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Identification and preparation of single phase 90 K oxide superconductor and structural determination by lattice imaging
Solid State Communications,Vo1.63,No.Z, pp.147-150, 1987. Printed in Great Britain.
Identification and Preparation of Siie
0038-1098/87 $3.00 + .OO Pergamon Journals Ltd.
Phase 90 K Oxide Superconductor
and Structoral Detemination
by Lattice Imaging
W.J. Gallagher, R.L. Sandstrom, T.R. Dinger, T.M. Shaw and D.A. Chance IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598 (Received 17 March 1987 by G. Burns) We have investigated various compositions of Ba-Y-Cu-0, resulting in the preparation of single phase 90 K superconducting material of composition (Ba,.,,Y,.,,)CuO,_s and the identification of the structure of this superconducting phase by lattice imaging as a layered perovskite. The single phase material had a resistive transition with a midpoint at 92 K and a 1.3 K width (lo%-90%) as well as a sharp low-field (H = 0.01 Oe) susceptibility transition.
Y,O, (325 mesh, 99.99% pure), and CuO (200 mesh, 99.9% pure). Approximately 1.75 g of each of the ground mixtures were pressed into 12 mm disks at a pressure of 4 13 MPa (60,000 psi). The firing conditions in air for this first set of mixtures were a ramp from 25 C to 1110 C in about 3 hours, sintering at 1110 C for 12 hours, and finally cooling to 25 C within the oven in 3.5 hours. After sintering, a diamond saw was used to cut strips from the disks for electrical testing. Two- and four-terminal dc resistances were measured at room temperature and four-terminal ac resistances were measured as a function of temperature using spring loaded pins for contacts. Typically, a current of 1mA was used in the measurements. Temperature was measured with a Si diode estimated to be accurate to + 0.5 K. After firing this first set of samples it was apparent that some of the disks had partially melted. In particular the disk with a nominal composition (Ba,,4Y0.6)0.67 CuO, had lost -40% of its initial mass. The sample with x = 3 was greenish, while those with x values ranging from 2. to 1.5 were a deep black with a greenish hue that became progressively less with lower x. A sample with x = 1.33 was greyish-black and samples with x = 1 and x = 0.67 had progressively lighter shades of grey. Upon storage in dry air, the surfaces of the samples with 1.5 I x < 2 seemed to pick up more of a greenish hue. The sample having compositions (Bq,4Y0,6)0_67C~Oywas highly resistive at low temperatures. All the other compositions in this series were high transition temperature superconductors. Midpoints of the resistive transitions varied from 86 K to 91.5 K, and zero resistance was achieved below 79.5 K to 89 K. The samples with x = 1.67 and x = 2 had the broadest and lowest transitions, with lo%-90% transition widths of 7.7 and 6.7 K, respectively. Three compositions with x= 1 (i.e., samples with the nominal compositions B~.~,YO,,,CuO,, Ba,,Y,,CuO,, and B~~~~Y~~~~CuO,) showed the sharpest transitions with (lo%-90%) resistive transition widths of 1.8K, 2.3K, and 1.8 K, respectively. The occurrence
There has been a flood of activity following the breakthrough discovery of high-T, perovskite-type superconductors by Bednorz and Mtilleri and the subsequent confirmation of this result with susceptibility measurements by Bednorz, et a1.2, Uchida et a1.3, and Chu et a1.4. Recently, Wu et a1.,5*6s7 and then Tarascon et al.* and Zhao et al.9 have reported superconductivity at -9OK in YBaCuO variants of the LaBaCuO perovskite-like mixture originally investigated by Bednorz and Mtiller. All three groups noted that the particular compositions they studied were multiphase mixtures, with Tarascon et al.* having identified two of those in their samples as Y,Cu,O, and Y,O, and having noted that their samples contained a third and possibly other phases as well. The samples were said to be green or to possess a greenish hue. In this paper we report on a study of a series of compositions of BaYCuO prepared under different conditions. Our studies have allowed us to isolate the T, N 90K phase in the BaYCuO family as (Ba,.,,Y,.,,)CuO,_, with a distorted layered perovskite structure and to identify some details of the BaO-Y,O,-CuO pseudo ternary phase diagram. After we had identified and prepared single phase samples of (Ba~~~~Y~~,,)CuO,~ y we learned of the work of Grant et al.10 and of that of Cava et al.11 who have also identified and prepared this single phase superconductor. The preparation details available in the Wu et al. papers526.7 were sketchy, though it was stated by Chu et al.’ that the > 90 K superconducting transitions were observed in mixtures with nominal compositions Ba,,Y,,CuO,_, and Ba,,Y,,,CuO,_,, with the latter having a sharper resistive transition. This led us to initially investigate a series of compositions near the line connecting these two points in the BaO-CuO-Y,O, ternary phase diagram. Specifically, we prepared nominal compositions (Ba,Y,.,),CuO, with z= 0.35, 0.4, and 0.45 and x ranging from 0.67 to 3. These compositions were prepared by grinding mixtures of the appropriate molar ratios of BaO (100 mesh, typically 99.5% pure), 147
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STRUCTURAL DETERMINATION BY LATTICE IMAGING
of narrower transition widths for x=1 than for x=2 is the reverse of what was true for Chu et al’s samples.’ The samples with the sharpest transitions also displayed the highest temperature transitions. The highest transition in this series occurred for the sample with nominal composition B~,,,Y,-,,,CuO, and is plotted in Fig. 1 (a) as the curve triangular symbols. It showed an onset at 94 K, a midpoint at 91K, and complete superconductivity by 88K. The 1.8K resistive transition widths observed for our sharpest transitions are similar to those we have achieved12 for La,,,,Sr,,,,Cu04_, samples that were prepared in a manner similar to the present sample, but these BaYCuO samples had more than ten-fold greater bulk resistivities as well as much higher contact resistances. The zero-field-cooled low field (H = 0.01 Oe) diamagnetic susceptibility and the fieldcooled susceptibility (Meissner effect) of this sample were measured’3 in a manner similar to that used by Maletta et a1.r4 and the data for these are plotted in Fig. 1 (b) with solid and open triangles respectively. In contrast to the resistive transition, the susceptibilities for this sample show a very broad transition with an onset just above 90 K. Furthermore the data suggests a possible second transition in the vicinity of -60 K, but we have observed no other indications of such a transition. Optical and scanning electron micrographs taken of the sample with nominal composition Ba,.,,Y,.,,CuO, revealed the presence of three phases. The microstructure of this material is shown in the optical micrograph
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of Fig. 2 (a) taken from a specimen which had been polished with 6 pm diamond paste and etched with a concentrated FeCl, solution to reveal grain boundaries and provide surface relief. Three phases are clearly observed as areas of different contrast or morphology. The major phase observed exhibited long lath-shaped grains and was continuous. This phase is seen in dark contrast as long grains extending from upper left to lower right in the micrograph. Energy dispersive x-ray spectroscopy (EDXS) identified this compound as being a Ba/Cu-rich compound with a minor amount of Y. The equiaxed (roughly spherical in most cases) grains shown in dark contrast in the micrograph were observed to be Y-rich. The third phase observed was the crystallized remnants of a eutectic liquid which had infiltrated the multi-grain channels of the specimen and can be seen as the light contrast phase in the optical micrograph. The EDXS data show this phase to be predominantly CuO; trace amounts of Y or Ba below the detectability limit of the EDXS technique may be present. The elongated lath-like morphology of the Ba/Cu-rich phase suggests that they crystallized from the eutectic liquid which was formed at the processing temperature. Having proven that the superconducting phase of BaYCuO would form over a wide range of compositions, we prepared a broader range of compositions to explore the extent of the compatibility region and to obtain samples with a higher percentage of the superconducting phase. The selection of compositions was
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Figure 1: (a) Resistive transitions of a multiphase sample with nominal composition Ba,.,,Y,,,,CuO, (triangles) and a single phase sample with composition Ba,,7Y,,,Cu0, (squares). (b) Magnetic susceptibilities in 0.01 G of the same samples as in (a). The solid symbols were measured by warming samples cooled in zero field; the open symbols are for samples cooled in 0.01 G (Meissner effect).
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STRUCTURAL DETERMINATION BY LATTICE IMAGING
Figure 2. (a) Optical micrograph illustrating the three phases present in a sample with nominal composition Ba,,,,Y,,,,CuO,. (b) Secondary electron image indicating the degree of porosity present in the single phase sample with composition B~.,,Y,,,,CuO,. biased towards the Ba-rich, following our suspicion that the superconducting phase was the Ba-rich textured phase. These samples were mixed, pressed, and sintered in an identical manner to those of the first batch. After the sintering at 1110 C, we observed considerable melting (and possibly evaporation) in the Ba-rich mixtures, particularly for the more Ba-rich mixtures. Most of these samples were either insulating or highly resistive. We therefore prepared another Ba-rich series of samples, and sintered these at the lower temperature of 950 C. After sintering it was observed that there was no significant loss of material, but that samples with nominal compositions (B~.5Y0.5)0.7SC~Oy, (Ba.,Yo.WuO,, and (Ba,,,Y,,,)CuO, were surrounded by a small frozen puddle. Apparently, 950 C exceeded the temperature at which the CuO-rich eutectic forms for these compositions and some of the eutectic liquid had flowed out of these samples. These samples had high contact resistance and high resistance at room temperature, and the resistance increased as the temperature was lowered. By contrast, the samples with nominal compositions Ba,,Y,&uO, and Ba,,,Y&uO, had very low contact resistances and low resistances, approaching the best values we have achieved for La,,,,Sr,,,,CuO,.,. The sample with nominal composition B~,,Y,,J!uO~_~ had the lowest resistivity at room temperature and the highest and sharpest superconducting resistive transition. The curve plotted with the square symbols in Fig. 1 (a) shows this sample’s resistive transition. The lo%-90% transition width is 1.3 K and the sample is completely superconducting by 90 K. The square symbols plotted
149
in Fig. 1 (b) show that the low field susceptibilities have almost ideally sharp shapes (10% - 90% transition widths less than 5 K) for both the zero-field cooled diamagnetism and the Meissner effect. However, the magnitudes of the effects are substantially less than 100 percent, the zero-field diamagnetism being 69 percent of the full value and the Meissner effect being 20 percent of this. Optical microscopy, like that performed on the multiphase sample in Fig. 2 (a), confiied our suspicions that the B~,,Y,,,CuO,., sample was the single phase -90 K superconductor. Figure 2(b) gives a secondary electron micrograph of a subsequently prepared sample with nominal composition B~,,,Y,,,,Cu04.,. The sample was prepared for this micrograph by polishing with 1 pm alumina. It shows the degree of porosity associated with our single-phase samples, which might account for the less than 100 percent Meissner effect and zero-field-cooled diamagnetism. No frozen liquid phase is evident and the grains are equiaxed indicating that the solid state reaction occurred from the component powders. By electron diffraction we identified the structure of the 90 K superconducting phase to have a distorted tetragonal unit cell with a-br4A and c-12angstrom. The transmission electron lattice image of the superconducting phase in Fig. 3 directly identifies the structure as a perovskite based superlattice. Lighter contrast in every thud vertical lattice plane indicates that the Y and Ba ions are ordered such that every third octahedral layer is yttrium rich to give the c axis repeat distance of 12A and the ideal stoichiometry of the 90 K superconducting phase as Bq,67Y,,,,Cu0, (in agreement with the determinations of Grant et al.10 and Cava et al.“). Electron diffraction patterns from the [OOl] zone axis show splitting of one set of the high order {l lo] reflections. This results from twinning of the crystals on the 11lOj type planes. The twinning confirms that a # b as proposed by Cava et al.11 and indicates that the
Figure 3. TEM lattice image showing the two dimensional structure of layered perovskite the Ba,,,Y,,,CuO,. The ordering of the Ba and Y planes is evident by the three-layer modulation of the regions of light contrast.
150
STRUCTURAL DETERMINATION BY LATTICE IMAGING
unit cell is in fact orthorhombic. A least squares refinement of the unit cell from x-ray data indicates the unit cell is in fact slightly smaller than the nominal unit cell size we determined by electron diffraction. Details of this distortion and imaging of the ordering of the cations will be reported in a later paper.15 In conclusion, we have identified and prepared in pure form the T,-90K phase in the Ba-Y-Cu-0 system as (B~.~,Y,.,,)CuQ, y . The resistive transition we achieve for this composition has a 92K midpoint and a 1.3 K (lo%-90%) width. Questions remain as to why our observed transition temperature is lower than the highest achieved by Chu et al.’ (midpoint N 96K). Our
Vol. 63, No. 2
initial experiments with rapid quenching from 950 C and oxygen annealing at 500 C have not resulted in significantly higher transition temperatures. We have begun other measurements on these high transitiontemperature superconductors and, in particular, have measured a superconducting energy gap A of about 16 meV (2A/k,T,-4.3 j- using a low-temperature scanning tunneling microscopei6. This work was supported in part by the Office of Naval Research Contract No. N00014-85-C-0361. We acknowledge measurements made by D.C. Cronemeyer, T.R. McGuire, and T. Penney III that helped confirm our results.
REFERENCES
5.
6.
7.
8. 9.
J.G. Bednorz and K.A. Mtiller, Z. Phys. B 64, 189 (1986). J.G. Bednorz, M. Takashige, and K.A. Mtiller, Europhysics Lett. (to be published February 1987). S. Uchida, H.Takagi, K. Kitazawa, and S. Tanaka, Jap. J. Appl. Phys. Lett. (to be published). C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, and Y.Q. Wang, Phys. Rev. Lett. z 405 (1987). M.K. Wu, J.R. Ashburn, C.J. Torng,P.H. Hor,R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, and C.W. Chu, Phys. Rev. Lett. 58,908 (1987). P.H. Hor, L. Gao, R.L. Meng, Z.J. Huang, Y.Q. Wang, K. Forster, J. Vassiliou, C.W. Chu, M.K. Wu, J.R. Ashburn, and C.J Torng, Phys. Rev. Lett. 5&911(1987). C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, J. Bechtold, D. Campbell, M.K. Wu, J. Ashburn, and C.Y. Huang, preprint. J.M. Tarascon, L.H. Greene, W.R. McKinnon, and G.W. Hull, Phys. Rev. Lett. (submitted). Z. Zhao, L. Chen, Q. Yang, Y. Huang, G. Chen, L. Wang, S. Guo, S. Li, and J. Bi, to be published, Kexue Tongbao, No. 6,1987.
10. P.M. Grant, R.B. Beyers, E.M. Engler, G. Lim, S.S.P. Parkin, M.L. Ranirez, V.Y. Lee, A. Nazzal, J.E. Vazquez, and R.J. Savoy Phys. Rev. Lett. (submitted). 11. R.J. Cava, B. Batlogg, R.B. van Dover, D.W. Murphy, S. Sunshine, TSiegrist, J.P. Remeika, E.A. Rietman, S. Zahurak, and G. P. Espinosa (preprint). 12. Our preparation conditions for LaSrCuO were similar to those reported here for BaYCuO. Our selection of the La:Sr:Cu ratio was based on the optimal achieved by J.M. Tarascon, L.H. Greene, W.K. McKinnon, G.W. Hull, and T.H. Geballe, Science (submitted). 13. D.C. Cronemeyer and A.P. Malozemoff (unpublished). 14. H. Maletta, A.P. Malozemoff, D.C. Cronemeyer, C.C. Tsuei, R.L. Greene, J.G. Bednorz, and K.A. Mtiller, Solid State Commun. (to be published). 15. R.B. Beyers, G. Lim, E.M. Engler, R.J. Savoy, T.M. Shaw, T.R. Dinger, W.J. Gallagher, and R.L. Sandstrom, (to be published). 16. J.R. Kirtley, W.J. Gallagher, Z. Schlesinger, R.L. Sandstrom, T.R. Dinger, and D.A. Chance, Phys. Rev. B (submitted).
Identification and Preparation of Siie
0038-1098/87 $3.00 + .OO Pergamon Journals Ltd.
Phase 90 K Oxide Superconductor
and Structoral Detemination
by Lattice Imaging
W.J. Gallagher, R.L. Sandstrom, T.R. Dinger, T.M. Shaw and D.A. Chance IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598 (Received 17 March 1987 by G. Burns) We have investigated various compositions of Ba-Y-Cu-0, resulting in the preparation of single phase 90 K superconducting material of composition (Ba,.,,Y,.,,)CuO,_s and the identification of the structure of this superconducting phase by lattice imaging as a layered perovskite. The single phase material had a resistive transition with a midpoint at 92 K and a 1.3 K width (lo%-90%) as well as a sharp low-field (H = 0.01 Oe) susceptibility transition.
Y,O, (325 mesh, 99.99% pure), and CuO (200 mesh, 99.9% pure). Approximately 1.75 g of each of the ground mixtures were pressed into 12 mm disks at a pressure of 4 13 MPa (60,000 psi). The firing conditions in air for this first set of mixtures were a ramp from 25 C to 1110 C in about 3 hours, sintering at 1110 C for 12 hours, and finally cooling to 25 C within the oven in 3.5 hours. After sintering, a diamond saw was used to cut strips from the disks for electrical testing. Two- and four-terminal dc resistances were measured at room temperature and four-terminal ac resistances were measured as a function of temperature using spring loaded pins for contacts. Typically, a current of 1mA was used in the measurements. Temperature was measured with a Si diode estimated to be accurate to + 0.5 K. After firing this first set of samples it was apparent that some of the disks had partially melted. In particular the disk with a nominal composition (Ba,,4Y0.6)0.67 CuO, had lost -40% of its initial mass. The sample with x = 3 was greenish, while those with x values ranging from 2. to 1.5 were a deep black with a greenish hue that became progressively less with lower x. A sample with x = 1.33 was greyish-black and samples with x = 1 and x = 0.67 had progressively lighter shades of grey. Upon storage in dry air, the surfaces of the samples with 1.5 I x < 2 seemed to pick up more of a greenish hue. The sample having compositions (Bq,4Y0,6)0_67C~Oywas highly resistive at low temperatures. All the other compositions in this series were high transition temperature superconductors. Midpoints of the resistive transitions varied from 86 K to 91.5 K, and zero resistance was achieved below 79.5 K to 89 K. The samples with x = 1.67 and x = 2 had the broadest and lowest transitions, with lo%-90% transition widths of 7.7 and 6.7 K, respectively. Three compositions with x= 1 (i.e., samples with the nominal compositions B~.~,YO,,,CuO,, Ba,,Y,,CuO,, and B~~~~Y~~~~CuO,) showed the sharpest transitions with (lo%-90%) resistive transition widths of 1.8K, 2.3K, and 1.8 K, respectively. The occurrence
There has been a flood of activity following the breakthrough discovery of high-T, perovskite-type superconductors by Bednorz and Mtilleri and the subsequent confirmation of this result with susceptibility measurements by Bednorz, et a1.2, Uchida et a1.3, and Chu et a1.4. Recently, Wu et a1.,5*6s7 and then Tarascon et al.* and Zhao et al.9 have reported superconductivity at -9OK in YBaCuO variants of the LaBaCuO perovskite-like mixture originally investigated by Bednorz and Mtiller. All three groups noted that the particular compositions they studied were multiphase mixtures, with Tarascon et al.* having identified two of those in their samples as Y,Cu,O, and Y,O, and having noted that their samples contained a third and possibly other phases as well. The samples were said to be green or to possess a greenish hue. In this paper we report on a study of a series of compositions of BaYCuO prepared under different conditions. Our studies have allowed us to isolate the T, N 90K phase in the BaYCuO family as (Ba,.,,Y,.,,)CuO,_, with a distorted layered perovskite structure and to identify some details of the BaO-Y,O,-CuO pseudo ternary phase diagram. After we had identified and prepared single phase samples of (Ba~~~~Y~~,,)CuO,~ y we learned of the work of Grant et al.10 and of that of Cava et al.11 who have also identified and prepared this single phase superconductor. The preparation details available in the Wu et al. papers526.7 were sketchy, though it was stated by Chu et al.’ that the > 90 K superconducting transitions were observed in mixtures with nominal compositions Ba,,Y,,CuO,_, and Ba,,Y,,,CuO,_,, with the latter having a sharper resistive transition. This led us to initially investigate a series of compositions near the line connecting these two points in the BaO-CuO-Y,O, ternary phase diagram. Specifically, we prepared nominal compositions (Ba,Y,.,),CuO, with z= 0.35, 0.4, and 0.45 and x ranging from 0.67 to 3. These compositions were prepared by grinding mixtures of the appropriate molar ratios of BaO (100 mesh, typically 99.5% pure), 147
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of narrower transition widths for x=1 than for x=2 is the reverse of what was true for Chu et al’s samples.’ The samples with the sharpest transitions also displayed the highest temperature transitions. The highest transition in this series occurred for the sample with nominal composition B~,,,Y,-,,,CuO, and is plotted in Fig. 1 (a) as the curve triangular symbols. It showed an onset at 94 K, a midpoint at 91K, and complete superconductivity by 88K. The 1.8K resistive transition widths observed for our sharpest transitions are similar to those we have achieved12 for La,,,,Sr,,,,Cu04_, samples that were prepared in a manner similar to the present sample, but these BaYCuO samples had more than ten-fold greater bulk resistivities as well as much higher contact resistances. The zero-field-cooled low field (H = 0.01 Oe) diamagnetic susceptibility and the fieldcooled susceptibility (Meissner effect) of this sample were measured’3 in a manner similar to that used by Maletta et a1.r4 and the data for these are plotted in Fig. 1 (b) with solid and open triangles respectively. In contrast to the resistive transition, the susceptibilities for this sample show a very broad transition with an onset just above 90 K. Furthermore the data suggests a possible second transition in the vicinity of -60 K, but we have observed no other indications of such a transition. Optical and scanning electron micrographs taken of the sample with nominal composition Ba,.,,Y,.,,CuO, revealed the presence of three phases. The microstructure of this material is shown in the optical micrograph
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of Fig. 2 (a) taken from a specimen which had been polished with 6 pm diamond paste and etched with a concentrated FeCl, solution to reveal grain boundaries and provide surface relief. Three phases are clearly observed as areas of different contrast or morphology. The major phase observed exhibited long lath-shaped grains and was continuous. This phase is seen in dark contrast as long grains extending from upper left to lower right in the micrograph. Energy dispersive x-ray spectroscopy (EDXS) identified this compound as being a Ba/Cu-rich compound with a minor amount of Y. The equiaxed (roughly spherical in most cases) grains shown in dark contrast in the micrograph were observed to be Y-rich. The third phase observed was the crystallized remnants of a eutectic liquid which had infiltrated the multi-grain channels of the specimen and can be seen as the light contrast phase in the optical micrograph. The EDXS data show this phase to be predominantly CuO; trace amounts of Y or Ba below the detectability limit of the EDXS technique may be present. The elongated lath-like morphology of the Ba/Cu-rich phase suggests that they crystallized from the eutectic liquid which was formed at the processing temperature. Having proven that the superconducting phase of BaYCuO would form over a wide range of compositions, we prepared a broader range of compositions to explore the extent of the compatibility region and to obtain samples with a higher percentage of the superconducting phase. The selection of compositions was
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Figure 1: (a) Resistive transitions of a multiphase sample with nominal composition Ba,.,,Y,,,,CuO, (triangles) and a single phase sample with composition Ba,,7Y,,,Cu0, (squares). (b) Magnetic susceptibilities in 0.01 G of the same samples as in (a). The solid symbols were measured by warming samples cooled in zero field; the open symbols are for samples cooled in 0.01 G (Meissner effect).
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STRUCTURAL DETERMINATION BY LATTICE IMAGING
Figure 2. (a) Optical micrograph illustrating the three phases present in a sample with nominal composition Ba,,,,Y,,,,CuO,. (b) Secondary electron image indicating the degree of porosity present in the single phase sample with composition B~.,,Y,,,,CuO,. biased towards the Ba-rich, following our suspicion that the superconducting phase was the Ba-rich textured phase. These samples were mixed, pressed, and sintered in an identical manner to those of the first batch. After the sintering at 1110 C, we observed considerable melting (and possibly evaporation) in the Ba-rich mixtures, particularly for the more Ba-rich mixtures. Most of these samples were either insulating or highly resistive. We therefore prepared another Ba-rich series of samples, and sintered these at the lower temperature of 950 C. After sintering it was observed that there was no significant loss of material, but that samples with nominal compositions (B~.5Y0.5)0.7SC~Oy, (Ba.,Yo.WuO,, and (Ba,,,Y,,,)CuO, were surrounded by a small frozen puddle. Apparently, 950 C exceeded the temperature at which the CuO-rich eutectic forms for these compositions and some of the eutectic liquid had flowed out of these samples. These samples had high contact resistance and high resistance at room temperature, and the resistance increased as the temperature was lowered. By contrast, the samples with nominal compositions Ba,,Y,&uO, and Ba,,,Y&uO, had very low contact resistances and low resistances, approaching the best values we have achieved for La,,,,Sr,,,,CuO,.,. The sample with nominal composition B~,,Y,,J!uO~_~ had the lowest resistivity at room temperature and the highest and sharpest superconducting resistive transition. The curve plotted with the square symbols in Fig. 1 (a) shows this sample’s resistive transition. The lo%-90% transition width is 1.3 K and the sample is completely superconducting by 90 K. The square symbols plotted
149
in Fig. 1 (b) show that the low field susceptibilities have almost ideally sharp shapes (10% - 90% transition widths less than 5 K) for both the zero-field cooled diamagnetism and the Meissner effect. However, the magnitudes of the effects are substantially less than 100 percent, the zero-field diamagnetism being 69 percent of the full value and the Meissner effect being 20 percent of this. Optical microscopy, like that performed on the multiphase sample in Fig. 2 (a), confiied our suspicions that the B~,,Y,,,CuO,., sample was the single phase -90 K superconductor. Figure 2(b) gives a secondary electron micrograph of a subsequently prepared sample with nominal composition B~,,,Y,,,,Cu04.,. The sample was prepared for this micrograph by polishing with 1 pm alumina. It shows the degree of porosity associated with our single-phase samples, which might account for the less than 100 percent Meissner effect and zero-field-cooled diamagnetism. No frozen liquid phase is evident and the grains are equiaxed indicating that the solid state reaction occurred from the component powders. By electron diffraction we identified the structure of the 90 K superconducting phase to have a distorted tetragonal unit cell with a-br4A and c-12angstrom. The transmission electron lattice image of the superconducting phase in Fig. 3 directly identifies the structure as a perovskite based superlattice. Lighter contrast in every thud vertical lattice plane indicates that the Y and Ba ions are ordered such that every third octahedral layer is yttrium rich to give the c axis repeat distance of 12A and the ideal stoichiometry of the 90 K superconducting phase as Bq,67Y,,,,Cu0, (in agreement with the determinations of Grant et al.10 and Cava et al.“). Electron diffraction patterns from the [OOl] zone axis show splitting of one set of the high order {l lo] reflections. This results from twinning of the crystals on the 11lOj type planes. The twinning confirms that a # b as proposed by Cava et al.11 and indicates that the
Figure 3. TEM lattice image showing the two dimensional structure of layered perovskite the Ba,,,Y,,,CuO,. The ordering of the Ba and Y planes is evident by the three-layer modulation of the regions of light contrast.
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unit cell is in fact orthorhombic. A least squares refinement of the unit cell from x-ray data indicates the unit cell is in fact slightly smaller than the nominal unit cell size we determined by electron diffraction. Details of this distortion and imaging of the ordering of the cations will be reported in a later paper.15 In conclusion, we have identified and prepared in pure form the T,-90K phase in the Ba-Y-Cu-0 system as (B~.~,Y,.,,)CuQ, y . The resistive transition we achieve for this composition has a 92K midpoint and a 1.3 K (lo%-90%) width. Questions remain as to why our observed transition temperature is lower than the highest achieved by Chu et al.’ (midpoint N 96K). Our
Vol. 63, No. 2
initial experiments with rapid quenching from 950 C and oxygen annealing at 500 C have not resulted in significantly higher transition temperatures. We have begun other measurements on these high transitiontemperature superconductors and, in particular, have measured a superconducting energy gap A of about 16 meV (2A/k,T,-4.3 j- using a low-temperature scanning tunneling microscopei6. This work was supported in part by the Office of Naval Research Contract No. N00014-85-C-0361. We acknowledge measurements made by D.C. Cronemeyer, T.R. McGuire, and T. Penney III that helped confirm our results.
REFERENCES
5.
6.
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