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New growth method of oxide crystals by PO2 change applied to SmBa2Cu3Ox single crystals
Journal of Crystal Growth 205 (1999) 503}509
New growth method of oxide crystals by PO change applied to SmBa Cu O single crystals V Yoshihiro Nishimura, Satoru Miyashita*, Stephen Duane Durbin, Toshitaka Nakada, Gen Sazaki, Hiroshi Komatsu Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received 19 January 1999; accepted 18 May 1999 Communicated by L.F. Schneemeyer
Abstract Reliable liquidus lines of SmBa Cu O in the SmBa Cu O }Ba Cu O system under various oxygen partial V V pressures were determined precisely by in situ observation. The dependence of the liquidus line on oxygen partial pressure was revealed: as the oxygen partial pressure increased, the liquidus shifted to higher temperature. Using this data, SmBa Cu O crystals were grown. This process was successfully observed in situ by high-temperature optical V microscopy. 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 74.72.Hs; 81.10.Dn; 81.30.Dz Keywords: High-¹ superconductor; SmBa Cu O ; In situ observation; Solution growth; Liquidus; Oxygen partial pressure V
1. Introduction The oxygen partial pressure (PO ) in the ambient atmosphere is one of the key factors for the growth of oxide crystals from high-temperature solutions. The value of PO strongly a!ects the phase equilib rium between a crystal and a liquid phase, and it also a!ects the viscosity and surface tension of the solution. The dependence of the pseudo-binary phase diagrams on PO have been reported, for example, for Y Fe O (YIG) [1], Bi Sr CaCu O V (BSCCO) [2], YBa Cu O (YBCO) [3], V * Corresponding author. Tel.: #81-22-215-2012; fax: #8122-215-2011. E-mail address: [email protected] (S. Miyashita)
NdBa Cu O (NdBCO) [4], Y Cu O (YCO) [5], V Gd CuO (GdCO) [6]. It was found that the peritectic and eutectic temperatures shift to higher temperature with increasing PO . However, the in#uence of PO on the crystal growth is not yet known, since it is di$cult to determine liquidus lines in detail by a conventional method. In general, crystal growth from solution utilizes a shift in the phase equilibrium where crystals coexist with solution. The conventional slow-cooling method, which uses temperature as a parameter, is based on this principle. Here, we suggest that PO could also be a parameter for the growth of oxide crystals, if a shift of the phase equilibrium occurs by a change of PO at a "xed temperature. This method, employing PO as a growth parameter,
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 2 6 8 - 7
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can be especially useful for growing large single crystals of high quality when the coexisting temperature range of solution and crystals in a phase diagram is so narrow that the slow-cooling method cannot be applied. Moreover, this method has the merit that thermal stress is not introduced into the growing crystals. The objectives of this study are to test the above-mentioned hypothesis and to propose a new method for the growth of oxide crystals from hightemperature solutions. This paper deals with the growth of SmBa Cu O (Sm123) crystals from V solution using BaO}CuO as a solvent. Here, we have determined the shift in the liquidus line induced by a change of PO in the environment by in situ observation and, based on these results, the crystal growth process was directly observed.
2. Experimental procedure In situ observation of the liquid/solid-phase equilibrium was carried out using a high-temperature optical microscope, consisting of an optical microscope with an objective lens of long working distance (OPTIPHOT, NIKON, magni"cation: x10, working distance: 10.5 mm) and an infrared heating furnace (MS-E1R, SHINKURIKO, INC., Japan) [7]. The furnace housed a gold-plated chamber in the shape of a paraboloid. A small amount of sample powder mixture was put on a cleaved thin plate (3;3;0.3 mm) of an MgO single crystal, which was set on the bottom of an alumina container in the chamber. The sample was heated by focusing the light of a circular 1 kVA halogen lamp, allowing it to be rapidly heated from room temperature to 12003C. The temperature of the container was measured by a thermocouple and controlled within $0.53C using a PID-controlled thermo-regulator. The temperature of the sample was calibrated by observing the melting points of Au (10643C), Ag (9613C), PbO (8863C), LiF (8483C), and Bi O (8243C). The error in the temperature measurement of a sample was estimated to be about $1.53C. The chamber had a volume of about 150 ml, and had gas inlet and outlet ports. The oxygen partial pressure in the chamber was controlled by adjust-
ing the mixing ratio of #owing oxygen and argon gases with mass#ow meters, and was monitored with an oxygen-meter (G-101, IIJIMA PRODUCTS M.F.G. Co. Ltd.), which could be read to 0.1%, using a galvanic cell-type sensor. The total #owrate of the gas mixture was about 100 ml/min. The total pressure was kept at atmospheric pressure, &0.1 MPa. Some runs were done in air (PO "21 kPa). Samples were prepared as follows: powders of Sm O , BaCO and CuO (3N) were weighed out and mixed to prepare the solute (Sm : Ba : Cu "1 : 2 : 3 in cation ratio) and the solvent (Ba : Cu "7 : 18). The powders were pressed into pellets and calcined in air at 900}9503C for 24 h with 3}4 intermediate grindings. Finally, appropriate amounts of the solute and solvent powders were weighed out in a given concentration, mixed using a planetary micro-mill, and used as samples. The sample concentration was de"ned as the molar percentage of the solute to the solution; samples with 5, 10, 20, 30, 35, 40, 45, 50% solute were prepared. The solvent was treated as having a nominal composition of `Ba Cu O a. The samples were heated in the furnace chamber and the entire process of crystal growth and dissolution were observed in situ with the optical microscope. The liquidus lines were determined at PO values of 0.1 MPa, 21 and 0.3 kPa, as described in the following section. On the basis of these liquidus lines, the growth of Sm123 at 9903C by changing PO was carried out. 3. Results and discussion 3.1. Inyuence of the oxygen partial pressure on the liquidus line In order to construct accurate liquidus lines, we observed in situ the dissolution process of Sm123 crystals. The sample in the chamber was heated at a rate of 503C/min up to a temperature above 9503C, until it melted into a droplet composed of many Sm123 micro-crystals and a liquid; it was held at this temperature. The liquidus line of the Sm123 phase for the SmBa Cu O }Ba Cu O V system was then determined as follows: "rst, only
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505
Table 1 The dissolution enthalpy and dissolution entropy obtained from the van't Ho! plot PO (kPa)
*H: dissolution enthalpy (kJ/mol)
*S: dissolution entropy (J/mol/K)
100 21 0.3
199 225 294
141 162 221
Fig. 1. The liquidus lines in part of the pseudo-binary phase diagram of the SmBa Cu O }Ba Cu O system for PO "0.3, V 21 kPa and 0.1 MPa.
one Sm123 crystal coexisting with the solution was prepared by using a self-seeding technique [7]. This crystal was then dissolved by slowly increasing the temperature step-wise by 1}23C every 10}30 min. Finally, the Sm123 crystal dissolved completely (without appearance of the Sm BaCuO phase) above a certain temperature; this temperature was de"ned as the liquidus temperature. In cases where the Sm BaCuO crystal appeared by incongruent melting of the Sm123 crystal, we also measured the peritectic temperature of the Sm123 crystal by the same procedure as for YBCO [7]. Fig. 1 shows three liquidus lines of the Sm123 phase in pure oxygen gas (PO "0.1 MPa), in air (PO "21 kPa) and in O /Ar mixed gas (PO "0.3 kPa). Some liquidus temperatures were measured both in air and in O /Ar mixed gas with the same PO as air; there was no di!erence in the results of the two measurements within the error of $1.53C. The liquidus compositions were found to range from 10 to 45 mol% for PO "0.1 MPa, from 5 to 40 mol% for PO "21 kPa and from 5 to 30 mol% for PO "0.3 kPa. It was deter mined that the peritectic points were between 45 and 50 mol% for PO "0.1 MPa, 40 and 45 mol% for PO "21 kPa, 30 and 35 mol% for PO "0.3 kPa. The temperatures of the peritectic points correspond to the peritectic temperatures.
Fig. 2. PO of the liquidus temperature at the starting composi tion of 30 mol% (Sm : Ba : Cu"1.00 : 18.3 : 45.0).
As PO was increased at a given temperature, the liquidus composition shifted to lower concentration of the Sm123 phase. Also, as PO was in creased, the peritectic temperature increased. From the data of these liquidus lines, the dissolution enthalpy *H and the dissolution entropy *S were obtained from the van't Ho! plot (log concentration vs. inverse of absolute temperature results in a linear expression where the slope and y-intercept correspond to *H and *S, respectively). It was found that both *H and *S increased as the oxygen partial pressure decreased, as shown in Table 1. The liquidus temperatures at 30 mol% concentration were measured in greater detail by changing PO incrementally from 0.3 to 0.1 MPa; the results are shown in Fig. 2. A ten-fold increase in PO resulted in an increase in the liquidus temperature of about 153C. The same tendency was reported for the peritectic temperature of YIG [1], BSCCO [2]
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and YBCO [3]. In addition, this composition of 30 mol% should allow growth of Sm123 crystals from the high-temperature solution under a wide PO range, from 0.3 to 0.1 MPa. For example, we have successfully used this composition for the solution-zone in the growth of Sm123 crystals by the traveling solvent #oating zone (TSFZ) method at 0.3}0.7 kPa. These crystals showed a superconducting transition below 95 K (¹ onset) after oxy gen annealing [8]. Within the range from 970 to 10003C, the solubility of the Sm123 phase monotonically decreased at a "xed temperature as PO was in creased from 0.3 kPa to 0.1 MPa. This suggests that the Sm123 crystals can be grown from the high-temperature solution by increasing PO . 3.2. Crystal growth by changing the oxygen partial pressure On the basis of the obtained liquidus lines, we next attempted to grow a single crystal under the microscope by changing PO . The starting concen tration of the sample was chosen to be 30 mol% (Sm : Ba : Cu"1 : 18.3 : 45 in molar ratio). At 0.4 kPa, a Sm123 crystal coexisting with the solution was prepared by the self-seeding method. The temperature was kept at 9903C, and PO was raised incrementally from 0.4 kPa up to 0.1 MPa, holding for 30 min at each PO value. Fig. 3 shows a sequence of optical micrographs at 0.4, 2.0, 19.6 kPa and 0.1 MPa, respectively. When PO was "rst increased to a new value, the Sm123 crystal started growing rapidly, keeping its square shape. This crystal growth almost ceased after 2}5 min while the PO was held constant. The total growth process illustrated in Fig. 3 took about 3.5 h. As might be expected, the crystal dissolved and became smaller when PO was reduced again. Thus, it is clear that Sm123 crystals can grow from a high-temperature solution by changing PO . We 䉴 Fig. 3. Optical micrographs of the growing Sm123 crystal as PO was increased. The temperature was kept at 9903C. The starting composition of the sample was Sm : Ba : Cu" 1.00 : 18.3 : 45.0. (a) PO "0.4 kPa, (b) PO "2.0 kPa, (c) PO "19.6 kPa, (d) PO "100 kPa.
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propose this method as a new means of growing of oxides from high-temperature solutions. We now propose a possible explanation for why Sm123 crystals grow by increasing PO . Consider that the Sm123 crystal, the solution and the ambient gas of a given PO are in equilibrium at a given temperature. This solution contains various ions or complexes, and the valency of the copper ion can be changed by changing PO [9,10]. Here, for simpli city, it is assumed that no complexes exist in the solution and only copper ions are in#uenced by PO changes. This phenomenon can then be ex plained on the basis of Le Chatelier's law. First, suppose two kinds of chemical reactions occur in the solution. One is the dissolution of the Sm123 crystals into the solution, and another is the reduction}oxidation of copper ions. These can be expressed as follows: SmBa Cu O (s) & Sm>(l)#2Ba>(l) V #3yCu>(l)#3(1!y)Cu>(l) #xO\(l), 10#3y )6.5, 6)x" 2
(1) (2)
(3) 2Cu>(l)#O\(l) & 2Cu>(l)#O (g), where (s), (l), and (g) mean solid, liquid, and gaseous state, respectively, and the relation between x and y is expressed in Eq. (2) [11,12]. Here, for simplicity, bare ion expressions are used to express the ions in the solution, although complexes may exist. Eq. (1) shows the equilibrium between the dissolution and growth of Sm123 crystals and the solution; Eq. (3) shows the redox equilibrium between Cu> and Cu> in the solution. The law of mass action for the reactions given by (1) and (3) can be expressed as follows: [Sm>(l)][Ba>(l)][Cu>(l)]W[Cu>(l)]\W[O\(l)]V [SmBa Cu O (s)] V "K , (4)
[Cu>(l)][O (g)] "K , [Cu>(l)][O\(l)]
(5)
where brackets represent concentration (mol/ volume), and K and K represent the equilibrium constants of Eqs. (1) and (3) at 9903C. These equi-
507
librium constants are independent of the concentration of each component when the temperature is "xed. Thus, when the concentrations of each element are changed, the equilibrium will shift on the basis of the Le Chateliers law in order to keep the K and K values constant. According to Henry's law, the concentration of O in the solution is proportional to the ambient PO , and it will increase as PO is increased. It follows from (5) that [Cu>(l)] and [O\(l)] must be increased and [Cu>(l)] must be decreased in order to keep the value of K constant. Thus, the equilibrium of Eq. (3) will shift to the left side, and the concentration of Cu> in the solution will be increased. This increase in the Cu> concentration leads to a shift in the equilibrium of Eq. (1) to the left side (due to the law of mass action and the principle of Le Chatelier). Therefore, the Sm123 crystal should grow when PO is raised, as ob served in the experiments. In other words, an increase in PO causes a decrease in the solubility of Sm123 phase. However, the details of the mechanism are not yet clari"ed, since the exact nature of the elementary reactions in the solution is unknown. We also observed the growth of BSCCO crystals by changing PO . As in case of Sm123, a BSCCO crystal was successfully grown as PO was increased. The growth process is shown in Fig. 4. The temperature was kept constant at 8823C and the starting composition of the sample was Bi : Sr : Ca : Cu"2.46 : 0.60 : 0.80 : 1.54. BSCCO was con"rmed to be the primary phase grown from the solution in air with this starting composition [13]. Fig. 4a shows a square-shaped BSCCO crystal coexisting with the solution at PO "28.6 kPa. As PO was increased from 28.6 to 35.5 kPa, this thin crystal grew larger, as shown in Figs. 4b and c. The well-developed crystal face was the (0 0 1) face. When PO was increased to above 40 kPa, this crystal grew much larger and "nally covered the liquid surface completely. The growth mechanism of the BSCCO may be explained by the same mechanism as for Sm123, since it also contains Cu(II) atoms. We expect that the growth method described here can also be applied to other oxide compounds which contain transition metal ions; the oxidation
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state of these ions can easily be increased by increasing PO . Then, the phase equilibrium between a crystal and a solution would be shifted to lower concentration of ions in the higher valence state in the solution. This means that the saturation concentration (solubility) of the crystal in the solution is decreased. This method has the advantage that the temperature of the system can be maintained constant during crystal growth. Thus, this method could be useful in high-temperature solution growth when the temperature range of the liquidus line is so narrow that crystals cannot be grown by a conventional slow-cooling method. In order to control crystal growth precisely with this method, however, the dependence of the liquidus line on PO must be known a priori.
4. Conclusions From in situ observation, the liquidus lines of the Sm123 phase were determined under various ambient oxygen partial pressures. The behavior of the liquidus line was found to be as follows: (1) The Sm123 phase coexisted with the solution at PO "0.1 MPa to 0.3 kPa. (2) As PO was increased at a given temperature, the liquidus composition shifted to a lower concentration of the Sm123 phase. Also, as PO was increased the peritectic temperature of Sm123 increased. This behavior of the liquidus line could be explained by a shift in the reduction}oxidation equilibrium of copper ions in the solution. (3) A Sm123 crystal was grown by increasing PO at 9903C using the liquidus data.
Acknowledgements
Fig. 4. Optical micrographs of the growing BSCCO crystal as PO was increased. The starting composition of the sample was Bi : Sr : Ca : Cu"2.46 : 0.60 : 0.80 : 1.54. The temperature was kept at 8823C. (a) PO "28.6 kPa, (b) PO "31.6 kPa, (c) PO "35.5 kPa.
The authors are grateful to Dr. Suzuki of the National Institute for Research in Inorganic Materials, Japan for valuable discussions. We also thank Mr. Shimizu of the Institute for Materials Research, Tohoku University for technical support. This study was supported by Nippon Sheet Glass Foundation for Materials Science and Engineering
Y. Nishimura et al. / Journal of Crystal Growth 205 (1999) 503}509
and Iwatani Naoji Foundation. S.M. acknowledges the support by the Science Research Grant-in-Aid from the Ministry of Education, Science and Culture, No. 09550004.
References [1] H.J. Van Hook, J. Am. Ceram. Soc. 46 (1963) 248. [2] H. Komatsu, Y. Kato, S. Miyashita, T. Inoue, The International Workshop on Superconductivity Co-Sponsored by ISTEC and MRS June 23}26, 1992, Honolulu S3-1, p. 93. [3] K.W. Lay, G.M. Renlund, J. Am. Ceram. Soc. 73 (1990) 1208. [4] M. Nakamura, M. Kambara, T. Umeda, Y. Shiohara, Physica C 266 (1996) 178.
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[5] W. Zhang, K. Osamura, Z. Metallkd. 81 (1990) 196. [6] W. Zhang, K. Osamura, Metall. Trans. B 22 (1991) 705. [7] Y. Nishimura, Y. Yasuhara, S. Miyashita, H. Komatsu, J. Crystal Growth 158 (1996) 255. [8] Y. Nishimura, Ph. D. Thesis, Faculty of Science, Tohoku University, Sendai, 1996. [9] W.D. Johnston, A. Chelko, J. Am. Ceram. Soc. 48 (1965) 184. [10] S. Banerjee, A. Paul, J. Am. Ceram. Soc. 57 (1974) 286. [11] T.B. Lindermer, J.F. Hunley, J.E. Gates, A.L. Sutton Jr., J. Brynestad, C.R. Hubbard, P.K. Gallagher, J. Am. Ceram. Soc. 72 (1989) 1775. [12] Jung-Sik Kim, D.R. Gaskell, J. Am. Ceram. Soc. 77 (1994) 753. [13] H. Komatsu, Y. Kato, S. Miyashita, T. Inoue, S. Hayashi, Physica C 190 (1991) 14.
New growth method of oxide crystals by PO change applied to SmBa Cu O single crystals V Yoshihiro Nishimura, Satoru Miyashita*, Stephen Duane Durbin, Toshitaka Nakada, Gen Sazaki, Hiroshi Komatsu Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received 19 January 1999; accepted 18 May 1999 Communicated by L.F. Schneemeyer
Abstract Reliable liquidus lines of SmBa Cu O in the SmBa Cu O }Ba Cu O system under various oxygen partial V V pressures were determined precisely by in situ observation. The dependence of the liquidus line on oxygen partial pressure was revealed: as the oxygen partial pressure increased, the liquidus shifted to higher temperature. Using this data, SmBa Cu O crystals were grown. This process was successfully observed in situ by high-temperature optical V microscopy. 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 74.72.Hs; 81.10.Dn; 81.30.Dz Keywords: High-¹ superconductor; SmBa Cu O ; In situ observation; Solution growth; Liquidus; Oxygen partial pressure V
1. Introduction The oxygen partial pressure (PO ) in the ambient atmosphere is one of the key factors for the growth of oxide crystals from high-temperature solutions. The value of PO strongly a!ects the phase equilib rium between a crystal and a liquid phase, and it also a!ects the viscosity and surface tension of the solution. The dependence of the pseudo-binary phase diagrams on PO have been reported, for example, for Y Fe O (YIG) [1], Bi Sr CaCu O V (BSCCO) [2], YBa Cu O (YBCO) [3], V * Corresponding author. Tel.: #81-22-215-2012; fax: #8122-215-2011. E-mail address: [email protected] (S. Miyashita)
NdBa Cu O (NdBCO) [4], Y Cu O (YCO) [5], V Gd CuO (GdCO) [6]. It was found that the peritectic and eutectic temperatures shift to higher temperature with increasing PO . However, the in#uence of PO on the crystal growth is not yet known, since it is di$cult to determine liquidus lines in detail by a conventional method. In general, crystal growth from solution utilizes a shift in the phase equilibrium where crystals coexist with solution. The conventional slow-cooling method, which uses temperature as a parameter, is based on this principle. Here, we suggest that PO could also be a parameter for the growth of oxide crystals, if a shift of the phase equilibrium occurs by a change of PO at a "xed temperature. This method, employing PO as a growth parameter,
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 2 6 8 - 7
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can be especially useful for growing large single crystals of high quality when the coexisting temperature range of solution and crystals in a phase diagram is so narrow that the slow-cooling method cannot be applied. Moreover, this method has the merit that thermal stress is not introduced into the growing crystals. The objectives of this study are to test the above-mentioned hypothesis and to propose a new method for the growth of oxide crystals from hightemperature solutions. This paper deals with the growth of SmBa Cu O (Sm123) crystals from V solution using BaO}CuO as a solvent. Here, we have determined the shift in the liquidus line induced by a change of PO in the environment by in situ observation and, based on these results, the crystal growth process was directly observed.
2. Experimental procedure In situ observation of the liquid/solid-phase equilibrium was carried out using a high-temperature optical microscope, consisting of an optical microscope with an objective lens of long working distance (OPTIPHOT, NIKON, magni"cation: x10, working distance: 10.5 mm) and an infrared heating furnace (MS-E1R, SHINKURIKO, INC., Japan) [7]. The furnace housed a gold-plated chamber in the shape of a paraboloid. A small amount of sample powder mixture was put on a cleaved thin plate (3;3;0.3 mm) of an MgO single crystal, which was set on the bottom of an alumina container in the chamber. The sample was heated by focusing the light of a circular 1 kVA halogen lamp, allowing it to be rapidly heated from room temperature to 12003C. The temperature of the container was measured by a thermocouple and controlled within $0.53C using a PID-controlled thermo-regulator. The temperature of the sample was calibrated by observing the melting points of Au (10643C), Ag (9613C), PbO (8863C), LiF (8483C), and Bi O (8243C). The error in the temperature measurement of a sample was estimated to be about $1.53C. The chamber had a volume of about 150 ml, and had gas inlet and outlet ports. The oxygen partial pressure in the chamber was controlled by adjust-
ing the mixing ratio of #owing oxygen and argon gases with mass#ow meters, and was monitored with an oxygen-meter (G-101, IIJIMA PRODUCTS M.F.G. Co. Ltd.), which could be read to 0.1%, using a galvanic cell-type sensor. The total #owrate of the gas mixture was about 100 ml/min. The total pressure was kept at atmospheric pressure, &0.1 MPa. Some runs were done in air (PO "21 kPa). Samples were prepared as follows: powders of Sm O , BaCO and CuO (3N) were weighed out and mixed to prepare the solute (Sm : Ba : Cu "1 : 2 : 3 in cation ratio) and the solvent (Ba : Cu "7 : 18). The powders were pressed into pellets and calcined in air at 900}9503C for 24 h with 3}4 intermediate grindings. Finally, appropriate amounts of the solute and solvent powders were weighed out in a given concentration, mixed using a planetary micro-mill, and used as samples. The sample concentration was de"ned as the molar percentage of the solute to the solution; samples with 5, 10, 20, 30, 35, 40, 45, 50% solute were prepared. The solvent was treated as having a nominal composition of `Ba Cu O a. The samples were heated in the furnace chamber and the entire process of crystal growth and dissolution were observed in situ with the optical microscope. The liquidus lines were determined at PO values of 0.1 MPa, 21 and 0.3 kPa, as described in the following section. On the basis of these liquidus lines, the growth of Sm123 at 9903C by changing PO was carried out. 3. Results and discussion 3.1. Inyuence of the oxygen partial pressure on the liquidus line In order to construct accurate liquidus lines, we observed in situ the dissolution process of Sm123 crystals. The sample in the chamber was heated at a rate of 503C/min up to a temperature above 9503C, until it melted into a droplet composed of many Sm123 micro-crystals and a liquid; it was held at this temperature. The liquidus line of the Sm123 phase for the SmBa Cu O }Ba Cu O V system was then determined as follows: "rst, only
Y. Nishimura et al. / Journal of Crystal Growth 205 (1999) 503}509
505
Table 1 The dissolution enthalpy and dissolution entropy obtained from the van't Ho! plot PO (kPa)
*H: dissolution enthalpy (kJ/mol)
*S: dissolution entropy (J/mol/K)
100 21 0.3
199 225 294
141 162 221
Fig. 1. The liquidus lines in part of the pseudo-binary phase diagram of the SmBa Cu O }Ba Cu O system for PO "0.3, V 21 kPa and 0.1 MPa.
one Sm123 crystal coexisting with the solution was prepared by using a self-seeding technique [7]. This crystal was then dissolved by slowly increasing the temperature step-wise by 1}23C every 10}30 min. Finally, the Sm123 crystal dissolved completely (without appearance of the Sm BaCuO phase) above a certain temperature; this temperature was de"ned as the liquidus temperature. In cases where the Sm BaCuO crystal appeared by incongruent melting of the Sm123 crystal, we also measured the peritectic temperature of the Sm123 crystal by the same procedure as for YBCO [7]. Fig. 1 shows three liquidus lines of the Sm123 phase in pure oxygen gas (PO "0.1 MPa), in air (PO "21 kPa) and in O /Ar mixed gas (PO "0.3 kPa). Some liquidus temperatures were measured both in air and in O /Ar mixed gas with the same PO as air; there was no di!erence in the results of the two measurements within the error of $1.53C. The liquidus compositions were found to range from 10 to 45 mol% for PO "0.1 MPa, from 5 to 40 mol% for PO "21 kPa and from 5 to 30 mol% for PO "0.3 kPa. It was deter mined that the peritectic points were between 45 and 50 mol% for PO "0.1 MPa, 40 and 45 mol% for PO "21 kPa, 30 and 35 mol% for PO "0.3 kPa. The temperatures of the peritectic points correspond to the peritectic temperatures.
Fig. 2. PO of the liquidus temperature at the starting composi tion of 30 mol% (Sm : Ba : Cu"1.00 : 18.3 : 45.0).
As PO was increased at a given temperature, the liquidus composition shifted to lower concentration of the Sm123 phase. Also, as PO was in creased, the peritectic temperature increased. From the data of these liquidus lines, the dissolution enthalpy *H and the dissolution entropy *S were obtained from the van't Ho! plot (log concentration vs. inverse of absolute temperature results in a linear expression where the slope and y-intercept correspond to *H and *S, respectively). It was found that both *H and *S increased as the oxygen partial pressure decreased, as shown in Table 1. The liquidus temperatures at 30 mol% concentration were measured in greater detail by changing PO incrementally from 0.3 to 0.1 MPa; the results are shown in Fig. 2. A ten-fold increase in PO resulted in an increase in the liquidus temperature of about 153C. The same tendency was reported for the peritectic temperature of YIG [1], BSCCO [2]
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and YBCO [3]. In addition, this composition of 30 mol% should allow growth of Sm123 crystals from the high-temperature solution under a wide PO range, from 0.3 to 0.1 MPa. For example, we have successfully used this composition for the solution-zone in the growth of Sm123 crystals by the traveling solvent #oating zone (TSFZ) method at 0.3}0.7 kPa. These crystals showed a superconducting transition below 95 K (¹ onset) after oxy gen annealing [8]. Within the range from 970 to 10003C, the solubility of the Sm123 phase monotonically decreased at a "xed temperature as PO was in creased from 0.3 kPa to 0.1 MPa. This suggests that the Sm123 crystals can be grown from the high-temperature solution by increasing PO . 3.2. Crystal growth by changing the oxygen partial pressure On the basis of the obtained liquidus lines, we next attempted to grow a single crystal under the microscope by changing PO . The starting concen tration of the sample was chosen to be 30 mol% (Sm : Ba : Cu"1 : 18.3 : 45 in molar ratio). At 0.4 kPa, a Sm123 crystal coexisting with the solution was prepared by the self-seeding method. The temperature was kept at 9903C, and PO was raised incrementally from 0.4 kPa up to 0.1 MPa, holding for 30 min at each PO value. Fig. 3 shows a sequence of optical micrographs at 0.4, 2.0, 19.6 kPa and 0.1 MPa, respectively. When PO was "rst increased to a new value, the Sm123 crystal started growing rapidly, keeping its square shape. This crystal growth almost ceased after 2}5 min while the PO was held constant. The total growth process illustrated in Fig. 3 took about 3.5 h. As might be expected, the crystal dissolved and became smaller when PO was reduced again. Thus, it is clear that Sm123 crystals can grow from a high-temperature solution by changing PO . We 䉴 Fig. 3. Optical micrographs of the growing Sm123 crystal as PO was increased. The temperature was kept at 9903C. The starting composition of the sample was Sm : Ba : Cu" 1.00 : 18.3 : 45.0. (a) PO "0.4 kPa, (b) PO "2.0 kPa, (c) PO "19.6 kPa, (d) PO "100 kPa.
Y. Nishimura et al. / Journal of Crystal Growth 205 (1999) 503}509
propose this method as a new means of growing of oxides from high-temperature solutions. We now propose a possible explanation for why Sm123 crystals grow by increasing PO . Consider that the Sm123 crystal, the solution and the ambient gas of a given PO are in equilibrium at a given temperature. This solution contains various ions or complexes, and the valency of the copper ion can be changed by changing PO [9,10]. Here, for simpli city, it is assumed that no complexes exist in the solution and only copper ions are in#uenced by PO changes. This phenomenon can then be ex plained on the basis of Le Chatelier's law. First, suppose two kinds of chemical reactions occur in the solution. One is the dissolution of the Sm123 crystals into the solution, and another is the reduction}oxidation of copper ions. These can be expressed as follows: SmBa Cu O (s) & Sm>(l)#2Ba>(l) V #3yCu>(l)#3(1!y)Cu>(l) #xO\(l), 10#3y )6.5, 6)x" 2
(1) (2)
(3) 2Cu>(l)#O\(l) & 2Cu>(l)#O (g), where (s), (l), and (g) mean solid, liquid, and gaseous state, respectively, and the relation between x and y is expressed in Eq. (2) [11,12]. Here, for simplicity, bare ion expressions are used to express the ions in the solution, although complexes may exist. Eq. (1) shows the equilibrium between the dissolution and growth of Sm123 crystals and the solution; Eq. (3) shows the redox equilibrium between Cu> and Cu> in the solution. The law of mass action for the reactions given by (1) and (3) can be expressed as follows: [Sm>(l)][Ba>(l)][Cu>(l)]W[Cu>(l)]\W[O\(l)]V [SmBa Cu O (s)] V "K , (4)
[Cu>(l)][O (g)] "K , [Cu>(l)][O\(l)]
(5)
where brackets represent concentration (mol/ volume), and K and K represent the equilibrium constants of Eqs. (1) and (3) at 9903C. These equi-
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librium constants are independent of the concentration of each component when the temperature is "xed. Thus, when the concentrations of each element are changed, the equilibrium will shift on the basis of the Le Chateliers law in order to keep the K and K values constant. According to Henry's law, the concentration of O in the solution is proportional to the ambient PO , and it will increase as PO is increased. It follows from (5) that [Cu>(l)] and [O\(l)] must be increased and [Cu>(l)] must be decreased in order to keep the value of K constant. Thus, the equilibrium of Eq. (3) will shift to the left side, and the concentration of Cu> in the solution will be increased. This increase in the Cu> concentration leads to a shift in the equilibrium of Eq. (1) to the left side (due to the law of mass action and the principle of Le Chatelier). Therefore, the Sm123 crystal should grow when PO is raised, as ob served in the experiments. In other words, an increase in PO causes a decrease in the solubility of Sm123 phase. However, the details of the mechanism are not yet clari"ed, since the exact nature of the elementary reactions in the solution is unknown. We also observed the growth of BSCCO crystals by changing PO . As in case of Sm123, a BSCCO crystal was successfully grown as PO was increased. The growth process is shown in Fig. 4. The temperature was kept constant at 8823C and the starting composition of the sample was Bi : Sr : Ca : Cu"2.46 : 0.60 : 0.80 : 1.54. BSCCO was con"rmed to be the primary phase grown from the solution in air with this starting composition [13]. Fig. 4a shows a square-shaped BSCCO crystal coexisting with the solution at PO "28.6 kPa. As PO was increased from 28.6 to 35.5 kPa, this thin crystal grew larger, as shown in Figs. 4b and c. The well-developed crystal face was the (0 0 1) face. When PO was increased to above 40 kPa, this crystal grew much larger and "nally covered the liquid surface completely. The growth mechanism of the BSCCO may be explained by the same mechanism as for Sm123, since it also contains Cu(II) atoms. We expect that the growth method described here can also be applied to other oxide compounds which contain transition metal ions; the oxidation
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state of these ions can easily be increased by increasing PO . Then, the phase equilibrium between a crystal and a solution would be shifted to lower concentration of ions in the higher valence state in the solution. This means that the saturation concentration (solubility) of the crystal in the solution is decreased. This method has the advantage that the temperature of the system can be maintained constant during crystal growth. Thus, this method could be useful in high-temperature solution growth when the temperature range of the liquidus line is so narrow that crystals cannot be grown by a conventional slow-cooling method. In order to control crystal growth precisely with this method, however, the dependence of the liquidus line on PO must be known a priori.
4. Conclusions From in situ observation, the liquidus lines of the Sm123 phase were determined under various ambient oxygen partial pressures. The behavior of the liquidus line was found to be as follows: (1) The Sm123 phase coexisted with the solution at PO "0.1 MPa to 0.3 kPa. (2) As PO was increased at a given temperature, the liquidus composition shifted to a lower concentration of the Sm123 phase. Also, as PO was increased the peritectic temperature of Sm123 increased. This behavior of the liquidus line could be explained by a shift in the reduction}oxidation equilibrium of copper ions in the solution. (3) A Sm123 crystal was grown by increasing PO at 9903C using the liquidus data.
Acknowledgements
Fig. 4. Optical micrographs of the growing BSCCO crystal as PO was increased. The starting composition of the sample was Bi : Sr : Ca : Cu"2.46 : 0.60 : 0.80 : 1.54. The temperature was kept at 8823C. (a) PO "28.6 kPa, (b) PO "31.6 kPa, (c) PO "35.5 kPa.
The authors are grateful to Dr. Suzuki of the National Institute for Research in Inorganic Materials, Japan for valuable discussions. We also thank Mr. Shimizu of the Institute for Materials Research, Tohoku University for technical support. This study was supported by Nippon Sheet Glass Foundation for Materials Science and Engineering
Y. Nishimura et al. / Journal of Crystal Growth 205 (1999) 503}509
and Iwatani Naoji Foundation. S.M. acknowledges the support by the Science Research Grant-in-Aid from the Ministry of Education, Science and Culture, No. 09550004.
References [1] H.J. Van Hook, J. Am. Ceram. Soc. 46 (1963) 248. [2] H. Komatsu, Y. Kato, S. Miyashita, T. Inoue, The International Workshop on Superconductivity Co-Sponsored by ISTEC and MRS June 23}26, 1992, Honolulu S3-1, p. 93. [3] K.W. Lay, G.M. Renlund, J. Am. Ceram. Soc. 73 (1990) 1208. [4] M. Nakamura, M. Kambara, T. Umeda, Y. Shiohara, Physica C 266 (1996) 178.
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[5] W. Zhang, K. Osamura, Z. Metallkd. 81 (1990) 196. [6] W. Zhang, K. Osamura, Metall. Trans. B 22 (1991) 705. [7] Y. Nishimura, Y. Yasuhara, S. Miyashita, H. Komatsu, J. Crystal Growth 158 (1996) 255. [8] Y. Nishimura, Ph. D. Thesis, Faculty of Science, Tohoku University, Sendai, 1996. [9] W.D. Johnston, A. Chelko, J. Am. Ceram. Soc. 48 (1965) 184. [10] S. Banerjee, A. Paul, J. Am. Ceram. Soc. 57 (1974) 286. [11] T.B. Lindermer, J.F. Hunley, J.E. Gates, A.L. Sutton Jr., J. Brynestad, C.R. Hubbard, P.K. Gallagher, J. Am. Ceram. Soc. 72 (1989) 1775. [12] Jung-Sik Kim, D.R. Gaskell, J. Am. Ceram. Soc. 77 (1994) 753. [13] H. Komatsu, Y. Kato, S. Miyashita, T. Inoue, S. Hayashi, Physica C 190 (1991) 14.