Chemical Problems Associated with the Preparation and

Chapter 7 C h e m i c a l P r o b l e m s Associated with the P r e p a r a t i o n and Characterization of Superconducting Oxides Containing Copper S. Davison, K. Smith, Y-C. Zhang, J-H. Liu, R. Kershaw, K. Dwight, P. H. Rieger, and A. Wold Chemistry Department, Brown University, Providence, RI 02912

Downloaded by FUDAN UNIV on February 9, 2017 | http://pubs.acs.org Publication Date: August 28, 1987 | doi: 10.1021/bk-1987-0351.ch007

The superconducting oxides La1.8Sr.2CuO4 and Ba2YCu3O7 were prepared by decomposition of mixed metal nitrates.

Thermogravimetric analysis and electron spin resonance measurements indicated the presence of Cu(I) and Cu(III) in the La1.8Sr.2CuO4 phase. The compound Ba2YCu3O7 prepared from the nitrates and subjected to an oxygen

anneal at 425°C gave a sharp superconducting transition at 92 K. The phase was stoichiometric but readily decomposed when kept in contact with moist air. Sr

u0

anc

In the last six months the compounds Lai .8 . 2^ 4 ^ BaYCu307 have received much attention in the literature because of their high temperature superconducting transitions. Lai.8 .2^ 4 reported to have a superconducting transition at 36 Κ (1-3). (See also Bednorz, J.G.; Takashige, M.; Muller, K.A.; to be published). A single crystal study has indicated that the compound crystallizes in the space group of I4/mmm (4). The coordination geometry around the copper atoms is a tetragonally elongated octahedron with four short copper-oxygen bonds d[Cu-0(l)] = 1.90 A and two long bonds d[Cu-0(2)] = 2.41 A. The high temperature superconducting transition has been attributed to the existence of mixed oxidation states of copper in the structure 3), but the valence and valence distribution have not been determined. The determination of copper valences in Lai,8 .2^ 4 could be a key for the understanding of the superconducting mechanism of the compound. The first part of this paper will deal with how the oxidation state of copper in Lai gSr Cu04 be determined. The compound BaYCu307 shows an even higher superconducting transition (y 93 K) and crystallizes as a defect perovskite. The structure of Ba2YCu3Û7 has been determined by neutron diffraction analysis. The space group is Pmmm with a = 3.8198, b = 3.8849 and c = 11.6762 A. Barium and yttrium are ordered on the A site to give a tripled cell along c and the oxygens occupy 7/9 of the anion sites. One third of the copper is in 4-fold coordination and 2/3 are five-fold coordinated (Gallagher, P.K.; O'Bryan, H.M.; Sunshine, S.A.; Murphy, D.W.; to be published). 2

Sr

Sr

u0

w a s

u0

c a n

e

#2

2

o

0097-6156/87/0351-0065$06.00/0 © 1987 American Chemical Society

Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS

66

Very l i t t l e has appeared i n the l i t e r a t u r e concerning the optimum conditions f o r the synthesis of Ba2YCu30y and only the above study has attempted to determine the degree and nature of the oxidation state of copper. The preparative conditions necessary to give pure Ba2YCu307 and the chemical i n s t a b i l i t y of the pure phase towards moist a i r w i l l be discussed i n the second part of t h i s paper. La?Cu04 and La-j gSr 2 ^ 0 4 β

#

Preparation of La?Cu04 and Lai ^gSr^CuCU* Samples of La2Cu04 and l . 8 . 2 ^ ^ 4 were prepared by codecomposition of the corresponding n i t r a t e s . The s t a r t i n g compounds were high purity copper metal (Matthey S.50250), La20 (Lindsay #528 99.999%) and SrC0 (Matthey 2H118, 99.999%). The copper was prereduced i n 85%Ar/15%H at 450°C for 6 hours. The La2Û3 was heated at 800°C f o r 8 hours to drive o f f adsorbed CO2 and water. The molecular weight of the SrC03 was analyzed as 147.6 by thermogravimetric analysis. A mixture of 254.2 mg copper metal, 1172.9 mg La2Û3 and 118.1 mg SrC03 was dissolved i n 6 ml of concentrated n i t r i c acid to convert a l l of the i n i t i a l compounds to n i t r a t e s . The solution was dried at 150°C f o r 12 hours and then predecomposed at 500°C f o r 4 hours. The sample was ground and heated at 970°C f o r 120 hours. During the heating the sample was taken out and ground 4 times. F i n a l l y , the sample was quenched to room temperature by removing i t from the furnace at the elevated temperature. The sample of pure La2Cu04 was prepared by the same procedure. L a

S r

u

3

3


2

Characterization of Products. X-ray powder d i f f r a c t i o n patterns of the samples were obtained using a P h i l i p s diffractometer and monochromated high i n t e n s i t y CuKai radiation (λ = 1.5405 A). The d i f f r a c t i o n patterns were taken i n the range 12° < 29 < 75° with a scan rate of 1° 26/min and a chart speed of 30 in/hr. Temperature programmed reduction of prepared samples was carried out using a Cahn System 113 thermal balance. The samples were purged in a stream of 85%Ar/15%H2 f o r 2 hours. Then the temperature was increased to 990°C at a rate of 50°C/hr. The flow rate of the gas mixture was 60 ml/min. Magnetic s u s c e p t i b i l i t y measurements were carried out using a Faraday Balance from 77 to 300 Κ with a f i e l d strength of 10.4 kOe. Honda-Owen ( f i e l d dependency) measurements were carried out at both 77 and 296 K. Electron spin resonance spectra of Lai ,8 .2^ 4 " La2Cu04 were recorded using a Bruker ER220D Spectrometer at room temperature. The frequency was v = 9.464 GHz, the microwave power was 200 mW, and the f i e l d modulation amplitude was 1 Gauss. Sr

u0

a n a

0

Results and Discussion. The x-ray powder d i f f r a c t i o n patterns of La2Cu04 and L a i . 8 . 2 4 given i n Figure 1(a) and (b). La2Cu04 shows a single phase which can be indexed on the basis of a distorted K2N1F4 type structure. The data obtained i s consistent with those of orthorhombic La2Cu04 reported by J . M. Longo (5). L a i . 8 . 2 ^ 4 also prepared as a single phase and could be indexed on the basis of an undistorted tetragonal K2N1F4 type structure. The tetragonal S r

C u 0

a r e

Sr


u0

w

a

s

DAVISON E T

Superconducting Oxides Containing Copper

AL.

(α )


χ

La Ci 2

1

h (b)

1

L

Ill ι

CuO

2 4

Iι (c)

II

I,.

. 1 reduction La 0 2

product 3

Cu

Ml 20

30

40

J,J 50

,

,i

.1

60

70

D i f f r a c t i o n Anglo 2Θ (deg) Figure 1. X-ray d i f f r a c t i o n patterns of (a) La2Cu04, (b) Lai.8 .2^ 4» ( ) reduction products of L a i , 8 , 2 ^ 4 Sr

u0

c

Sr

u0


CHEMISTRY O FHIGH-TEMPERATURE SUPERCONDUCTORS

68

phase i s the superconducting phase with a t r a n s i t i o n reported to be about 36 Κ (1-3). The average oxidation state of copper i n the Lai.8^ .2^ ^4 determined from the TPR r e s u l t s shown i n Figure 2. X-ray d i f f r a c t i o n confirmed that L a i . 8 $ . 2 ^ 4 decomposed completely during the reduction, the detectable reduction products being La 03 and metallic copper (Figure 1(c)). The reduction proceeded according to the following equation: r

u

w

a

s

u 0

T

2

10Lai Sr CuO(2.90+0.5x) e8

+

5

e2

H

* 2 * 9La 0 2

3

+ 2SrO + lOCu + 5xH 0 2

where χ i s the average oxidation state of the copper ions. From the r a t i o of f i n a l and i n i t i a l weights the average valence of the copper ions was determined as 2.00(^.04). There are several p o s s i b i l i t i e s consistent with these r e s u l t s . The copper could remain a l l Cu^ , disproportionate to Cu and C u , or be a mixture of a l l three valencies. In order to help determine the most probable oxidation state, an electron spin resonance study was carried out. In the La Cu04 structure, the C ion has an elongated octahedral coordination with respect to oxygen (_3, 5). The lengths of the four short planar CuO bonds are 1.90 A and the two long Cu-0 bonds along the +z and -z d i r e c t i o n are 2.43 A. Hence, the copper i s a c t u a l l y i n a square planar coordination with respect to oxygen. Wang et a l . have reported that L a i . 8 5 . 1 5 4 tetragonal K NiF4 structure with space group I4/mmm. The four short [Cu-O(l)] bonds and the two long [Cu-0(2)] bonds are 1.90 A and 2.41 A, respectively (4). They found that copper ions were also i n a tetragonally distorted octahedral s i t e . The copper ions i n L a i . 8 5 . 1 5 ^ 4 l primarily square planar coordinated with respect to oxygen. Since there i s an apparent structural s i m i l a r i t y of Cu i n the two compounds, Cu^ i n La2Cu04 can be used as a standard (6) f o r the ESR study of L a i , 8 . 2 ^ 4 * The electron spin resonance spectrum of La2Cu04 ( v = 9.464GHz, room temperature) i s shown i n Figure 3(a). The spectrum can be interpreted with


+

+

3 +

2

ô

Sr

Cu0

n

a

s a

2

Sr

u0

a

r

e

a

s

o

+

Sr

u0

0

g

|(

= 2.310*0.002, g 4

1

Aj_ = (132±2)xl0" cm- ,

and

= 2.062*0.002, =

4

(21±)xl0- cm~

1

Such parameters are t y p i c a l of Cu(II) i n a square planar or tetragonally distorted octahedral s i t e . ESR spectra of L a i g S r C u 0 4 show a feature with the same g-value as the perpendicular features seen i n the La2Cu04 spectrum. However, the size of t h i s resonance decreases with sample p u r i f i c a t i o n and appears to be due to a trace of the La2Cu04 phase. The spectrum of one such sample, shown i n Figure 3(b) (same experimental conditions as the spectrum of Figure 3(a)) has an i n t e n s i t y about 1% that of pure La2Cu04 This fact means that only about 1% of the copper atoms i n the Lai sSr 2Cu04 heated at 970°C f o r 120 hours are i n the Cu(II) state. Considering that copper i n Lai.8^ .2^* 4 has average valence of 2.0, i t i s proposed that the copper has disproportionated into Cu(I) and Cu(III). Cu(II) i s generally the most stable copper ion; therefore, the disproportionation i s a unique c h a r a c t e r i s t i c of e

#2

#

#

r

u0

a

n


DAVISON


E TA L .

102

100 A


n CD

200

400

600

Temperature

800

1000

C°C)

Figure 2. Temperature programmed reduction p r o f i l e of 1.8 .2 4 85%Ar/15%H . L a

S r

C u 0

i

n

2

V

2600

2800

Magnetic

3000

Field

3200

3400

(gauss)

Figure 3. X-band ESR spectra of (a) La Cu04 and (b) Lax gSr Cu04.(Microwave power, 200 mW, f i e l d modulation amplitude, 1 Gauss.) 2

#

#2



70 Sr

u0

T

n

i

s

Lai.8 .2^ 4* unique property i s probably related to the observed superconductivity at high temperature. The magnetic s u s c e p t i b i l i t y of L a i . 8 ^ . 2 ^ 4 plotted against temperature i n Figure 4. The absence of any temperature dependence demonstrates Pauli paramagnetism over the temperature range from 77 to 300 K. Cu(II) 3d^ electrons are usually l o c a l i z e d and characterized by Curie-Weiss behavior; such results were obtained by Ganguly and Rao f o r La Cu04 (7). Since the Pauli-paramagnetic behavior of L a i . 8 . 2 4 consistent with delocalized electrons, t h i s would also indicate a high probability f o r the existence of Cu(I)-Cu(III) formed as a result of disproportionation of Cu(II). r

u0

i s

2

S r

C u 0

i s


Ba2YCus07 Attempted Preparation of Ba2Y&i307 from BaCOg, Y2O3 and CuO. Sample s were prepared by the usual ceramic techniques using predried BaCC3 (400°C), Y 03 heated at 800°C f o r 6 hours and analyzed CuO. The mixture of the carbonate and oxides was heated at 900°C f o r 24 hours. 2

Preparation of BagYCusOy from the Nitrates. The n i t r a t e samples were prepared from stoichiometric quantities of BaC03, Y 03 and Cu metal. The appropriate weights were put into a porcelain crucible and dissolved by the addition of concentrated n i t r i c acid (20 ml/g Cu). The excess acid was boiled o f f and the product was heated at 130°C for 6 hours and then at 400°C f o r 3 hours i n order to decompose most of the copper and yttrium n i t r a t e s . The product was then removed, ground and transferred to a platinum or s t a b i l i z e d zirconia c r u c i b l e . The powder was heated to 800°C f o r 12 hours and cold-pressed into p e l l e t s at 6 Kbar. The p e l l e t s were heated to 950°C f o r 12 hours and then annealed i n oxygen at 425°C f o r 6 hours. They were f i n a l l y cooled to room temperature at 50°C/hr. Additional powder samples, used f o r TGA studies, were prepared by p a r t i a l decomposition of the n i t r a t e mixture at 400°C followed by a second heating at 900°C f o r 24 hours. 2

Characterization of Products: Chemical Analysis. Determination of the stoichiometry of Ba YCu3Py was performed using a Cahn System 113 thermal balance. An atmosphere of of 85%Ar/15%H2 was used f o r reduction i n these studies. The gas was predried by passing i t through a P 2 P 5 column. The sample was heated at 50°C/hr u n t i l no further weight change was noted. 2

Characterization of Products: E l e c t r i c a l Measurements. The van der Pauw method was used to measure the e l e c t r i c a l r e s i s t i v i t i e s . Contacts were made by the ultrasonic soldering of indium d i r e c t l y onto the sintered discs of Ba2YCu3py, and t h e i r ohmic behavior was established by measuring t h e i r current-voltage c h a r a c t e r i s t i c s . The sample and a thermocouple were mounted i n a small cavity inside a massive copper body which was f i r s t cooled to 77 Κ and then allowed to warm very slowly. Results and Discussion. Ba2YCu30y was prepared both by the reaction of BaC03, Y2O3 and CuO according to published procedures and by the


7.

DAVISON

ET


AL.

71

0. 5


0. 4

0.

3

0.

2 o

o

Û ο

0

°

ο

ο

0

ο

Ο

Ο

ο

0. 1

•. ϋ 100

200

Temperature

300

(Κ)

Figure 4. Temperature dependence of the magnetic s u s c e p t i b i l i t y of L a i . 8 . 2 4 S r

C u 0

decomposition of the mixed metal n i t r a t e s . The changes i n weight are plotted as functions of temperature f o r both reaction mixtures i n Figure 5. Copper n i t r a t e and yttrium n i t r a t e are completely converted to oxides by 400°C, so that the TGA results shown i n Figure 5 represent the ease of decomposition of barium n i t r a t e compared to barium carbonate. It can r e a d i l y be seen that barium n i t r a t e i s decomposed by 650 °C whereas the carbonate does not decompose substantially u n t i l 900°C. It should therefore be possible to prepare Ba2YCu30y more r e a d i l y v i a the decomposition of the mixed metal n i t r a t e s . From Figure 6 i t i s seen that the phase Ba2YCu30y i s not completely formed even a f t e r a 24 hour heat treatment of barium carbonate, yttrium oxide and copper(II) oxide at 900°C. However, from Figure 7 the product i s r e a d i l y formed from the nitrates a f t e r being heated at 900°C f o r 24 hours. An important part of the preparative procedure i s annealing the phase formed at 900-950°C i n a flowing oxygen atmosphere at 425°C. This step assures a product which closely approaches the stoichiometric composition of Ba YCu30y. Samples which were annealed at 425°C i n oxygen and allowed to cool slowly at 50°C/hr were 2


CHEMISTRY

72

OF HIGH-TEMPERATURE

SUPERCONDUCTORS

140

D (_) >OJ • CD O

130

120

JO


CO

1 10

H

100 CD

on 200

400

600

800

Temperature

1000

(°C)

Figure 5. Decomposition i n a i r of a mixture of BaC03, Y 03 and CuO compared with that of a mixture of the n i t r a t e s . 2

analyzed by reduction i n an 85%Ar/15%H atmosphere. The results are plotted i n Figure 8. As determined from the measured weight loss, the formula may be represented as Ba YCu307 oi(2)· The e l e c t r i c a l r e s i s t i v i t y and magnetic s u s c e p t i b i l i t y of these samples were measured as functions of temperature and the results are plotted i n Figures 9 and 10. As can be seen from Figure 9, the r e s i s t i v i t y data defines an extremely sharp t r a n s i t i o n at a temperature of 92 K. The superconducting nature of the compound below t h i s temperature i s v e r i f i e d by the Meissner effect evident i n the s u s c e p t i b i l i t y data shown i n Figure 10. The pure samples of Ba YCu307 prepared from the nitrates were placed on a watch glass which was transferred to a desiccator containing water i n place of the desiccant. After 48 hours, the sample was removed and subjected to visual and x-ray analysis. The black product contained numerous aggregated white p a r t i c l e s . The x-ray analysis of the product i s shown i n Figure 11 and indicates the formation of BaC03» These results would indicate that the product i s attacked by moist a i r and the i n s t a b i l i t y of Ba YCu30y w i l l present problems f o r any p r a c t i c a l device application. 2

2

#

2

2


DAVISON

ET

AL.


12 Hrs at 900°C

— Oxd ie phases —

X -P

BaC0

-

3

(/)

c OJ -P c

l.l:

1—1

L l l l l ,

1 1 1 II 1 ..

1

i. II

QJ

>

•H r — ι1 • t


•r-i

24 Hrs at 900°C

— Oxd ie phases

QJ

Λ 20

• llltU. 30

l, . 1. II, U L k • 40

50

60

D i f f r a c t i o n AnglQ 2Θ (deg) Figure 6. X-ray d i f f r a c t i o n patterns of a mixture of BaC03, Y2O3 and CuO heated at 900°C showing the presence of unreacted BaC03 after 12 hrs and a multiphase product a f t e r 24 hrs.



. . . . 1 . 12 Hrs at 900°C — Oxide phases

(/) c OJ -P c

1.

à.

u. . . , 1

.In,

QJ >

•H r • t


X -P

24 Hrs at 900°C — B^YCUgO,

OJ

Λ 20

• « , . 30

40

50

60

D i f f r a c t i o n Angle 2Θ (deg) Figure 7. X-ray d i f f r a c t i o n patterns of a mixture of the n i t r a t e s heated at 900°C f o r 12 and 24 hrs showing the f i n a l product to be single-phase Ba2YCu30y.


DAVISON

+>

E T AL.

100.

2

1Q0.

0


9 9 . 8 -|

CO QJ


£

99.

6

99.

4

99.

2

cu or

99. G

0

200

400

600

Temperature

80G

1000

(°C)

Figure 8. Analysis of Ba2YCu307 by temperature-programmed reduction i n 85%Ar/15%H2. The observed weight loss corresponds to the formula Ba2YCu307 Ql(2)· e

0. 8

0. 6 H

0. 4

cu

0. 2

0. 0 100

150

Temperature

200

250

300

(K)

Figure 9. R e s i s t i v i t y of Ba2YCu3Û7 as a function of temperature.



76

CHEMISTRY

ο Ε

OF HIGH-TEMPERATURE

SUPERCONDUCTORS

-2

D

GJ Γη

-4

b

-6 X

-8 -10 -12 100

150

TemporaturQ

200

250

300

(K)

Figure 10. Magnetic s u s c e p t i b i l i t y of Ba2YCu30y as a function of temperature.


DAVISON

ET

AL.


. . I —

I 1 I 1

1

Ba YCu 0 2

3

7

as

prQpared


χ

li

il

II,

•

—

Ba YCu 0

—

BaC0

2

3

7

after

48

hr

3

QJ

(ill

, ι.,ϋ 20

30

l.Ui,., 40

D i f f r a c t i o n Angle 2Θ

50

60

(deg)

Figure 11. X-ray d i f f r a c t i o n patterns of Ba2YCu30y as prepared for the nitrates and a f t e r 48 hrs of exposure to moist a i r , showing p a r t i a l decomposition of the sample.


CHEMISTRY OFHIGH-TEMPERATURE SUPERCONDUCTORS

78 Acknowledgments

This research was p a r t i a l l y supported by the Office of Naval Research and by NSF Grant. No. DMR-820-3667. The authors also express t h e i r appreciation f o r the use of Brown University's Materials Research Laboratory which i s funded by the National Science Foundation.

Literature Cited 1. 2.


3. 4.

5. 6. 7.

Rao, C.N.R.; Ganguly, P. Current Science 1987, 56(2), 47. Bednorz, J.G.; Muller, K.A.; Takashige, M. Science 1987, 236, 73. Cava, R.J.; van Dover, R.B.; Batlogg, B.; Rietman, E.A., Phys. Rev. Lett. 1987, 58(4), 408. Wang, H.H.; Geiser, M.; Thorn, R.J.; Carlson, K.D.; Beno, M.A.; Monaghan, M.R.; Allen, T . J . ; Proksch, R.B.; Stupka, D.L.; Kwok, W.K.; Crabtree, G.W.; William, J.M. Inorg. Chem. 1987, 26, 1190. Longo, J.M.; Raccah, P.M. J . Solid State Chem. 1973, 6, 526. Jorgensen, J.D.; Schüttler, H-B.; Hinks, D.G.; Capone II, D.W.; Zhang, K.; Brodsky, M.B.; Scalapino, D.J. Phys. Rev. Letts. 1987, 58(10), 1024. Ganguly, P.; Rao, C.N.R. J . Solid State Chem. 1984, 53, 193.

RECEIVED

July 6,

1987


Chemical Problems Associated with the Preparation and

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