- Email: [email protected]
New family of superconducting copper oxides
PhysicaC 198 (1992) North-Holland
PHYSICA Ii
l-6
New family of superconducting copper oxides Bi cuprates
of type
12 12
A. Ehmann a, S. Kemmler-Sack a, S. LSsch a, M. Schlichenmaier ‘, W. Wischert a, P. Zoller T. Nissel b and R.P. Huebener b a Institut ftir Anorganische Chemie der Vniversitiit Tiibingen, Auf der Morgenstelle 18. D- 7400 Tiibingen, Germany
a,
b Physikalisches Institut der Vniversitiit Tiibingen, Lehrstuhlftir Experimentalphysik II, Auf der Morgenstelle 14, D-7400 Tiibingen, Germany Received
14 May 1992
Bi cuprates of type 12 12 have been synthesized above 970” C from the proper starting materials. The oxides of idealized composition (Bi,&u,,,)Sr2YCu20,_,crystallize in the tetragonal space group P4/mmm (a=3.815(5), c= 11.73( 1) A) and show solid solution behaviour. The as-prepared materials are non-superconducting above 5 K. However, post-treatment in flowing oxygen at moderate temperature ( = SOO’C) introduces superconductivity. The highest superconducting transition temperature is situated at 68 K for a sample of nominal composition (Bi&uO.,) - Sr2Y0.8Cu2.206.95.
1. Introduction Bi,Cu(l)
Bi cuprate superconductors of the 22 12 type have severe drawbacks for high-current applications at liquid nitrogen temperature, especially in the presence of a high magnetic field. This is due to the extreme two-dimensionality of the flux line lattice and has been attributed to the weak coupling of the superconducting charge-carrier wavefunctions across the (BiO) double layer [ 11, whilst the Tl- and Pbbased superconductors of the 12 12 type show strong flux pinning at 77 K owing to their thinner single layers of Tl or Pb ions separating the superconducting perovskite-like slabs [ 2,3 1. These differences are easily explainable by the strong structural correlation between the 12 12 structure and the 123 structure of YBCO (fig. 1). In fact, YBCO-123 is an analog and special form of the 12 12 structure. The only difference is that the single plane of rocksalt contiguration in the 12 12 structure is replaced by the CuO chain in the 123 structure. This replacement decreases the symmetry of the structure from tetragonal (space group P4/mmm; 1212 phase) to orthorhombic (space group Pmmm; 123 phase). From that comparison follows, additionally, that 12 12 is superior to 123 owing to its higher crystalline sym0921-4534/92/$05.00
0 1992 Elsevier Science Publishers
Bi,Cu(l) 1212 structure Comparison Fig. 1. (Bi0.5Cu0.5)Sr2Y-Cu20,
between 1212 structure and 123 structure of YBazCu90,.
of
metry and the absence of a structural transformation from a non-superconducting, tetragonal high-temperature material into the orthorhombic superconductor. In practice, a significant drawback of the 12 12 materials is that only thallium- and/or lead-containing materials are known [4]. However, the toxicity of lead oxides and especially of thallium oxides has limited their widespread use, and the synthesis of bismuth-based 1212 materials would be of consid-
B.V. All rights reserved.
2
A. Ehmann et al. /New family of superconducting copper oxides
erable practical importance. By adjusting the conditions of preparation we have been able to synthesize the first Bi cuprates of the 1212 type with a range of composition around (Bi, Cu)Sr,YCuzO,. The as-prepared materials are non-superconducting. However, superconductivity is obtained by posttreatment in flowing O2 at about 500°C [ 5 1, with the highest transition temperature situated at 68 K for a sample of nominal composition (B& - CU~.~)Sr2Yo.8Cu2.206.95.
2. Experimental Samples of nominal composition (Bi,_,Cu,)Sr2_xM,(Y1_,M,)Cu2-0,_, (M=Cu, Ba) were prepared via a solid state reaction from Bi(N03)3.5H20, Sr(N03)2, Ba(N03)2 (all p.a.; Merck), CuO (puriss., P.A.; Fluka) and Y203 (99.99%; Rhone-Poulenc). After an initial decomposition of the nitrates at x 700°C ( 1 h) in alumina crucibles (Degussit A123), the samples were ground in an agate mortar and heated in air at 960”-985°C. The reaction was interrupted several times for regrindings and for X-ray analysis (Philips powder diffractometer, Cu Ku radiation, Au standard). The heat-treatment was stopped after no further change was detected in the X-ray diagram. This stage was obtained after a total reaction time of lo-25 h. All samples were air quenched. Aliquots of these as-prepared materials were subsequently submitted to several post-treatments: quenching with liquid N2 from the reaction temperature, annealing in flowing Ar or flowing oxygen at the reaction temperature, heattreatment in flowing O2 at 500°C for 1-16 h and cooling down to room temperature at a rate of z 50’ C/h. The samples were characterized by XRD, SEM, EDXA, DC-susceptibility (SQUID) measurements and redox titration for the determination of the oxygen content [ 61.
3. Results and discussion 3.1. XRD and SEM investigations The XRD patterns of as-prepared crushed samples of nominal composition (Bio.SCuo.s)Sr2YCu20,_,
indicate the presence of a nearly homogeneous 12 12 phase (fig. 2). The unit cell is tetragonal with the lattice constants aE3.815 (5) and c=11.73 (1) A. SEM investigations of the surface as well as of fractured cross-sections of a pellet indicate a plate-like texture (fig. 3), a typical observation for a layered crystal structure. Especially, the images of the surface area show a well-developed preferred orientation of the platelets with the c-axis normal to the surface plane, likewise easily deducible from the XRD pattern of pressed pellets by the pronounced increase of the intensity for the (001) reflections. However, from a comparison with the SEM images of Bi-based cuprates of type 2212 with two rocksalt-like layers between the perovskite blocks (e.g. the SEM images in refs. [ 7,8 ] ), it follows that in the present 12 12 type the mica-like plate texture is less pronounced. Thus, the single rocksalt layer of the 12 12 type is likely to mediate a stiffer connection between the perovskite blocks than the double layer case of 2212. EDXA conducted on different areas as well as on different grains indicate a nearly homogeneous distribution of the elements Bi, Sr, Y and Cu. From an analysis of the X-ray powder diffraction data follows the conclusion that the crystal structure of (Bio.SCu0.s)Sr2YCu20,_, is isotypic with the 1212 prototype of space group P4/mmm [ 9 1. The relined atomic coordinates and thermal displacement parameters are given in table 1. Table 2 shows the observed and calculated intensities. From a comparison of the distribution of the elements in the Bi cuprate of 12 12 type and YBCO- 123 in fig. 1, it follows that the Y-atoms similarly occupy the central position accompanied above and below by slightly wavy Cu( 2) 0 ( 1 )2 sheets. The nearly square planar oxygen coordination of the Cu (2) is completed by 0( 2) to an elongated quadratic pyramid with Cu(2)-0( 1)=4x 1.92 8, and Cu(2)-O(2)= 1 x 2.43 A. The rocksalt-like single layer, situated in the basal plane, is occupied by the statistically distributed Bi and Cu( 1) atoms. From the structure determination by neutron powder diffraction of the Pbcontaining 1212 type (Pbo.&uo.35)Sr(Yo.,Cao.~)[ 91, it is known that the rocksalt-like arcu207+, rangement is highly idealized and a better description is given by a split of the ideal la and 1c sites into 41 and 4n. Pointing in the same direction are the actually observed relatively high thermal parameters
A. Ehmann et al. /New family of superconducting copper oxides
I
60
53
50
45
LO
35
30
25
20
cze
15
Fig. 2. Section of the XRD pattern of a powdered sample of ( Bio,5Cuo.s)SrzYCu20,_.. The reflections of the 1212 phase are indicated. for Bi and Cu( 1), combined with the reduced occupancy on the corresponding ideal la site. The presence of modulation can be quoted as a possible explanation, having its origin in the mismatch between the enlarged rocksalt- and the narrowed perovskite-like section of the lattice. However, owing to reduced accuracy, the X-ray powder data were not further refined and neutron diffraction and TEM studies are planned.
3.2. Magnetic susceptibility and solid solution behaviour composition materials of As-prepared (Bi,&uo.s)SrzYCuz07_, are non-superconducting above 5 K. Since in other families of cuprates superconductivity is developed after different methods of post-treatment, similar experiments were performed with the present materials. However, posttreatment in flowing 02, as well as in flowing Ar, at the reaction temperature (975”C/l h) unfortunately results in a complete decomposition of the 12 12 structure. Quenching with liquid N2 from 975°C does not affect the phase purity, but no transition to superconductivity above 5 K is observed. Actually, the best method for the introduction of superconductivity is prolonged heating in flowing O2
at 500’ C and subsequent slow cooling to room temperature. However, note that these conditions are not yet optimized. A second attempt at the introduction of superconductivity is based on the solid solution behaviour of the Bi-based 1212 material. In fact, several substitution routes have been tested and materials with a predominant 12 12 content were obtained for a partial substitution of Bi for Cu on the la site, for Cu on 2h (e.g. (Bi,,sCu0.s)Sr,.8Cu0.2YCu207-r) and/or Id ( (Bi,,,CuO.S) -SrZY0.8Cu2.207--r), for Ba on 2h and/or Id ( (Bio.sCuo.s)Sr1.8~Ba0.2YCu207--z, (Bi,,,Cu0.s)Sr2Ba0.8Y0.2CuZ07_z), as well as for Ln in 1d. However, irrespective of the charge of the substituting element, the as-prepared substituted materials are non-superconducting above 5 K. But, similarly to the unsubstituted case of (Bio,,Cuo.s)Sr2YCuZ07_-r, post-annealing in flowing 0, at 500” C introduces superconductivity. Actually, the highest transition temperature of T,=68 K is found for a material of nominal composition ( Bi,,sCuo.s)Srz - Y0.8Cu2.207_-z after a posttreatment in flowing O2 at SOO’C/ 16 h and slow cooling to room temperature. The corresponding x versus T dependence is shown in fig. 4. The diamagnetism is still comparatively weak, equivalent to only a few per cent of a perfect superconductor of the
A. Ehmann et al. /New family of superconducting copper oxides
Fig. 3. SEM of a pellet of (Bio,5Cuo.s)Sr2YCu207_-lr(a) surface, (b) fractured cross-section.
same volume. However, from the observation that the diagmagnetism is even weaker after a shorter time of post-annealing in flowing 02, we deduce a correlation between the post-treatment and the superconducting volume fraction. A similar observation holds for the Pb-based 12 12 type [ 31, likewise exhibiting a superconducting fraction of only a few per For 1212 cent. the Bi-based cuprates
(B~&uo.s Pr1.8YCu2.207--z
and (Bio,,-Cu0.5)Sr,,8Ba0.ZYCuZ07_r, the transition temperatures of 58 K and 50 K are situated at slightly inferior values after post-treatment in flowing oxygen. The introduction of superconductivity by oxygen annealing is most probably due to hole doping. From a comparison of the average oxygen content of, e.g., the non-superconducting as-prepared material
A. Ehmann et al. /New family of superconducting copper oxides Table 1 Atomic coordinates and isotropic thermal mmm (~1); R,,,=~:ll,-I,l/~1,=13.81
Table 2 Observed
displacement
parameters
R (A’) for (Bi,&u,,
,)Sr,YCuZO,
5
in the tetragonal
Atom
Site
X
Y
z
Occupancy
B
Bi Cu(l) Sr Y Cu(2) O(1) O(2) O(3)
la la 2h Id 2g 4i 2g lc
0 0 t I
0 0 t 4 0 t 0 t
0 0 0.1985 t 0.3570 0.3730 0.1500 0
0.44 0.30 1.00 1.00 1.oo 1.00 1.00 1.00
2.5 2.5 0.0 0.0 1.5 0.0 0.5 0.0
(lo) and calculated
intensities
hkl
; 0 0 f
space group P4/
(I,) for Bi0.5Cu0.&ZYCu207 IO
4
001 002
12 6
33 9
003 100 101 102 004 103 110 111 112 005 104 > 113 105 114 006 200 201 202
3 10 88 95 0
1 4 48 84 4 1000 683 29 3 75 154 34 1 82 78 369 2 1
1602 13 0 299 34 6 82 83 316 13 0
10
hkl
(Bio.5Cu0.5)SrZY0.8Cu2.206.74 and the 02-treated 68 K superconductor ( Bio.sCuo.s)Sr2Y0.8Cu2.206.95, follows an increase of the average oxidation state of Cu from + 2.07 to +2.22. In the latter case, the wellknown threshold value for the development of superconductivity in hole-doped cuprates of about 0.2 holes/Cu ion is just reached. Note that the above values have been calculated with the assumption of fixed valencies for Sr( +2), Y( +3) and Bi( +3). The crystal structure of the 12 12 material is not affected by the O2 post-treatment and the lattice constants of the tetragonal unit cell are identical within
115 106
66 3
203 210/120 211/121 007 1221212 204 116 1231213 107 205 1241214 008 117 1251215 108 206 220
the
L
31 9 36 19 6 147 322 74 115 18 45 0 89 70
limit
of
the
52 0 1 5 9 23 21 5 135 324 50 54 64 6 24 0 27 78 97
experimental
error (e.g. a=3.811(5), (Bio.sCuo.s )SrzYo.8Cuz.z07-.: c=11.73(1) 8, (as-prepared); a=3.814(5), c= 11.72 ( 1) A ( O2 post-annealed) ). Consequently, no phase transformation takes place.
4. Conclusions Superconducting Bi cuprates of type 12 12 are existent with a range of composition around (Bi, Cu)Sr,YCuzO,. The crystal structure is tetragonal
A. Ehmann et al. /New family ofsuperconducting copper oxides 120
temperature obtainable.
of about
100 K should
be likewise
Ii1
Acknowledgements
-0.00035
This work was supported by the Bundesministerium fur Forschung und Technologie (FKZ 13N5842 ), the Forschungsschwerpunkt “Supraleitung” des Landes Baden-Wiirttemberg and the Verband der Chemischen Industrie. The help of E. Niquet is much appreciated.
1
Fig. 4. ,y vs. T for a zero field cooled powder sample of (F.C.)
(Z.F.C.) and field cooled oxygen post-treated
(Bio.sCuo.s)SrzYo.sCu2.20,-,.
References [ 1] K.E. Gray and D.H. Kim, Physica C 180 ( 199 1) 139.
(space group P4/mmm) and, contrary to the case of YBCO-123, the development of superconductivity is not combined with a transformation to a lower lattice symmetry. However, similarly to YBCO-123, the transition to superconductivity depends on the hole concentration of the material and the well-known threshold value of 0.2 holes/Cu ion seems likewise suitable. The major difference between the Bi-based materials of types 12 12 and 22 12 lies in the reduced two-dimensionallity of the former, easily expressed by the stiffer connection between the perovskite slabs via the single rocksalt layer. At present, however, we have not fully determined the optimum synthesis conditions for the largest superconducting fraction and the highest superconducting temperature. But, from a comparison with isostructural Tl- and Pbbased 1212 materials, it follows that a transition
T. Kano, T. Doi, A. Soeta, T. Yuasa, N. Inoue, K. Aihara and S. Matsuda, Appl. Phys. Lett. 59 ( 1991) 3 186. 1T.P. Beales, C. Dineen, W.G. Freeman, S.R. Hall, M.R. Harrison, D.M. Jacobson and S.J. Zammattio, Supercond. Sci. Technol. 5 ( 1992) 47. Z.Z. Sheng, Y.F. Li and D.O. Pederson, Proc. 5th Annual Conf. of Superconductivity and its Applications, Buffalo, N.Y., 199 1, preprint. S. Lasch and P. Zoller, 1A. Ehmann, S. Kemmler-Sack, Karlsruher HTSL-Informationstreffen, 9 April 1992. W. Schafer, J. Maier-Rosenkranz, S. Lijsch, R. Kiemel, W. Wischert and S. Kemmler-Sack, J. Less-Common Met. 142 (1988) L5. M. Schlichenmaier, S. L&h, S. Kemmler-Sack, D. Kiille, M. Hartmann and R.P. Huebener, J. Less-Common Met. 160 (1990) Ll. S. Losch, H. Budin, 0. Eible, M. Hartmann, T. Rentschler, M. Rygula, S. Kemmler-Sack and R.P. Huebener, Physica C 177 (1991) 271. 1T. Maeda, K. Sakyama, F. Izumi, H. Yamauchi, H. Asano and S. Tanaka, Physica C I75 ( 199 1) 39 1 and refs. therein.
PHYSICA Ii
l-6
New family of superconducting copper oxides Bi cuprates
of type
12 12
A. Ehmann a, S. Kemmler-Sack a, S. LSsch a, M. Schlichenmaier ‘, W. Wischert a, P. Zoller T. Nissel b and R.P. Huebener b a Institut ftir Anorganische Chemie der Vniversitiit Tiibingen, Auf der Morgenstelle 18. D- 7400 Tiibingen, Germany
a,
b Physikalisches Institut der Vniversitiit Tiibingen, Lehrstuhlftir Experimentalphysik II, Auf der Morgenstelle 14, D-7400 Tiibingen, Germany Received
14 May 1992
Bi cuprates of type 12 12 have been synthesized above 970” C from the proper starting materials. The oxides of idealized composition (Bi,&u,,,)Sr2YCu20,_,crystallize in the tetragonal space group P4/mmm (a=3.815(5), c= 11.73( 1) A) and show solid solution behaviour. The as-prepared materials are non-superconducting above 5 K. However, post-treatment in flowing oxygen at moderate temperature ( = SOO’C) introduces superconductivity. The highest superconducting transition temperature is situated at 68 K for a sample of nominal composition (Bi&uO.,) - Sr2Y0.8Cu2.206.95.
1. Introduction Bi,Cu(l)
Bi cuprate superconductors of the 22 12 type have severe drawbacks for high-current applications at liquid nitrogen temperature, especially in the presence of a high magnetic field. This is due to the extreme two-dimensionality of the flux line lattice and has been attributed to the weak coupling of the superconducting charge-carrier wavefunctions across the (BiO) double layer [ 11, whilst the Tl- and Pbbased superconductors of the 12 12 type show strong flux pinning at 77 K owing to their thinner single layers of Tl or Pb ions separating the superconducting perovskite-like slabs [ 2,3 1. These differences are easily explainable by the strong structural correlation between the 12 12 structure and the 123 structure of YBCO (fig. 1). In fact, YBCO-123 is an analog and special form of the 12 12 structure. The only difference is that the single plane of rocksalt contiguration in the 12 12 structure is replaced by the CuO chain in the 123 structure. This replacement decreases the symmetry of the structure from tetragonal (space group P4/mmm; 1212 phase) to orthorhombic (space group Pmmm; 123 phase). From that comparison follows, additionally, that 12 12 is superior to 123 owing to its higher crystalline sym0921-4534/92/$05.00
0 1992 Elsevier Science Publishers
Bi,Cu(l) 1212 structure Comparison Fig. 1. (Bi0.5Cu0.5)Sr2Y-Cu20,
between 1212 structure and 123 structure of YBazCu90,.
of
metry and the absence of a structural transformation from a non-superconducting, tetragonal high-temperature material into the orthorhombic superconductor. In practice, a significant drawback of the 12 12 materials is that only thallium- and/or lead-containing materials are known [4]. However, the toxicity of lead oxides and especially of thallium oxides has limited their widespread use, and the synthesis of bismuth-based 1212 materials would be of consid-
B.V. All rights reserved.
2
A. Ehmann et al. /New family of superconducting copper oxides
erable practical importance. By adjusting the conditions of preparation we have been able to synthesize the first Bi cuprates of the 1212 type with a range of composition around (Bi, Cu)Sr,YCuzO,. The as-prepared materials are non-superconducting. However, superconductivity is obtained by posttreatment in flowing O2 at about 500°C [ 5 1, with the highest transition temperature situated at 68 K for a sample of nominal composition (B& - CU~.~)Sr2Yo.8Cu2.206.95.
2. Experimental Samples of nominal composition (Bi,_,Cu,)Sr2_xM,(Y1_,M,)Cu2-0,_, (M=Cu, Ba) were prepared via a solid state reaction from Bi(N03)3.5H20, Sr(N03)2, Ba(N03)2 (all p.a.; Merck), CuO (puriss., P.A.; Fluka) and Y203 (99.99%; Rhone-Poulenc). After an initial decomposition of the nitrates at x 700°C ( 1 h) in alumina crucibles (Degussit A123), the samples were ground in an agate mortar and heated in air at 960”-985°C. The reaction was interrupted several times for regrindings and for X-ray analysis (Philips powder diffractometer, Cu Ku radiation, Au standard). The heat-treatment was stopped after no further change was detected in the X-ray diagram. This stage was obtained after a total reaction time of lo-25 h. All samples were air quenched. Aliquots of these as-prepared materials were subsequently submitted to several post-treatments: quenching with liquid N2 from the reaction temperature, annealing in flowing Ar or flowing oxygen at the reaction temperature, heattreatment in flowing O2 at 500°C for 1-16 h and cooling down to room temperature at a rate of z 50’ C/h. The samples were characterized by XRD, SEM, EDXA, DC-susceptibility (SQUID) measurements and redox titration for the determination of the oxygen content [ 61.
3. Results and discussion 3.1. XRD and SEM investigations The XRD patterns of as-prepared crushed samples of nominal composition (Bio.SCuo.s)Sr2YCu20,_,
indicate the presence of a nearly homogeneous 12 12 phase (fig. 2). The unit cell is tetragonal with the lattice constants aE3.815 (5) and c=11.73 (1) A. SEM investigations of the surface as well as of fractured cross-sections of a pellet indicate a plate-like texture (fig. 3), a typical observation for a layered crystal structure. Especially, the images of the surface area show a well-developed preferred orientation of the platelets with the c-axis normal to the surface plane, likewise easily deducible from the XRD pattern of pressed pellets by the pronounced increase of the intensity for the (001) reflections. However, from a comparison with the SEM images of Bi-based cuprates of type 2212 with two rocksalt-like layers between the perovskite blocks (e.g. the SEM images in refs. [ 7,8 ] ), it follows that in the present 12 12 type the mica-like plate texture is less pronounced. Thus, the single rocksalt layer of the 12 12 type is likely to mediate a stiffer connection between the perovskite blocks than the double layer case of 2212. EDXA conducted on different areas as well as on different grains indicate a nearly homogeneous distribution of the elements Bi, Sr, Y and Cu. From an analysis of the X-ray powder diffraction data follows the conclusion that the crystal structure of (Bio.SCu0.s)Sr2YCu20,_, is isotypic with the 1212 prototype of space group P4/mmm [ 9 1. The relined atomic coordinates and thermal displacement parameters are given in table 1. Table 2 shows the observed and calculated intensities. From a comparison of the distribution of the elements in the Bi cuprate of 12 12 type and YBCO- 123 in fig. 1, it follows that the Y-atoms similarly occupy the central position accompanied above and below by slightly wavy Cu( 2) 0 ( 1 )2 sheets. The nearly square planar oxygen coordination of the Cu (2) is completed by 0( 2) to an elongated quadratic pyramid with Cu(2)-0( 1)=4x 1.92 8, and Cu(2)-O(2)= 1 x 2.43 A. The rocksalt-like single layer, situated in the basal plane, is occupied by the statistically distributed Bi and Cu( 1) atoms. From the structure determination by neutron powder diffraction of the Pbcontaining 1212 type (Pbo.&uo.35)Sr(Yo.,Cao.~)[ 91, it is known that the rocksalt-like arcu207+, rangement is highly idealized and a better description is given by a split of the ideal la and 1c sites into 41 and 4n. Pointing in the same direction are the actually observed relatively high thermal parameters
A. Ehmann et al. /New family of superconducting copper oxides
I
60
53
50
45
LO
35
30
25
20
cze
15
Fig. 2. Section of the XRD pattern of a powdered sample of ( Bio,5Cuo.s)SrzYCu20,_.. The reflections of the 1212 phase are indicated. for Bi and Cu( 1), combined with the reduced occupancy on the corresponding ideal la site. The presence of modulation can be quoted as a possible explanation, having its origin in the mismatch between the enlarged rocksalt- and the narrowed perovskite-like section of the lattice. However, owing to reduced accuracy, the X-ray powder data were not further refined and neutron diffraction and TEM studies are planned.
3.2. Magnetic susceptibility and solid solution behaviour composition materials of As-prepared (Bi,&uo.s)SrzYCuz07_, are non-superconducting above 5 K. Since in other families of cuprates superconductivity is developed after different methods of post-treatment, similar experiments were performed with the present materials. However, posttreatment in flowing 02, as well as in flowing Ar, at the reaction temperature (975”C/l h) unfortunately results in a complete decomposition of the 12 12 structure. Quenching with liquid N2 from 975°C does not affect the phase purity, but no transition to superconductivity above 5 K is observed. Actually, the best method for the introduction of superconductivity is prolonged heating in flowing O2
at 500’ C and subsequent slow cooling to room temperature. However, note that these conditions are not yet optimized. A second attempt at the introduction of superconductivity is based on the solid solution behaviour of the Bi-based 1212 material. In fact, several substitution routes have been tested and materials with a predominant 12 12 content were obtained for a partial substitution of Bi for Cu on the la site, for Cu on 2h (e.g. (Bi,,sCu0.s)Sr,.8Cu0.2YCu207-r) and/or Id ( (Bi,,,CuO.S) -SrZY0.8Cu2.207--r), for Ba on 2h and/or Id ( (Bio.sCuo.s)Sr1.8~Ba0.2YCu207--z, (Bi,,,Cu0.s)Sr2Ba0.8Y0.2CuZ07_z), as well as for Ln in 1d. However, irrespective of the charge of the substituting element, the as-prepared substituted materials are non-superconducting above 5 K. But, similarly to the unsubstituted case of (Bio,,Cuo.s)Sr2YCuZ07_-r, post-annealing in flowing 0, at 500” C introduces superconductivity. Actually, the highest transition temperature of T,=68 K is found for a material of nominal composition ( Bi,,sCuo.s)Srz - Y0.8Cu2.207_-z after a posttreatment in flowing O2 at SOO’C/ 16 h and slow cooling to room temperature. The corresponding x versus T dependence is shown in fig. 4. The diamagnetism is still comparatively weak, equivalent to only a few per cent of a perfect superconductor of the
A. Ehmann et al. /New family of superconducting copper oxides
Fig. 3. SEM of a pellet of (Bio,5Cuo.s)Sr2YCu207_-lr(a) surface, (b) fractured cross-section.
same volume. However, from the observation that the diagmagnetism is even weaker after a shorter time of post-annealing in flowing 02, we deduce a correlation between the post-treatment and the superconducting volume fraction. A similar observation holds for the Pb-based 12 12 type [ 31, likewise exhibiting a superconducting fraction of only a few per For 1212 cent. the Bi-based cuprates
(B~&uo.s Pr1.8YCu2.207--z
and (Bio,,-Cu0.5)Sr,,8Ba0.ZYCuZ07_r, the transition temperatures of 58 K and 50 K are situated at slightly inferior values after post-treatment in flowing oxygen. The introduction of superconductivity by oxygen annealing is most probably due to hole doping. From a comparison of the average oxygen content of, e.g., the non-superconducting as-prepared material
A. Ehmann et al. /New family of superconducting copper oxides Table 1 Atomic coordinates and isotropic thermal mmm (~1); R,,,=~:ll,-I,l/~1,=13.81
Table 2 Observed
displacement
parameters
R (A’) for (Bi,&u,,
,)Sr,YCuZO,
5
in the tetragonal
Atom
Site
X
Y
z
Occupancy
B
Bi Cu(l) Sr Y Cu(2) O(1) O(2) O(3)
la la 2h Id 2g 4i 2g lc
0 0 t I
0 0 t 4 0 t 0 t
0 0 0.1985 t 0.3570 0.3730 0.1500 0
0.44 0.30 1.00 1.00 1.oo 1.00 1.00 1.00
2.5 2.5 0.0 0.0 1.5 0.0 0.5 0.0
(lo) and calculated
intensities
hkl
; 0 0 f
space group P4/
(I,) for Bi0.5Cu0.&ZYCu207 IO
4
001 002
12 6
33 9
003 100 101 102 004 103 110 111 112 005 104 > 113 105 114 006 200 201 202
3 10 88 95 0
1 4 48 84 4 1000 683 29 3 75 154 34 1 82 78 369 2 1
1602 13 0 299 34 6 82 83 316 13 0
10
hkl
(Bio.5Cu0.5)SrZY0.8Cu2.206.74 and the 02-treated 68 K superconductor ( Bio.sCuo.s)Sr2Y0.8Cu2.206.95, follows an increase of the average oxidation state of Cu from + 2.07 to +2.22. In the latter case, the wellknown threshold value for the development of superconductivity in hole-doped cuprates of about 0.2 holes/Cu ion is just reached. Note that the above values have been calculated with the assumption of fixed valencies for Sr( +2), Y( +3) and Bi( +3). The crystal structure of the 12 12 material is not affected by the O2 post-treatment and the lattice constants of the tetragonal unit cell are identical within
115 106
66 3
203 210/120 211/121 007 1221212 204 116 1231213 107 205 1241214 008 117 1251215 108 206 220
the
L
31 9 36 19 6 147 322 74 115 18 45 0 89 70
limit
of
the
52 0 1 5 9 23 21 5 135 324 50 54 64 6 24 0 27 78 97
experimental
error (e.g. a=3.811(5), (Bio.sCuo.s )SrzYo.8Cuz.z07-.: c=11.73(1) 8, (as-prepared); a=3.814(5), c= 11.72 ( 1) A ( O2 post-annealed) ). Consequently, no phase transformation takes place.
4. Conclusions Superconducting Bi cuprates of type 12 12 are existent with a range of composition around (Bi, Cu)Sr,YCuzO,. The crystal structure is tetragonal
A. Ehmann et al. /New family ofsuperconducting copper oxides 120
temperature obtainable.
of about
100 K should
be likewise
Ii1
Acknowledgements
-0.00035
This work was supported by the Bundesministerium fur Forschung und Technologie (FKZ 13N5842 ), the Forschungsschwerpunkt “Supraleitung” des Landes Baden-Wiirttemberg and the Verband der Chemischen Industrie. The help of E. Niquet is much appreciated.
1
Fig. 4. ,y vs. T for a zero field cooled powder sample of (F.C.)
(Z.F.C.) and field cooled oxygen post-treated
(Bio.sCuo.s)SrzYo.sCu2.20,-,.
References [ 1] K.E. Gray and D.H. Kim, Physica C 180 ( 199 1) 139.
(space group P4/mmm) and, contrary to the case of YBCO-123, the development of superconductivity is not combined with a transformation to a lower lattice symmetry. However, similarly to YBCO-123, the transition to superconductivity depends on the hole concentration of the material and the well-known threshold value of 0.2 holes/Cu ion seems likewise suitable. The major difference between the Bi-based materials of types 12 12 and 22 12 lies in the reduced two-dimensionallity of the former, easily expressed by the stiffer connection between the perovskite slabs via the single rocksalt layer. At present, however, we have not fully determined the optimum synthesis conditions for the largest superconducting fraction and the highest superconducting temperature. But, from a comparison with isostructural Tl- and Pbbased 1212 materials, it follows that a transition
T. Kano, T. Doi, A. Soeta, T. Yuasa, N. Inoue, K. Aihara and S. Matsuda, Appl. Phys. Lett. 59 ( 1991) 3 186. 1T.P. Beales, C. Dineen, W.G. Freeman, S.R. Hall, M.R. Harrison, D.M. Jacobson and S.J. Zammattio, Supercond. Sci. Technol. 5 ( 1992) 47. Z.Z. Sheng, Y.F. Li and D.O. Pederson, Proc. 5th Annual Conf. of Superconductivity and its Applications, Buffalo, N.Y., 199 1, preprint. S. Lasch and P. Zoller, 1A. Ehmann, S. Kemmler-Sack, Karlsruher HTSL-Informationstreffen, 9 April 1992. W. Schafer, J. Maier-Rosenkranz, S. Lijsch, R. Kiemel, W. Wischert and S. Kemmler-Sack, J. Less-Common Met. 142 (1988) L5. M. Schlichenmaier, S. L&h, S. Kemmler-Sack, D. Kiille, M. Hartmann and R.P. Huebener, J. Less-Common Met. 160 (1990) Ll. S. Losch, H. Budin, 0. Eible, M. Hartmann, T. Rentschler, M. Rygula, S. Kemmler-Sack and R.P. Huebener, Physica C 177 (1991) 271. 1T. Maeda, K. Sakyama, F. Izumi, H. Yamauchi, H. Asano and S. Tanaka, Physica C I75 ( 199 1) 39 1 and refs. therein.