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Influence of flux composition on the superconducting properties of single crystal Bi2CaSr2Cu2O8
Volume 9. number 4
MATERIALS LETTERS
INFLUENCE OF FLUX COMPOSITION OF SINGLE CRYSTAL Bi2CaSr2Cu20s
ON THE SUPERCONDUCTING
February 1990
PROPERTIES
P.D. HAN and D.A. PAYNE Department of Materials Science and Engineering, Materials Research Laboratory, and Science and Technology Center for Superconductivity* University of Illinois at Urbana-Champaign,
Urbana, II. 6 1801, USA
Received 29 December 1989
The influence of starting composition (i.e. variations in the Ca and Cu content, and with Pb additions) on the properties of flux grown superconducting Bi,CaSr,Cu,O, single crystals was investigated. Excess Cu promoted 2-O-2-1 intergrowths, whereas excess Ca gave more “pure” 2-I-2-2 crystals with less inte~ro~hs. Pb additions suppressed the growth of 2-l-2-2 and yielded more 2-O-2-1, despite the presence of excess Ca. The optimum composition was evaluated from XRD and SQUID results.
1. Introduction
Since Michel et al. [ 1) reported the discovery of superconductivity in Bi2SrZCu106 at 22 K, two additional superconducting phases in the Bi-Ca-SrCu-0 system have been identified [2,3]. The three phases are: (i) BizSrzCu,OG (2-O-2-1 ) a single perovskite layered compound with a 7’, near 20 K, (ii) Bi,Ca, Sr,CuzO, ( 2- l-2-2 ) with two CuOZ layers and a T, near 80 K and, (iii) BizCazSrzCu3010 (2-2-23 ) with three CuO, layers and a T, near 105 K [ 4,5 1. Crystal growth of the lower Tc phases (2-l-2-2 and 2-O-2-1) has been reported by several methods. Todate, all attempts to grow the high T, phase (2-2-23) from the melt have been unsuccessful. Some of the methods used in crystal growth for 2-l-2-2 are: growth from off-stoichiometric starting compositions [ 6-8 1, use of alkali-halide fluxes [ 91, and crystallization by a laser-heated floating-zone process [ 10 1. Problems encountered in the crystal growth of superconductors include: chemical instability, incongruent melting, chemical attack of the crucible, etc. The extreme anisotropy in growth habit (resulting in thin plate-like crystals), and the propensity for chemical intergrowths [ 111, are further complications. Lack of reliable phase diagrams (which may be used as a guide for crystal growth), and a paucity of information on directional solidification for incongruent melting materials, are also impedi0167-577x/90/$
ments for the growth of large single crystals. Due to these reasons, it has not been possible to grow large and thick single-phase single crystals (i.e. free from syntactic intergrowths). This has caused difficulty in the determination of intrinsic properties, and hindered the development of an understanding of the role of defects (such as, stacking faults, twins, dislocations, etc. ) on the properties of the material. For example, it is not clear why 2-l-2-2 has a broader superconducting transition and a smalfer Meissner fraction than Y-Ba-Cu-0 superconductors. In this Letter we report on a systematic investigation of the influence of starting composition on the properties of superconducting 2-l-2-2 crystals grown by flux methods. The effect of variations in starting compositions is discussed in the context of phase development (i.e. intergrowths) and superconducting properties.
2. Experimental Crystals were grown by slow cooling of melts in a vertical furnace with a temperature gradient of 5 ‘C/ cm. The starting compositions were mixtures of Bi203, CaCO,, SrCOj and CuO. All the materials were greater than 99.9% pure. The experiments were carried out with 100 g batches and in high-purity alumina crucibles. Slow cooling was from 890 to 790°C
03.50 0 Eisevier Science Publishers B.V. (North-Holland
)
157
Volume 9, number 4
MATERIALS LETTERS
Table 1 Starting compositions, with variations in Cu, Ca and Pb No.
Al A2 A3 A4 Bl B2 B3 B4 Cl c2 c3 c4
Starting composition Pb
Bi
Ca
Sr
CU
0.3 0.3 0.3
2 2 2 2 2 2 2 2 2 1.8 1.8 1.8
1 1 1 1 1 1.1 1.2 1.5 1 1 1.2 1.5
2 2 2 2 2 2 2 2 2 2 2 2
4 2.6 2.2 2 2.6 2.6 2.6 3 2.4 2.4 2.4 2.1
at 2”C/h after a 6 h soak at 980°C. In order to investigate the effect of starting composition on the intergrowth content, and thereby determine the optimum starting composition for pure 2-l-2-2 single crystals (i.e. free of 2-O-2-1 intergrowths), three groups of experiments (A, B and C) were designed (table 1). The crystals had a micaceous habit, and were separated from the solidified flux by cleavage. Bu this method, single crystals greater than 5 x 5 mm and thicker than 50 urn were obtained. In some cases “free crystals” were obtained from cavities within the resolidified flux or from the external surface. XRD data were obtained with a Rigaku D-Max diffractometer. Superconducting properties were measured on a SQUID operated under field-cooling conditions. A field of 10 Oe was applied parallel to the (00 1) face of the crystal.
February 1990
Three specimens from the same batch were examined to minimize sampling errors. Multiple scans (with additive data-acquisition software) were used to detect 2-0-2-I diffractions and to improve the resolution. Results for the three groups are given in figs. l-3. Starting compositions are indicated in the figures. Excess Cu promoted 2-O-2-1 intergrowths (fig. 1), whereas excess Ca gave more “pure” 2-l-2-2 crystals with fewer intergrowths (fig. 2). If the Cu content was increased further, a greater amount of Ca was required to neutralize the effect of increased Cu (see fig. 2d). Pb additions increased the content of 2-02-1 intergrowths (fig. 3). Excess Cu combined with Pb additions substantially suppressed 2-l-2-2 formation and yielded pure 2-O-2-1, despite the presence of excess Ca (see fig. 3d). The above experiments indicated that only the stoichiometric composition (fig. Id) and small amounts of Ca and Cu in excess (fig. 2c) were necessary for phase-pure 2-l-2-2 crystals. In fact, large amounts ( > 20%) of Ca resulted in smaller crystals. Fig. 4 illustrates a sharp diamagnetic transition,
2122 0 2021
Big Co Srg Cu4 Ox
l
. .
a .
. 0
I
.
.
B$Co
0
1
Sr2 CIJ~.~O~
I
3. Results and discussion XRD was initially used for the determination of intergrowth content. For simple comparison purposes, between different crystals, the (001) faces were oriented so that only (001) peaks were observed. Crystals were carefully selected under an optical microscope so as to make sure that no additional second phases were present. The chosen single crystals were cleaved again through use of doublesided tape, and mounted on glass slides for XRD. 158
60 4’0 28 (deg) Fig. 1. XRD data for 2-l-2-2, with excess copper, which increased the “2-0-2-1” content. Composition d gave pure 2-l-2-2.
MATERIALS LE-M’ERS
Volume 9, number 4
February 1990
Field cooling at 10 Oe parallel to (001)
-0.5i
I
20
0
40
60
TE~~~~~
80
100
fK>
Fig. 4. Temperature dependence of diamagnetic susceptibility for 2-I-2-2 single crystals grown from composition 2 : I .2 : 2 : 2.6. 3
40
60
ZWdeg) Fig. 2. XRD data for 2-l-2-2, with excess calcium, which decreased the “2-0-2-1” content. Composition c gave pure 2-l-Z-2.
with an onset temperature near 86 K and a Meissner faction of 47% for a sin@e crystal of 2-I -2-2 grown from the starting composition 2 : 1.2 : 2 : 2.5. A single crystal grown from starting composition 2 : 1: 2 : 2 had 7 1W Meissner fraction, Large single crystals up to 13 mm in length were obtained from the starting composition 2 : 1.2 : 2 : 2.6, whereas the stoichiometric -0.0 -0.2
2
-0.4
0
$
Q 33 o?&. * Bl 5% *
c3
24%
20
40 -rEMrmm
*
+
-0.6 -0.8 -1.0 0
2Bfdegf
Fig. 3. XRD data for 2-l-2-2, with lead additions, which increased the “2-0-2-1” content. Composition d gave “2-0-2-1”.
60
80
too
(K)
Fig. 5. Temperature dependence of diamagnetic susceptibility ratio x/xl0 K for 2-l-2-2 crystals, with different amounts of “2-O-21” intergrowths.
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Volume 9, number 4
MATERIALS LETTERS
composition 2 : 1: 2 : 2 gave single crystals less than 5 mm in length. The presence of intergrowths caused a degradation in superconducting properties. The transition temperature was lowered and broadened with increasing intergrowth content (fig. 5). The intergrowth content was scaled according to the ratio of intensities for 2-O-2-1 (0010) and 2-l-2-2 (0010). The propet-ties could not be improved by annealing in air or oxygen at 820°C for up to 11 days.
4. Conclusion A strong correlation between starting composition, 2-O-2-1 intergrowth content, and superconducting properties for 2-l-2-2 single crystals grown from the flux was determined. Excess Cu promoted 2-O-2-1 intergrowths whereas small contents of Ca in excess of stoichiometry gave more “pure” 2-l-2-2 crystals. Contrary to the results for polycrystalline ceramic specimens [ 12 1, Pb additions stimulated 20-2-1 intergrowths, and caused a degradation in superconducting properties. The best starting composition for large single crystals with a sharp transition was 2 : 1.2: 2: 2.6. The largest Meissner fraction (7 1%) was obtained from smaller single crystals grown from the stoichiometric melt (2 : 1: 2 : 2).
Acknowledgement
The research was supported by U.S. DOE DMR DE-AC02-76ERO1198 and NSF-STC-88-09854. The
160
February 1990
assistance of A. Asthana, Z.K. Xu and L. Chang is gratefully acknowledged. References [ I] C. Michel, M. Hervieu, M.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Physik B 68 ( 1987) 421. [2]R.M. Hazen, C.T. Prewitt, R.J. Angel, N.L. Ross, L.W. Finger, C.G. Hadidacos, D.R. Veblen, P.J. Heaney, P.H. Hor, R.L. Meng, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, Z.J. Huang, L. Gao, J. Bechtold and C.W. Chu, Phys. Rev. Letters 60(1988) 1174. [3] M.A. Subramanian, CC. Torardi, J.S. Calabrese, J. Gopalakrishnan, K.J. Morissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 239 (1988) 1015. [4] H. Maeda, Y. Tanaka, M. Fukotami and T. Asano, Japan. J. Appl. Phys. 27 (1988) 209. [ 5] J.M. Tarascon, Y. le Page, L.H. Greene, B.G. Bagley, P. Barboux, D.M. Hwang, G.W. Hull, W.R. McKinnon and M. Giroud, Phys. Rev. B 38 (1988) 2504. [6] J.Z. Liu, G.W. Crabtree, L.E. Rehn, U. Geiser, W.K. Kwok, P.M. Baldo, J.M. Williams and D.J. Lam, Phys. Letters A 127 (1988) 444. [ 71 H. Takagi, H. Eisaki, S. Uchida, A. Maeda, S. Tajima, K. Uchinokura and S. Tanaka, Nature 332 (1988) 236. [8] T.F. Ciszek, J.P. Goral, CD. Evans and H. KatayamaYoshida, J. Crystal Growth 91 (1988) 312. [9] L.F. Schneemeyer, R.B. van Dover, S.H. Glarum, S.A. Sunshine, R.M. Flemming, B. Batlogg, T. Siegrist, J.H. Marshall, J.V. Waszczak and L.W. Rupp, Nature 332 ( 1988) 422. [lo] H.D. Brody, J.S. Haggerty, M.J. Cima, M.C. Fleming, R.L. Barns Gyorgy, D.W. Johnson, W.W. Rhodes, W.A. Sunder and R.A. Laud&, J. Crystal Growth 96 (1988) 225. [ 111 A.W. Sleight, M.A. Subramanian and C.C. Torardi, MRS Bull. 14 (1989) 45. [ 121 R.J. Cava, presented at the Spring Meeting of the American Physical Society, New Orleans, March 1988.
MATERIALS LETTERS
INFLUENCE OF FLUX COMPOSITION OF SINGLE CRYSTAL Bi2CaSr2Cu20s
ON THE SUPERCONDUCTING
February 1990
PROPERTIES
P.D. HAN and D.A. PAYNE Department of Materials Science and Engineering, Materials Research Laboratory, and Science and Technology Center for Superconductivity* University of Illinois at Urbana-Champaign,
Urbana, II. 6 1801, USA
Received 29 December 1989
The influence of starting composition (i.e. variations in the Ca and Cu content, and with Pb additions) on the properties of flux grown superconducting Bi,CaSr,Cu,O, single crystals was investigated. Excess Cu promoted 2-O-2-1 intergrowths, whereas excess Ca gave more “pure” 2-I-2-2 crystals with less inte~ro~hs. Pb additions suppressed the growth of 2-l-2-2 and yielded more 2-O-2-1, despite the presence of excess Ca. The optimum composition was evaluated from XRD and SQUID results.
1. Introduction
Since Michel et al. [ 1) reported the discovery of superconductivity in Bi2SrZCu106 at 22 K, two additional superconducting phases in the Bi-Ca-SrCu-0 system have been identified [2,3]. The three phases are: (i) BizSrzCu,OG (2-O-2-1 ) a single perovskite layered compound with a 7’, near 20 K, (ii) Bi,Ca, Sr,CuzO, ( 2- l-2-2 ) with two CuOZ layers and a T, near 80 K and, (iii) BizCazSrzCu3010 (2-2-23 ) with three CuO, layers and a T, near 105 K [ 4,5 1. Crystal growth of the lower Tc phases (2-l-2-2 and 2-O-2-1) has been reported by several methods. Todate, all attempts to grow the high T, phase (2-2-23) from the melt have been unsuccessful. Some of the methods used in crystal growth for 2-l-2-2 are: growth from off-stoichiometric starting compositions [ 6-8 1, use of alkali-halide fluxes [ 91, and crystallization by a laser-heated floating-zone process [ 10 1. Problems encountered in the crystal growth of superconductors include: chemical instability, incongruent melting, chemical attack of the crucible, etc. The extreme anisotropy in growth habit (resulting in thin plate-like crystals), and the propensity for chemical intergrowths [ 111, are further complications. Lack of reliable phase diagrams (which may be used as a guide for crystal growth), and a paucity of information on directional solidification for incongruent melting materials, are also impedi0167-577x/90/$
ments for the growth of large single crystals. Due to these reasons, it has not been possible to grow large and thick single-phase single crystals (i.e. free from syntactic intergrowths). This has caused difficulty in the determination of intrinsic properties, and hindered the development of an understanding of the role of defects (such as, stacking faults, twins, dislocations, etc. ) on the properties of the material. For example, it is not clear why 2-l-2-2 has a broader superconducting transition and a smalfer Meissner fraction than Y-Ba-Cu-0 superconductors. In this Letter we report on a systematic investigation of the influence of starting composition on the properties of superconducting 2-l-2-2 crystals grown by flux methods. The effect of variations in starting compositions is discussed in the context of phase development (i.e. intergrowths) and superconducting properties.
2. Experimental Crystals were grown by slow cooling of melts in a vertical furnace with a temperature gradient of 5 ‘C/ cm. The starting compositions were mixtures of Bi203, CaCO,, SrCOj and CuO. All the materials were greater than 99.9% pure. The experiments were carried out with 100 g batches and in high-purity alumina crucibles. Slow cooling was from 890 to 790°C
03.50 0 Eisevier Science Publishers B.V. (North-Holland
)
157
Volume 9, number 4
MATERIALS LETTERS
Table 1 Starting compositions, with variations in Cu, Ca and Pb No.
Al A2 A3 A4 Bl B2 B3 B4 Cl c2 c3 c4
Starting composition Pb
Bi
Ca
Sr
CU
0.3 0.3 0.3
2 2 2 2 2 2 2 2 2 1.8 1.8 1.8
1 1 1 1 1 1.1 1.2 1.5 1 1 1.2 1.5
2 2 2 2 2 2 2 2 2 2 2 2
4 2.6 2.2 2 2.6 2.6 2.6 3 2.4 2.4 2.4 2.1
at 2”C/h after a 6 h soak at 980°C. In order to investigate the effect of starting composition on the intergrowth content, and thereby determine the optimum starting composition for pure 2-l-2-2 single crystals (i.e. free of 2-O-2-1 intergrowths), three groups of experiments (A, B and C) were designed (table 1). The crystals had a micaceous habit, and were separated from the solidified flux by cleavage. Bu this method, single crystals greater than 5 x 5 mm and thicker than 50 urn were obtained. In some cases “free crystals” were obtained from cavities within the resolidified flux or from the external surface. XRD data were obtained with a Rigaku D-Max diffractometer. Superconducting properties were measured on a SQUID operated under field-cooling conditions. A field of 10 Oe was applied parallel to the (00 1) face of the crystal.
February 1990
Three specimens from the same batch were examined to minimize sampling errors. Multiple scans (with additive data-acquisition software) were used to detect 2-0-2-I diffractions and to improve the resolution. Results for the three groups are given in figs. l-3. Starting compositions are indicated in the figures. Excess Cu promoted 2-O-2-1 intergrowths (fig. 1), whereas excess Ca gave more “pure” 2-l-2-2 crystals with fewer intergrowths (fig. 2). If the Cu content was increased further, a greater amount of Ca was required to neutralize the effect of increased Cu (see fig. 2d). Pb additions increased the content of 2-02-1 intergrowths (fig. 3). Excess Cu combined with Pb additions substantially suppressed 2-l-2-2 formation and yielded pure 2-O-2-1, despite the presence of excess Ca (see fig. 3d). The above experiments indicated that only the stoichiometric composition (fig. Id) and small amounts of Ca and Cu in excess (fig. 2c) were necessary for phase-pure 2-l-2-2 crystals. In fact, large amounts ( > 20%) of Ca resulted in smaller crystals. Fig. 4 illustrates a sharp diamagnetic transition,
2122 0 2021
Big Co Srg Cu4 Ox
l
. .
a .
. 0
I
.
.
B$Co
0
1
Sr2 CIJ~.~O~
I
3. Results and discussion XRD was initially used for the determination of intergrowth content. For simple comparison purposes, between different crystals, the (001) faces were oriented so that only (001) peaks were observed. Crystals were carefully selected under an optical microscope so as to make sure that no additional second phases were present. The chosen single crystals were cleaved again through use of doublesided tape, and mounted on glass slides for XRD. 158
60 4’0 28 (deg) Fig. 1. XRD data for 2-l-2-2, with excess copper, which increased the “2-0-2-1” content. Composition d gave pure 2-l-2-2.
MATERIALS LE-M’ERS
Volume 9, number 4
February 1990
Field cooling at 10 Oe parallel to (001)
-0.5i
I
20
0
40
60
TE~~~~~
80
100
fK>
Fig. 4. Temperature dependence of diamagnetic susceptibility for 2-I-2-2 single crystals grown from composition 2 : I .2 : 2 : 2.6. 3
40
60
ZWdeg) Fig. 2. XRD data for 2-l-2-2, with excess calcium, which decreased the “2-0-2-1” content. Composition c gave pure 2-l-Z-2.
with an onset temperature near 86 K and a Meissner faction of 47% for a sin@e crystal of 2-I -2-2 grown from the starting composition 2 : 1.2 : 2 : 2.5. A single crystal grown from starting composition 2 : 1: 2 : 2 had 7 1W Meissner fraction, Large single crystals up to 13 mm in length were obtained from the starting composition 2 : 1.2 : 2 : 2.6, whereas the stoichiometric -0.0 -0.2
2
-0.4
0
$
Q 33 o?&. * Bl 5% *
c3
24%
20
40 -rEMrmm
*
+
-0.6 -0.8 -1.0 0
2Bfdegf
Fig. 3. XRD data for 2-l-2-2, with lead additions, which increased the “2-0-2-1” content. Composition d gave “2-0-2-1”.
60
80
too
(K)
Fig. 5. Temperature dependence of diamagnetic susceptibility ratio x/xl0 K for 2-l-2-2 crystals, with different amounts of “2-O-21” intergrowths.
159
Volume 9, number 4
MATERIALS LETTERS
composition 2 : 1: 2 : 2 gave single crystals less than 5 mm in length. The presence of intergrowths caused a degradation in superconducting properties. The transition temperature was lowered and broadened with increasing intergrowth content (fig. 5). The intergrowth content was scaled according to the ratio of intensities for 2-O-2-1 (0010) and 2-l-2-2 (0010). The propet-ties could not be improved by annealing in air or oxygen at 820°C for up to 11 days.
4. Conclusion A strong correlation between starting composition, 2-O-2-1 intergrowth content, and superconducting properties for 2-l-2-2 single crystals grown from the flux was determined. Excess Cu promoted 2-O-2-1 intergrowths whereas small contents of Ca in excess of stoichiometry gave more “pure” 2-l-2-2 crystals. Contrary to the results for polycrystalline ceramic specimens [ 12 1, Pb additions stimulated 20-2-1 intergrowths, and caused a degradation in superconducting properties. The best starting composition for large single crystals with a sharp transition was 2 : 1.2: 2: 2.6. The largest Meissner fraction (7 1%) was obtained from smaller single crystals grown from the stoichiometric melt (2 : 1: 2 : 2).
Acknowledgement
The research was supported by U.S. DOE DMR DE-AC02-76ERO1198 and NSF-STC-88-09854. The
160
February 1990
assistance of A. Asthana, Z.K. Xu and L. Chang is gratefully acknowledged. References [ I] C. Michel, M. Hervieu, M.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Physik B 68 ( 1987) 421. [2]R.M. Hazen, C.T. Prewitt, R.J. Angel, N.L. Ross, L.W. Finger, C.G. Hadidacos, D.R. Veblen, P.J. Heaney, P.H. Hor, R.L. Meng, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, Z.J. Huang, L. Gao, J. Bechtold and C.W. Chu, Phys. Rev. Letters 60(1988) 1174. [3] M.A. Subramanian, CC. Torardi, J.S. Calabrese, J. Gopalakrishnan, K.J. Morissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 239 (1988) 1015. [4] H. Maeda, Y. Tanaka, M. Fukotami and T. Asano, Japan. J. Appl. Phys. 27 (1988) 209. [ 5] J.M. Tarascon, Y. le Page, L.H. Greene, B.G. Bagley, P. Barboux, D.M. Hwang, G.W. Hull, W.R. McKinnon and M. Giroud, Phys. Rev. B 38 (1988) 2504. [6] J.Z. Liu, G.W. Crabtree, L.E. Rehn, U. Geiser, W.K. Kwok, P.M. Baldo, J.M. Williams and D.J. Lam, Phys. Letters A 127 (1988) 444. [ 71 H. Takagi, H. Eisaki, S. Uchida, A. Maeda, S. Tajima, K. Uchinokura and S. Tanaka, Nature 332 (1988) 236. [8] T.F. Ciszek, J.P. Goral, CD. Evans and H. KatayamaYoshida, J. Crystal Growth 91 (1988) 312. [9] L.F. Schneemeyer, R.B. van Dover, S.H. Glarum, S.A. Sunshine, R.M. Flemming, B. Batlogg, T. Siegrist, J.H. Marshall, J.V. Waszczak and L.W. Rupp, Nature 332 ( 1988) 422. [lo] H.D. Brody, J.S. Haggerty, M.J. Cima, M.C. Fleming, R.L. Barns Gyorgy, D.W. Johnson, W.W. Rhodes, W.A. Sunder and R.A. Laud&, J. Crystal Growth 96 (1988) 225. [ 111 A.W. Sleight, M.A. Subramanian and C.C. Torardi, MRS Bull. 14 (1989) 45. [ 121 R.J. Cava, presented at the Spring Meeting of the American Physical Society, New Orleans, March 1988.