Influence of Nd substitution on the superconducting properties of ceramics in the 2212 system Bi2−wPbwSr2−xCa1−yNdx+yCu2O8+z

Physica C 200 (1992) North-Holland

287-295

Influence of Nd substitution on the superconducting properties of ceramics in the 2212 system Bi2-_,,,PbwSrZ-_xCal_yNdx+yCu208+z T. Rentschler a, S. Kemmler-Sack a, M. Hartmann b, R.P. Hiibener b, P. Kessler ’ and H. Lichte ’ a Institut ftir Anorganische Chemie der Universitiit Tiibingen, Auf der Morgenstelle 18. W- 7400 Tiibingen, Germany b Physikalisches Institut der Universitiit, Lehrstuhlfiir Experimentalphysik II, Auf der Morgenstelle 14, W-7400 Tiibingen, Germany ’ Institut ftirdngewandte Physik der Universitiit, Auf der Morgenstelle 10, W-7400 Tiibingen, Germany Received 5 June 1992 Revised manuscript received

15 July 1992

In the system Bi,_,Pb,Sr,_,Ca,_,Nd,,,Cu~08+r different fractions of Nd are substituted on either Sr of Ca sites in order to introduce intrinsic insulating pinning centres. It is shown that a Nd concentration around x or y=O.2 is likewise favourable with an average Nd-Nd distance in the range of the coherence length in the a, b-plane. However, clear evidence of flux pinning is only present for charge compensation with Pb*+. A simultaneous substitution of the B&based 2212 superconductor with moderate amounts of Nd3+ and Pb’+ improves the superconducting properties by strengthening the flux pinning forces.

1. Introduction One of the striking features of Bi-based high-temperature superconductors is the large superconducting anisotropy mainly due to the large distance between the CuOz layers compared to the coherence length. Recently, these phenomena have been theoretically anticipated by Kes et al. [ 1 ] and Clem [ 21 and, after this work was done, checked experimentally by Kim et al. [ 31 focusing on the two-dimensional pancake vortex model and its relationship with the distance between the CuOz sheets. The smaller pinning energy for pinning by two-dimensional pancake vortices instead of complete flux lines has important implications for the technical application of Bi-based cuprates at liquid nitrogen temperature. The purpose of the present investigation is to study the effect of Nd substitution on the pinning behaviour of Bi-based cuprate superconductors of type 2212 (system (Bi, Pb)2Sr2CaCu208+z). It is well known that in this structure a substitution of rare earth ions Ln3+ for Ca2+ is possible and that a complete substitution (series (Bi, Pb)zSr2LnCu20s+,) results in the formation of insulating compounds. 0921-4534/92/$05.00

0 1992 Elsevier Science Publishers

Consequently, by a partial Ln3+ substitution, insulating regions can be created in the lattice of the pure superconductor which are likely to act as pinning centres. Additionally, the distances between these intrinsic pinning centres can be adjusted to the coherence length via the average Ln-Ln distances by choosing the proper Ln3+ concentration. For example, for a Ln3+ concentration of 20 mol% the average Ln-Ln distance is about 13 A, decreasing to about 10 8, for 40 mol% (calculated according to ref. [ 41). To further study the influence of the lattice site on the pinning properties, the Ln3+ substitution will be done for either Sr2+ or Ca2+ in the system Biz_,Pb,Sr2_,Ca,_,-Nd,,,Cu20~+z. Owing to the good correspondence between the ionic radii (CN 8; [5]) of Sr2+ (1.28 A), Ca2+ (1.12 A) and Nd3+ ( 1.109 A), the relatively large Nd3+ was chosen for Ln3+. The increase in positive charge on the alkaline earth positions is simultaneously compensated by a decrease on the Bi sites via the substitution Pb2++Bi3+. It is shown that a Nd concentration around x or y=O.2 is likewise favourable, with an average NdNd distance in the range of the coherence length in

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the a, b-plane (20 A, [ 6 ] ). However, for the maintenance of the charge balance, a simultaneous substitution of Pb for Bi is necessary. A moderate codoping of the Bi-based 2212 superconductor with Nd’+ and Pb2+ improves the superconducting properties by strengthening the flux pinning forces.

2. Experimental Samples of composition the Biz_,Pb,Srz_,YCa, _,,Nd,+,VCu208+_ were prepared according to ref. [ 71. In the rest of the paper the short notation w/x/y is used for different 22 12 materials; for instance, ~~0.2, x=0.0, y=O.2 is denoted as 0.21 010.2. The products were air quenched and examined by XRD diffraction (Cu Ku radiation, Au standard), TEM, SEM, EDXA, DC susceptibility (SQUID) measurement and redox titration in order to determine the oxygen content 181.

3. Results and discussion 3.1. XRD, TEM SEM and EDXA investigations According to the present results (table 1) and the data in ref. [ 7 1, materials with w < x and w < y as well as w=xand w=y (x, ~~0.6) consistofsingle-phase 22 12 type. The same holds for Pb-rich samples (w>x, y) up to a Pb amount of w=O.4. However, materials with a higher Pb amount of w=O.6 invar-

iably contain a small admixture of a phase of CazPb04 type. Generally, the lattice constants for the 2212 subcell the system of Biz_.,Pb,Sr~_.,Ca,_?,Nd,+,,Cu208+_ are strongly influenced by the different manners of substitution (e.g. fig. 1 for w=x (~~0.6) and fig. 2 for O< ~~0.6, x=0.2), thus indicating that the corresponding ions do really enter the lattice. From a comparison of the data several tendencies can be deduced: ( 1) an increasing Nd substitution results in the well-known decrease of c, while the II- and b-axes increase linearly for 0.2
Table 1 Values for w, x, y, cell dimensions of the average 22 12 unit cell a), volume of the unit cell ( V) and average formal valence of copper for the case of Nd excess of the system Bi,_,Pb,Sr,_,Ca, _,Nd,+,Cu,O,+,

a’ 50.005

w

X

Y

a

Cl 0 0.2 0.2 0.2 0.2 0.4 0.4

0.2 0 0.4 0 0.6 0 0.6 0

0 0.2 0 0.4 0 0.6 0 0.6

5.431 5.419 5.436 5.459 5.453 5.465 5.431 5.430

8, (a, b), kO.05 A (c)

[Al

b [Al

c [Al

V IA’1

ox

5.440

30.68 30.71 30.66 30.78 30.48 30.70 30.48 30.61

904.93 901.82 906.01 917.26 906.33 916.89 899.03 904.19

+2.21 l )-2.22 +2.11 +2.14 +2.10 +2.12 +2.13 +2.12

(Ox)

289

T. Rentschler et al. /Influence of Nd substitution on 2212 ceramics

30.0 C

[Al 30.6

34.4

/b

34.2

,.,t 0

5.39

v 0.2

I

5.30

93

0.4

0.6

0.0

i

5.37

\a

I 0

0.2

0.4

Y, 0.5

T

0.0

30

1

W W

Fig. I. Lattice constants vs. w for w=x.

Fig. 2. Lattice constants vs. w for x=0.2.

Fig. 3. TEM of a typically homogeneous region of 0.6/O/0.2.

the sample. The (00 1) CuOz layers are interrupted by the boundaries, but coupling of the charge carriers across these boundaries is necessary to transport electrical currents. SEM investigations of the surface as well as of fractured cross-sections from pellets of the present

system show the typical mica-like platy texture of Bibased 2212 superconductors, whereas the level of substitution on either Bi or alkaline earth sites is without influence on the morphology. Especially on images of the surface area (fig. 5 for 0.6/O/0.2 and 0.4/O/0.2), the well-developed preferred orienta-

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et al. /Influence

qf Nd substitution

on 2212 ceramrcs

Fig. 4. TEM of a tilt boundary in the [ 1 IO] pole of 0.6/b,‘J.2

tion of platelets with the c-axis normal to the surface plane can be observed, likewise easily deduced from the XRD pattern of pressed pellets. EDXA conducted on different areas as well as on different grains of samples with ~~0.4 indicated a practically homogeneous distribution of the elements Bi, Pb, Sr, Ca, Nd and Cu. It should be noted that no deviations in the Bi: Pb ratio could be detected. 3.2. Oxygen content The oxygen content was determined via the formal valence of the constituting cations and calculated with the fixed valences of +2 for Pb, Sr, Ca and + 3 for Bi, Nd. The experimental values for the two cases of a formal Nd+Sr and Nd-+Ca substitution are uniformely close together (table 1 and ref. [71). For the fully charge compensated samples (IV= 0.2, 0.4,0.6) the oxygen content is situated near Os.Z, thus indicating that the intended charge compensation is indeed realized. The corresponding average oxidation state of copper is near +2.20+0.03. Corre-

spondingly, the hole concentration amounts to about 0.2 holes per Cu ion, a value which is close to the threshold value for the development of superconductivity. Partial charge compensation in Nd-rich samples w 0 by a small decrease in hole concentration with an average oxidation state of copper of + 2.12 & 0.03. This observation can be explained by the assumption that the oxygen uptake of the lattice is limited, most probably by the oxygen incorporation in the modulated (Bi, Pb)O double layers. On the other hand, an excess of Pb in materials with w>x, y (0.4/0.2/ 0 and 0.4/O/0.2) is not compensated by an increase in the formal valence of copper, with the latter value lying invariantly near + 2.2 [ 7 1. This behavi- .lr is in agreement with an increase of the modulation wavelength with increas ing Pb content [ lo], synonymous with a reduced oxygen incorporation in the (Bi, Pb)O double layers.

T. Rentschler et al. /Injluence OfNdsubstitution on 2212 ceramics

Fig. 5. SEM of the surface of d pellet of nominal composition (a) 0.4/O/0.2 and (b) 0.6/O/0.2.

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T. Rentschler et al. /Influence o.fNd substitutmn on 2212 ceramiu

292

Table 2 T, and Meissner fraction at 20 K for samples Bi,_,Pb,Sr,_,Ca,_,Nd,+,Cu,O,+z Composition W

T, X

I’

0.2 0 0.4 0 0.6 0

0 0.2 0 0.4 0 0.6

lK1

of the system

Meissner fraction [%I

w=xorw=y 0.2 0.2 0.4 0.4 0.6 0.6

88 87 83 87 %50 -70

42 43 33 31 6 5

0.2 0 0.4 0 0.6 0 0.6 0

0 0.2 0 0.4 0 0.6 0 0.6

88 89 81 85 -50 -75 ~56 -70

34 26 19 15 1 2 4 6

0.2 0 0.2 0 0.4 0

0 0.2 0 0.2 0 0.4

87 87 87 89 84 86

48 48 47 42 39 31

w>xorw>y 0.4 0.4 0.6 0.6 0.6 0.6

.I

.s 0

3.3. Magnetic susceptibility density

and critical current

In table 2 the values of T, (onset of the Meissner signal) and the Meissner fraction (calculated at 20 K with x=-M/(H,,V’); H,ff=HoX1/(l-n,); n,,,= 4 and compared with a perfect diamagnet x= - 1/4x) are listed. In the fully charge-compensated case (w=x or w=y) up to ~~0.4, T, is situated as high as ~87 K (fig. 6 for w= y). For the higher substitution levels of 0.6/0.6/O and 0.6/O/0.6, a drastic decrease takes place. The same holds for the Nd-rich partially charge-compensated samples 0.2/0.6/O, 0.2/O/0.6, 0.4/0.6/O and 0.4/O/0.6. From the x versus T curves (e.g. fig. 6), it follows additionally that the superconducting volume fraction diminishes with increasing Nd content due to the nonsuperconducting area around each Nd atom. Furthermore, the steepness of

FC

-0.005

g -0.006 2 i -0.007+

-0.008~

w
,x-o.004 .-

ZFC

i/*

Fig. 6. x vs. T for powdered materials with w=y=O.2. (B= 10 G; ZFC: zero-field-cooled, FC: field-cooled ).

0.4. 0.6

the signal is reduced with increasing Nd content, most probably due to inhomogeneities in the distribution between the Nd and the alkaline earth atoms. Compared with the fully charge-compensated case, partial charge compensation with an excess of Nd ( LV<.Y, v) does not meaningfully affect the position of T,; however, the superconducting volume fraction is reduced. With the exception of the cases O/0.2/0 and O/O/0.2, this effect is easily explainable by the observed decrease in the hole concentration (cf. section 3.2). In contrast to the unfavourable case of Nd excess, an excess of Pb’+ exerts a beneficial role for medium Nd contents of x or y=O.2 and 0.4. In all cases, T, lies between 84 and 89 K, whereas the superconducting volume fraction of 0.4/0.2/O, 0.4/O/0.2. 0.6/0.2/O and 0.6/0.4/O has grown in comparison with the fully charge-compensated materials. In addition, the steepness of the diamagnetic signal is increased, as can be seen in fig. 7 from a comparison of the x versus T signals for the three materials of identical Nd amount but increasing Pb content O/ 0.210, 0.2/0.2/O and 0.6/0.2/O. From the behaviour of the Pb-rich samples follows a higher magnetic homogeneity of the materials, whereas in the X-ray diagrams of Pb-rich samples 0.6/0.2/O, 0.6/O/0.2, 0.6/0.4/O and 0.6/O/0.4 small fractions of a phase of CazPbO, type are just detectable (cf. section 3. I ). The critical current density j,, has been determined by magnetization measurements for post-annealed pellets using the relation &,,, = 3M,/ ( 2r x V),

T. Rentschler et al. /Influence

T WI

-0.001 t

?

i

-0.002 -0.003

A.2

t

-0.004 x C .= -0.005 g $--0.006

*w=o

z: (jj-0.007

--w = 0.2

1

-0,008 t

+v

= 0.6

-o.oosl

Fig. 7. x vs. T for powdered materials O/0.2/0. 0.2/0.2/O and 0.6/0.2/O (B= 10 G; ZFC: zero-field-cooled, FC: field-cooled).

where M, is f of the width of the irreversible magnetization loop, V the sample volume ( V= a/m) and r is the average sample radius [ 111. For calculations the latter has been estimated from SEM investigations as 4x 10e6 m. From a comparison of the j,, values in table 3 with the critical current densities j, determined by magnetic and resistivity measurements for melt-textured 22 12 layers on Ag tape [ 12 1, it follows that the data of our Nd-free samples are situated in the same range, thus being typical for intragrain critical current densities of the present material. From the values of table 3 several trends can be detected: ( 1) for a fixed Pb content (w= 0.6), an increasing Nd amount generally reduces jcm (c.f. 0.6/O/O, 0.61 O/0.2,0.6/0/0.4 and 0.6/O/0.6). However, note that

Table 3 Critical current

density jcm (A/cm*)

Composition

for post-annealed

X

Y

5K

0.6 0.6 0.4 0.2 0 0.6 0.6

0 0 0 0 0 0 0

0 0.2 0.2 0.2 0.2 0.4 0.6

2.0x 1.2x 1.3x 0.9x 0.5x 0.6x 2.8X

T

4

-4 -6000

/-ol

w

-3

-4000

-2000

Magnetic Fig. 8. M-Hloops

at 30 Kfor

0

2000

Field

[G]

4000

6000

(a) 0.6/O/O and (b) 0.4/O/0.2.

BiZ_SbWSrZ_Fa, _YN&+YCu~O~+,

0.2 T

lo6 106 106 lo6 lo6 lo6 lo4

293

for 0.6/O/0.2 the value at 30 K, 0 T is exceptionally superior to the Nd-free case 0.6/O/O. (2) For a fixed Nd content (y= 0.2), an increasing Pb amount results in an increase of j,, (cf. O/O/ 0.2, 0.2/O/0.2, 0.4/O/0.2). For 0.4/O/0.2 a maximum is reached and j,, for 0.6/O/0.2 is uniformly reduced. Note that at 30 K, for the sample 0.4/O/0.2 with a moderate Nd content, higher j,, values are obtainable at 0, 0.2 and 0.4 T than in the Nd-free case 0.6/O/O. The corresponding M(H) loops are shown in fig. 8. Interestingly, contrary to the 5 K case, all 30 K hysteresis loops of Nd-containing materials are characterized by well-defined kinks near H=O T, typical for flux jumps, probably due to a collective depinning of pinned pancake vortices. The jumps are absent in the Nd-free material (e.g. fig. 8 ). The fa-

pellets of the system

OT

w

ofNd substitution on 2212 ceramics

0.4 T

30K

5K

30 K

5K

30 K

2.3x lo5 4.1x10’ 5.6x IO5 4.0x lo5 0.9x 10S l.lX105 1.0x lo3

1.8X lo6 1.2x 106 1.2x 106 0.8x lo6 0.6x lo6 0.4x lo6 2.3~10~

2.1x105 1.6x lo5 2.2x 105 1.1x105 0.5x 10’ 5 x10’ -

1.4x lo6 0.9x 106 0.9x 106 0.7x lo6 0.5x lo6 0.3x lo6 7.7x103

1.0x 105 0.9x los 1.3x 10’ 0.5x 105 0.2x 105 9 x10* -

294

T Rentschler

et al. /Influence

0.25 -(a)

,

0(b)

-100

0

100

Magnetic

Field

[G]

200

300

Fig. 9. M-Hloops at 77 K for (a) 0.6/O/O and (b) 0.6/O/0.2.

vourable role of a moderate Pb substitution of w< 0.6 for y=O.2 follows generally from the observed uniform increase of j,, at 30 K, 0 T in comparison to the Nd-free case. Furthermore, Pb-containing materials with y=O.2 are characterized by a minor reduction of j,, in the presence of a magnetic field. In order to investigate the possible influence of doping with Nd as a method of introducing intrinsic pinning centres at 77 K, additional measurements were performed with the samples 0.6/O/0.2 and 0.61 O/O. From the magnetization hysteresis loops in fig. 9 the result at H=O T for the Nd-free case 0.6/O/O is an intragrain critical current densityj,, = 1.4~ 1O4 A/cm*, whereas the corresponding value of the Ndsubstituted material 0.6/O/0.2 is more than twice as high ci,, = 3.6 x 1O4A/cm’). These observed differences between j,, are reflected in the experimentally observed transport critical current densities j,, of 260 A/cm2 for 0.6/O/0.2 and < 5 A/cm2 for 0.6/O/O.

ofNd substitution

on 2212 ceramics

the incorporated Nd amount, and a relative small Nd concentration of x or y=O.2 is likewise favourable. This effect is easily explainable by the resulting average Nd-Nd distance in the range of the coherence length in the a, b-plane. However, without an additional charge compensation by a simultaneous substitution of Pb2+-+Bi3+, the average hole concentration is below the favourable value of 0.2 holes per copper ion and the total superconducting volume fraction remains small. For cases of co-doping the Bi-based 2212 system with Nd3+ and a similar volume fracamount of Pb’+, the superconducting tion is enhanced and, simultaneously, clear evidence of flux pinning is obtained by measurements of magnetization hysteresis loops as well as of transport critical current densities. Additionally, a slight excess of Pb’+ is beneficial and actually the best results are obtained for the 2212 material of composition 0.4/o/0.2.

Acknowledgements The authors wish to thank the Bundesministerium fur Forschung und Technologie (FKZ 13 N 5842)) the Forschungsschwerpunkt Supraleitung des Landes Baden-Wiirttemberg and the Verband der Chemischen Industrie for supporting this investigation. T. Rentschler is very grateful to S. Haidlinger, Daimler Benz Forschungszentrum Ulm for the measurements of transport critical current densities and thanks the Landesgraduiertenfdrderung Baden-Wiirttemberg for granting a scholarship. The help of A. Ehmann and E. Niquet is much appreciated.

References 4. Conclusions In the system Bi2_ ,Pb,Sr,_,Ca, _.,Nd,+,,Cu20s+,, different fractions of Nd are substituted for the alkaline earth elements in order to introduce insulating lattice regions, which are likely to act as intrinsic pinning centres. According to the present results, the superconducting properties are indeed strongly influenced by

[ 1] P.H. Kes, J. Aarts. V.M. Vinokur and C.J. van der Beek. Phys. Rev. Lett. 64 (1990) 1063. [2] J.R. Clem, Phys. Rev. B 43 (1991) 7837. [3]D.H. Kim, K.E. Gray, R.T. Kampwirth, J.C. Smith, D.S. Richeson, T.J. Marks and H. Eddy, Physica C 177 ( 199 I ) 43 1. [4] H. Tachiante, A. Arbus, M.T. Foumier and J.C. Cousseins, J. Less. Common Met. 112 (1992) 83. (51R.D. Shannon, Acta Crystallogr. A 32 ( 1976) 75 1. [6] W.C. Lee, J.H. Cho and D.C. Johnston, Phys. Rev. B 43 (1991) 457.

T. Rentschler et al. /Influence ofNd substitution on 2212 ceramics [ 71 T. Rentschler and S. Kemmler-Sack, Physica C 176 ( 199 1) 445. [ 81 W. Schafer, J. Maier-Rosenkranz, S. Lijsch, R. Kiemel, W. Wischert and S. Kemmler-Sack, J. Less. Common. Met. 142 (1988) L5. [ 91 S. Lijsch, H. Budin, 0. Eibl, M. Hartmann, T. Rentschler, M. Rygula, S. Kemmler-Sack and R.P. Huebener, Physica C 177 (1991) 271.

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[lO]O.Eibl,PhysicaC 175 (1991) 419. [ 111 S.Elschner and S. Gauss, Supercond. Sci. Technol. 5 ( 1992) 300. [ 121 H.-W. Neumtlller, H. Assmann, B. Kress and G. Riess, 4th bit. Symp. on Superconductivity, ISS 91,Oct. 14-17, 1991, Tokyo, Japan.