Crystal structure and superconductivity in REBaSrCu3Ox

PHYSICA

Physiea C 200 (1992) 12-16 North-Holland

Crystal structure and superconductivity in REBaSrCu3Ox X.Z. Wang, B. H e l l e b r a n d a n d D. B~iuerle Angewandte Physik, Johannes-Kepler Universit?it Linz, A-4040 Linz, Austria Received 5 May 1992 Revised manuscript received 3 July 1992

The preparation, crystal structure and physical properties of REBaSrCu3Ox compounds have been studied (Re ~ La, Pr, Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm or Lu). These compounds are found to be superconducting with a transition temperature within the range 54 K < Tc< 86 K, except for RE = Pr, which is semiconducting. The maximum transition temperature is observed with RE = Gd and Dy where the lattice structure changes from tetragonal to orthorhombic.

1. Introduction

At present YBa2CusO7 (Y-123) is among the most extensively studied high-To superconducting materials [ 1,2]. In the Y-123 system, the superconducting transition temperature, To, depends on the oxygen content and is insensitive to the substitution of Y by magnetic rare-earth ions, except Pr [ 3-5 ]. In the REBaSrCu3Ox system the compounds with RE = La, Gd, Y and Yb are known to belong to the Y- 123 family [ 6-8 ]. It is interesting to note that with an oxygen content of x -~ 7 the material is tetragonal or orthorhombic, depending on the particular rareearth ion. In this paper we report a systematic investigation of REBaSrCu3Ox compounds (RE-= La, Ce, Pr, Nd, Sin, Eu, Gd, Tb, Dy, Y, Ho, Er, T m and Lu) with respect to both their crystal structure and their transition temperature to the superconducting state.

2. Experimental Most of the rare-earth oxides employed as starting materials have the composition RE203 with RE = La, Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, T m and Lu. The other materials employed were Pr6OI 1, CeO2, Tb4OT, BaCO3, SrCO3 and CuO. The purity of all these materials was better than 99.9%. Samples of REBa-

S r C u 3 0 x w e r e prepared by mixing and grinding stoichiometric amounts of RE,,O,, BaCO3, SrCO3 and CuO. 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 ground for about one h, and the powders were pressed into pellets of 13 m m 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/h. The highest superconducting transition temperature for LaBaSrCu3Ox was obtained when heating the calcined pellets to 1080°C for 24 h in oxygen atmosphere. Here, the same cooling procedure was employed as with the other samples. The X-ray powder diffraction ( X R D ) analysis was carried out at room temperature by using Cu Ka radiation. The instrument was calibrated with silicon powder. The lattice parameters were calculated by a least squares refinement. The oxygen content of all samples was determined by iodometric titration analysis using a similar procedure to that reported in ref [9]. The temperature dependence of the electrical resistance down to 20 K was measured by employing a standard DC four-point probe technique. The currents used in the measurements were around 4 to 10 mA. The radii of the RE 3÷ ions (coordination number

0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.

X.Z. Wang et a L I Crystal structure and superconductivity in REBaSrCu30~

eight) considered in this paper were taken from ref. [10].

3. Results

The different REBaSrCu3Ox compounds investigated are isostructural with Y B a 2 C u 3 0 7 , except for RE=Ce, Tb and Lu, for which no Y-123 phase has been formed. The compounds with rare-earth elements that have an ionic radius smaller than that of erbium, r~E < rEr, contain the so-called green phase, RE2BaCuOs, as impurity. This has been found by the powder X-ray diffraction analysis. It should be noted, however, that single-phase LuBaSrCu3Ox has recently been synthesized in the form of thin films by means of pulsed-laser deposition [ 11 ]. The data used in the present paper for LuBa2Cu307 and LuBaSrCu30~ are taken from this reference. For all singlephase REBaSrCu30~ compounds the oxygen content, determined by iodometric titration experiments, was between x - 6 . 9 3 and 6.94. As shown in fig. 1, samples with ionic radius /'RE are orthorhombic, while those with rRE are tetragonal. At the phase boundary, i.e. with DyBaSrCu3Ox, both the tetragonal and the orthorhombic phases have been obtained, depending on <~/'Dy

....

'l' '

• , ....

1 ....

~a

1.74

0

m rj

1.68

I

I

tO

>ttt~

(020) (006)

__tb I

I

/+L~

I

I

L,8

t~6 20 [degrees]

50

,

Fig. 2. Partial X-ray powder diffraction spectra for (a) tetragonal GdBaSrCu3Ox and (b) orthorhombic DyBaSrCu3Ox.

I"1 REBa2Cu307

0 REBaSrCu307 []

o4 ,...,

11.7

~

11.6

[] []

0

I-] F']

0

(

O0

o°

o

o

11.5

Trr 1.66

I

O

0 0

1.70

I

O I

1.7~.

o I>

I

11.8

T

2115

I

(200)

! 0 +

I

T

0

1.76

o.~

%

i

i ....

REBaSrCu307

'I

the particular thermal treatment of the sample. This has been reported elsewhere [12]. The structural changes are most clearly seen from the intensity changes in (006), (020) and (200) X-ray reflections. Representative X-ray spectra indicating the phase change from tetragonal GdBaSrCu30~ to orthorhombic DyBaSrCu30~ are shown in figs. 2(a) and (b), respectively. The length of the c-axis of different REBaSrCu3Ox compounds is shown in fig. 3 as a function of ionic

>/'Dy

1.78

13

,

O.g5

•

,

.

Eu Nd )y Gd Sm Pr

~o .

.

.

I

1.00 RE "+

•

t.05 -

.

,

J

t

,

1.10

RADXUS

,

,

Lo ,

J

,

1.15

.

.

r~

,

1.20 ,

Fig. 1. Unit cell volume and the phase formation of REBaSrCu30~ compounds as a function of ionic radius of RE 3+ ions. The abbreviations denote O = orthorhombic, T = tetragonal, and 2115 = RE2BaCuOs.

Yb Er Y Gd Sm Trn Ho Dy Eu Nd Pr ........................

11.4 0.95

1.00

1.05

1.10

RE a+ _ RADIUS

La 1.15

[ ,~ ]

1.20

•

Fig. 3. Length o f c-axis for REBaSrCu3Ox (circles) and REBa2Cu307 (squares) as a function o f the radius o f the RE 3+ ions.

14

X.Z. Wang et al. / Crystal structure and superconductivity in REBaSrCusOx

radius. The figure reveals that the c-axis first decreases from R E - La to Dy, and then increases again. The cell volume shows a similar dependence (fig. 1 ). This b e h a v i o u r differs from that o f the REBaECU307 c o m p o u n d s which show a continuous increase o f the c-axis with increasing radius the RE 3+ ion. The corresponding d a t a included in the figure were taken from refs. [ 3-5,11 ]. Here, small deviations from the continuous increase o f the c-axis observed for Pr a n d Lu. The increases o f the cell v o l u m e and c-axis observed for the REBaSrCu3Oz c o m p o u n d s with rRE
100

[] ~

D DC)

O

[]

90

I

©

[]

k

oo

80

© ©

70

[7 REBo2Cu307 60

0 REBaSrCu307

0

© 50

Ho Lu ....

40

0.05

Yb Er Y Trn Dy ' .... 1.00

Srn Gd Eu ' ....

l.Ofl

Nd i ....

1.10

REa+ - RADIUS

Lo i .... 1.15

[~]

1.20

•

Fig. 4. Dependence of the superconducting transition temperature, To, for REBaSrCu3Ox ( circles ) and REBaECU307 (squares) on the radius of RE3+ ions.

ions show only very small changes in Tc [3-5,11,14].

4. D i s c u s s i o n

W h e n YBa2Cu307 is heated to above 1000°C, it decomposes into Y2BaCuO5 a n d other phases [ 15 ]. In the Y-123 structure the RE 3+ ion is c o o r d i n a t e d by 8 oxygen ions, while in RE2BaCuO5 it is coor-

Table 1 Room temperature lattice parameters [A ] and maximum transition temperature Tc(R = O) [K] for REBaSrCu3Ox.The estimated standard deviations (in parentheses) refer to the last digit printed. The ionic radii (coordination number eight) for RE 3+ ions are taken from ref. [10] RE

a

La Pr Nd Sm Eu Gd Dy Dy Y Ho Er Tm Lu

3.877(1) 3.859(1) 3.870(1) 3.851(1) 3.844(1) 3.835(1) 3.828(1) 3.816(1) 3.791(1) 3.789(1) 3.791(1) 3;821(2)

a) a and b have not been determined.

b

3.838(1) 3.845(1) 3.851(1) 3.847(1) 3.836(2)

c

RE3+

v

T~

11.728(3) 11.551(3) 11.622(3) 11.591(2) 11.579(2) 11.554(2) 1t.533(2) 11.542(4) 11.542(4) 11.546(3) 11.560(4) 11.550(5) 11.61 a)

1.160 1.126 1.109 1.079 1.066 1.053 1.027 1.027 1.019 1.015 1.004 0.994 0.977

176.3 172.05 173.2 171.9 171.1 170.0 168.0 168.0 168.2 168.5 168.6 169.3 -

57 74 80 80 86 81 83 79 79 78 70 54

Re£

[8] [12] [12]

[11]

x . z . Wang et al. / Crystal structure and superconductivity in REBaSrCu30x

d i n a t e d by only 7 oxygen ions [ 16 ]. Thus, the 8-fold a n d 7-fold oxygen c o o r d i n a t i o n s o f the RE 3+ ions c o m p e t e in these systems. The results shown in fig. 1, in particular the occurrence o f the RE2BaCuO5 phase o b s e r v e d with rRE~
5. Conclusion We have shown that the REBaSrCu3Ox comp o u n d s are superconductors with transition temperatures between 54 a n d 86 K, d e p e n d i n g on the particular rare-earth element. A n exception is PrBaSrCu3Ox, which is s e m i c o n d u c t i n g down to 20 K. These c o m p o u n d s show a structural phase transition from tetragonal to o r t h o r h o m b i c when the ra-

15

dius o f the rare-earth ion decreases. The m a x i m u m transition t e m p e r a t u r e is observed near the phase b o u n d a r y which occurs for RE = G d and Dy.

Acknowledgements We wish to thank E. Stangl a n d S. Proyer for their help in preparing LaBaSrCu3Ox samples. We also acknowledge the financial support by the "Forschungsf6rderungsfonds for die Gewerbliche Wirtschaft in t~sterreich"

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X.Z. Wang et al. / Crystal structure and superconductivity in REBaSrCu~O~

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