High-Tc superconductivity in GdBaSrCu3O7

High-Tc superconductivity in GdBaSrCu3O7

Physica C 176 (1991) 507-510 North-Holland High- Tc superconductivity in GdBaSrCu307 X.Z. Wang and D. B~iuede Angewandte Physik, Johannes-Kepler-Univ...

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Physica C 176 (1991) 507-510 North-Holland

High- Tc superconductivity in GdBaSrCu307 X.Z. Wang and D. B~iuede Angewandte Physik, Johannes-Kepler-UniversitdtLinz, A-4040 Linz, Austria Received I 1 March 1991

The new compoundGdBaSrCu307shows high-Tosuperconductivitywith zero-resistancetemperature of about 86 K. The material seemsto have a tetragonal structure.

1. Introduction It is well-known that YBa2Cu307_6 with ~ 0 is superconducting below about 92 K and characterized by double CuO2 layers and CuO chains [ 1-4 ]. The CuO2 layers are oriented along the a - b plane, and the CuO chains along the b direction. It is generally believed that the CuO2 planes are responsible for carrying the supercurrent, while the CuO chains provide a charge reservoir for these planes [ 5,6 ]. A reduction of the oxygen content results in a destruction of the CuO chains and of the superconductivity [4,7-10]. In the tetragonal phase with ~< 1, the oxygen in the basal plane is randomly distributed along a and b directions. It has been argued that in compounds of the type YBa2Cu307_~ the orthorhombic structure favors higher transition temperatures, Tc [ l l - 1 6 ] . This point requires more experimental evidence. In this paper we report on the new compound GdBaSrCu3OT_ 6. It belongs to the same family of superconductors as YBa2Cu307_a. From the powder X-ray analysis we suggest that this compound has tetragonal structure for both ~ = 0 and ~=0.6.

2. Experimental Samples of GdBaSrCu307_~ were prepared by mixing and grinding stoichiometric quantities of Gd(NOa)a'6H20 (purity >99.9%), BaCO3 ( > 99.9%), SrCO3 ( > 99.9%) and CuO ( > 99.99%).

The mixture was pressed into pellets, calcined in air at 940°C for 24 h, and furnace cooled to room temperature. The calcined pellets were grinded in an automatic mill for about one hour, and the powders were pressed into pellets of 13 mm diameter at 800 MPa. The second calcination, carried out under flowing oxygen, was performed at 940°C for 24 h. Subsequently, the samples were cooled to 600°C. Further cooling to room temperature, still under flowing oxygen, was accomplished at a rate of 60 ° C per hour. With these samples we henceforth assume ~=0. To change the oxygen concentration, these samples were subjected to two different thermal treatments: (a) Samples were heated in air to 940°C for two hours and quenched to room temperature by removing the sample from the furnace. (b) Samples (a) were reheated to 650°C for two hours and subsequently cooled to room temperature at a rate of 80 ° C/h. This treatment was performed under flowing argon (purity > 99.995%). The oxygen content of samples (a) and (b) was determined from the weight loss, immediately after thermal treatment. X-ray diffraction (XRD) of the powder was carried out at room temperature by using Cu K~ radiation. The instrument was calibrated with silicon powder. The XRD intensity was measured from 6 ° to 100 ° (20) at a speed of 0.25 °/min. The temperature dependence of the electrical resistance down to 20 K was measured by employing a standard DC four-point probe technique. The cur-

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508

A\Z. Wang, D. Bgiuerle I HTSC in GdBaSrCusO;

rents used for the m e a s u r e m e n t s were a r o u n d 4 to l 0 mA.

I

I

T

I

I

{ 200)

,1 3. R e s u l t s

¢--

and discussion

The main results o f the analysis o f the GdBa-

,

/~(200)

S r C u 3 0 7 _ 6 samples are s u m m a r i z e d in table 1. F o r

air-quenched and argon-annealed samples values of 5 equal to 0.4 and 0.6, respectively, have been obtained. The structural changes related to the different thermal treatments o f samples are most clearly seen from X R D - i n t e n s i t y changes in (006), (200) and (020) peaks at a r o u n d 2 0 = 4 7 ° . These spectra are shown in fig. 1. W i t h 6 = 0 . 4 the p o w d e r X R D spectrum shows a pattern similar to that o f o r t h o r h o m b i c YBa2Cu3OT. Samples with 5 = 0 and 6 = 0 . 6 , however, show X R D spectra that suggest tetragonal structures. F o r 5 = 0.6 the peaks shift to lower angles. This is a c o m m o n feature observed with phases of low oxygen content. The r o o m - t e m p e r a t u r e lattice p a r a m e t e r s included in table 1 were calculated by least-squares refinement using 21 observed d values. The e s t i m a t e d uncertainties in lattice parameters, written in parentheses, refer to the last digit printed. The observed d values, d e n o t e d by do, are c o m p i l e d for the case 5 = 0 in table 2. The overall consistency o f the analysis is d e m o n s t r a t e d by calculating the d values from the lattice parameters. These calculated values, d e n o t e d by de, are included in table 2. W i t h i n the accuracy o f the X-ray measurements, no impurity phase has been detected for the samples investigated. Observed and calculated X R D intensities for GdBaSrCu307 are also included in table 2. The cal-

Z

~1 45

I

t

47 20 [ d e g r e e s ]

I

49 ,

Fig. 1. Detail of powder X-ray diffraction (XRD) spectra for GdBaSrCu3Ov 6for 5=0, 0.4, and 0.6.

culated intensities assume tetragonal s y m m e t r y with the space group P 4 / m m m (D4~h). Two sets o f a t o m positions have been considered in the calculations: In the first case it was assumed that G d is placed exclusively on l d ( D 4 h ) sites, and Ba a n d Sr are rand o m l y distributed on 2h (C4v) sites. The oxygen in the basal plane is assumed to occupy one half o f the 2f (D2h) sites. In the second case it was assumed that 26% o f the G d ions are on Sr sites and equal amounts o f Sr ions on G d sites. The X R D - i n t e n s i t y calculations were p e r f o r m e d by employing the p r o g r a m C M I N T . The calculated intensities, Ic, are included in table 2 for both cases. The c o m p a r i s o n with the observed X R D intensities shows that the assumptions m a d e for the second case yield better agreement with the experimental data than those m a d e for the first case.

Table 1 Oxygen content, lattice parameters, unit cell volumes, and electrical resistivities for GdBaSrCu307 a at 300 K Sample

5

a (h)

Annealed in 02 Airquenched Annealed in Ar

0

3.835 ( 1)

0.4

3.827( 1)

0.6

3.855( 1)

b (A)

3.849( 1)

p ( m ~ c m -j )

c (A)

u (A3) 3

7c ( K )

11.554(2 )

170.0

86

1.2

11.574(4)

170.5

24

3.4

11.581 (3)

172.1

178

509

X.Z. Wang, D. Bduerle / HTSC in GdBaSrCu30z

Table 2 Observed values do and calculated values de for different planes with index numbers (h k 1). Io are observed XRD intensities while I~ and I¢2 have been calculated by assuming no site-mixture and 16% site-mixture between Gd and Sr, respectively

h

k

l

do

dc

to

0 0

0 0

1 3 ]>

11.556 3.840

11.554 3.851 3.835 3.640 2.718 2.712 2.311 2.307 2.217 1.926 1.918 1.721 1.717 1.715 1.570 1.567 1.478 1.475 1.359 1.356 1.352 1.217 1.213 1.213 1.157 1.109 1.106 1.105 1.028 1.026 1.025

101 137

1

0

0J

1 1

0 0

1 3]>

3.640 2.715

1

1

0

0J

1

0

0

4J

5 ]>

2.310

1 0 2

1 0 0

3 6 0

2.215 1.926 1.917

1

0

6}

2 2

0 1

3 0

1.714

1 2

1 1

6~ 3J

1.567

2 2 2

0 1 0

5~ 4J 6

1.475 1.360

2

2

0~

1.357

3 3 3 2

0 1 1 2

1.213

1

0

2 2 3 3

1 1 1 2

3~ 0J 3 6 10~ 8 J 9 6 3

1 1

0 0

I

8J 9

I

I

{

1.218

1.157 1.109 1.106 1.029 1.027 1.024

I

f

TEMPERATURE

200 [K]

300 ,

119 174 421 72 577 68

227

196 39 95

228

z~

173 ~ 45 L 88 25 ~ 1000 [ 508 ~ 30 [ 86 149 f 88 [ 289 { 21 23 23 ~ 148 [ 326 ~ 22 [ 16 { 92 74 18 { 29 61 64 29 42 20 17 { 34 63 72

104 { 45 89 11 1000 509 36 75 149 88 290

{2o 23 23 147 326 26 28

74 21 61 64 30 42 19 20 62 72

The temperature dependence of the electrical resistance of the GdBaSrCu3OT_6 samples is shown in fig. 2 for values of 6=0, 0.4, and 0.6. The corresponding room-temperature resistivities are included in table 1. For 6 = 0 a metallic behavior of the resistance is observed for temperatures down to about I00 K. Zero resistance is obtained at a temperature of around Tc (0) ~ 86 K. The drop in resistance from the linear behavior to 90% occurs at around 89 K, and to 10% at around 86.7 K. For values of 6=0.4

I

GdBaSrCu307_ 6

100

23 1511

z~,



Fig. 2. Resistance of ceramic GdBaSrCu307_~ as a function of temperature for 6=0, 0.4 and 0.6. For 6=0 zero resistance is observed at around To(0) ~ 86 K.

510

x.z. Wang, D. Biiuerle / HTSC in GdBaSrCu x07

t h e r e s i s t a n c e first d e c r e a s e s d o w n t o a t e m p e r a t u r e o f a b o u t 120 K, t h e n slightly i n c r e a s e s , a n d d r o p s to z e r o at T o ( 0 ) ~ 2 4 K. T h e s a m p l e s w i t h t h e l o w e s t o x y g e n c o n t e n t , 8 = 0 . 6 , s h o w s e m i c o n d u c t i n g beh a v i o r d o w n to 20 K. It m a y b e i n t e r e s t i n g to n o t e t h a t Y B a S r C u 3 0 7 h a s o r t h o r h o m b i c s t r u c t u r e [ 13 ] a n d a lower t r a n s i t i o n t e m p e r a t u r e , T o ( 0 ) ~ 80 K, t h a n G d B a S r C u 3 0 7 .

4. S u m m a r y We have synthesized a new compound, GdBaSrCu307_6. This compound has tetragonal structure in t h e o x y g e n - r i c h p h a s e w i t h 8 = 0 , a n d o r t h o r h o m bic s t r u c t u r e i n t h e o x y g e n - d e f i c i e n t p h a s e w i t h 8 = 0.4. A f u r t h e r d e c r e a s e in oxygen c o n t e n t to c~= 0.6 r e s u l t s a g a i n in t e t r a g o n a l s t r u c t u r e . T h e a n a l y s i s o f p o w d e r X R D - i n t e n s i t y m e a s u r e m e n t s suggest t h e p o s s i b i l i t y o f s i t e - m i x t u r e s b e t w e e n G d a n d Sr.

Acknowledgements W e w i s h to t h a n k Prof. H. B o l l e r for v a l u a b l e discussions and the "'Fonds zur F6rderung der wissens c h a f t l i c h e n F o r s c h u n g in O s t e r r e i c h " f o r f i n a n c i a l support.

References [ 1 ] M.K. Wu, J.R. Ashburn, C.T. Tong, P.H. Hor, R.L. Meng, L Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908.

[2] R.J. Cava, B. Batlogg, R.B. van Dover, D.W. Murphy, S. Sunshine, T. Siegrist, J.P. Remeika, E.A. Rietman, S. Zahuark and G.P. Espinosa, Phys. Rev. Len. 58 (1987) 1676. [3] Y. LePage, W.R. McKinnon, J.M. Tarascon, L.H. Greene, G.W. Hull and D.M. Huang, Phys. Rev. B 35 (1987) 7245. [4] A. Santoro, S. Miraglla, F. Beech, S.A. Sunshine, D.W. Murphy, L.F. Schneemeyer and J.W. Waszczak, Mat. Res. Bull. 22 (1987) 1007. [5] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [ 6 ] R.J. Cava, Science 247 ( 1990 ) 656. [7] P.K. Gallagher, H.M. O'Bryan, S.A. Sunshine and D.W. Murphy, Mat. Res. Bull. 22 (1987) 995. [8] J.M.D. Coey and X.Z. Wang, Sol. Star. Chem. 77 (1988) 204. [9] P. Strobel, J.J. Capponi, C. Chaillout, M. Marezio and J.k. Tholence, Nature 327 (1987) 306. [ 10] J.D. Jorgensen, B.W. Veal, W.K. Kwok, G.W. Crabtree, A. Umezawa, L.J. Nowicki and A.P. Pualikas, Phys. Rev. B 36 (1987) 5731. [ 1 I ] A.M. Umarji, P. Somasundaram and C.N.R. Rao, Physica CI53-155 (1988)497. [12] K. Kinoshita, A. Matsuda, T. lshii, N. Suzuki, H. Shibata, T. Watanabe and T. Yamada, Jpn. J. Appl. Phys. 27 ( 1988 ) L795. [ 13 ] Y. Takeda, R. Kanno, O. Yamamoto, M. Takano, Z. Hiroi, Y. Bando, M. Shimada, H. Akinaga and K. Takita, Physica C157 (1989) 358. [14IT. Wada, N. Suzuki, T. Maeda, A. Maeda, S. Uchida, K. Uchinokura and S. Tanaka, Appl. Phys. Lett. 52 (1988) 1989. [ 15 ] X.Z. Wang, M. Henry and J. givage, Solid State Commun. 64 (1987) 881. [ 16 ] R. Yoshizaki, H. Sawada, T. lwazumi, Y. Saito, Y. Abe, H. Ikeda, K. lmai and I. Nakai, Jpn. J. Appl. Phys. 26 (1987) L1703.