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Single crystal growth of pure and Y-substituted NdAlO3 by the floating zone (FZ) method
jo..... o~ C R Y S T A L GROWTH
Journal of Crystal Growth 128 (1993) 680-683 North-Holland
Single crystal growth of pure and Y-substituted NdA10 3 by the floating zone (FZ) method Isao Tanaka, Muneyoshi Kobashi and Hironao Kojima Institute of Inorganic Synthesis, Faculty of Engineering, Yamanashi University, Miyamae 7, Kofu, Yamanashi 400, Japan
It was found that NdAIO3 has a reversible phase transition at a temperature close to its melting point. Substitution of Y for Nd in NdA10 3 was effective in stabilizing the high temperature phase. Crystals of Ndl_xYxA10 3 (0 < x < 0.25) were grown by the floating zone (FZ) method using an infrared furnace. The crystals had some cracks, subgrain boundaries and domain structure, and also were not uniform in Y concentration due to supercooling.
1. Introduction Single crystals of some oxides are being used as substrates for the epitaxial growth of high-Tc superconductors. LnAIO 3 and L n G a O 3 (Ln: La, Nd, Pr) have been found to be useful for these substrates because they have a perovskite-type structure and their lattice parameters are close to those of the superconducting oxides. However, crystals of these materials exhibit twins which result from phase transitions [1,2]. The twin structures lead to poorer quality superconductor thin films. In an attempt to reduce twinning, impurities were doped into these oxides [3]. NdA10 3 has a rhombohedral symmetry with the lattice parameters of a R = 5 . 2 9 1 A and a R = 6 0 . 4 3 °. Thus NdA10 3 is regarded as having a pseudocubic symmetry in which the unit cell of the cubic perovskite structure is contracted along (111), because the a R value of the rhombohedral NdA10 3 cell is close to 60 ° [4]. It was also reported that NdA10 3 transforms from rhombohedral to cubic symmetry above ll00°C. Stabilization of the cubic phase by doping an impurity into NdAIO 3 was expected to prevent twinning in NdA10 3 crystals. In this report, single crystals of pure and Ysubstituted NdA10 3 were grown by the FZ
method, and it was found that in the pure NdAIO 3 the twin structure disappeared at a temperature close to its melting point. Substitution of Y for Nd in NdA10 3 was effective in stabilizing the cubic phase.
2. Experimental procedure Raw materials for preparation of the feed rods were NdzO 3, T - A I 2 0 3 and Y203 of 99.9% purity. Stoichiometric amounts of the raw materials were mixed in ethanol, dried, and calcined at 1200°C for 2 h. The reacted powder was formed to a cylindrical shape of 8 mm in diameter and 70 mm long, and pressed at a hydrostatic pressure of about 300 MPa. The rod was sintered at 1600°C for 2 h in air, and then used as a feed rod. Part of the feed rod was also used for determining the lattice parameters. The FZ apparatus used for crystal growth had infrared heaters of the double ellipsoidal type (Nichiden Machinery Ltd.) with two 3.5 kW halogen lamps as the heat source. The crystal growth was performed in air at a growth rate of 1 to 5 mm/h. The lattice parameters of Y-substituted NdA10 3 were determined by the powder X R D
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
I. Tanaka et al. / Single crystal growth of pure and Y-substituted NdAIO~ by FZ method
681
method using sintered rods. The domain structure was studied by optical microscopy and the Y concentration analyzed using electron probe microanalysis ( J E O L Ltd. model JXA-8600M).
3. Results and discussion 3.1. Crystal growth and phase transition in NdAlO~
Fig. 1 shows a typical as-grown crystal of pure NdAIO 3. This crack-free, transparent crystal was 5 m m in diameter and 30 m m in length, and was obtained independent of the growth rates. However, these crystals were found to have the typical domain structure with width of about 0.2 m m as shown in fig. 2. Mizuno et al. [4] reported that NdA10 3 transformed between the rhombohedral and cubic symmetry above ll00°C from high temperature X R D results. However, we could not detect the phase transition by D T A analysis. Therefore, we decided to study the twin formation in as-grown crystals by observing them during heating and cooling in the infrared growth furnace. The results are shown in fig. 3. Domains were observed in cut and polished vertical sections of an asgrown crystal at a lamp power of 3.2 kW. The domains disappeared at 4.0 kW, and the grown crystals were melted at 4.4 kW. The domains appeared again when the crystals were cooled down after melting. This result suggested that NdA10 3 indeed has a phase transition at a temperature close to the melting point of NdA10 3. Therefore, one method of preparing domain-free
Fig. 1. As-grown boule of NdAIO3.
Fig. 2. Microphotograph of domains in NdAIO3 grown crystals.
Fig. 3. Observation of domains in NdAIO3 grown crystals during heating: (a) at lamp power of 3.2 kW; (b) at lamp power of 4.0 kW.
I. Tanaka et al. / Single crystal growth of pure and Y-substituted NdAIO 3 by FZ method
682
3, 770 ~
3. 750 3.
740
"~ 3.
730
Table 1 Analysis result of the Ndl_xYxA10 3 crystals grown
aH
~ - ~ - ~ ~ , . . . . Q
-
I
I
I
I
I
10
20
30
40
50
Y content
(at%)
Fig. 4. Lattice constants versus Y concentration of Nd 1 _xYx A103.
crystals of temperature
NdA10
3
Feeds
Grown crystals
20 at% Y 25 at% Y
14_+0.2 at% Y 20_+5 at% Y 49+_7 a t % Y
at% Y were equal. This result shows that the rhombohedral form of pure N d A 1 0 3 c a n be changed to the cubic structure by substituting at least 20 at% Y for Nd, and thereby stabilizing the high temperature phase.
would be to stabilize the high
phase.
3.2. Crystal growth of Y-substituted N d A I O 3 Fig. 4 gives the lattice parameters for sintered Nd~_xY~AIO 3 samples having different Y concentrations. The transformation from hexagonal lattice constants (a H and c H) to the cubic perovskite structure (aH/v~ and cH/2x/-3) with compositions can be clearly seen. For pure NdAIO,j , all~X~2 is about 0.8% larger than cH/2x/3. As the Y content of Ndl_~YxA103 increased from 0 to 10 at% Y, a and c changed only slightly. However, in the range of 10-15%, they rapidly approached each other, and at 20
1
b
,
lOmm
! ........................
........ * ; ' . . . . . . . . ~L_2-'_': :-'-2.-2 ~.LLLSf-L2 -~Zi12 L: :: : : : : : : : :: :
Fig. 5. As-grown boule of Nd~_xYxAIO3: (a) x = 0.20; (b) x = 0.25.
Ir Fig. 6. Microphotograph of Nd I_xYxAIO3 grown crystals: (a) x = 0.20; (b) x = 0.25.
I. Tanaka et al. / Single crystal growth o f pure and Y-substituted NdAIO 3 by F Z method
683
The crystal growth of Y-substituted NdAlO 3 (Ndl_xYxA103; x = 0.15, 0.20, 0.25) was attempted using the FZ method described above. Fig. 5 shows two as-grown crystals of Ndl_xYxAIO3 with x = 0.20 and 0.25. They were opaque and had many cracks. Some subgrain boundaries were found in the crystals, and also irregular domain structures were observed, as shown in fig. 6. The Y concentration of these crystals was found to be lower than that of the feed rods, as shown in table 1. The Ndl_xYxA103 crystals grown using a feed rod containing an Y content of x = 0.25 had two phases of 20 at% Y and 49 at% Y. These results suggest that the distribution coefficient of Y in NdA10 3 is lower than unity, and also that supercooling occurs easily during growth of N d l _ x Y x A 1 0 3 . Therefore, the constant composition and temperature of the molten zone should be maintained during growth to obtain uniform solid solutions of Nd 1_xYxA10 3. The T S F Z (traveling solvent floating zone) method using an Y excess solvent coexisting with Nd~_xYxA10 3 may be effective to prevent the constitutional supercooling.
mains disappeared at a temperature close to the melting point, and reappeared again upon cooling. Substitution of 20 at% Y for Nd in NdA103 causes the structure to change from the rhombohedral to the cubic phase. However, the quality of Ndl_xYxA103 crystals grown by the FZ method was poor and contained some cracks, subgrain boundaries, and the domain structure was not eliminated. Y concentration in these crystals was not uniform, indicating that the distribution coefficient of Y in NdAIO 3 was less than unity.
4. Conclusion
References
Crystals of pure and Y-substituted NdA103 can be grown by the FZ method using infrared heating. Crack-free, transparent crystals of pure NdA103, 5 mm in diameter and 30 mm long, were grown, but contained the usual domain structure. By optically observing these crystals at elevated temperatures, it was found that the do-
[1] G.W. Berkstresser, A.J. Valentio and C.D. Brandle, J. Crystal Growth 109 (1991) 457. [2] G.W. Berkstresser, A.J. Valentio and C.D. Brandle, J. Crystal Growth 109 (1991) 467. [3] D. Mateika, H. Kohler, H. Laudan and E. V61kel, J. Crystal Growth 109 (1991) 447. [4] M. Mizuno, T. Yamada and T. Noguchi,Yogyo-Kyokai-Shi 85 (1977) 91.
Acknowledgments The authors wish to thank Professor R.S. Feigelson of Stanford University for many helpful discussions as well as critical reading of the manuscript. This work was partly supported by a Grant-in Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.
Journal of Crystal Growth 128 (1993) 680-683 North-Holland
Single crystal growth of pure and Y-substituted NdA10 3 by the floating zone (FZ) method Isao Tanaka, Muneyoshi Kobashi and Hironao Kojima Institute of Inorganic Synthesis, Faculty of Engineering, Yamanashi University, Miyamae 7, Kofu, Yamanashi 400, Japan
It was found that NdAIO3 has a reversible phase transition at a temperature close to its melting point. Substitution of Y for Nd in NdA10 3 was effective in stabilizing the high temperature phase. Crystals of Ndl_xYxA10 3 (0 < x < 0.25) were grown by the floating zone (FZ) method using an infrared furnace. The crystals had some cracks, subgrain boundaries and domain structure, and also were not uniform in Y concentration due to supercooling.
1. Introduction Single crystals of some oxides are being used as substrates for the epitaxial growth of high-Tc superconductors. LnAIO 3 and L n G a O 3 (Ln: La, Nd, Pr) have been found to be useful for these substrates because they have a perovskite-type structure and their lattice parameters are close to those of the superconducting oxides. However, crystals of these materials exhibit twins which result from phase transitions [1,2]. The twin structures lead to poorer quality superconductor thin films. In an attempt to reduce twinning, impurities were doped into these oxides [3]. NdA10 3 has a rhombohedral symmetry with the lattice parameters of a R = 5 . 2 9 1 A and a R = 6 0 . 4 3 °. Thus NdA10 3 is regarded as having a pseudocubic symmetry in which the unit cell of the cubic perovskite structure is contracted along (111), because the a R value of the rhombohedral NdA10 3 cell is close to 60 ° [4]. It was also reported that NdA10 3 transforms from rhombohedral to cubic symmetry above ll00°C. Stabilization of the cubic phase by doping an impurity into NdAIO 3 was expected to prevent twinning in NdA10 3 crystals. In this report, single crystals of pure and Ysubstituted NdA10 3 were grown by the FZ
method, and it was found that in the pure NdAIO 3 the twin structure disappeared at a temperature close to its melting point. Substitution of Y for Nd in NdA10 3 was effective in stabilizing the cubic phase.
2. Experimental procedure Raw materials for preparation of the feed rods were NdzO 3, T - A I 2 0 3 and Y203 of 99.9% purity. Stoichiometric amounts of the raw materials were mixed in ethanol, dried, and calcined at 1200°C for 2 h. The reacted powder was formed to a cylindrical shape of 8 mm in diameter and 70 mm long, and pressed at a hydrostatic pressure of about 300 MPa. The rod was sintered at 1600°C for 2 h in air, and then used as a feed rod. Part of the feed rod was also used for determining the lattice parameters. The FZ apparatus used for crystal growth had infrared heaters of the double ellipsoidal type (Nichiden Machinery Ltd.) with two 3.5 kW halogen lamps as the heat source. The crystal growth was performed in air at a growth rate of 1 to 5 mm/h. The lattice parameters of Y-substituted NdA10 3 were determined by the powder X R D
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
I. Tanaka et al. / Single crystal growth of pure and Y-substituted NdAIO~ by FZ method
681
method using sintered rods. The domain structure was studied by optical microscopy and the Y concentration analyzed using electron probe microanalysis ( J E O L Ltd. model JXA-8600M).
3. Results and discussion 3.1. Crystal growth and phase transition in NdAlO~
Fig. 1 shows a typical as-grown crystal of pure NdAIO 3. This crack-free, transparent crystal was 5 m m in diameter and 30 m m in length, and was obtained independent of the growth rates. However, these crystals were found to have the typical domain structure with width of about 0.2 m m as shown in fig. 2. Mizuno et al. [4] reported that NdA10 3 transformed between the rhombohedral and cubic symmetry above ll00°C from high temperature X R D results. However, we could not detect the phase transition by D T A analysis. Therefore, we decided to study the twin formation in as-grown crystals by observing them during heating and cooling in the infrared growth furnace. The results are shown in fig. 3. Domains were observed in cut and polished vertical sections of an asgrown crystal at a lamp power of 3.2 kW. The domains disappeared at 4.0 kW, and the grown crystals were melted at 4.4 kW. The domains appeared again when the crystals were cooled down after melting. This result suggested that NdA10 3 indeed has a phase transition at a temperature close to the melting point of NdA10 3. Therefore, one method of preparing domain-free
Fig. 1. As-grown boule of NdAIO3.
Fig. 2. Microphotograph of domains in NdAIO3 grown crystals.
Fig. 3. Observation of domains in NdAIO3 grown crystals during heating: (a) at lamp power of 3.2 kW; (b) at lamp power of 4.0 kW.
I. Tanaka et al. / Single crystal growth of pure and Y-substituted NdAIO 3 by FZ method
682
3, 770 ~
3. 750 3.
740
"~ 3.
730
Table 1 Analysis result of the Ndl_xYxA10 3 crystals grown
aH
~ - ~ - ~ ~ , . . . . Q
-
I
I
I
I
I
10
20
30
40
50
Y content
(at%)
Fig. 4. Lattice constants versus Y concentration of Nd 1 _xYx A103.
crystals of temperature
NdA10
3
Feeds
Grown crystals
20 at% Y 25 at% Y
14_+0.2 at% Y 20_+5 at% Y 49+_7 a t % Y
at% Y were equal. This result shows that the rhombohedral form of pure N d A 1 0 3 c a n be changed to the cubic structure by substituting at least 20 at% Y for Nd, and thereby stabilizing the high temperature phase.
would be to stabilize the high
phase.
3.2. Crystal growth of Y-substituted N d A I O 3 Fig. 4 gives the lattice parameters for sintered Nd~_xY~AIO 3 samples having different Y concentrations. The transformation from hexagonal lattice constants (a H and c H) to the cubic perovskite structure (aH/v~ and cH/2x/-3) with compositions can be clearly seen. For pure NdAIO,j , all~X~2 is about 0.8% larger than cH/2x/3. As the Y content of Ndl_~YxA103 increased from 0 to 10 at% Y, a and c changed only slightly. However, in the range of 10-15%, they rapidly approached each other, and at 20
1
b
,
lOmm
! ........................
........ * ; ' . . . . . . . . ~L_2-'_': :-'-2.-2 ~.LLLSf-L2 -~Zi12 L: :: : : : : : : : :: :
Fig. 5. As-grown boule of Nd~_xYxAIO3: (a) x = 0.20; (b) x = 0.25.
Ir Fig. 6. Microphotograph of Nd I_xYxAIO3 grown crystals: (a) x = 0.20; (b) x = 0.25.
I. Tanaka et al. / Single crystal growth o f pure and Y-substituted NdAIO 3 by F Z method
683
The crystal growth of Y-substituted NdAlO 3 (Ndl_xYxA103; x = 0.15, 0.20, 0.25) was attempted using the FZ method described above. Fig. 5 shows two as-grown crystals of Ndl_xYxAIO3 with x = 0.20 and 0.25. They were opaque and had many cracks. Some subgrain boundaries were found in the crystals, and also irregular domain structures were observed, as shown in fig. 6. The Y concentration of these crystals was found to be lower than that of the feed rods, as shown in table 1. The Ndl_xYxA103 crystals grown using a feed rod containing an Y content of x = 0.25 had two phases of 20 at% Y and 49 at% Y. These results suggest that the distribution coefficient of Y in NdA10 3 is lower than unity, and also that supercooling occurs easily during growth of N d l _ x Y x A 1 0 3 . Therefore, the constant composition and temperature of the molten zone should be maintained during growth to obtain uniform solid solutions of Nd 1_xYxA10 3. The T S F Z (traveling solvent floating zone) method using an Y excess solvent coexisting with Nd~_xYxA10 3 may be effective to prevent the constitutional supercooling.
mains disappeared at a temperature close to the melting point, and reappeared again upon cooling. Substitution of 20 at% Y for Nd in NdA103 causes the structure to change from the rhombohedral to the cubic phase. However, the quality of Ndl_xYxA103 crystals grown by the FZ method was poor and contained some cracks, subgrain boundaries, and the domain structure was not eliminated. Y concentration in these crystals was not uniform, indicating that the distribution coefficient of Y in NdAIO 3 was less than unity.
4. Conclusion
References
Crystals of pure and Y-substituted NdA103 can be grown by the FZ method using infrared heating. Crack-free, transparent crystals of pure NdA103, 5 mm in diameter and 30 mm long, were grown, but contained the usual domain structure. By optically observing these crystals at elevated temperatures, it was found that the do-
[1] G.W. Berkstresser, A.J. Valentio and C.D. Brandle, J. Crystal Growth 109 (1991) 457. [2] G.W. Berkstresser, A.J. Valentio and C.D. Brandle, J. Crystal Growth 109 (1991) 467. [3] D. Mateika, H. Kohler, H. Laudan and E. V61kel, J. Crystal Growth 109 (1991) 447. [4] M. Mizuno, T. Yamada and T. Noguchi,Yogyo-Kyokai-Shi 85 (1977) 91.
Acknowledgments The authors wish to thank Professor R.S. Feigelson of Stanford University for many helpful discussions as well as critical reading of the manuscript. This work was partly supported by a Grant-in Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.