Enhancement of orthorhombicity and superconductivity in argon preheated EuSrBaCu3O6+z

PHYSICA Physica C 225 (1994) 105-110

El ~SEVIER

Enhancement of orthorhombicity and superconductivity in argon preheated EuSrBaCu306+z R.

Suryanarayanan *, A. Nafidi

~

Laboratoire de Physique des Solides de Bellevue, CNRS, 92195 Meudon, France

Received 7 January 1994; revised manuscript received 10 March 1994

Abstract

We have studied the structural and superconducting properties of the superconductor EuSrBaCu306 +~. This compound when annealed in oxygen at 450 °C showed an orthorhombic structure in contradiction to the earlier published data. When the same sample was heated in argon followed by oxygen annealing, the orthorhombicity ( b - a ) / ( b + a) increased from 2.86 X 10-3 to 6.96 X 10 -3 and Tc from 80 to 87 K. Further, there was an enhancement in the shielding and the intergranular critical current. A combination of several factors such as changes in the Cu 1-apical oxygen distance, chain oxygen ordering and hole density may qualitatively account for the observed data.

1. Introduction

The structural and superconducting properties o f YBa2Cu306+~ have been well documented [ 1 ]. There are at least four distinct crystallographic sites (excluding that o f oxygen) - Y, Ba, Cu plane a n d Cu chain - which can be substituted with different elements. The effect o f such substitutions on the structural and superconducting properties has been extensively investigated in order to u n d e r s t a n d the m e c h a n i s m o f the occurrence o f superconductivity at high temperatures [ 1 ]. Concentrating on the Y site, it has been established that single-phase LnBa2fu306+z can be p r e p a r e d with the superconducting transition t e m p e r a t u r e T¢ close to 92 K except in the case o f L n = C e , P r and Tb. All these c o m p o u n d s show an o r t h o r h o m b i c a l l y - d i s t o r t e d oxygen-deficient tripled-perovskite structure a n d both the ortho* Corresponding author. i Permanent address: D6partement de Physique, Facult6 des Sciences, Universit6 Ibnou-Zohr, 8000 Agadir, Morocco.

rhombic distorsion and Tc d e p e n d sensitively on the oxygen content (6 + z ) . We recall that in the case o f YBa2_kSrkCU306+z, it was shown that Tc decreased from 92 to 83 K when k increased from 0 to 1 and that for k > 1.2, the perovskite structure was not stable [ 2 ]. Though the o r t h o r h o m b i c distorsion was always observed [ 2 - 4 ] for fully oxygenated samples ( z > 0 . 8 ) for k = l , the o r t h o r h o m b i c to tetragonal transition temperature 7", decreased as k increased [ 5 ] and further Tt d e p e n d e d on the oxygen pressure [ 6 ]. It is interesting to check if an isovalent substitution o f Ba by Sr would modify some o f the results discussed above when Y is replaced by other rare earths. W i t h these in m i n d several authors [ 7 - 1 0 ] have studied the structural and superconducting properties o f Y~_xLaxSrBaCu306+z. However, we came across a few contradicting results. F o r example, in the case o f EuSrBaCu306+z, Wang et al. [ 8 ] reported a tetragonal structure with a = 3.844/k and c = 11.579 /~ a n d To=80 K, whereas Badri et al. [9] observed a = 3.845 A and c = 11.59 A but with a lower T¢=60 K. Whereas Wang et al. [8 ] cooled the samples in

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R. Suryanarayanan, A. Nafidi / Physica C 225 (1994) 105-110

flowing oxygen from 900°C to 600°C and then to room temperature at a rate of 6 0 ° C / h , Badri et al. [9] annealed the samples in oxygen at 450°C for more than 48 h. Further, no magnetic measurements were reported. In order to resolve this controversy, we report here on the preparation, X-ray diffraction (XRD), and AC susceptibility of EuSrBaCu306 +z. In addition we examine the effect of argon heat treatment followed by oxygen annealing that has considerable influence on the structural and superconducting properties.

2. Experimental techniques

The polycrystalline samples have been prepared by solid state sintering of the respective oxides or carbonates. The chemicals were of 99.999% purity except in the case of Eu203 which was 99.9% pure. Eu203, SrCO3, BaCOa, and CuO were thoroughly mixed in required proportions and calcined at 950 °C in air for a period of 12-18 h. The resulting product was ground, mixed, pelletized and heated in air at 980°C for a period of 16-24 h. This was repeated twice. The pellets were annealed in oxygen at 450 °C for a period of 60-72 h and furnace cooled. This sample is denoted as [O ]. XRD data of the samples were collected with Philips diffractometer fitted with a secondary beam graphite monochromator and using Cu K a (40 kV/20 mA) radiation. The angle 20 was varied from 20 ° to 120 ° in steps of 0.025 ° and the counting time per step was 10 sec. Superconducting transitions were also checked by measuring both the real (Z') and the imaginary (Z") parts of the AC susceptibility as a function of temperature in a field of 0.11 Oe and at a frequency of 1500 Hz. In addition, X' and X" were measured in a static field (0 < H < 150 Oe) superimposed on the AC field of 0.11 Oe. The same sample [ O ] was then heated in argon at 850 °C for about 12 h , cooled to 20°C and oxygen was allowed to flow instead of argon and the sample was annealed at 450°C for about 72 h. This sample is denoted as [AO]. XRD and susceptibility measurements were done on a part of this sample.

3. Results 3.1. X-ray diffraction

The XRD patterns of the samples [O] and [AO] are shown respectively in Figs. 1 and 2. The calculated and observed d values for these samples with the corresponding Miller indices are collected in Table 1. Both the samples showed the characteristic orthorhombic splitting of the peaks (006), (002) and (020). The splitting, however, increased in the case of the sample [AO]. Similarly well resolved (116) and (213 ) peaks were observed for this sample. Some impurity peaks (marked by crosses in Figs. 1 and 2) were seen and some of them disappeared after the argon treatment. The lattice parameters were for the sample [O] a=3.826 A; b=3.848 A; c=11.577 A;

O

20

30

40

50 60 70 20 Fig. 1. X-ray (Cu Kct) diffraction pattern of the sample EuSrBaCu306+zannealed in oxygendenoted as [O] in the text. (x= impurity peaks.)

AO

20

30

40

50 60 70 20 Fig. 2. X-ray (Cu Ka) diffraction pattern of the sample EuSrBaCu306+z heated in argon followed by oxygen annealing denoted as [AO] in the text. (x = impurity peaks.)

R, Suryanarayanan, A. Nafidi /Physica C 225 (1994) 105-110

107

Table 1 Miller indices, calculated and observed interplanar (d) spacings in (A) and relative intensities for the samples EuSrBaCu306+z [O ] and [AO]. [O] =oxygen annealed at 450°C for 72 h. [AO] =preheated in argon, followed by oxygen annealing at 450°C for 72 h EuSrBaCu306+~ [O]

EuSrBaCu306+= [AO]

h kl

d (obs)

d (cal)

I/Io

0 10

3.852

3.850

9.7

10 1 0 13

3.640 2.719

3.627 2.721

3.5 100

0 14 113 10 5 006 200 1 16

2.314 2,221 1.981 1.930 1.913 1.725

2.312 2.218 1.979 1.928 1.910 1.724

9.4 15.5 3.8 17.7 7.7 4.4

12 2 1 16

1.656 1.573

1.648 1.572

3.2 27.8

025

1.479

1.478

4.3

026 206

1.362 1.358

1.362 1.357

7.1 9.6

h kl

d (obs)

D (cal)

I/Io

0 10 10 0 10 1 0 13

3.857 3.804 3.613 2.726 2.711 2,316 2,219 1,979 1,931 1,902 1,725 1,707 1.654 1.573 1.561 1.482 1.470 1.366 1.356

3.859 3.806 3.616 2.730 2.711 2.317 2.218 1.980 1.932 1.903 1.726 1.707 1.650 1.573 1.5 61 1.483 1.470 1.365 1.356

13.6 5.6 4.4 61.5 100 11.4 18.4 4.4 29.3 10.8 5.4 4 4.4 36 13.2 4.5 4 7 11.3

10 3

a n d for the s a m p l e [ A O ] a = 3 . 8 0 5 A; b = 3 . 8 5 8 A; c = 11.592 A. T h e o r t h o r h o m b i c i t y d e f i n e d by the ratio (b-a) / (b+ a) i n c r e a s e d r e m a r k a b l y from 2.8 6 X 1 0 - 3 for the sample [ O ] to 6.9 6 X 1 0 - 3 for the sample [AO ]. T h e latter value s h o u l d be c o m p a r e d with 5.87 X 10 -3 r e p o r t e d [ 9 ] for the c o n v e n t i o n a l l y o x i d i z e d s a m p l e Y S r B a C u 3 0 6 +z. All these values are m u c h smaller c o m p a r e d to 10 X 1 0 - 3 r e p o r t e d [ 1 1 ] for the c o n v e n t i o n a l l y oxidized sample YBa2Cu306+z.

0 14 1 13 10 5 006 200 0 16 2 10 12 2 1 16 2 13 025 2 14 026 206

~ /

AO

O

-1

70

I

I

80

I

I

90

T (K) 3.2. Real part of the A C susceptibility and the

shielding effect. T h e real part o f the AC susceptibility o f the samples [ O ] a n d [ A O ] m e a s u r e d in a n AC field o f 0 . 1 1 Oe is s h o w n in Fig.3. T h e d i a m a g n e t i c onset Tc for the s a m p l e [ O ] was f o u n d to occur at 80 K in agreem e n t with that r e p o r t e d b y W a n g et al. [ 8 ] f r o m the resistivity d a t a b u t m u c h higher c o m p a r e d to 60 K (also f r o m the resistivity d a t a ) r e p o r t e d by Badri et al. [ 9 ]. It is i n t e r e s t i n g to note, however, that the Tc o f the s a m p l e [AO ] i n c r e a s e d r e m a r k a b l y by 7 to 87 K. T h e shielding effect, which is n o t h i n g b u t the real

Fig. 3. Real part of the AC susceptibility of the sample EuSrBaCu306+z annealed in oxygen [O] and heated in argon followed by oxygen annealing [AO]. part o f the susceptibility was also f o u n d to increase after the argon t r e a t m e n t . This was m e a s u r e d at three different t e m p e r a t u r e s 50, 60 a n d 70 K in the presence of an externally applied D C field, H (Fig. 4 ). T h e shielding effect S was set arbitrarily equal to 1 for the s a m p l e [ A O ] at 50 K a n d for H = 0 . At all t e m p e r a tures a n d at all fields, the sample [ A O ] showed a higher value o f S. Further, at T = 70 K, S decreased n e a r l y by 70% w h e n H i n c r e a s e d from 0 to 125 Oe in

R. Suryanarayanan, .4. Nafidi /Physica ( 225 (I~94) l~J5 I i0

108

. . . . . . .

1.00

3"

•

D

•

•

~

+

D

[]

[]

4 A

co

+

<

.........

•

+

•

0,60

O

&

OL

A

0,20

1

0

I

50

,, . . . . . . . . 50 60

i 70

80

: 5(

I O0

H (Oe) Fig. 4. Shielding effect (in arbitraD' units) of the samples [()] and [AO] as a function of externally applied DC field at different temperatures. Sample [O ]: ( + ) 50 K, (4) 60 K, I ,~ t v0 K: sample [AO]: ({Z) 5 0 K , ( I ) 6 0 K , ( A I 7 0 K

Fig. 6. l m a g i n a o ' part o f the -\£ s u s c e p t i b i l i b oi the s a m p l e i \~ as a f u n c t i o n o f t e m p c r a t u r c at t o u r d i f f e r e n t D(" ) ] e l d s t w i d i n c r e a s i n g f r o m right ln left (I 2 = s % . 7 . ! 2 6 . 5 O c .

a

1

•

i,i}

II

[

"1-

2J(l ~ •

ii

O

I

60

I

I

70

T

L

I

80

90

e

:~

t

(K)

Fig. 5. Imaginary pan of the AC susceptibility of the sample [{)] as a function of temperature at four different DC fields. Field increasing from right to left 0, 27.5, 56.7, 90.80e. the case of the s a m p l e [ O ] whereas a decrease o f o n b 10% was n o t e d in the case o f the s a m p l e [AO ].

3, 3. Imaginary part of the AC susceptibihty and the irreversibility line T h e i m a g i n a r y part o f the AC susceptibility o f the samples [ O ] a n d [ A O ] as a f u n c t i o n of t e m p e r a t u r e at selected a p p l i e d fields is shown in Figs. 5 a n d 6 respectively. T h e width o f the t r a n s i t i o n was smaller in the case o f the s a m p l e 1 A O ] at all fields a n d the peak Tp shifted less with the a p p l i e d field c o m p a r e d to that in the case o f the s a m p l e [O ]. In fact, w h e n t t is p l o t t e d as a f u n c t i o n of t = Tp/T~ in Fig. 7 ( a ) , an e n h a n c e m e n t o f the irreversibility line was o b s e r v e d due to argon treatment. The data can be analyzed with

ii0

.

.

.

.

i

t:ig. ? I;| ) g"= I p I as a | u n c t i o n o l t t fi)r the t w o s a m p l e s {{ ) i a n d [~X() ]. ( b ) log t l as a f u n c t i o n o f log ( I - : ) fol t h e tv,.~ s a m pies [ O ] a n d [ A O ] .

the help of the following relation t t = k q l .-ft Straight line plots ( Fig, 7 ( b ) ) were o b t a i n e d when log t t was plotted against log( l - t ) . T h e value of K was e s t i m a t e d to be 1590 a n d 31500 Oe respectively for the samples [O] a n d [ A O ] . Following an earlier result [ l 2 ], K m a y be interpreted as the field necessary' to reduce the i n t e r g r a n u l a r critical c u r r e n t to zero. We note thai the argon t r e a t m e n t c o n s i d e r a b l y

R. Suryanarayanan, A. Nafidi / Physica C 225 (1994) 105-110

increases the value of K indicating an improvement in the pinning properties.

4. Discussion

As mentioned earlier, the prototype superconductor YBa2Cu306+z with To= 92 K possesses an orthorhombic crystal symmetry. Both the structure and the value of T¢ are retained when other rare earths (except Ce, Pr, and Tb) are substituted for Y, keeping z close to 0.9. The orthorhombic symmetry is further retained when one atom of Ba is replaced by one atom of Sr though Tc reduces to 83 K. In analogy with the RBa2fu306+z system, one would expect similar results when other rare earths are substituted for Y in YSrBaCuaO6+ z - that is retention of the orthorhombic symmetry with no reduction in Tc from 83 K. We did find that when Y was replaced by Eu, the orthorhombic symmetry was retained in disagreement with the data obtained by Wang et al. [ 8 ] and Badri et al [9]. However, a tetragonal structure was obtained [8-10] when Y was replaced by Sm whose ionic radius is only slightly greater than that of Eu. Wang et al. [ 13 ] have earlier reported the effect of preparation procedure on the symmetry of DySrBaCuO6+z. The exact reason for this change is not known. It is possible that a tetragonal structure may be obtained from an orthorhombic structure by rearranging the oxygen vacancies in the AB-plane. Thus one can have an ordering of oxygen along both the a and b directions. It is interesting to note that such a possibility has been suggested [ 14 ] in the case of YBaCuO. In the present case, the argon treatment seemed to favor not only the orthorhombic symmetry but also increased the ratio ( b - a) / (b + a). It is possible that this resulted in an increased ordering of chain oxygen. This could enhance the transfer of charge between the chains and the planes resulting in an increase in the hole density. Such an increase would lead to optimum superconducting properties and could account for the observed increase in T¢, shielding and irreversibility line. Such an argument was recently discussed [ 15 ]. These authors pointed out that the hole transfer into CuO2 planes is strongly connected with the mechanism of oxygen filling in the basal Cu 1 plane. There it was shown that the carrier concentration (and To) would increase with the probability of

109

finding a given number of Cu 1-O chain fragments. At this point, it is interesting to recall at least two related reports concerning the orthorhombicity and the irreversibility line both as a function of oxygen content. In the case of YSrBaCu306+z, the structure could be changed from tetragonal to orthorhombic and back to tetragonal as a function of oxygen pressure [ 3 ]. Thus, for 0.2 < z < 0.6, the structure was tetragonal, for 0.7 < z < 0.97, it was orthorhombic and for 1 < z < 1.3, it was tetragonal again. In particular, for z > 1, these authors observed a transition from orthorhombic to a pseudotetragonal structure with microdomains, which were resolved by TEM images. There was also a decrease in Tc for z > 0.9. However, results obtained in the present study are somewhat different. Our samples were prepared in 1 atm of oxygen. Further, the argon treatment did not sensibly change the total oxygen constant which was around 6.95+0.04 from iodometry measurements but increased the To. This was also pointed out recently [ 16 ] in the case ofNdSrBaCu306 ÷z samples that were treated in a similar fashion. Concerning the irreversibility line, we would like to make short comments. Both the imaginary part of the AC susceptibility and the DC magnetization are used to establish such a line, some favoring the latter technique [17]. Some authors have used both the techniques on the same sample and have shown that in the case of a single crystal [ 18 ] of YBaCuO or Bi (2212), the irreversibility line obtained from the AC technique (at frequencies of the order of 1000 Hz) coincided with that obtained from the DC technique whereas in the case of polycrystals, the irreversibility line obtained from the AC technique was found to lie lower in the H - T plane [ 18,19 ]. However, any enhancement in the irreversibility line of a given sample due to substitution or heat treatment was reflected in both types of measurements [ 19 ]. It is only this aspect - enhancement in the irreversibility line due to heat treatment - that we wanted to emphasize. Vanacken et al. [20] have measured the fieldcooled and zero-field-cooled DC magnetization of YBa2Cu306+z as a function o f z to establish the irreversibility line and analyzed their data using the relation H = K ( I - t ) n. They showed that n was independent of z but Tc and K (and also the orthorhombicity) increased steadily with z indicating possibly the role played by an increase in oxygen

110

R. Suryanarayanan, A. Nafidi /Physica C 225 (1994) 105-110

ordering. Slight differences in oxygenation might also induce changes in Cu 1-6 oxygen apical distance and Tc as was discussed elsewhere [ 10,21 ]. The observed increase in the shielding properties and pinning may also be explained qualitatively by the factors discussed above. We wish to make a brief comment on the relation between ionic size of the rare earth and Tc as was suggested by Wang et al. [ 8]. These authors have observed a very small increase in T¢, from 79 to 80 K when Y was replaced by Eu in YSrBaCu306+z. Not only our value of T¢ of YSrBaCu306+z is different (83 K see Ref. [ 4 ] ) from that reported by Wang et al. but also the variation in To, as Y was replaced by Eu, is different. In fact we found an opposite behavior - T~ decreased from 83 to 80 K . Hence we are tempted to believe that the changes (increase or decrease) observed in T~ need not be related only to the ionic size of the rare earth but rather to a combination of several factors such as changes in the Cu l-apical oxygen distances, oxygen disorder, hole density etc.

5. Conclusions We have shown that EuSrBaCu306÷z processed by conventional oxygen annealing at 450°C crystallizes with an orthorhombic symmetry in contrary to what was reported earlier [ 8,9 ]. Both the orthorhombicity and Tc increased as a result of preheating the sample in argon prior to oxygen annealing. Further, AC susceptibility measurements indicated increased shielding and an enhancement in the irreversibility line as a result of this heat treatment. Several factors such as changes in the Cu 1-apical oxygen distances, oxygen disorder, hole density etc would qualitatively account for our data. The present studies indicate a simple heat treatment procedure to optimize superconducting properties that should be examined in the case of other members of the LnSrBaCu306+z system.

Acknowledgement One of us (AN) would like to thank E1 Mohtadi, Dean of the faculty of Sciences, Agadir, for granting him the leave of absence.

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