- Email: [email protected]
Crystal growth of REBa2Cu3O7−y (RE=Y, La, Pr, Nd and Sm) by the travelling-solvent floating-zone method
ELSEVIER
Physica C 227 (1994) 77-84
Crystal growth of REBa2Cu307_-y (RE =Y, La, Pr, Nd and Sm) by the travelling-solvent floating-zone method Kunihiko Oka *, Toshimitsu Ito EiectrotechnicalLaboratory,l-l-4, Umezono, Tsukuba. 305. Japan Received 15 March 1994
Abstract Single crystals of REBa2Cu307_, (RE = Y, La, Pr, Nd and Sm) have been grown successfully by the travelling-solvent floatingzone (TSFZ) method. The maximum dimensions of the grown boules were 6 mm 0 x 27 mm. It is shown that the density of the feed rod and atmosphere are the crucial variables for a successful growth procedure. The grown crystals were identified to be single-crystal REBa2Cus07_, by Laue X-ray measurements and X-ray powder diffraction. The maximum area of the grains was about 1 X 1 mm’ for the Y system, 1.5 X 1.5 mm* for the La, Pr and Nd systems and 1.5 x 3 mm2 for the Sm system, respectively. Superconducting transitions of crystals annealed in oxygen typically occur at 9 1.5 K, 90 K, 9 1 K and 94 K for the Y, La, Nd and Sm compounds, respectively.
1. Introduction
The discovery of 90 K superconductivity in REBa2Cua07_, (RE=rare-earth elements and Y) have led to many studies in recent years. The interest in studies of the superconducting mechanism and application to the substrate of homoepitaxial growth of thin films has resulted in the requirement of highquality single crystals of REBa2Cu30,_, with large dimensions in all crystal axes. Many studies have been carried out on the preparation of REBa&&O,_, crystals such as the slow-cooling [ l-l 81 and the topseeded solution-growth (TSSG) methods [ 19-2 11. The slow-cooling method has the disadvantage of growing a large number of thin-plate crystals because of uncontrollable nucleation. Also, the TSSG method has the problem of contamination from the crucible materials. The travelling-solvent floating-zone (TSFZ) method is a valuable growth technique to * Correspondingauthor.
produce high-quality and large single crystals along the c-axis. However, the TSFZ method has not been applied to REBa&&O,_, compounds because the solvent composition must be kept in a narrow range [ 22,23 1. Several efforts have been made without success. As the viscosity of the solvent composed of BaO-CuO is low, the solvent easily falls down onto the seed and penetrates deeply into the feed rod. It is difficult to keep the molten zone during growing. Our experiments were performed in air atmosphere at the early stage of the crystal growth of YBa2Cu307_, NdBa2CuJ07_, and PrBa2Cu307_-y, and we have not succeeded in getting single crystals due to the unstable molten zone. The necessary conditions for stable crystal growth of REBa2Cu307_, by the TSFZ method are that the solvent does not penetrate into the feed rod and does not easily fall down onto the seed to keep the molten zone stable, and that in the wide compositional region, only two phases, liquid and REBa2Cu307_,,, co-
0921-4534/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZO921-4534(94)00213-Y
78
K. Oka, T. Ito / Physica C 227 (1994) 77-84
exist to precipitate REBa2Cu@_, crystals. Concerning the problem of the penetration of solvent into the feed rod, we found that the “twice-scanning” technique is useful to avoid solvent penetration into the feed rod for the crystal growth of Laz_Sr,CuO, [ 241. This phenomenon is very sensitive to temperature, so we have to grow the REBa2Cu307_-y crystal at temperatures as low as possible. Concerning the problem of the solvent falling down onto the seed, we have earlier found that at lower oxygen pressures the problem is not serious for the crystal growth of (NdCe)zCuOd by the TSFZ method
[251. As for the problem of the narrow range of primary crystallization fields, liquidus lines were shown to be extended and lowered with smaller oxygen partial pressure in the YBCO phase relation, by Lay et al. [ 261 and Lee et al. [ 27 1. Also the concentration width of the liquidus line of the SmBazCu30,_, and NdBazCu30,_, system were shown to be wider by Katsui et al. [ 7 ] and Oka et al. [ 28 1. This implies that there is a possibility of obtaining high-quality crystals with ease. In addition, we found that in the La-Sm-Sr-Cu0 system, unlike the La-Sr-Cu-0 system, the solvent neither climbs up the crucible nor penetrates into the feed rod for crystal growth of La,,8Sm,.0Sr0.2Cu04 with the T* structure by the TSSG and TSFZ methods [28]. Therefore, as a first step in growing single crystals, we tried to grow SmBazCu307_, by the TSFZ method with the twice-scanning technique at lower oxygen pressure. Then, we tried crystal growth of other REBa2Cu30,_, In this paper, we describe the crystal growth of REBa2Cu307_-y (RE =Y, La, Pr, Nd, Sm, Gd, Dy, Er and Yb) by the TSFZ method. Some results of the characterization and magnetic measurements on the grown crystals are also presented.
2. Experimental
Nd203,Sm203,G&03, DY,% Er203,yb20~,BaC03 and CuO of 99.9% purity. The feed rod and solvent zone consist of a RE, Ba and Cu mixture at 1: 2 : 3 and 1: 4 : 6-l : 29 : 66, respectively. The well-mixed powder in the required atom ratios was first calcined at 880°C for 15 h, then after grinding and milling it was formed into a cylindrical shape of 8 mm in diameter and 10 cm in length by pressing a metal mold system for making the rod. The feed rod and the solvent rod were then sintered in air for 15 h at 9501030’ C and 900-920” C, respectively. The “twice-scanning” method for crystal growth has been applied. The high-density crystallized feed rod was made using the “first scanning” procedure. About 1.O- 1.5 g of solvent was connected to the sintered feed rod. First, the feed rod was suspended from the hook of the upper shaft; the polycrystalline sintered rod as the seed, was held at the seed holder of the lower shaft. The feed rod and the growing crystal were rotated at about 25 rpm in opposite directions. The molten-zone was attached to the top of the seed. In order to densify the feed rod, the molten zone was passed through the feed rod at a relatively high rate of about 2.0-15.0 mm/h. The regular growing procedure was, then, carried out using the once-scanned crystallized rod as the feed rod to which the solvent of about 0.3-0.5 g was connected, and using the slow-cooling grown crystal or sintered rod as the seed. The growth rate was in the range of 0.4-1.0 mm/h. Both scans were performed in argon mixed with oxygen at pressures of 1O-*- 10p4 atm. Some experimental results are summarized in Table 1. The as-grown crystals had tetragonal symmetry and showed no superconductivity. These as-grown crystals were annealed at 500°C for 25 h, cooled at 250”C/h, held at 400°C for 60 h in an oxygen atmosphere, and cooled down slowly to room temperature. They were then characterized by powder X-ray diffraction measurements, Laue X-ray measurements, polarization microscope observation and magnetization measurements.
technique
Single crystals of REBa2Cu307_-y were grown in an infrared radiation furnace with a 750 W halogen lamp as the radiation source. The starting feed and solvent materials were prepared from Yz03, Laz03, Pr601 I,
3. Result and discussion Crystal boules grown by the TSFZ method are shown in Fig. 1. These crystals have dimensions of
K. Oka, T. Ito / Physica C 227 (1994) 77-84 Table 1 Experimental Experimental no.
12 13 33 34 36 31 18 19 21 1 2 6 9 8 10 20 30 26 24 27 37
details for the crystal-growth Rare-earth element RE
Y Y Y Y Y La Pr Nd Nd Sm Sm Sm Sm Sm Sm Gd Gd Er Yb Yb Yb
experiments
Composition RE:Ba:Cu
1: 19:42.7 1: 19:42.7 1:19:42.7 1: 19:42.7 1:29:66 1:4.0:6.0 1:6.0: 13 1: 13:21.3 1: 13:21.3 1: 19.4:31.7 1: 19.4:31.7 1:19.4:31.7 1:19.4:31.7 1:19.4:31.7 1: 16.8: 27.4 1:15:22 1:15:22 1:18:25 1: 15:22 1: 18:25 1: 15:22
of solvent
19
of REBa#&O,_, Sintering (“C)
[15
temperature
hl
feed rod
solvent rod
950 950 950 950 950 950 950 950 950 1030 L 1030” 1030” 1030 a 1030 B 1030 B 950 950 950 950 950 950
900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900
Atmosphere
Feed speed
W/G)
(mm/h)
99.011.0 99.9fO.l 100/o (N2= 100) 99.910.1 99.910.1 99.910.1 99.9lO.l (N,= 100) air 93.Ol7.0 99.0/1.0 100/o 99.0/ 1.0 99.910.1 (N,/02=99.0/l.0) 99.910.1 99.910.1 99.910.1 99.910.1 100/o
1st scan
2nd scan
2-3 3-5 3-5 2-3 5-6 5-15 5-10 3-5 3-5
1.0 1.0 0.5-1.0 0.5-1.0 1.0 -
5-6 3-5 2-5 3-5 3-6 3-5 3-5 2-5 5-10
1.0 1.0 1.0 1.0
a [2 h].
4-6 mm in diameter, and 15-30 mm in length. Some results of crystals are summarized in Table 2. 3.1. YBatCu,O,_,
crystals
The maximum size of the YBa2Cu30,_, crystal was about 4 mm 0 x 15 mm. The X-ray Laue analysis indicated it was a single crystal. Small crystals were pulverized and used for powder X-ray diffraction. The observed pattern showed mainly YBa2Cuj0,_, phase and a small amount of YzBaCuO, phase. Magnetization measurements down to 4.2 K showed a T, onset at 9 1.5 K, the transition width was not so sharp. The grain was observed by an optical microscope under polarized light. The maximum size of the grain was about 1x 1 mm’. Attempts to grown crystals at high temperature, using a solvent with composition of Y: Ba: Cu= 1: 19: 42, resulted in mixing of the Y2BaCuOS phase into the crystals. The BaCu202 phase crystal was often grown by the use of a solvent with composition
Y: Ba: Cu = 1: 29: 66 at the oxygen pressure of 10e3 atm. The YBCO system was difficult to crystallize. In the first scan, the solvent penetrated deeply into the feed rod in spite of high-speed scans such as 10 mm/ h, because it was hard to get the high-density sintered feed rod. A 7.5 mm 0 sintered feed rod always expands due to the penetration of the solvent becoming 8 mm as shown in Fig. 2. Finally, a 6 mm 0 crystallized feed rod was obtained by “first scanning”. 3.2. (La, Pr, Nd and Sm) Ba#Zu,O,_,
crystals
The maximum size of the (La, Pr, Nd and Sm) Ba2Cuj0,_,, crystals were about 6 mm 0x27 mm, 4.5 mm 0x 16 mm, 5 mm 0x 15 mm and 5 mm 0 x 15 mm, respectively. The X-ray diffraction pattern was observed using the Cu Ka line as incident radiation. The La system showed mainly LaBa2Cu30,_, phase and a very small amount of second phase. The Pr system showed only
80
K. Oka. T Ro / Physica C 227 (1994) 77-84
Fig. 1. REBa2Cu307_, crystal boule grown by the travelling-solvent PrBa2Cu307_, (d) SmBa2Cu307_, and (e) NdBa2Cu30,_,
PrBa2Cu307,y phase as shown in Fig. 3. The Nd and Sm system showed mainly NdBa&u@_, and SmBazCu30,_, phase and a small amount of NdzBaCuO, and SmzBaCuO, phase as shown in Fig. 4. For the Sm system, a high growth temperature leads to the growth of SmlBaCu05 phase crystals. We ob-
floating-zone
method: (a) YBa2Cu30,_,,
(b) LaBa2Cu30,_,
(c)
served the SmzBaCuOs phase in our crystal obtained by high-temperature growth at more than 320 W lamp power. The domain structures of the cross section perpendicular and parallel to the growth direction were observed by an optical microscope under polarized light.
K. Oka, T. Ito / Physica C 227 (I 994) 77-84
81
Table 2 Characterization of the REBazCu~O,_, crystals Experimental no.
13
31
18
19
6
21
RE element involved Size of crystal boule (mm 0Xmm) Onset T, Secondary phase
Y
La
Pr
Nd
Sm
Yb
4x15
6x12
45x16
5x15
5x15
5x12
91.5 weak
90 very weak
none none
91 very weak
94 very weak
none none
(BaCW2 )
I
,
SmBa,Cu,O,., TSFZ crystal
sintered feed rod
0, annealed
penetrated part of feed rod
L
molten-zone crystallized feed rod
/
30
Ji I
I
40
50
tD
28 (degree) Fig. 2. Schematic figure of the “first scanning” to make a crystallized feed rod of YBazCusO,_, by the travelling-solvent floatingzone method. I
20
30
I
40
I
50
60
28 (degree) Fig. 3. X-ray powder diffraction pattern of an as-grown PrBazCusO,_, crystal grown by the travelling-solvent floating-zone method.
We observed several grains oriented at random directions. The largest grains perpendicular to the growth direction were found to be 1.5 mm in each dimension on the La, Pr and Nd systems. Also the largest grains
Fig. 4. X-ray powder diffraction pattern of an oxygen-annealed SmBazCusO,_, crystal grown by the travelling-solvent floatingzone method.
parallel to the growth direction were found to be 3 x 1.5 mm2 on the Sm system and 1.5 mm in each dimension on the other REBa2Cu@_, systems. The as-grown crystals had tetragonal symmetry and showed no superconductivity. Measurements of the temperature dependence of the magnetization at 5 G were performed on oxygen-annealed (5OO”C, 25 h, +4OO”C, 60 h) crystals. The oxygen-annealed LaBa2Cu~0,_, and NdBa2Cu307_, crystals showed a Tconsetat 90 K and 91 K, respectively. The result of the SmBa2Cu307_, crystal showed a sharp superconducting transition with an onset temperature of 94 K as shown in Fig. 5. 3.3. (Gd, Dy, Er, Yb) Ba2Cu307_, crystals In the (Gd, Dy, Er and Yb) Ba2Cu307_-y systems we have succeeded in keeping the molten zone, but we could not get the (Gd, Dy, Er and Yb) Ba2Cuj0,_, crystals. We have made many trials
82
K. Okz, T. Ito / Physica C 22 7 (I 994) 77-84
0
I
-12’ 0
lution penetrates deeply into the feed rod and easily falls down onto the seed. This phenomenon depends very much on the oxygen partial pressure of the ambient atmosphere, the density of the feed rod, the solvent materials, the growth temperature and the heating time or the feed speed of the solvent. In the YBa2Cu307_-y and PrBazCu30,_, systems, the solvent is apt to fall down easily onto the seed in 1% oxygen mixed argon gas. However, the molten zone remains stable in 0.1% oxygen mixed argon gas.
SmBo2 C1~307-~ 02 annealed
20
40 Temperature
I 60 (K)
80
100
Fig. 5. Measurements of the temperature dependence of the magnetization of oxygen-annealed (SOO’C, 25 h+400”C, 60 h) SmBa2Cu,0,_, crystal at H=5 G. I
I
L
20
30
/
BaCu,O,
40
crystal as-grown
TSFZ
50
3.5. Composition of the solvent Fig. 7 is a triangular coordinate diagram showing the composition of the solvent for the crystal growth of REBa2Cu30,_, by the TSFZ method in an atmosphere of low oxygen pressure. With larger ionic radius of the RE atoms, the composition of the solvent becomes near to the composition of the crystal, in other words, the extent of the liquidus line becomes wider. So the REBa2Cu307_-y system shows a similar tendency to the RE2Cu04 system [ 30 1. LaBazCuSO,_, and PrBa2Cu307_-y crystals were
60
28 (degree)
Fig. 6. X-ray powder diffraction pattern of an as-grown BaCuzOl crystal grown by the travelling-solvent floating-zone method.
by changing the solvent composition, atmosphere and growth rate. We grew a single crystal of BaCu202, when we tried the crystal growth of YbBazCuSO,_, with using a solvent composition by Yb: Ba: Cu = 1: 15 : 22 in argon atmosphere. The Xray diffraction pattern showed only BaCuzOz phase as shown in Fig. 6. 3.4. Atmosphere We have experimented with air, argon, nitrogen and argon mixed with oxygen (Ar/0,=93/7,99/1,99.9/ 0.1 and 99.99/0.0 1) as the atmosphere. Generally, it is difficult to keep the molten zone and grow a crystal in air and mixed gas of Ar/Oz = 9 3 / 7 because the so-
Fig. 7. The composition of the solvent for the growth of REBa2Cux0,_, crystals (solid squares) and BaCuzOZ crystals (open squares) by the TSFZ method from RE203-BaO-CuO, solution.
K. Oka. T. Ito / Physica C 227 (I 994) 77-84
easy to grow, because the composition of the solvent is near the crystal composition [ 3 1,2 11.
4. Conclusion The penetration of the solvent into the feed rod and the low viscosity of the molten zone are serious problems for the crystal growth of REBa2Cu307_, The twice-scanning technique has been applied in order to avoid the penetration into the feed rod. A low oxygen pressure atmosphere can stabilize the molten zone. It has been possible to keep the stable molten zone growing with and continue crystals the REBa2Cu30,_, system by using a low oxygen pressure atmosphere and a twice-scanning procedure. Also, it has been possible to obtain YBa2Cu307_,, LaBa2Cu30,_,, PrBa2Cu30,_,,, NdBazCu30,_, and SmBa2Cu30,_, crystals containing several randomly oriented grains by the TSFZ method. We also developed a method of growing large single crystals of REBa2Cu307_-y by the TSFZ method. Furthermore we have carried out many trials by changing the composition of the solution, species of atmosphere and growth rate for each REBa2Cu30,_, system. However, further trials and phase diagrams under low oxygen pressure atmosphere will be necessary to prepare crystals of good quality.
Acknowledgements The authors would like to thank H. Unoki of superconductivity Research Laboratory for attracting their attention to the application of the TSFZ method to the crystal growth of REBa2Cu307_, and for discussions. They also thank F. Iga for measurements of magnetization, and Y. Ohashi for his help in the crystal growth. They would also like to thank S. Waki and K. Kajimura for their useful comments and encouragement during this study.
References [ I] Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda, A. Katsui and T. Murakami, Jpn. J. Appl. Phys. Lett. 26 (1987) L726.
a3
[2] S. Takekawa and N. Iyi, Jpn. J. Appl. Phys. Lett. 26 ( 1987) L851. [ 31 H. Takei, H. Takeya, Y. Iye, T. Tamegai and F. Sakai, Jpn. J. Appl. Phys. Lett. 26 (1987) L1425. [4] K. Hayashi, K. Murata, K. Takahashi, M. Tokumoto, H. Ihara, M. Hirabayashi, N. Terada, N. Koshizuka and Y. Kimura, Jpn. J. Appl. Phys. Lett. 26 (1987) L1240. [5] L.F. Schneemeyer, J.V. Waszczak, T. Siergrist, R.B. van Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D. W. Murphy, Nature (London) 328 (1987) 601. [6] S. Hayashi, H. Komatsu, T. Inoue, T. Ono, K. Sasaki, Y. Koike and T. Fukase, Jpn. J. Appl. Phys. Lett. 26 (1987) L1197. 17I A. Katsui, Y. Hidaka and H. Ohtsuka, Jpn. J. Appl. Phys. Lett. 26 (1987) L1521. ]8 M.J.V. Menken and A.A. Menovsky, J. Cryst. Growth 91 (1988) 264. ]9 1M.J.V. Menken, K. Kadowaki and A.A. Menovsky, J. Cryst. Growth 96 (1989) 1002. [ lo] J.A. Campa, J.M.G. Salazar, E.G. Puebla, M.A. Monge, I. Rasines and C.R. Valero, J. Cryst. Growth 91 (1988) 334. [ 111 M. Hikita, Y. Tajima, A. Katsui, Y. Hidaka, T. Iwata and S. Tsurumi, Phys. Rev. B 36 (1987) 7199. [ 121 T. Iwata, M. Hikita, Y. Tajima and S. Tsurumi, J. Cryst. Growth85 (1987) 661. [ 131 Y. Tajima, M. Hikita, A. Katsui, Y. Hidaka, T. Iwata and S. Tsurumi, J. Cryst. Growth 85 (1987) 665. [ 141 R.C.J. Draper, G.A. Saunders, B. Chapman, W. Hong, H. Perrot, R.N. Hampton and R.M. Bush, J. Mater. Sci. Lett. 7 (1988) 1281. [15] R.X. Wan, G.M. Zhao, X.M. Tang, W.Z. Li, S.Z. Hu and H.N. Yao, J. Appl. Phys. 64 (1988) 3754. [ 161 T. Iwata, Y. Tajima and M. Hikita, J. Cryst. Growth 91 (1988) 274. [ 171 T. Inoue, S. Hayashi, M. Shimizu and H. Komatsu, J. Cryst. Growth 91 (1988) 287. [ 181 S. Hayashi, T. Ohno, T. Inoue and H. Komatsu, J. Cryst. Growth91 (1988) 331. [ 191 K. Oka, M. Saito, M. Ito, K. Nakane, M. Murata, Y. Nishihara and H. Unoki, Jpn. J. Appl. Phys. Lett. 28 ( 1989) L1521. [20] Y. Yamada and Y. Shiohara, Physica C 217 (1993) 182. [ 211 M. Tagami, M. Sumida, C. Krauns, Y. Yamada, T. Umeda and Y. Siohara, Int. Symp. Supercond. 1993 Proc., to be published. 122 K. Oka, K. Nakene, M. Ito, M. Saito and H. Unoki, Jpn. J. Appl. Phys. Lett. 27 (1988) L1065. ~23 M. Maeda, M. Kadoi and T. Ikeda, Jpn. J. Appl. Phys. 29 (1989) 1417. 124 K. Oka, M.J.V. Menken, Z. Tamawski, A.A. Menovsky, A.M. Moe, T.S. Han, H. Unoki, T. Ito and Y. Ohashi, J. Cryst. Growth, to be published. 125 K. Oka and H. Unoki, Jpn. J. Appl. Phys. Lett. 29 (1990) L1807. 126 K.W. Lay and G.M. Renlund, J. Am. Ceram. Sot. 73 ( 1990) 1208. 127 B.J. Lee and D.N. Lee, J. Am. Ceram. Sot. 74 ( 199 I ) 78.
84 [28
K. Oka, T. Ito / Physica C 227 (1994) 77-84
] K. Oka and H. Unoki, Jpn. J. Appl. Phys. Lett. 28 (1989)
L937. [29] K. Oka and H. Unoki, J. Cryst. Growth 99 ( 1990) 922. [30] K. Oka, H. Unoki, A.M. Moe, F. Iga, K. Kaneko, D.H. Ha
and T.S. Han, Advances in Superconductivity V, eds. Y Bando and H. Yamauchi (Springer, Tokyo, 1993) p. 383. [ 311 H. Eisaki, private communication (1992).
Physica C 227 (1994) 77-84
Crystal growth of REBa2Cu307_-y (RE =Y, La, Pr, Nd and Sm) by the travelling-solvent floating-zone method Kunihiko Oka *, Toshimitsu Ito EiectrotechnicalLaboratory,l-l-4, Umezono, Tsukuba. 305. Japan Received 15 March 1994
Abstract Single crystals of REBa2Cu307_, (RE = Y, La, Pr, Nd and Sm) have been grown successfully by the travelling-solvent floatingzone (TSFZ) method. The maximum dimensions of the grown boules were 6 mm 0 x 27 mm. It is shown that the density of the feed rod and atmosphere are the crucial variables for a successful growth procedure. The grown crystals were identified to be single-crystal REBa2Cus07_, by Laue X-ray measurements and X-ray powder diffraction. The maximum area of the grains was about 1 X 1 mm’ for the Y system, 1.5 X 1.5 mm* for the La, Pr and Nd systems and 1.5 x 3 mm2 for the Sm system, respectively. Superconducting transitions of crystals annealed in oxygen typically occur at 9 1.5 K, 90 K, 9 1 K and 94 K for the Y, La, Nd and Sm compounds, respectively.
1. Introduction
The discovery of 90 K superconductivity in REBa2Cua07_, (RE=rare-earth elements and Y) have led to many studies in recent years. The interest in studies of the superconducting mechanism and application to the substrate of homoepitaxial growth of thin films has resulted in the requirement of highquality single crystals of REBa2Cu30,_, with large dimensions in all crystal axes. Many studies have been carried out on the preparation of REBa&&O,_, crystals such as the slow-cooling [ l-l 81 and the topseeded solution-growth (TSSG) methods [ 19-2 11. The slow-cooling method has the disadvantage of growing a large number of thin-plate crystals because of uncontrollable nucleation. Also, the TSSG method has the problem of contamination from the crucible materials. The travelling-solvent floating-zone (TSFZ) method is a valuable growth technique to * Correspondingauthor.
produce high-quality and large single crystals along the c-axis. However, the TSFZ method has not been applied to REBa&&O,_, compounds because the solvent composition must be kept in a narrow range [ 22,23 1. Several efforts have been made without success. As the viscosity of the solvent composed of BaO-CuO is low, the solvent easily falls down onto the seed and penetrates deeply into the feed rod. It is difficult to keep the molten zone during growing. Our experiments were performed in air atmosphere at the early stage of the crystal growth of YBa2Cu307_, NdBa2CuJ07_, and PrBa2Cu307_-y, and we have not succeeded in getting single crystals due to the unstable molten zone. The necessary conditions for stable crystal growth of REBa2Cu307_, by the TSFZ method are that the solvent does not penetrate into the feed rod and does not easily fall down onto the seed to keep the molten zone stable, and that in the wide compositional region, only two phases, liquid and REBa2Cu307_,,, co-
0921-4534/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZO921-4534(94)00213-Y
78
K. Oka, T. Ito / Physica C 227 (1994) 77-84
exist to precipitate REBa2Cu@_, crystals. Concerning the problem of the penetration of solvent into the feed rod, we found that the “twice-scanning” technique is useful to avoid solvent penetration into the feed rod for the crystal growth of Laz_Sr,CuO, [ 241. This phenomenon is very sensitive to temperature, so we have to grow the REBa2Cu307_-y crystal at temperatures as low as possible. Concerning the problem of the solvent falling down onto the seed, we have earlier found that at lower oxygen pressures the problem is not serious for the crystal growth of (NdCe)zCuOd by the TSFZ method
[251. As for the problem of the narrow range of primary crystallization fields, liquidus lines were shown to be extended and lowered with smaller oxygen partial pressure in the YBCO phase relation, by Lay et al. [ 261 and Lee et al. [ 27 1. Also the concentration width of the liquidus line of the SmBazCu30,_, and NdBazCu30,_, system were shown to be wider by Katsui et al. [ 7 ] and Oka et al. [ 28 1. This implies that there is a possibility of obtaining high-quality crystals with ease. In addition, we found that in the La-Sm-Sr-Cu0 system, unlike the La-Sr-Cu-0 system, the solvent neither climbs up the crucible nor penetrates into the feed rod for crystal growth of La,,8Sm,.0Sr0.2Cu04 with the T* structure by the TSSG and TSFZ methods [28]. Therefore, as a first step in growing single crystals, we tried to grow SmBazCu307_, by the TSFZ method with the twice-scanning technique at lower oxygen pressure. Then, we tried crystal growth of other REBa2Cu30,_, In this paper, we describe the crystal growth of REBa2Cu307_-y (RE =Y, La, Pr, Nd, Sm, Gd, Dy, Er and Yb) by the TSFZ method. Some results of the characterization and magnetic measurements on the grown crystals are also presented.
2. Experimental
Nd203,Sm203,G&03, DY,% Er203,yb20~,BaC03 and CuO of 99.9% purity. The feed rod and solvent zone consist of a RE, Ba and Cu mixture at 1: 2 : 3 and 1: 4 : 6-l : 29 : 66, respectively. The well-mixed powder in the required atom ratios was first calcined at 880°C for 15 h, then after grinding and milling it was formed into a cylindrical shape of 8 mm in diameter and 10 cm in length by pressing a metal mold system for making the rod. The feed rod and the solvent rod were then sintered in air for 15 h at 9501030’ C and 900-920” C, respectively. The “twice-scanning” method for crystal growth has been applied. The high-density crystallized feed rod was made using the “first scanning” procedure. About 1.O- 1.5 g of solvent was connected to the sintered feed rod. First, the feed rod was suspended from the hook of the upper shaft; the polycrystalline sintered rod as the seed, was held at the seed holder of the lower shaft. The feed rod and the growing crystal were rotated at about 25 rpm in opposite directions. The molten-zone was attached to the top of the seed. In order to densify the feed rod, the molten zone was passed through the feed rod at a relatively high rate of about 2.0-15.0 mm/h. The regular growing procedure was, then, carried out using the once-scanned crystallized rod as the feed rod to which the solvent of about 0.3-0.5 g was connected, and using the slow-cooling grown crystal or sintered rod as the seed. The growth rate was in the range of 0.4-1.0 mm/h. Both scans were performed in argon mixed with oxygen at pressures of 1O-*- 10p4 atm. Some experimental results are summarized in Table 1. The as-grown crystals had tetragonal symmetry and showed no superconductivity. These as-grown crystals were annealed at 500°C for 25 h, cooled at 250”C/h, held at 400°C for 60 h in an oxygen atmosphere, and cooled down slowly to room temperature. They were then characterized by powder X-ray diffraction measurements, Laue X-ray measurements, polarization microscope observation and magnetization measurements.
technique
Single crystals of REBa2Cu307_-y were grown in an infrared radiation furnace with a 750 W halogen lamp as the radiation source. The starting feed and solvent materials were prepared from Yz03, Laz03, Pr601 I,
3. Result and discussion Crystal boules grown by the TSFZ method are shown in Fig. 1. These crystals have dimensions of
K. Oka, T. Ito / Physica C 227 (1994) 77-84 Table 1 Experimental Experimental no.
12 13 33 34 36 31 18 19 21 1 2 6 9 8 10 20 30 26 24 27 37
details for the crystal-growth Rare-earth element RE
Y Y Y Y Y La Pr Nd Nd Sm Sm Sm Sm Sm Sm Gd Gd Er Yb Yb Yb
experiments
Composition RE:Ba:Cu
1: 19:42.7 1: 19:42.7 1:19:42.7 1: 19:42.7 1:29:66 1:4.0:6.0 1:6.0: 13 1: 13:21.3 1: 13:21.3 1: 19.4:31.7 1: 19.4:31.7 1:19.4:31.7 1:19.4:31.7 1:19.4:31.7 1: 16.8: 27.4 1:15:22 1:15:22 1:18:25 1: 15:22 1: 18:25 1: 15:22
of solvent
19
of REBa#&O,_, Sintering (“C)
[15
temperature
hl
feed rod
solvent rod
950 950 950 950 950 950 950 950 950 1030 L 1030” 1030” 1030 a 1030 B 1030 B 950 950 950 950 950 950
900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900 900
Atmosphere
Feed speed
W/G)
(mm/h)
99.011.0 99.9fO.l 100/o (N2= 100) 99.910.1 99.910.1 99.910.1 99.9lO.l (N,= 100) air 93.Ol7.0 99.0/1.0 100/o 99.0/ 1.0 99.910.1 (N,/02=99.0/l.0) 99.910.1 99.910.1 99.910.1 99.910.1 100/o
1st scan
2nd scan
2-3 3-5 3-5 2-3 5-6 5-15 5-10 3-5 3-5
1.0 1.0 0.5-1.0 0.5-1.0 1.0 -
5-6 3-5 2-5 3-5 3-6 3-5 3-5 2-5 5-10
1.0 1.0 1.0 1.0
a [2 h].
4-6 mm in diameter, and 15-30 mm in length. Some results of crystals are summarized in Table 2. 3.1. YBatCu,O,_,
crystals
The maximum size of the YBa2Cu30,_, crystal was about 4 mm 0 x 15 mm. The X-ray Laue analysis indicated it was a single crystal. Small crystals were pulverized and used for powder X-ray diffraction. The observed pattern showed mainly YBa2Cuj0,_, phase and a small amount of YzBaCuO, phase. Magnetization measurements down to 4.2 K showed a T, onset at 9 1.5 K, the transition width was not so sharp. The grain was observed by an optical microscope under polarized light. The maximum size of the grain was about 1x 1 mm’. Attempts to grown crystals at high temperature, using a solvent with composition of Y: Ba: Cu= 1: 19: 42, resulted in mixing of the Y2BaCuOS phase into the crystals. The BaCu202 phase crystal was often grown by the use of a solvent with composition
Y: Ba: Cu = 1: 29: 66 at the oxygen pressure of 10e3 atm. The YBCO system was difficult to crystallize. In the first scan, the solvent penetrated deeply into the feed rod in spite of high-speed scans such as 10 mm/ h, because it was hard to get the high-density sintered feed rod. A 7.5 mm 0 sintered feed rod always expands due to the penetration of the solvent becoming 8 mm as shown in Fig. 2. Finally, a 6 mm 0 crystallized feed rod was obtained by “first scanning”. 3.2. (La, Pr, Nd and Sm) Ba#Zu,O,_,
crystals
The maximum size of the (La, Pr, Nd and Sm) Ba2Cuj0,_,, crystals were about 6 mm 0x27 mm, 4.5 mm 0x 16 mm, 5 mm 0x 15 mm and 5 mm 0 x 15 mm, respectively. The X-ray diffraction pattern was observed using the Cu Ka line as incident radiation. The La system showed mainly LaBa2Cu30,_, phase and a very small amount of second phase. The Pr system showed only
80
K. Oka. T Ro / Physica C 227 (1994) 77-84
Fig. 1. REBa2Cu307_, crystal boule grown by the travelling-solvent PrBa2Cu307_, (d) SmBa2Cu307_, and (e) NdBa2Cu30,_,
PrBa2Cu307,y phase as shown in Fig. 3. The Nd and Sm system showed mainly NdBa&u@_, and SmBazCu30,_, phase and a small amount of NdzBaCuO, and SmzBaCuO, phase as shown in Fig. 4. For the Sm system, a high growth temperature leads to the growth of SmlBaCu05 phase crystals. We ob-
floating-zone
method: (a) YBa2Cu30,_,,
(b) LaBa2Cu30,_,
(c)
served the SmzBaCuOs phase in our crystal obtained by high-temperature growth at more than 320 W lamp power. The domain structures of the cross section perpendicular and parallel to the growth direction were observed by an optical microscope under polarized light.
K. Oka, T. Ito / Physica C 227 (I 994) 77-84
81
Table 2 Characterization of the REBazCu~O,_, crystals Experimental no.
13
31
18
19
6
21
RE element involved Size of crystal boule (mm 0Xmm) Onset T, Secondary phase
Y
La
Pr
Nd
Sm
Yb
4x15
6x12
45x16
5x15
5x15
5x12
91.5 weak
90 very weak
none none
91 very weak
94 very weak
none none
(BaCW2 )
I
,
SmBa,Cu,O,., TSFZ crystal
sintered feed rod
0, annealed
penetrated part of feed rod
L
molten-zone crystallized feed rod
/
30
Ji I
I
40
50
tD
28 (degree) Fig. 2. Schematic figure of the “first scanning” to make a crystallized feed rod of YBazCusO,_, by the travelling-solvent floatingzone method. I
20
30
I
40
I
50
60
28 (degree) Fig. 3. X-ray powder diffraction pattern of an as-grown PrBazCusO,_, crystal grown by the travelling-solvent floating-zone method.
We observed several grains oriented at random directions. The largest grains perpendicular to the growth direction were found to be 1.5 mm in each dimension on the La, Pr and Nd systems. Also the largest grains
Fig. 4. X-ray powder diffraction pattern of an oxygen-annealed SmBazCusO,_, crystal grown by the travelling-solvent floatingzone method.
parallel to the growth direction were found to be 3 x 1.5 mm2 on the Sm system and 1.5 mm in each dimension on the other REBa2Cu@_, systems. The as-grown crystals had tetragonal symmetry and showed no superconductivity. Measurements of the temperature dependence of the magnetization at 5 G were performed on oxygen-annealed (5OO”C, 25 h, +4OO”C, 60 h) crystals. The oxygen-annealed LaBa2Cu~0,_, and NdBa2Cu307_, crystals showed a Tconsetat 90 K and 91 K, respectively. The result of the SmBa2Cu307_, crystal showed a sharp superconducting transition with an onset temperature of 94 K as shown in Fig. 5. 3.3. (Gd, Dy, Er, Yb) Ba2Cu307_, crystals In the (Gd, Dy, Er and Yb) Ba2Cu307_-y systems we have succeeded in keeping the molten zone, but we could not get the (Gd, Dy, Er and Yb) Ba2Cuj0,_, crystals. We have made many trials
82
K. Okz, T. Ito / Physica C 22 7 (I 994) 77-84
0
I
-12’ 0
lution penetrates deeply into the feed rod and easily falls down onto the seed. This phenomenon depends very much on the oxygen partial pressure of the ambient atmosphere, the density of the feed rod, the solvent materials, the growth temperature and the heating time or the feed speed of the solvent. In the YBa2Cu307_-y and PrBazCu30,_, systems, the solvent is apt to fall down easily onto the seed in 1% oxygen mixed argon gas. However, the molten zone remains stable in 0.1% oxygen mixed argon gas.
SmBo2 C1~307-~ 02 annealed
20
40 Temperature
I 60 (K)
80
100
Fig. 5. Measurements of the temperature dependence of the magnetization of oxygen-annealed (SOO’C, 25 h+400”C, 60 h) SmBa2Cu,0,_, crystal at H=5 G. I
I
L
20
30
/
BaCu,O,
40
crystal as-grown
TSFZ
50
3.5. Composition of the solvent Fig. 7 is a triangular coordinate diagram showing the composition of the solvent for the crystal growth of REBa2Cu30,_, by the TSFZ method in an atmosphere of low oxygen pressure. With larger ionic radius of the RE atoms, the composition of the solvent becomes near to the composition of the crystal, in other words, the extent of the liquidus line becomes wider. So the REBa2Cu307_-y system shows a similar tendency to the RE2Cu04 system [ 30 1. LaBazCuSO,_, and PrBa2Cu307_-y crystals were
60
28 (degree)
Fig. 6. X-ray powder diffraction pattern of an as-grown BaCuzOl crystal grown by the travelling-solvent floating-zone method.
by changing the solvent composition, atmosphere and growth rate. We grew a single crystal of BaCu202, when we tried the crystal growth of YbBazCuSO,_, with using a solvent composition by Yb: Ba: Cu = 1: 15 : 22 in argon atmosphere. The Xray diffraction pattern showed only BaCuzOz phase as shown in Fig. 6. 3.4. Atmosphere We have experimented with air, argon, nitrogen and argon mixed with oxygen (Ar/0,=93/7,99/1,99.9/ 0.1 and 99.99/0.0 1) as the atmosphere. Generally, it is difficult to keep the molten zone and grow a crystal in air and mixed gas of Ar/Oz = 9 3 / 7 because the so-
Fig. 7. The composition of the solvent for the growth of REBa2Cux0,_, crystals (solid squares) and BaCuzOZ crystals (open squares) by the TSFZ method from RE203-BaO-CuO, solution.
K. Oka. T. Ito / Physica C 227 (I 994) 77-84
easy to grow, because the composition of the solvent is near the crystal composition [ 3 1,2 11.
4. Conclusion The penetration of the solvent into the feed rod and the low viscosity of the molten zone are serious problems for the crystal growth of REBa2Cu307_, The twice-scanning technique has been applied in order to avoid the penetration into the feed rod. A low oxygen pressure atmosphere can stabilize the molten zone. It has been possible to keep the stable molten zone growing with and continue crystals the REBa2Cu30,_, system by using a low oxygen pressure atmosphere and a twice-scanning procedure. Also, it has been possible to obtain YBa2Cu307_,, LaBa2Cu30,_,, PrBa2Cu30,_,,, NdBazCu30,_, and SmBa2Cu30,_, crystals containing several randomly oriented grains by the TSFZ method. We also developed a method of growing large single crystals of REBa2Cu307_-y by the TSFZ method. Furthermore we have carried out many trials by changing the composition of the solution, species of atmosphere and growth rate for each REBa2Cu30,_, system. However, further trials and phase diagrams under low oxygen pressure atmosphere will be necessary to prepare crystals of good quality.
Acknowledgements The authors would like to thank H. Unoki of superconductivity Research Laboratory for attracting their attention to the application of the TSFZ method to the crystal growth of REBa2Cu307_, and for discussions. They also thank F. Iga for measurements of magnetization, and Y. Ohashi for his help in the crystal growth. They would also like to thank S. Waki and K. Kajimura for their useful comments and encouragement during this study.
References [ I] Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda, A. Katsui and T. Murakami, Jpn. J. Appl. Phys. Lett. 26 (1987) L726.
a3
[2] S. Takekawa and N. Iyi, Jpn. J. Appl. Phys. Lett. 26 ( 1987) L851. [ 31 H. Takei, H. Takeya, Y. Iye, T. Tamegai and F. Sakai, Jpn. J. Appl. Phys. Lett. 26 (1987) L1425. [4] K. Hayashi, K. Murata, K. Takahashi, M. Tokumoto, H. Ihara, M. Hirabayashi, N. Terada, N. Koshizuka and Y. Kimura, Jpn. J. Appl. Phys. Lett. 26 (1987) L1240. [5] L.F. Schneemeyer, J.V. Waszczak, T. Siergrist, R.B. van Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D. W. Murphy, Nature (London) 328 (1987) 601. [6] S. Hayashi, H. Komatsu, T. Inoue, T. Ono, K. Sasaki, Y. Koike and T. Fukase, Jpn. J. Appl. Phys. Lett. 26 (1987) L1197. 17I A. Katsui, Y. Hidaka and H. Ohtsuka, Jpn. J. Appl. Phys. Lett. 26 (1987) L1521. ]8 M.J.V. Menken and A.A. Menovsky, J. Cryst. Growth 91 (1988) 264. ]9 1M.J.V. Menken, K. Kadowaki and A.A. Menovsky, J. Cryst. Growth 96 (1989) 1002. [ lo] J.A. Campa, J.M.G. Salazar, E.G. Puebla, M.A. Monge, I. Rasines and C.R. Valero, J. Cryst. Growth 91 (1988) 334. [ 111 M. Hikita, Y. Tajima, A. Katsui, Y. Hidaka, T. Iwata and S. Tsurumi, Phys. Rev. B 36 (1987) 7199. [ 121 T. Iwata, M. Hikita, Y. Tajima and S. Tsurumi, J. Cryst. Growth85 (1987) 661. [ 131 Y. Tajima, M. Hikita, A. Katsui, Y. Hidaka, T. Iwata and S. Tsurumi, J. Cryst. Growth 85 (1987) 665. [ 141 R.C.J. Draper, G.A. Saunders, B. Chapman, W. Hong, H. Perrot, R.N. Hampton and R.M. Bush, J. Mater. Sci. Lett. 7 (1988) 1281. [15] R.X. Wan, G.M. Zhao, X.M. Tang, W.Z. Li, S.Z. Hu and H.N. Yao, J. Appl. Phys. 64 (1988) 3754. [ 161 T. Iwata, Y. Tajima and M. Hikita, J. Cryst. Growth 91 (1988) 274. [ 171 T. Inoue, S. Hayashi, M. Shimizu and H. Komatsu, J. Cryst. Growth 91 (1988) 287. [ 181 S. Hayashi, T. Ohno, T. Inoue and H. Komatsu, J. Cryst. Growth91 (1988) 331. [ 191 K. Oka, M. Saito, M. Ito, K. Nakane, M. Murata, Y. Nishihara and H. Unoki, Jpn. J. Appl. Phys. Lett. 28 ( 1989) L1521. [20] Y. Yamada and Y. Shiohara, Physica C 217 (1993) 182. [ 211 M. Tagami, M. Sumida, C. Krauns, Y. Yamada, T. Umeda and Y. Siohara, Int. Symp. Supercond. 1993 Proc., to be published. 122 K. Oka, K. Nakene, M. Ito, M. Saito and H. Unoki, Jpn. J. Appl. Phys. Lett. 27 (1988) L1065. ~23 M. Maeda, M. Kadoi and T. Ikeda, Jpn. J. Appl. Phys. 29 (1989) 1417. 124 K. Oka, M.J.V. Menken, Z. Tamawski, A.A. Menovsky, A.M. Moe, T.S. Han, H. Unoki, T. Ito and Y. Ohashi, J. Cryst. Growth, to be published. 125 K. Oka and H. Unoki, Jpn. J. Appl. Phys. Lett. 29 (1990) L1807. 126 K.W. Lay and G.M. Renlund, J. Am. Ceram. Sot. 73 ( 1990) 1208. 127 B.J. Lee and D.N. Lee, J. Am. Ceram. Sot. 74 ( 199 I ) 78.
84 [28
K. Oka, T. Ito / Physica C 227 (1994) 77-84
] K. Oka and H. Unoki, Jpn. J. Appl. Phys. Lett. 28 (1989)
L937. [29] K. Oka and H. Unoki, J. Cryst. Growth 99 ( 1990) 922. [30] K. Oka, H. Unoki, A.M. Moe, F. Iga, K. Kaneko, D.H. Ha
and T.S. Han, Advances in Superconductivity V, eds. Y Bando and H. Yamauchi (Springer, Tokyo, 1993) p. 383. [ 311 H. Eisaki, private communication (1992).