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Preparation of superconducting crystals of YBCO
308
Journal of Crystal Growth 91(1988) 308—311 North-Holland, Amsterdam
PREPARATION OF SUPERCONDUCTING CRYSTALS OF YBCO C.N.W. DARLINGTON and D.A. O’CONNOR Department of Physics, University of Birmingham, Birmingham B]5 2TT, UK
and CA. HOLLIN Department of Metallurgy and Materials, University ofBirmingham, Birmingham B15 2 TT, UK Received 14 April 1988; manuscript received in final form 18 May 1988
Single crystals of YBCO that become superconducting at 92.4 K have been grown using a flux growth technique; the elevated transition temperature was achieved through an in-situ anneal — i.e. very slow cooling in air from the growth temperature to room temperature. Some of the crystals, contaminated with aluminium, were tetragonal at room temperature and yet showed diamagnetic behaviour with Onset at around 70 K. Annealing these crystals in oxygen at 5500 C following the growth run did not raise the transition temperature to the superconducting state. Others grown in platinum lines crucibles were orthorhombic with sharp onset to a superconducting state at 92.4 K, and did not require further annealing. We present evidence that in crystals with orthorhomic symmetry the superconductivity is a bulk phenomon.
I. Introduction Single crystals of YBCO with a sharp transition to a superconducting state have been grown using a flux growth technique. The starting materials of Y2O3, BaCO3 and CuO were mixed to give a cation ratio Y: Ba: Cu of 1: 4: 10. An earlier report [1], based on the phase diagram published by Roth and co-workers [2], indicates that this cornposition can produce good single crystals of 1: 2: 3 compound after suitable heat treatment. The 1: 4 : 10 mixture was ball-milled under cyclohexane for 30 mm, dried in a desiccator and placed in a boat-shaped alumina crucible. mitially, the crucible was unlined, though for later runs a platinum lining was used. 2. Results using unlined alumina crucibles The following temperature cycle was employed and performed in air:
(1) Heat from room temperature to 1000°Cat a constant rate of 50°C/h. (2) Soak at 1000°Cfor 15 h. (3) Cool slowly to 910°C at a constant rate of 6°C/h. (4) Soak at 910°Cfor 3 days. (5) Cool slowly to room temperature at a constant rate of 12°C/h. Examination of the contents of the crucible after the thermal cycle described above revealed the following. The central part was composed of hard sintered material which had not melted; analysis of its composition by X-ray fluorescence showed virtually no yttrium. Surrounding this hard, central portion was material which clearly had melted. Embedded in this were plate-like crystals of third YBCO, sometimes as large 2, with dimension typically 0.1 as mm.2 XNo2 mm yttrium could be detected, from X-ray fluorescence, in the material in which the 1: 2: 3 crystals were embedded. In addition, long needles of CuO were present.
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.N. W. Darlington el a!.
/ Preparation ofsuperconducting crystals of YBCO I
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TEMPERATURE/K 3) with tetragonal
Fig. 1. Temperature dependence of AC magnetic susceptibility of 15 small crystals (typically 0.5 X 0.5 x 0.1 mm symmetry measured at 1 kHz. The measuring field was 500 mOe.
Many runs with small variations in heating rates, hold times and cooling rates produced essentially the same results. However, it is our expenence that significantly longer holds at 1000 and 910°Care detrimental to the production of YBCO crystals. We believe that this occurs because of progressive loss of oxygen from the melt. All 1: 2: 3 crystals selected from runs using unlined alumina crucibles possessed tetragonal symmetry with lattice parameters at room temperature of a 3.863 ±0.005 A, c 11.701 ±0.014 A. Following the literature (e.g, ref. [1]), we associated the occurrence of tetragonal symmetry with oxygen deficiency, and attempted to induce a phase change to orthorhombic symmetry by annealing in flowing oxygen at several different ternperatures around 550°C. Each anneal at a constant temperature lasted at least a weak, but after each attempt the symmetry was still tetragonal. Analysis of the crystals showed contamination with aluminium (— 2%) and traces of silicon (< 1%). =
=
However, a weak Meissner effect with onset at about 70 K in crystals that had not been annealed in flowing oxygen indicated superconducting behaviour (fig. 1).
3. Runs using platinum foil as a crucible liner The alumina crucible was lined with platinum foil, 0.001 inch thick, filled with the 1 : 4: 10 mixture, placed in the same furnace and subjected to an identical temperature cycle as described above. The appearance of the charge was quite different this time. The content appeared uniform and was easy to crumble into smaller fragments, YBCO crystals with largest dimension typically 0.1 to 2 mm were relatively easy to isolate from the surrounding solid. Some fragments turned out to be clumps of platelets of YBCO crystals, all in the same orientation, weakly held together, weighing 10—20 mg. All the YBCO crystals examined from this run, and all other runs performed with platinum lined —
—
C. N. LV Darlington et aL
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/ Preparation
of superconducting crystals of YBCO
crucibles. were found to be orthorhombic with lattice parameters at room temperature of: a 3.897 ±0.005 A. h 3.824 ±0.005 A, c 11.714 ±0.015 A. When unpolarized light was shone at 45° to the straight edges of the platelets, colourful striations perpendicular to the incident beam appeared on the surface of the crystals. These striations are parallel to either [110] or [1101pseudo-tetragonal directions. They appear because of mimetic twinfling. The domains on either side of the walls have interchanged [100] and [010] orthorhombic directions. The walls are about I jim apart. The AC susceptibility versus temperature is shown in fig. 2, showing a very sharp transition to a superconducting state at 92.4 K. We found little or no difference in susceptibility with measuring field applied parallel and perpendicular to the c-axis. We believe the crystals are superconducting throughout their bulk for two reasons: (1) We cleaved some crystals and found that the
lattice parameters of the inner portions were identical with those measured on the large crystal before cleaving. This is strong evidence that the oxygen stoichiometry is uniform throughout the crystal. since it is well established [3] that the orthorhombic distortion the difference between the repeat distances a and h decreases significantly with loss of oxygen. The reciprocal of the absorption coefficient for Cu Ks X-rays is 9.1 jim, and so measurement of lattice parameters allows one to profile the oxygen content as a function of depth. The transition temperature to the superconducting state remains around 90 K provided the oxygen content is greater than about 6.8 per formula unit, and from our lattice parameter measurements we believe that the oxygen concent of these crystals is close to 7 per formula unit throughout the crystal. (2) Measurement of the AC magnetic susceptibility on crystals that had been broken up into smaller pieces still showed an onset to a diamagnetic state at 92.4 K. although the transition was
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Fig. 2. Temperature dependence of AC magnetic susceptibility of 6 small crystals (typically 0.7 x 0.7 x 0.1 mm symmetry measured at I kHz. The measuring field was 50 mOe. The crystals weighed 2 fig.
C.N. LV Darlington eta!.
/ Preparation of superconducting crystals
more rounded than that occurring in larger crystals. The magnitude of the susceptibility did not change from 88 K down to 4 K in these t 1 crys a 5. Slow cooling is essential to the production of crystals with high transition temperature to the superconducting state since we find that quenching from 800°C results in an oxygen content of about 6.4 per formula unit deduced from lattice parameter measurements and a transition temperature of 58 K. It appears that in single crystals it is more difficult to increase the oxygen content of a partially-ordered, deficient array of oxygens than to raise the oxygen content while ordering is taking place for the first time. For some runs contamination of the YBCO with strontium occurred, because of impure starting materials, and these showed a much weaker Meissner effect starting at 92 K. .
.
of YBCO
311
Acknowledgement
We wish to thank Dr. J.S. Abell for allowing us the use of facilities in the Department of Metal1 urgy.
—
—
References [1] L.F. Schneemeyer, J.V. Waszczak, T. Siegrist, R.B. van Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D.W. Murphy, Nature 320 (1987) 401. [2] R.S. Roth, J.R. Dennis and K.C. Davis, Advan. Ceram. Mater. 2 (1987) 303. [3] R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Nature 329 (1987) 423.
Journal of Crystal Growth 91(1988) 308—311 North-Holland, Amsterdam
PREPARATION OF SUPERCONDUCTING CRYSTALS OF YBCO C.N.W. DARLINGTON and D.A. O’CONNOR Department of Physics, University of Birmingham, Birmingham B]5 2TT, UK
and CA. HOLLIN Department of Metallurgy and Materials, University ofBirmingham, Birmingham B15 2 TT, UK Received 14 April 1988; manuscript received in final form 18 May 1988
Single crystals of YBCO that become superconducting at 92.4 K have been grown using a flux growth technique; the elevated transition temperature was achieved through an in-situ anneal — i.e. very slow cooling in air from the growth temperature to room temperature. Some of the crystals, contaminated with aluminium, were tetragonal at room temperature and yet showed diamagnetic behaviour with Onset at around 70 K. Annealing these crystals in oxygen at 5500 C following the growth run did not raise the transition temperature to the superconducting state. Others grown in platinum lines crucibles were orthorhombic with sharp onset to a superconducting state at 92.4 K, and did not require further annealing. We present evidence that in crystals with orthorhomic symmetry the superconductivity is a bulk phenomon.
I. Introduction Single crystals of YBCO with a sharp transition to a superconducting state have been grown using a flux growth technique. The starting materials of Y2O3, BaCO3 and CuO were mixed to give a cation ratio Y: Ba: Cu of 1: 4: 10. An earlier report [1], based on the phase diagram published by Roth and co-workers [2], indicates that this cornposition can produce good single crystals of 1: 2: 3 compound after suitable heat treatment. The 1: 4 : 10 mixture was ball-milled under cyclohexane for 30 mm, dried in a desiccator and placed in a boat-shaped alumina crucible. mitially, the crucible was unlined, though for later runs a platinum lining was used. 2. Results using unlined alumina crucibles The following temperature cycle was employed and performed in air:
(1) Heat from room temperature to 1000°Cat a constant rate of 50°C/h. (2) Soak at 1000°Cfor 15 h. (3) Cool slowly to 910°C at a constant rate of 6°C/h. (4) Soak at 910°Cfor 3 days. (5) Cool slowly to room temperature at a constant rate of 12°C/h. Examination of the contents of the crucible after the thermal cycle described above revealed the following. The central part was composed of hard sintered material which had not melted; analysis of its composition by X-ray fluorescence showed virtually no yttrium. Surrounding this hard, central portion was material which clearly had melted. Embedded in this were plate-like crystals of third YBCO, sometimes as large 2, with dimension typically 0.1 as mm.2 XNo2 mm yttrium could be detected, from X-ray fluorescence, in the material in which the 1: 2: 3 crystals were embedded. In addition, long needles of CuO were present.
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.N. W. Darlington el a!.
/ Preparation ofsuperconducting crystals of YBCO I
I
I
40
60
80
309
C
...
>-
S.....
S
0
~
20
100
TEMPERATURE/K 3) with tetragonal
Fig. 1. Temperature dependence of AC magnetic susceptibility of 15 small crystals (typically 0.5 X 0.5 x 0.1 mm symmetry measured at 1 kHz. The measuring field was 500 mOe.
Many runs with small variations in heating rates, hold times and cooling rates produced essentially the same results. However, it is our expenence that significantly longer holds at 1000 and 910°Care detrimental to the production of YBCO crystals. We believe that this occurs because of progressive loss of oxygen from the melt. All 1: 2: 3 crystals selected from runs using unlined alumina crucibles possessed tetragonal symmetry with lattice parameters at room temperature of a 3.863 ±0.005 A, c 11.701 ±0.014 A. Following the literature (e.g, ref. [1]), we associated the occurrence of tetragonal symmetry with oxygen deficiency, and attempted to induce a phase change to orthorhombic symmetry by annealing in flowing oxygen at several different ternperatures around 550°C. Each anneal at a constant temperature lasted at least a weak, but after each attempt the symmetry was still tetragonal. Analysis of the crystals showed contamination with aluminium (— 2%) and traces of silicon (< 1%). =
=
However, a weak Meissner effect with onset at about 70 K in crystals that had not been annealed in flowing oxygen indicated superconducting behaviour (fig. 1).
3. Runs using platinum foil as a crucible liner The alumina crucible was lined with platinum foil, 0.001 inch thick, filled with the 1 : 4: 10 mixture, placed in the same furnace and subjected to an identical temperature cycle as described above. The appearance of the charge was quite different this time. The content appeared uniform and was easy to crumble into smaller fragments, YBCO crystals with largest dimension typically 0.1 to 2 mm were relatively easy to isolate from the surrounding solid. Some fragments turned out to be clumps of platelets of YBCO crystals, all in the same orientation, weakly held together, weighing 10—20 mg. All the YBCO crystals examined from this run, and all other runs performed with platinum lined —
—
C. N. LV Darlington et aL
310
/ Preparation
of superconducting crystals of YBCO
crucibles. were found to be orthorhombic with lattice parameters at room temperature of: a 3.897 ±0.005 A. h 3.824 ±0.005 A, c 11.714 ±0.015 A. When unpolarized light was shone at 45° to the straight edges of the platelets, colourful striations perpendicular to the incident beam appeared on the surface of the crystals. These striations are parallel to either [110] or [1101pseudo-tetragonal directions. They appear because of mimetic twinfling. The domains on either side of the walls have interchanged [100] and [010] orthorhombic directions. The walls are about I jim apart. The AC susceptibility versus temperature is shown in fig. 2, showing a very sharp transition to a superconducting state at 92.4 K. We found little or no difference in susceptibility with measuring field applied parallel and perpendicular to the c-axis. We believe the crystals are superconducting throughout their bulk for two reasons: (1) We cleaved some crystals and found that the
lattice parameters of the inner portions were identical with those measured on the large crystal before cleaving. This is strong evidence that the oxygen stoichiometry is uniform throughout the crystal. since it is well established [3] that the orthorhombic distortion the difference between the repeat distances a and h decreases significantly with loss of oxygen. The reciprocal of the absorption coefficient for Cu Ks X-rays is 9.1 jim, and so measurement of lattice parameters allows one to profile the oxygen content as a function of depth. The transition temperature to the superconducting state remains around 90 K provided the oxygen content is greater than about 6.8 per formula unit, and from our lattice parameter measurements we believe that the oxygen concent of these crystals is close to 7 per formula unit throughout the crystal. (2) Measurement of the AC magnetic susceptibility on crystals that had been broken up into smaller pieces still showed an onset to a diamagnetic state at 92.4 K. although the transition was
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92 94 TEMPERATURE/K 3) with orthorhombic
Fig. 2. Temperature dependence of AC magnetic susceptibility of 6 small crystals (typically 0.7 x 0.7 x 0.1 mm symmetry measured at I kHz. The measuring field was 50 mOe. The crystals weighed 2 fig.
C.N. LV Darlington eta!.
/ Preparation of superconducting crystals
more rounded than that occurring in larger crystals. The magnitude of the susceptibility did not change from 88 K down to 4 K in these t 1 crys a 5. Slow cooling is essential to the production of crystals with high transition temperature to the superconducting state since we find that quenching from 800°C results in an oxygen content of about 6.4 per formula unit deduced from lattice parameter measurements and a transition temperature of 58 K. It appears that in single crystals it is more difficult to increase the oxygen content of a partially-ordered, deficient array of oxygens than to raise the oxygen content while ordering is taking place for the first time. For some runs contamination of the YBCO with strontium occurred, because of impure starting materials, and these showed a much weaker Meissner effect starting at 92 K. .
.
of YBCO
311
Acknowledgement
We wish to thank Dr. J.S. Abell for allowing us the use of facilities in the Department of Metal1 urgy.
—
—
References [1] L.F. Schneemeyer, J.V. Waszczak, T. Siegrist, R.B. van Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D.W. Murphy, Nature 320 (1987) 401. [2] R.S. Roth, J.R. Dennis and K.C. Davis, Advan. Ceram. Mater. 2 (1987) 303. [3] R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Nature 329 (1987) 423.