted 100 X with solution E (table I ). ( 2 ) 10 - 4M Na7 [ Cu ( IO6 ) 2 ] in solution E (sample 4, table I ). d a r d a d d i t i o n s ( s a m p l e 4 ) o r e m p l o y i n g t h e calib r a t i o n c u r v e d e p i c t e d i n fig. 2 b y t h e e x p e r i m e n t a l p o i n t s . T h e scale o f t h e c o o r d i n a t e a x e s w a s c h o s e n to d e m o n s t r a t e t h e a g r e e m e n t b e t w e e n t h e s p e c t r o photometric and polarographic determinations. The

r e d u c t i o n w a v e o f C u lIl at t h e d r o p p i n g m e r c u r y e l e c t r o d e is d i s t u r b e d b y excess i o d a t e i n s o l u t i o n , w h i c h is r e d u c e d a t a m o r e p o s i t i v e p o t e n t i a l [ 19 ]. The extract of sample 1 employing the tellurate c o m p l e x r e a c t e d at t h e d r o p p i n g m e r c u r y e l e c t r o d e

160

F.P, Dousek / Cum in YBa2Cu j07_ x superconductors

10-6 10-5

5.10-5

10-4

c (M/[ Cu3÷)

I[im

E

(~A) -15

1

-10

0.5 -5

J ~"

0

0

"--~ iti m = f(c)

L

I

/

1.10-3

2.10-3

I

c (M/t Cu3+}

3.10-3

Fig. 2, Comparison of spectrophotometric and polarographic determination o f C u m in the iodate complex. Full curve: dependence of the absorbance E = - l o g I/Io of a solution of NaT [ Cu (IO6)2 ] (in solution E, table I ) on the concentration of Cu 3+. Experimental points: dependence of the limiting current &m for the reduction of Cu 3+ at a platinum vibrating electrode on the depolarizer concentration.

to yield a very well d e v e l o p e d reduction wave, C u 3 + + e-~ Cu 2+, followed by the two-electron wave for the reduction Cu 2÷ + 2e ~ Cu (n~/2 = - 420 m V ) . Tellurate d i d not interfere even when present in a large excess in solution, as it is r e d u c e d in this med i u m at n l / 2 = - 1400 inV. Thus, b o t h forms o f copper can be d e t e r m i n e d quantitatively in solution. Typical waves for the reduction o f Cu n~ a n d Cu H are depicted in fig. 3 together with the curve for the base electrolyte ( 2 M K O H ) . The anodic wave visible on the p o l a r o g r a m (at a potential o f about - 1 6 5 m V ) was identified as the a d s o r p t i o n prewave p r o d u c e d by the f o r m a t i o n o f the insoluble complex o f tellurate with mercury. It was found by s t a n d a r d a d d i t i o n o f Cu 2+ to this solution that [ C u 3 + ] = 2 X 1 0 - 4 M and [Cu2+]= 10-4M so that the ratio [ C u Z + ] / [ C u 3 + ] = 0 . 5 , while this ratio equalled 2.7 in sample 1. This difference, which was c o m m o n l y found for samples after long extraction times, is a result o f the considerably higher solubility o f the Cu 3+ complex. The given ratio, 2.7, could be found s p e c t r o p h o t o m e t r i c a l l y after short extraction times for sample 1, where the concentration o f the Cu 2+ complex in solution had not yet attained saturation values. The kinetics o f the

-3 i tim ([JA) -2

-'1

i

-0.2

-0.3

0.4

i

t

-0S

-0.6

E IV)

Fig. 3. Polarographic wave for the reduction of Cu 1|1 and CU II in the tellurate complex at a dropping mercury electrode in a nitrogen atmosphere, H g / H g O reference electrode, scan rate 1 m V / s . ( 1 ) 2M KOH. (2) Extract of 80.7 mg of sample 1 in 2M KOH saturated with K2TeO4 (solution B, table I).

dissolution o f the superconductor in alkaline m e d i a would certainly warrant a more detailed quantitative study, as would the simultaneous evolution o f oxygen. Extraction o f samples 2 and 3, which do not con-

F.P. Dousek / Cu 1tl in YBa2Cu3Ov_xsuperconductors

tain C u Ill, by an alkaline K I O 4 solution, leads only to a blue colour o f the solution, similar to extraction with a solution o f K O H alone. Oxygen is not evolved. In contrast to C u Ill complexes, however, these solutions do not have unlimited stability [20,26]. Typical spectrophotometric transmittance curves are depicted in fig. 4. Curves 1, 2 and 3 correspond to the hydroxo complexes of Cu n (~-min = 640-650 nm), while curve 4 is characteristic for the iodate complexes, for which the colour shifts to green ()]'min = 621 nm). Standard Cu H solutions in the given medium were employed to determine the extinction coefficient ~62~= 4 5 , which is 290 times lower than for the [ Cum ( IO6 ) 2 ] 7- complex. These results explain why extraction of sample 1 by an alkaline iodate solution (or tellurate solution ), where Cu n is present in a 2.7fold excess over Cu HI, yields a yellow solution and the presence of blue components is not observed. The limited solubility o f the Cu n complexes also contributes to this effect. Polarography is more sensitive than spectrophotometry for the detection of complexes o f divalent copper but the opposite is true for the complexes of trivalent copper. Similar effects were observed during extraction by a tellurate solution in KOH. Dissolving of samples 2 and 3 (Cu ~) occurs somewhat more slowly than with iodate, apparently reflecting the effect of the redox potential of the systems on the dissolution rate

161

of the Cu component, which was also observed in electrochemical studies [ 27 ]. 3.2. S o m e aspects o f corrosion o f YBaCuO superconductors

The chemical processes occurring during extraction in an alkaline solution of the periodate KIO4 can be described as follows: the action o f K O H on KIO4 produces K5IO6 (potassium orthoperiodiate) in solution: KIO4 + 4KOH--.K5 IO6 + 2 H 2 0 ,

( 1)

which can dissolve the compounds of trivalent copper to form the stable bisperiodate complex: 4K5 I06 at-Cu2 03 q- 3 H 2 0 --.2K7 [Cum(IO6)2 ] + 6 K O H .

(2)

It can be seen from eq. (2) that oxygen is not split off during dissolution of the oxide of trivalent copper, in agreement with our observations (only traces o f oxygen were observed, probably as a result of the parallel reaction (6) (see below), or as a result of oxidation of water). The extraction with alkaline solutions o f the tellurate is completely analogous; K2YeO6, potassium orthotellurate, is formed: K2 TeO4 + 4 K O H ~ K6 TeO6 + 2 H 2 0 .

(3)

I

%T 80

60

40

20 2 0

I 350

I 400

I 450

I 500

I

I

600

700

900 ), (nrn)

Fig. 4. Spectrophotometric transmittance curves of the CuIIcomplex. ( 1) Extract of 500 mg of sample 1 in 20 ml of 7.1M KOH (solution D, table I). (2) Ditto, sample 2. (3) Ditto, 188 mg of sample 3. (4) Extract of 80.7 mg of sample 2 in solution A (table I).

162

F.P. Dousek/ Cum in Y B a 2 C u s O 7 _ x superconductors

Subsequent reaction with Cum then yields the bistellurato cuprate complex: 4K6TeO6 +Cu203 + 3H20

---~2K9 [CulIl(TeO6)2 ] + 6 K O H .

(4)

No gas evolution, even in traces, was observed in this extraction. On the other hand, the following reaction apparently occurs during extraction with a solution of KOH alone, where the formation of a blue solution of the hydroxo complex of Cu 2÷ and the evolution of oxygen were observed: Cu2 O3 + 2KOH + 3H2 O--*2K [ Cure (OH)4 ] .

(5)

The unstable hydroxo complex ofCu m formed is immediately decomposed with evolution of oxygen: 2K[Cum(OH)4] +2KOH --~2K2 [Cun(OH)4] + H z O + ½02 •

(6)

The simultaneous formation of the Cu u hydroxo complex leads to the blue colour of the solution. It was found by using material enriched in the ~80 isotope [7] that the oxygen evolved during the dissolution of YBaCuO superconductor in an acidic medium comes from the superconductor lattice and not from solution. This would probably correspond to the reaction Cu2 03 + 4H + -~ 2Cu 2+ + 2H20 + ½0 2 ,

(7)

in agreement with the properties described for the compounds of trivalent copper [ 20 ]; if the YBaCuO lattice contained peroxo compounds of divalent copper, as stated in refs. [ 7,8,23 ], then the action of an acidic medium should lead to the release of hydrogen peroxide into solution [ 20 ]. This was not demonstrated in any of our studies, similarly to the other works [ 9,10 ]. On the other hand, we have shown that hydrogen peroxide reduces trivalent copper in alkaline solutions of the iodate or tellurate complex (with the formation of the blue Cu n complexes) and is oxidized to form oxygen. These conclusions also follow from the differences in the redox potential values for the two systems [ 18 ]. It is thus obvious that the assumed existence of these two redox systems in the superconductor lattice described in ref. [23 ] for YBaCuO would require a further critical

approach. In fact, the system would consist of a "mixture" of two redox substances in an electrically conductive skeleton (at room temperature, the YBaCuO superconductor has metallic conductivity), i.e. of a shorted galvanic cell (corrosion cell), in which self-discharging would occur, i.e. electrochemical reaction of the components. In this connection, it should be pointed out that similar corrosion processes must necessarily occur on contact of superconductors with other materials that can be oxidized by trivalent copper, e.g. various metals acting as electric current collectors. The contact will apparently involve interaction leading to oxidation of the contacting metal (with simultaneous reduction of Cu "I to Cu ") through an electrochemical mechanism in the solid phase, whose kinetics obey a parabolic law [24]. The reaction rate is relatively low and decreases with time (the diffusion of the oxygen anion in the solid phase is a limiting factor) but penetrates to depth and, as demonstrated for analogous reactions, can lead to complete depletion of one of the reaction components under suitable conditions [25 ]. This can have an unfavourable effect both on the metal-superconductor transition resistances, as the first oxide layers at the interface are formed practically immediately after contact, and also on the superconductivity itself through loss of Cu nI in the superconductor. As the compounds of Cum have a very positive redox potential (see below), it can be expected that there are very few materials that would not undergo this oxidative reaction on contact with a superconductor of the YBaCuO type. This must necessarily also be true of the other superconductive oxygen-containing compounds, containing, e.g. Ti, Bi or Cu in a high oxidation state [3]. 3.3. Electrochemical behaviour of YBaeCU3OT_x in the solid phase Figure 5 depicts typical chronopotentiometric curves for the samples, obtained in 7.1M KOH at 25°C. When the individual samples were immersed in the electrolyte, they exhibited identical corrosion behaviour, as described above, including evolution of oxygen for sample 1 (Cu In) (table I). Sample 1 yielded a very positive rest potential ( + 920-960 mV), which was up to 200 mV more positive than the potential of the oxygen electrode in

F.P. Dousek / Cum in YBaeCu~Oz_x superconductors

E IV vs Cu/Cu20

163

1 I

I

I

I

I

I 0

I 2

I 4

I 6

I 8

0.8 0.6 0.4 0.2

-0.2

t (hi

Fig. 5. Chronopotentiometric curves of YBaCuO superconductor and reference materials. 640 mg of sample, 7.1M KOH, 25 ° C, Cu/ Cu20 reference electrode (potential - 360 mV vs. SHE). ( 1 ) Sample 1, i = 1 mA. (2) Sample 2, i = 0.1 mA. ( 3 ) Sample 3, i = 0.1 mA. (a) E O2/4OH-, +760 mV. (b) E Cu(OH)2/Cu20, +278 inV.

the same medium. Thus thermodynamic conditions were established for the YBaCuO superconductor to release gaseous oxygen from water. However, the electrochemical evolution of oxygen, similar to the electrode reactions of hydrogen peroxide, usually occurs with a large overpotential, with the exception of some catalytic substances, e.g. silver and platinum [28,29]. All the evidence indicates that this overpotential is also large for YBaCuO: it followed from our research that oxygen is evolved spontaneously in alkaline media only with simultaneous conversion of Cu "~ to Cu", while simple extraction of Cu Ill into solution with no change in valence is not accompanied by oxygen evolution (see reactions ( 1 ) to (7) ). An increased overvoltage for the evolution of oxygen on the YBaCuO superconductor in 0.1M NaOH of almost 200 mV was found compared to platinum [ 8 ], and a value of 380 mV in 5M KOH was found compared to the oxygen electrode in the same medium [6 ]. Obviously, the origin of the spontaneous evolution of gaseous oxygen must be sought elsewhere than in oxidation of water, most likely in the superconductor lattice. This conclusion is in agreement with the results found for acidic media [ 7 ]. As corrosion processes occur on an electrode of sample 1 (in addition to dissolution of Cu, certainly

also oxidation of the Ni current collector), it is obvious that the measured rest potential must be more negative than the actual redox potential of the sample itself. We measured a value of about + 960 mV vs. Cu/Cu20 in 7.1M KOH, i.e. +602 mV vs. SHE, in good agreement with the value found in 1M NaOH ( + 6 6 3 mV vs. SHE [5]) and in 5M NaOH ( + 6 0 0 mV vs. SHE [6 ] ). In the cathodic polarization of a working electrode of the superconductor (sample 1 ) by a constant current of 1 mA, reduction process I (fig. 5, curve 1 ) began to occur at the electrode and the potential remained at values more positive than that for the oxygen electrode for a period of 2.5 h. With a short disconnection of the polarization current, the potential rapidly returned practically to the initial value of the rest potential. As the oxides Y 2 0 3 and BaO are electrochemically stable under the given experimental conditions [ 18 ], this process can be correlated only to the reduction of the Cu-oxide component. It is highly improbable that this reduction would involve peroxo compounds of divalent copper, for the following reasons: (i) The reduction of the peroxidic substances is irreversible and occurs with an overvoltage t h a t is one of the largest known in electrochemistry;

164

F.P. D o u s e k / C u lIl in YBa2Cu 3O T_ x superconductors

it usually occurs at potentials that are several hundred millivolts more negative than that for the above process (I). (ii) It has been found [ 5 ] that reduction processes occurring in alkaline media in the same potential region as here (process I) are reversible, i.e. reversal of the polarizing current repeatedly returned the electrode to the initial state. If the reduction of peroxidic substances were involved, such reversibility would be impossible. (iii) As mentioned above, neither we not other authors [ 9,10 ] could demonstrate the presence of hydrogen peroxide in solutions of the alkaline or acidic extracts of the superconductor, indicating that peroxo compounds of divalent copper are not present [20]. Consequently, electrode process I, occurring at the electrode (sample 1 ) at the most positive potentials, can be unambiguously equated with the reduction of trivalent copper to divalent copper in the superconductor lattice. After three-hour polarization, a step appeared on the chronopotentiometric curve, indicating a transition to another electrode process, i.e. to CuZ++e~Cu +. This process(II) (fig. 5, curve 1) began at the electrode before all the Cu 3+ was reduced to Cu 2+ (after 3.3 h of passage of a current of 1 mA, 7.8 mg of Cu 3+ was reduced, i.e. 15.5% of the total amount of Cu 3+ present). This observation is a result of the widely known fact that only a certain fraction of the electroactive substance can undergo electrochemical reaction at a given current density in a porous electrode system. The reduction efficiency can be increased by decreasing the current density, but it is very difficult in principle to attain an efficiency of 100%, similar to the extraction of samples with complexing agents. Even at this stage, a short disconnection of the charging current led to a return of the potential to positive values, indicating that Cu 3+ is still present in the sample. For the same reason the plateau on our curve occurred at more positive potentials than the standard potential of process II ( - 80 mV, vs. SHE, i.e. + 2 7 8 mV vs. the C u / C u 2 0 reference electrode). As soon as Cu + appears on the electrode during the polarization, dismutation reactions such as Cu 3+ -I-Cu + --~2Cu 2+ must be taken into account. It

should be pointed out that there is indication of a further process at the potential step towards another electrode process (III) on the reproducible curves; this was not present in samples of Cu 2+ (samples 2 and 3). No interpretation of this process is yet possible because of the complexity of the whole system, including, e.g. evolution of oxygen on the electrode, the solubility of the Cu 2+ and Cu + forms, overvoltage in the electrode reactions, dismutation reactions and also a number of chemical reactions of Y203 and BaO components with K O H and H 2 0 and a change in the conductivity of the working electrode skeleton (see below ). A further potential step appeared after 7 h polarization, ending discontinuously on a plateau where the reaction Cu + + e-~Cu(metal) occurs (process III, fig. 5, curve 1 ). The discontinuity reflects the complex conditions at the electrode, which gradually became less conductive (loss of Cu 3+) and then became more conductive with the appearance of metallic copper; however, this copper began to react in dismutation reactions with the remaining Cu 3+ and Cu 2+, e.g. Cu 3+ + Cu-~Cu 2+ + C u +, Cu 2+ + C u ~ 2 C u +. When the current was turned off for a short time in this stage, the potential of the working electrode initially returned to values even more positive than the potential of the C u 2 + / C u + system and, only after depletion of Cu 3+ and Cu 2+, did it attain rest potential values identical with that of the reference electrode. The final process in this system (not visible in fig. 5) is the evolution o f hydrogen on metallic copper, 2H20 + 2e--~2 O H - + H2, which also occurs with an overvoltage at a working electrode potential of about - 6 0 0 mV. Details of this process and also on electrode processes II and III have already been published [ 15 ]. The chronopotentiometric curves of YBa2Cu306.13 (sample 2) and CuO (sample 3) were more or less identical (fig. 5, curves 2 and 3). Their expected rest potentials unambiguously indicated the presence of the Cu(OH)2/Cu20 system, as did the shapes of the chronopotentiometric curves. Electrode process II occurred first, followed by process III, both with a certain overvoltage. The electrode reactions occurring in these processes are identical with those described for sample 1. The polarization current was chosen ten times less than for sample 1, particularly because o f the lower conductivities o f samples 2 and

F.P. Dousek / Cu TM in YBa2Cu 307_ x superconductors

3. The changes in the electrode potential (sample 2 ) with time after disconnecting the current source are also depicted (for time t = 3.4 h ): after 15 h, the electrode potential returned to a value close to the initial rest potential; the "S" shape of the curve is typical for the reaction C u + C u 2 + - ~ 2 C u + and its effect on the electrode potential [ 15 ].

4. Conclusions The fact that extraction of the Cu component from the superconductor Y B a 2 C u 3 0 7 _ x (0~
165

and also on the superconductivity itself as a result of the decrease in the Cum content.

Acknowledgements I am very grateful to Dr. M. Heyrovsk~, for carrying out the polarographic measurements, for useful discussions and for his interest in this subject. I would also like to thank Dr. L. Jen~ovsk~ for providing the chemicals and for advice on trivalent copper, and Dr. E. Pollert for preparing the YBaCuO sample.

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166

F.P. Dousek / Cum in YBaeCu307_ x superconductors

[20] H. Remy, Lehrbuch der Anorganischen Chemie, 12th ed. (Akademische Verlagsgesellschaft Geest und Portig K.-G., Leipzig, 1965 ). [21]F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 2nd ed. (Interscience, New York, 1966). [22]V.M. Magee, J.M. Rosamilia, T.Y. Kometani, L.F. Schneemeyer, J.V. Waszczak and B. Miller, J. Electrochem. Soc. 135 (1988) 3026. [23]J. Nov~ik, P. Vyhlidka, D. Zemanovti, E. Pollert and A. T~'iska, Physica C 157 (1989) 346.

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