The nonstoichiometry of the high-Tc superconductor Y2Ba4Cu7O15±x (14 K⩽Tc⩽68 K)

Physica C 161 (1989) 618-625 North-Holland

T H E N O N S T O I C H I O M E T R Y O F T H E HIGH-To S U P E R C O N D U C T O R Y2Ba4Cu7Ols+x

(14K~< Tc~<68 K) J. K A R P I N S K I , S. RUSIECKI, B. B U C H E R , E. KALDIS a n d E. JILEK

Laboratorium J~r Festkrrperphysik ETH-Hrnggerberg, CH-8093 Zfirich, Switzerland Received 28 September 1989 Revised manuscript received 9 October 1989

In this paper we present a study of the thermodynamic, structural and physical properties ofYEBa4CUTOls+x,the new high-To superconductorwe have discovered recently.The nonstoichiometryof YEBa4Cu7Ols+xhas been varied between -0.70
1. The high-To superconductor Y2Ba4Cu7Ols+x In the course of our first investigations of the phase diagram of the pseudobinary system YBaECU307_xO at high OE-pressures (P~< 3 0 0 0 b a r ) and temperatures ( T~< 1200 ° C) [ l ], we have observed the appearance of a mixture of several new phases. Growth of single crystals of these phases was possible using a high-pressure flux method [2 ]. Electron diffraction and EDAX studies (2) showed the existence of a new phase with long c-axis, more Cu than in the "123" phase and rather low Tc (approx. 4 0 K ) . A single-crystal X-ray structure d e t e r m i n a t i o n revealed a most interesting hybrid of the "123" and "124" formed by alternating unit blocks of these two structures (fig. 1 ) a n d havin.g A m m m s y m m e t r y [3 ]. Due to the A-centered unit cell, the double chains of the "124" structure blocks are shifted by b / 2 along the b-axis and, therefore, four blocks are necessary to form the unit cell. Thus, the c-axis is very long c - 50.3 A, whereas am 3.85 A and b ~ 3.87 A are very similar to those o f the "123" a n d " 124" phases. Based on this structure (fig. 1 ) the chemical formula was determined to be Y2Ba4Cu7O14+x [3]. After the present study of the nonstoichiometry, the formula 0921-4534/89/$03.50 © Elsevier SciencePublishers B.V. ( North-Holland )

Y2Ba4Cu7Ols_+x seems more appropriate. For reasons of direct and easy comparison with the other Y - B a - C u - O phases, we use also the half cell formula YBaECU3.50 7+x or "123.5". An i m p o r t a n t c o n t r i b u t i o n of the structure refinem e n t was the d e t e r m i n a t i o n of very low occupancies of the oxygen sites in the chains Os and 09 ( 10% a n d 20% respectively), indicating a possible reason for the rather low To==40 K of the investigated crystal [ 3 ]. Variation of the P a n d T p a r a m e t e r s allowed the synthesis of a mixture of nonstoichiometric samples showing susceptibility steps at 52, 72 a n d 80 K [4]. Later, it was possible to change Tc as a function of the oxygen content between 40 a n d 65 K [5 ] and recently we have published a variation of Tc between 14 and 68 K as a function of the orthorhombicity [ 6 ]. A n i m p o r t a n t property of this c o m p o u n d is that single crystals are not t w i n n e d [ 7 ], because contrary to "123" there is no phase transition as a function of nonstoichiometry.

2. Thermodynamic stability The investigation of the Cu-rich part of the phase

J. Karpinski et al. / The nonstoichiometry o f Y2Ba~Cu 7015 +x

619

fig. 2. The stability range of the "123.5" phase which we could determine up to now extends between 8 ~ P < 2 9 0 0 b a r O2 and 860°C~ T ~ 1100°C. The formation of the "123.5" phase at elevated pressures and temperatures can be easily monitored T (°c) 1200 1100

1000

SO0

800

700

4000

(bar)

1000

I--

.<

100

bi

10

Fig. 1. Structure of Y2Ba4Cu~O=5_~ after [3]. The structure is formed from alternating blocks of the "123" and "124" structures. Due to a b/2 shift of the double chains lib-axis, a very long four block unit cell results with c ~ 50.3 ,~ and Ammm symmetry. The depicted half cell ( c / 2 ~ 2 5 A ) has the formula YBa2Cu3.5OT.5+r For easy comparison with the other phases the abbreviation "123.5" is used. 7

diagram

of

pseudobinary system for n = 1;2 gave the thermodynamic stability range of the three Y - B a - C u - O phases ( n = 0 " 1 2 3 + C u O " ; n = l "123.5"; n = 2 "124") in the pressure range 1-3000 bar O2 and 300°C~< T~< 1200°C [6]. The results are shown in

7.5

8

8.5

9

9.5

10

l o 4 / T (K)

10.5

the

Y2Ba4Cu6+nOIg+,_x-O

Fig. 2. P - T phase diagram (log P - 1/ T) of the CuO-rich part of the pseudobinary system Y2Ba4Cu6+,O14+,_~-O ( n = 1;2). L denotes the thermodynamic stability range of the melt, broken lines arc proposed phase boundaries. Starting material YBa2Cu307 + CuO with stoichiometry either YBa2Cu4Oa (n = 2 ) or YBa2Cu3.sOv.s ( n = 1 ).

620

J. K a r p i n s k i

e t al. / T h e n o n s t o i c h i o m e t r y

in our two-chamber high-purity autoclave [ 8 ]. Fig. 3 shows a P - T r u n during the synthesis of the "123.5" phase at the rather low pressure of 17 bar O2, starting from a mixture of " 1 2 Y ' + l / 2 C u O . At T ~ 8 0 0 ° C and 14 bar O2 the "124" is formed (byproduct should be "123", but it could not be detected up to now), at T~ 900 ° C and 15 bar the decomposition o f " 124" to "123.5" starts and at T - 1000°C and 18 bar the pure "123.5" phase appears. During cooling, usually a small part of the sample converts black to "124". Therefore, a fast cooling is required. Quenching is, however, questionable if one wishes to investigate as perfect as possible samples (compare fig. 8a). The run shown in fig. 3 leads to an oxygen content of 14.91 per formula unit, as determined with the highaccuracy volumetric oxygen analysis (Ax= _+0.002) we have described in the past [9]. The transition temperature of this sample is also very narrow. To vary the oxygen content of these samples we have annealed them in the thermobalance. Annealing temperatures as a function of oxygen content are shown in fig. 4. The atmosphere was Ar with Po~ = 4 × 10- 6 bar, the annealing time 10-15 h, the

500

of Y2Ba4CuTOzs+x

ANNEALING TEMPERATURE PC) ~

[

480 . . . .

':Z~

~i

!

& 460 ........

~ ............ ~ . . . . . . . ! . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

? 4 4 0 . . . . . . . ; .......... ~---

~

...... :' ........... ~. . . . . . ~. . . . . . . . . . . . . . . . . .

i 400

.........~-L - - - - i÷ . . . . . . . . . .

a....

420 .............. i . . . . . . . . . . ; ~

: ..........................

i

i

i ;-

i ii [ i

I 0.55

i 0.6

,

/x 360 0.3

, 0.35

i 0.4

1 0.45

I 0.5

I 0.65

0.7

X

Fig. 4. C h a n g e o f t h e o x y g e n c o n t e n t o f Y 2 B a 4 C u 7 O l s _ x s y n t h e sized in t h e e x p e r i m e n t o f fig. 3, b y c o n t r o l l e d a n n e a l i n g in A r a t m o s p h e r e ( w i t h Po2 = 4 × 1 0 - 6 b a r ) at v a r i o u s t e m p e r a t u r e s . S l o w h e a t i n g a n d c o o l i n g r a t e s (0.1 ° C / h ) . T h e c o m p o s i t i o n o f the s t a r t i n g m a t e r i a l w a s 14.91 o x y g e n a t o m s / f o r m u l a unit.

18

_

ill

(n gg a.

UJ

i

17

16

15

124

123.51

13

12

I

P

TEMPERATURE ('~:)

Fig. 3. P - T e x p e r i m e n t in t h e t w o - c h a m b e r a u t o c l a v e s h o w i n g the f o r m a t i o n o f " 1 2 4 " a n d " 1 2 3 . 5 " p h a s e s . T h e s t a r l i n g m a t e rial is a m i x t u r e o f " 1 2 3 " + 1 / 2 C U O with stoichiometry Y2Ba4CuTOis_x. " 1 2 3 " is expected as a b y p r o d u c t in " 1 2 4 " fields h u t it c o u l d n o t h e d e t e c t e d u p to n o w w i t h E D A X , X - r a y s o r susceptibility m e a s u r e m e n t s .

oxygen content of the starting material 14.91. Using this method, the oxygen nonstoichiometry of the "123.5" was varied between 14.28 and 14.91 ( - 0.09 ~>x>~ - 0 . 7 2 ) . In view of the two different kinds of chains (double chains in the "124" blocks and single chains in the " I 2 Y ' blocks), it is interesting to strip the oxygen out of the "123.5" structure in a controlled way. Such information can be useful for understanding the role of chains in the mechanism of superconductivity. For this purpose we use two sets of measurements: a) Thermogravimetric data, which give the stoichiometry changes as a function of temperature (slow heating rate). This we can do with high absolute accuracy, because we know exactly the starting oxygen content of our samples due to the volumetric analysis [9 ]. b) Lattice parameter measurements (Guinier method, standardized with NBS Silicon Standard) made on samples analyzed after cooling. During cooling, however, the samples absorb oxygen and, therefore, the stoichiometries received from this

621

J. Karpinski et al. / The nonstoichiometry o f Y2Ba+CuzO is +_x

determination showed, the occupancy of 0 8 site was 10% and that of 0 9 site 20%, i.e. only 30% of the oxygen sites in the single chains were occupied. This indicates that in the region B oxygen is removed from the chains of the "123" blocks. Removing of oxygen from the single chains ( Hb-axis) of the "123" blocks should decrease the orthorhombicity. Fig. 6 shows that this is actually the case. With decreasing oxygen content in the range 14.90-14.30 (0.1 ~
method are systematically higher than those obtained with method (a). As various samples were cooled with different rates, this systematic shift of the oxygen contents is different for various groups of samples. However, what is important is that both measurements show a sequence of rather well defined regimes as a function of oxygen content, so that it is reasonable to assume that a correspondence of these stages is existing in the two sets of measurements. Of course a detailed neutron diffraction study is necessary to finally clarify this problem. Fig. 5 shows the results obtained in an atmosphere of argon with low oxygen partial pressure (Po2-4 × 10-6bar) and slow heating rate (0.1 °C/min between 250-660°C). Four different slopes appear during decomposition, A: 250-340°C, B: 340550°C, C: 550-600°C, and D: >600°C. The structure refinement [3 ] was performed with a single crystal containing approx. 14.30 oxygen atoms in the formula unit. This was, therefore, material corresponding to the region B of fig. 5. As the structure

100.2 100.0

14.91

CHEMICAl ANAlY

99.8

I

99.5 99.4

A

I-I-

[

FU.____~YSTRI pPED ,P+

I--

96.2 99.0

LU

I

x- Y A YS,S sI+,E cRYs , sl,+E

-

98.8 _.EJ.lJJ-YSTRIPPI~ DOUBLE CHAINS

[-

[

98.5

i

98.4

A

B

C +

98.2 98.0 i

I

100.0 150.0 annealing In I bar argon

I

200.0

/ 250.0

I

II

I

300.0

350.0

400.0

I

I

450.0 500.0 TEMPERATUR

I1

t

I

550.0 600.0 E (°C)

650.0

Fig. 5. Controlled decomposition of Y2Ba4CuTO~s_x(thermobalance run) by slow heating (0.1°C/h) in Ar atmosphere (Po2=4× 10-6bar). We attribute the ranges A and B to loss of oxygenfrom the single chains ("123" blocks), and the range C to stripping of oxygenboth from double chains ("124" blocks) and single chains. In range D, decompositionof the material to "123"+CUO takes place.

J. Karpinski et al. / The nonstoichiometry of Y2Ba,CuzOt5+x

622 lO00x(b-a)/(b+a)

oxygen from single chains ("123" blocks) and double chains (" 124" blocks) (compare section 3). The X-ray powder pattern of the material remains in A, B and C the same (with the exception of the decreasing orthorhombicity). Table I gives the indexing of the powder pattern. Up to now we have found that the nonstoichiometric range of

4.5

\ z .

.

.

.

YBazCu4Os+x

.

A -~-~-

B

~

C

\ 3.5

\

L_.

~

is - 0 . 1 4 < x <

+0.10(4).

If we as-

the same nonstoichiometric range for the individual blocks of the Y 2 B a 4 C u 7 O l s _ + x structure, then the change of weight should be A x ~ 0.24. This compares favourably with Ax=0.28 found experimentally (see section 3). We can conclude, therefore, that sume

i

3

2.6

0

0.1

I

I

I

I

J

I

0.2

0.3

0.4

0.5

0.6

0.7

0.8

X

Fig. 6. Orthorhombicity as a function of the oxygen content. With decreasing oxygen content the orthorhombicity decreases in three consecutive stages, which we attribute to structural changes similar to those of the regions A, B, and C of fig. 5. For discussion see the text.

Table I Indexed X-ray powder diagram of Y2Ba4CUvOls_x for x = 0 . 0 9 (Cu K1 ). 2theta

Indices

Intensity

2theta

Indices

Intensity

6.966 10.626 14.076 21.156 23.006 23.186 23.456 24.246 24.616 24.736 25.516 26.096 27.196 27.996 29.216 31.526 31.886 32.596 32.836 33.236 34.046 35.146 35.556 36.816 38.126 39,216

00 4 00 6 00 8 0 0 12 0 1 1 10 0 10 2 10 4 0 1 5 0 0 14 10 6 0 1 7 10 8 0 1 9 1 0 10 1 0 12 0 0 18 0 1 13 1 1 1 1 1 3 11 5 11 7 0 0 20 1 0 16 0 1 17 0 0 22

medium v. weak weak weak medium weak weak weak weak weak medium v. weak strong weak weak strong v. weak v. strong v. strong medium v. strong medium v. weak medium medium medium

40.356 45.046 46.776 46.776 47.276 50.506 52.756 53.166 53.996 53.996 54.896 56.586 57.466 58.596 59.076 59.846 60.946 62.256 62.616 63.016 63.246 67.496 68.266 68.726 69.456

1 1 13 1 1 17 0 2 0 0 0 26 20 0 1 1 21 12 0 2 1 1 0 2 14 2 0 14 12 8 1 1 25 1 2 12 2 1 13 1 2 14 1 1 27 1 2 16 2 1 17 0 2 22 2 0 22 1 1 29 1 0 34 0 2 26 22 0 0 1 35

strong v. weak v. strong v. strong v. strong medium v. weak v. weak v. weak v. weak weak medium medium v. strong v. strong v. strong medium weak medium medium medium weak strong v. strong v. weak

Space group A m m m . a = 3.842/~; b = 3 . 8 8 1 ~.; c = 50.50/~.

J. Karpinski et al. / The nonstoichiometry of Y2Ba4Cu7Ols+_x 70 Tc (K)

60 50

[]

[]

*

40

"

,

[]

_i ...........

D -

[]

=

30 20 10

:

!

i

i

I

f

I

L

I

I

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

623

creases to T c ( 1 / 2 ) = 2 6 K but also the Tc of the accompanying "124" phase decreases from 80 to 65 K. Heating above 600°C leads to the gradual decomposition of the material, the products being tetragonal "123" (due to the low Po2) and CuP. Obviously, in the low-temperature region A (fig. 5 ) the most unstably bound oxygen escapes. We consider as a possible candidate defects in the single chains which become mobile already at 250°C. The occupation of 0 9 sites in the structure [3] can be considered to result from the existence of single chains oriented vertically to the double chains (and the b-axis) [ 10]; as this takes place inside the unit cell the crystals are not twinned. It seems reasonable, therefore, to assume that these disordered chains decompose at the lowest temperature. Neutron diffraction work is on the way now [ 10] in order to find out if the above oxygen stripping model is correct.

X Fig. 7. Tc (from AC susceptibility measurements) as a function of the oxygen nonstoichiometry. Onset; Tc( 1/2 ). Note the large

3. Structural and physical characterization

range of To, 14-68 K, that can be achieved in this compound.

probably in the region C double chains and the remaining single chains are loosing oxygen. This is supported by the fact that for samples treated at 600°C for 1 h not only the T¢ of the "123.5" de-

As we have mentioned in section 1 a most a t tractive aspect of Y 2 B a 4 C u 7 O l s + x is the possibility of a wide variation of the Tc by means of the n o n stoichiometry [ 4 6 ] Fig 7 shows that this d e p e n dence is very strong the up to now existing experi

i

Y2Ba4CuzOls'°2

010.~

r,

i

20

40

60 T(K)

80

100

o

10

4o

T(K)

,oo

Fig. 8. AC susceptibility ( H = 10 Oe, 86 Hz ) vs. temperature: (a) Sample synthesized ( P = 10 bar Oz, T = 1000 oC ), then "quenched" to 400 °C in the autoclave and then annealed 2 h at 400 ° C. This treatment eliminates the "124" phase but increases the transition temperature width ( ~ 10 K). (b) The sample with oxygen content 14.910 was synthesized in the autoclave ( P = 17 bar O2, T = 1000°C) and cooled with 2°C/min. Note the great difference in the transition temperature width ( ---2 K) as compared to fig. 8a. The other samples were of the same batch but subsequently annealed in argon atmosphere (conditions in fig. 4). All these samples show small amounts of "124" phase in the susceptibility, but not in the X-ray investigation.

J. Karpinski et al. I The nonstoichiometry o f Y2Ba~Cu~)j5 ±x

624 a(A)

b(A) 3.88

3.851

-

i............

~

.........

3.879 3.849---

,. . . . . . . ~. . . . . . . . . . .

=

................. 3.87d

3.847

i

_

:

_2 .........

~

.

.

.

.

.

.

.

.

.

3.877

i 3.845 ....

3.843--

~ - ........i . . . . .

/

',-

~

........

3.876

; ............. i - - - ~ v _ _ .

3.841

.....~........ ....

3.875

i

0

0.1

0.2

..........

i

0.3

0.4

0.5

0.6

. . . . . . . . . . . . . .

3.874 0.7

0.8

, 0.1

0

i. . . . .

i 0.2

:

~

...........

f 0.3

i 0.4

T ......................

p 0.5

~ 0.6

X

50.75

i 0.7

0.8

X

ctA)

CELL V O L U M E (A 3) !

757

i

756

50.7

50.65

755

50.6

754

50.65

753 ..... J

752

50.5

0

0.1

0.2

0.3

0.4

0.5

0.6 X

0.7

0.8

0

J 0.1

--

i 0.2

J 0.3

i 0.4

J 0.5

~ 0.6

i 0.7

0.8

X

Fig. 9. Lattice p a r a m e t e r s o f Y2Ba4Cu7Ols_x as a function o f n o n s t o i c h i o m e t r y ( a ) a increases with decreasing oxygen content, ( b ) b is decreasing with decreasing oxygen content, (c) c is increasing w i t h oxygen content a n d ( d ) the u n i t cell v o l u m e increases w i t h increasing oxygen content. In all these figures, three regions with different slopes a p p e a r at the same oxygen concentrations.

J. Karpinski et al. / The nonstoichiometry of Y2Ba~CuzOls+_x

ments showing the range 1 4 K ~ T c ~ 6 8 K . Fig. 8 shows a typical susceptibility curve o f a sample synthesized at P = 10 b a r O2 and T = 1000 ° C. Quenching in the autoclave gave a pure phase but with a wide transition t e m p e r a t u r e range ( 10 K ) . N o r m a l cooling in the autoclave ( 2 ° C / m i n ) results in a step at T = 8 0 K (fig. 8b) due to a small a m o u n t o f " 1 2 4 " phase f o r m e d during crossing the " 1 2 4 " stability range (fig. 2). The width o f the transition is very narrow (2 K ) . The existing data indicate that the T¢ ( 1 / 2 ) changes rather linearly with the oxygen content in the range 0.1 ~
625

[ 12 ] the strong pressure d e p e n d e n c e o f Tc f o u n d in " 1 2 4 " [13]. The "123.5" phase shows a strong Tc pressure d e p e n d e n c e as well [ 14], and lately it has been shown [ 15 ] that also this p h e n o m e n o n is followed by a strong contraction o f the C u 2 - O l distance.

References [ 1] J. Karpinski and E. Kaldis, Nature 331 (1988) 242. [2] J. Karpinski, C. Beeli, E. Kaldis, A. Wisard and E. Jilek, Physica C 153-155 (1988) 830. [3] P. Bordet, C. ChaiUout, J. Chenavas, J.L. Hodeau, M. Marezio, J. Karpinski and E. Kaldis, Nature 334 (1988) 596. [4] J. Karpinski, E. Kaldis, S. Rusiecki, E. Jilek, P. Fischer, P. Bordet, C. ChaiUout, J. Chenavas, J.L. Hodeau and M. Marezio, J. Less Comm. Metals 150 (1989) 129. [ 5 ] J. Karpinski, E. Kaldis and E. Jilck, Abstract 4A-5, Abstract Book M2S-ATSC,Stanford 23-28 ( 1989 ). Submission date Feb. 24, 1989. [ 6 ] J. Karpinski, S. Rusiecki, E. Kaldis, B. Buchcr and E. Jilek, Physica C 160 (1989) 449. [7] C. Chaillout, P. Bordet, J. Chenavas, J.L. Hodeau, M. Marezio, J. Karpinski, E. Kaldis and S. Rusiecki, Solid State Commun. 70 (1989) 275. [8 ] J. Karpinski and E. Kaldis, J. Crystal Growth 79 (1986 ) 47. [ 9 ] K. Conder, S. Rusiecki and E. Kaldis, Mater. Res. Bull. 24 (1989) 581. [ l0 ] A. Hewat, P. Fischer, E. Kaldis, J. Karpinski, S. Rusiecki et al., to be published. [ll]R.J. Cava, B. Batlogg, S.A. Sunshinc, T. Siegrist et al., Physica C 153 (1988) 560. [ 12] E. Kaldis, P. Fischer, A. Hewat, E.A. Hewat, J. Karpinski and S. Rusiecki, Physica C 159 ( 1989 ) 668. [ 13 ] B. Bucher, J. Karpinski, E. Kaldis and P. Wachter, Physica C 157 (1989) 478. [ 14] B. Bucher, J. Karpinski, S. Rusiecki, E. Kaldis, P. Wachter et al., Proc. AIRAPT 89 Paderborn. [ 15 ] P. Fischcr, A. Hcwat, E. Kaldis, J. Karpinski and S. Rusiecki, Proc. Int. Conf. SUPRA-Conductivitd, Paris 23-24 November 1989.