JPH0471876B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a lanthanum cuprate (La ₂ CuO ₄ ) single crystal by a solution drawing method.

[Prior Art] La ₂ CuO ₄ is an oxide with a layered perovskite crystal structure, and when La atoms are replaced with small amounts of Sr or Ba atoms, it exhibits superconductivity at a low temperature of 30 to 40 K. Future applications of this material are considered as cryogenic devices, which will require large, high-quality single crystals, and the development of a manufacturing method is desired.

Conventionally, as a means of obtaining a single crystal of a substance that undergoes a peritectic reaction, which decomposes and dissolves at high temperatures and cannot be grown as a single crystal by ordinary pulling methods, it is usually done by mixing the target substance during dissolution and then cooling the mixed solution. Therefore, a flux method is used in which the mixed solution is made to have a supersaturation concentration or higher and the target substance is precipitated as a single crystal from the melt. When La ₂ CuO ₄ is heated to high temperature,
Because it decomposes and melts at around 1050°C, it is not possible to produce a single crystal from raw materials with the same composition as a single crystal.
Therefore, the Czyochralski method cannot be applied. Therefore, the flux method is usually applied, but the maximum
It is only 8mm x 2mm.

[Problems to be Solved by the Invention] As mentioned above, the single crystal grown by the flux method cannot be said to be very large. Moreover, it has the disadvantage that crystal growth requires a long time of 3 to 7 days.
Also, after crystal growth, only the solution can be removed with chemicals, or
There was a problem in that the single crystal could not be separated unless the crucible was removed from the furnace during slow cooling and the unsolidified solvent was discarded or the single crystal floating on the surface of the solution was scooped out.

[Means for Solving the Problems] The present invention was made in view of the above points, and includes at least one of lanthanum oxide and lanthanum carbonate within a composition range that can precipitate the target product, La ₂ CuO ₄ . By mixing copper oxide with copper oxide and cooling the melt, La ₂ precipitates in the melt.
CuO ₄ microcrystals are crystallized on seed crystals that are in contact with the melt, and then La ₂ CuO ₄ is crystallized by the solution pulling method.
This is a method for producing single crystals, and unlike the flux method, it uses seed crystals to produce high-quality single crystals of any size in a desired direction in a short time.

[Operation] First, the principle of this invention will be described. Figure 1 was drawn from the results of differential thermal analysis and rapid cooling/heating method.
It is a phase equilibrium diagram of the La ₂ O ₃ −Cu0 system. In the figure, the black dots indicate the measurement results. For example, when La ₂ CuO ₄ having a composition of 50 mol % La ₂ O ₃ and 50 mol % Cu0 is heated and heated, it decomposes and melts at around 1050°C. Therefore, the Czyochralski method, which pulls up crystals with the same composition as the melt, cannot be applied. Next, after heating and melting the raw materials mixed at a composition ratio between liquidus line A and B, that is, 28.9 to 7.1 mol% of La ₂ O ₃ and 71.1 to 92.9 mol% of Cu0 at about 1330 to 1040 °C, When the temperature of the melt is gradually lowered, the composition of the melt shifts toward the Cu0 side of the diagram along the liquidus line A-B.
La ₂ CuO ₄ becomes a solid phase and precipitates.

If the composition of the melt is closer to Cu0 than the eutectic point B, Cu0 will precipitate first during cooling, and the composition of the melt will be closer to Cu0 than the eutectic point B.
If it is on the La ₂ O ₃ side, crystals on the La ₂ O ₃ side precipitate first during cooling, and La ₂ CuO ₄ cannot precipitate and grow in either case. In this phase equilibrium diagram, when a small amount of some kind of different element is mixed at the La atom and Cu atom positions, if the phase equilibrium diagram does not change in characteristics, the same La ₂ CuO ₄ solid solution will precipitate as a solid phase. In this invention, these crystals are made of the same La ₂ CuO ₄
Alternatively, a La ₂ CuO ₄ single crystal precipitated on the same layered perovskite seed crystal is grown and pulled up.

[Examples] The present invention will be explained in detail below using Examples.

Example 1 A La ₂ CuO ₄ single crystal was produced by a solution pulling method.

Figure 2 shows the single crystal pulling device used.
In the figure, 1 is a water-cooled shaft, 2 is a platinum shaft, 3 is a heat insulator, 4 is a high-frequency heating coil, 5 is a thermocouple, 6 is a crucible support, 7 is a seed crystal, 8 is a grown single crystal, and 9 is a starting material. The raw material 10 is a platinum crucible.

Mix La ₂ O ₃ and Cu0 at a molar ratio of 15:85,
200 g of the mixture was placed in a platinum crucible 10 with a diameter of 50 mm and a height of 35 mm, which also served as a heating element, and heated to approximately 1200°C by induction heating using a high-frequency heating coil ₄ to melt it. ₄ Bring the single crystal 7 into contact with the surface of the melt. When the temperature of the melt is gradually lowered, La ₂ CuO ₄ microcrystals gradually precipitate at the interface of the melt that is in contact with the seed crystal with the lowest temperature in the melt, and crystallize and grow on the seed crystal 7. . The single crystal 8 grown in this way is gradually pulled up from the melt. That is, while cooling the melt, the grown single crystal is pulled up at the same time. The manufacturing conditions at this time are La ₂ CuO ₄
The single crystal 8 is pulled up at a speed of 0.3 to 1 mm/hr, a melt cooling rate of 2 to 10°C/hr, a crystal rotation speed of 10 to 30 rpm,
The atmosphere is in the air. The time required to complete the growth of La ₂ CuO ₄ single crystal is 9 x 7 x 4 mm,
It took only 9 to 10 hours to obtain a single crystal weighing 1 g. Single crystal growth is possible even in an oxygen atmosphere.

Example 2 A solid solution single crystal having the composition (La _0.9 Ba _0.1 ) ₂ Cu0 ₄ was produced by a solution pulling method. 90 mol%
A mixture with the composition La ₂ O ₃ + 10 mol% 2BaCO ₃ and
Cu0 was mixed in a molar ratio of 15:85, and the same procedure as in Example 1 was carried out to create an 8×7×3 mm
A (La _0.9 Ba _0.1 ) ₂ Cu0 ₄ solid solution single crystal was obtained. In the production method of this invention, the position of La atoms in the crystal is
_{Even if it is replaced by}
A single crystal was produced.

Similarly, if the phase equilibrium diagram does not qualitatively change when a small amount of a different element is mixed at the position of La and Cu atoms in the crystal, this different element is mixed in using the same method and conditions as above. La ₂
It is also possible to produce CuO ₄ solid solution single crystals.

When producing a La ₂ CuO ₄ single crystal, lanthanum carbonate (La ₂ (CO ₃ ) ₃ ) was used instead of lanthanum oxide (La ₂ O ₃ ).
can also be used as a starting material. La ₂
(CO ₃ ) ₃ undergoes the following reaction during heating: La ₂ (CO ₃ ) ₃ →La ₂ O ₃ +3CO ₂ ↑ and changes to La ₂ O ₃ . Therefore, even if La ₂ (CO ₃ ) ₃ and Cu0 are used as starting materials, a phase equilibrium diagram completely similar to that shown in FIG. 1 is obtained as a result, and La ₂ CuO ₄ single crystals and single crystals in which part of La was replaced with Ba or Sr were obtained.

[Effect of the invention] As explained above, according to this invention, La ₂
CuO ₄ single crystals can be produced by a pulling method. Furthermore, a single crystal containing a small amount of Ba and Sr can be produced. As described above, the present invention has made it possible to produce an oxide superconducting single crystal exhibiting superconductivity at 30 to 40K.

[Brief explanation of the drawing]

Figure 1 is La ₂ for explaining the principle of this invention.
A phase equilibrium diagram of the O ₃ -Cu0 system, FIG. 2 is a block diagram of a La ₂ CuO ₄ single crystal manufacturing apparatus for explaining an embodiment of the present invention. 1...Water cooling shaft, 2...Platinum shaft, 3
... Heat insulation material, 4 ... High frequency heating coil, 5 ... Thermocouple, 6 ... Crucible support, 7 ... La ₂ CuO ₄ seed crystal, 8 ... La ₂ CuO ₄ single crystal, 9 ... Starting material ,1
0...Platinum crucible.

Claims (1)

[Claims] 1. After mixing copper oxide with at least one of lanthanum oxide and lanthanum carbonate and heating and melting the mixture, the temperature of the melt is lowered, and microcrystals represented by the general formula La ₂ CuO ₄ are placed on a seed crystal. A method for producing a lanthanum cuprate single crystal, which comprises precipitating and growing a lanthanum cuprate single crystal. 2. The mixing ratio of at least one of the lanthanum oxide and lanthanum carbonate and the copper oxide is 28.9 to 28.9.
The method for producing a lanthanum cuprate single crystal according to claim 1, characterized in that the ratio is 7.1 mol % to 71.1 to 92.9 mol %. 3. The method for producing a lanthanum cuprate single crystal according to claim 1, wherein the lanthanum cuprate contains a small amount of a different element.