JP2833920B2 - Method for producing barium titanate single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for optical application equipment such as a phase conjugate mirror, a laser resonator, and an optical image analyzer, and has a high photorefractive effect, such as barium titanate (BaTiO ₃ ). The present invention relates to a method for producing a crystal.

[0002]

2. Description of the Related Art As a method for producing a BaTiO ₃ single crystal,
Potassium fluoride (KF) or barium chloride (BaC
l ₂ ) etc. as a flux is known (JP Remeika et al .; J. Am. chem. So
c., Volume 76, 1954, p. 940). The BaTiO ₃ single crystal produced by this method is called a butterfly type crystal and is a triangular flake having a maximum thickness of about 1 mm. In this manufacturing method, a large and thick BaTiO ₃ single crystal usable for optical application equipment cannot be manufactured.

Thereafter, BaTiO ₃ is crystallized in a seed crystal (BaTiO ₃ single crystal) while gradually cooling a raw material melt having a composition of titanium dioxide (TiO ₂ ) in excess of BaTiO ₃ , thereby obtaining BaTiO _3. Melt pulling method to produce single crystal (TSSG method; Top Seeded Solution Growth)
Method was developed (A. Linz et al .; J. Electro. Chem.
Soc., 60C, 1965, p. 112).

[0004] BaTiO ₃ produced by this method
The single crystal can have a desired bulk shape and is less contaminated by impurities from the flux or the like, and thus has better optical characteristics than the flux method. For this reason, research on using BaTiO ₃ single crystals as photorefractive crystals for optical application equipment has been actively conducted (Kitayama, The 95th Research Meeting of the Crystal Engineering Subcommittee of the Japan Society of Applied Physics). Association Textbook, 1991, p.13).

[0005] Recently, from the field of utilizing this type of photorefractive crystal, it has been demanded to further enhance the photorefractive effect of BaTiO ₃ single crystal. The photorefractive property of the BaTiO ₃ single crystal is based on crystal defects in the single crystal such as oxygen deficiency,
Alternatively, it is considered to depend on the content of an impurity element such as a transition metal element existing as an impurity in the single crystal. Therefore, conventionally, as a method for enhancing the photorefractive effect of BaTiO ₃ single crystal, a method of the BaTiO ₃ single crystal grown in the atmosphere low oxygen partial pressure or heated in a reducing atmosphere (P.
G. Schunemann et al .; J. Opt. Soc. Am. B. 5, 1988
And p. 1702) and in BaTiO ₃ single crystal.
doping with a transition metal element such as r (D. Rytz et al .;
J. Opt. Soc. Am. B. 7, 1990, p. 2234) has been proposed. Thus, BaTi grown in the atmosphere
Heat treatment of the O ₃ single crystal in a low oxygen partial pressure or a reducing atmosphere aims to further increase oxygen vacancies in the single crystal, and doping with impurity elements such as Fe and Cr is The purpose is to increase the transition metal element content in a single crystal. B produced by these methods
The aTiO ₃ single crystal shows a relatively large photorefractive effect.

[0006]

However, the photorefractive effect exhibited by the BaTiO ₃ single crystal manufactured by the above-mentioned conventional method is still insufficient in practical use. Further, there are still many unclear points regarding the relationship between the above-described heat treatment in the reducing atmosphere and the doping amount of the transition metal element and the photorefractive effect of the BaTiO ₃ single crystal. For this reason, it is difficult to stably produce a BaTiO ₃ single crystal exhibiting a large photorefractive effect by the above-described conventional method.

The present invention has been made in view of the above problems, and provides a method for producing a barium titanate single crystal capable of stably producing a BaTiO ₃ single crystal exhibiting an excellent photorefractive effect. The purpose is to do.

[0008]

According to the present invention, there is provided a method for producing a barium titanate single crystal, comprising the steps of: starting from a mixture of titanium dioxide and an oxide or carbonate of barium; and heating and melting the mixture. Step and contacting a seed crystal of BaTiO ₃ with the obtained melt under a growth atmosphere having an oxygen partial pressure of 0.02 atm or less, then gradually cooling the melt to grow a single crystal on the surface of the seed crystal It is characterized by comprising a growing step and a heat treatment step of heating the single crystal after the growth.

[0009]

The inventors of the present invention have proposed a method of producing B having an excellent photorefractive effect.
In developing the aTiO ₃ single crystal, the facts or findings shown in the following (1) to (5) were based. (1) It is photo carriers composed of holes and electrons that contribute to the photorefractive effect of BaTiO ₃ single crystal. (2) In order to enhance the photorefractive effect of BaTiO ₃ single crystal, first, it is necessary to increase the photocarrier density of the single crystal. (3) Further, when the photocarrier that controls the photorefractive effect of the BaTiO ₃ single crystal is a hole, it is necessary to increase the (hole / electron) ratio in order to further enhance the photorefractive effect. Conversely, if the photocarriers that govern the photorefractive effect are electrons, to increase the photorefractive effect, it is necessary to use (hole / electron)
Needs to be reduced. (4) The hole or electron species forming the photocarrier are crystal defects such as oxygen vacancies. Therefore, in order to increase the photocarrier density and increase the number of crystal defects, growing at a low oxygen partial pressure has more oxygen deficiency in the oxide than growing in the air, which has been conventionally performed. Therefore, it is more effective. (5) In addition, after the growth step, to a heat treatment at different oxygen partial pressure and the oxygen partial pressure in the growth atmosphere is, BaTiO ₃
An oxygen reduction reaction (for example, a valence change in which the impurity transition metal element becomes M ⁿ⁻ or M ^{(n−1) +} ) may occur in the single crystal, and (hole /
This is effective for changing the ratio of electrons.

Based on the above concept, the inventors of the present application have
From a melt composed of titanium dioxide (TiO ₂ ) and an oxide or carbonate of barium such as BaO, the BaTiO 3 is mixed in an atmosphere having an oxygen partial pressure of 0.02 atm or less, which is lower than the atmospheric oxygen partial pressure. _{When the} single crystal was grown, the single crystal grown under low oxygen partial pressure (as-grown; as-grown without heat treatment) was replaced with the single crystal grown in air (as-grown).
It has been found that the photorefractive effect is larger than that of n), and that the photorefractive effect is dominated by holes. In this case, it was also found that when the single crystal grown under a low oxygen partial pressure was subjected to a heat treatment under an oxygen partial pressure higher than the oxygen partial pressure of the growth atmosphere, the hole-dominated photorefractive effect was further enhanced. Thus, the present invention has achieved the object of the present invention by appropriately setting conditions such as the growth atmosphere.

Next, the reasons for limiting the manufacturing conditions defined in the claims of the present invention will be described.

[0012]Oxygen partial pressure (PO ₂ ) in the growing atmosphere
0.02 atm or less Ba grown in an oxygen partial pressure atmosphere higher than 0.02 atm
TiO_ThreeSingle crystals are grown in the atmosphere conventionally manufactured.
Only the photorefractive effect of the same size as the formed single crystal
Not shown. For this reason, the oxygen partial pressure in the growing atmosphere is
It is necessary to make it 0.02 atmosphere or more.

Oxygen in heat treatment atmosphere after growing step
The partial pressure can be higher than the oxygen partial pressure in the growing atmosphere.
BaTiO ₃ single crystals grown under a preferable oxygen partial pressure of 0.02 atm or less (as-grown) show a hole-dominated photorefractive effect. In addition, the BaTiO ₃ single crystal (as-grown after growing) under an oxygen partial pressure higher than the oxygen partial pressure in the growing atmosphere.
When heat treatment is performed on n), an oxidation reaction of the impurity transition metal element contained in the BaTiO ₃ single crystal (M ^{(n-1) +} → M ^{n +} )
Progresses. Therefore, the BaTiO ₃ single crystal (after the heat treatment) heat-treated under an oxygen partial pressure higher than the oxygen partial pressure in the growth atmosphere has a (hole / grown) single crystal as compared with the as-grown single crystal. The electron (electron) ratio increases, and the hole-dominated photorefractive effect also increases. Further, Ba heat-treated under the same oxygen partial pressure as the oxygen partial pressure in the growing atmosphere.
The TiO ₃ single crystal exhibited the same level of photorefractive effect as compared to the as-grown single crystal.
Conversely, a BaTiO ₃ single crystal that has been heat-treated under an oxygen partial pressure lower than the oxygen partial pressure in the growth atmosphere has a (a)
The hole-dominated photorefractive effect was smaller compared to s-grown) single crystals.

[0014] The heating temperature in the heat treatment step after the heat treatment temperature growth step in the heat treatment step, in the growth after the (as-grown) BaTiO ₃ impurity transition metal elements contained in the single crystal, the progress of the oxidation reaction It is necessary to set the temperature at which the temperature becomes possible. If heated at high temperature,
Although the oxidation reaction proceeds in a short time, defects such as cracks are likely to occur in the single crystal in a cooling process after the heat treatment. On the other hand, when heating is performed at a low temperature, the possibility of occurrence of defects such as cracks during the cooling process is reduced, but it is necessary to lengthen the heat treatment time for promoting the oxidation reaction. For this reason, the preferable heating temperature is 600 to 1250 ° C., and in this case, the heating time is 1 to 24 hours.

V, Cr, Mn, Fe, Co, Ni and
At least selected from the group of transition metal elements consisting of
In addition , the transition metal element plays an important role in the crystal defects generated during the growth in which the total content of one element is 2 ppm or more and the oxidation-reduction reaction during the heat treatment.
This has a great effect on the photorefractive effect exhibited by the O ₃ single crystal. Therefore, it is preferable to include a certain amount or more of the transition metal element in the BaTiO ₃ single crystal in order to further enhance the photorefractive effect.

In the growing melt of the starting material which is usually available,
When the transition metal element is contained in an amount of 2 ppm or more, the BaTiO ₃ single crystal produced by the method of the present invention exhibits a higher photorefractive effect.

[0017]

Next, the present invention will be described in more detail.
In the present invention, first, a mixture of titanium dioxide and barium oxide or carbonate is used as a starting material, and this mixture is heated to a temperature of 1330 ° C. or higher to be melted. FIG. 1 shows Ba
FIG. 3 is an O-TiO ₂ phase equilibrium phase diagram (DE Rase and R.
Roy; J. Am. Ceram., 38, 110, 1955). As shown in this state diagram, the melt cannot be obtained stably unless the temperature is 1330 ° C. or higher.

Further, since a hexagonal crystal is grown from a melt having a composition of TiO ₂ : BaO = 1: 1, it is necessary to use a melt having an excess composition of TiO ₂ to obtain a tetragonal crystal. Next, after a seed crystal of BaTiO ₃ is brought into contact with the obtained melt, the melt is gradually cooled to grow a single crystal on the surface of the seed crystal.

Next, an apparatus for growing this BaTiO ₃ single crystal (barium titanate single crystal) will be described. FIG. 2 is a sectional view showing the growing apparatus. The heat insulating material 3 is provided with a heating space vertically penetrating from the center of the upper surface to the lower surface, and the heater 2 is embedded in the heat insulating material 3 around the heating space. Insulation material 3
Is provided with an observation window 5 that reaches a substantially central portion in the vertical direction of the heating space from an appropriate position on the peripheral portion of the upper surface thereof, and through this window 5, the growth state of the single crystal can be observed. I have.

A stage 12 is provided near the lower end of the heating space of the heat insulating material 3, and a muffle 10 is arranged on the stage 12 so as to pass through the heating space of the heat insulating material 3. This muffle 10 is a bottomed cylindrical container made of quartz glass or the like, and its upper end is closed by a lid 11 made of quartz glass or the like. A gas outlet 4 is provided at the edge of the lid 11, and a gas outlet 14 is provided at the bottom of the muffle 10. The gas outlet 4 is provided through the gas outlet 4 and the gas outlet 14. The atmosphere for growing the single crystal therein is adjusted.

A table 13 is arranged in the muffle 10, and the crucible 1 in which the raw material melt 6 is stored is placed on the table 13. A hole through which the seed rod 8 is inserted is provided at the center of the lid 11, and the lower half of the seed rod 8 is directly above the crucible 1 in the muffle 10 through the hole provided in the lid 11. Is located. The seed rod 8 is a double pipe composed of an inner pipe and an outer pipe, and the upper end thereof is fixed to an ascending / descending head 9. Cooling gas is supplied to the seed rod 8 from the outer system into the inner pipe. After flowing through the inner pipe, the cooling gas flows from the lower end to the gap between the outer pipe and the inner pipe. And is discharged through a gas exhaust port 8a provided in the upper part of the outer tube through this gap. Thereby, the seed rod 8 is cooled.

The ascending / descending head 9 is driven up and down by a driving device (not shown), and the seed rod 8 moves up or down as the head 9 moves up and down. Seed stick 8
Is provided with a seed crystal attaching portion at the lower end thereof, and the seed crystal 7 is attached to this attaching portion.

Next, BaTiO ₃ single crystals according to Examples and Comparative Examples of the present invention were manufactured using the growing apparatus configured as described above, and the results of comparing the characteristics thereof will be described. Table 1 below shows the content of transition metal elements in the melts, which are the starting materials of the examples and comparative examples. This content was measured by cooling the melt to room temperature, sampling, and then performing mass spectrometry. In addition, in Table 1, ND is a case where it is below the detection limit.

[0024]

[Table 1]

Next, the manufacturing conditions of each of the examples and comparative examples will be described.

Example 1 First, a TiO ₂ powder and a BaCO ₃ powder were mixed in a molar ratio of 65:35, and further, a transition metal element shown in the column of Example 1 in Table 1 was added to obtain a mixture. The mixed powder was used as a starting material.
Next, the raw material powder was charged into the crucible 1, and the crucible 1 was placed in the muffle 10. Then, the raw material powder is melted by heating by the heater 2 to thereby obtain the raw material melt 6.
I got The raw material melt 6 is heated by the heater 2 to 1400
Maintained at a temperature of ° C.

Next, barium titanate (BaTiO ₃ ) seed crystal 7 was attached to the lower end of seed rod 8. Thereafter, the ascending / descending head 9 was lowered to bring the seed crystal 7 into contact with the melt 6. Then, the temperature of the melt 6 is lowered at a rate of 5 ° C./hour, and after confirming that crystals are crystallized on the surface of the seed crystal 7, the temperature lowering rate of the melt 6 is changed to 0.3 ° C./hour. And
The seed rod 8 was raised at a speed of 0.4 mm / hour.

During crystal growth, an argon gas is supplied into the muffle 10 from the gas vent 4 and the oxygen partial pressure (P _O2 ) of the gas collected from the gas outlet 14 is measured using a zirconia limiting current type oxygen analyzer. And measured. As a result, the oxygen partial pressure of the growing atmosphere was 0.01 atm. The crystal after growth is
Heat treatment was performed at 1200 ° C. for 24 hours under atmospheric pressure.

Thereafter, the BaTiO ₃ single crystal after the heat treatment was cut into hexahedrons along the (100) plane or the (001) plane, and all the planes were mirror-polished. After that, a cubic BaTi having a side of 3.5 mm is formed by performing a single domaining process.
O ₃ single crystal was obtained.

Example 2 As shown in Table 1, except that a starting material powder having a total content of transition metal elements of 2 ppm was used, B
An aTiO ₃ single crystal was produced.

Example 3 A BaTiO ₃ single crystal was obtained in the same manner as in Example 1 except that the oxygen partial pressure in the growth atmosphere was changed to 0.02 atm.

Example 4 In the heat treatment step after growing, the same procedure as in Example 1 was performed except that the heating atmosphere was a mixed gas atmosphere of argon gas and oxygen gas, and the partial pressure of oxygen gas was limited to 0.1 atm. Thus, a BaTiO ₃ single crystal was produced.

Example 5 In the heat treatment step after growing, the heating temperature was set to 600 ° C.
A BaTiO ₃ single crystal was produced in the same manner as in Example 1, except that

Example 6 A BaTiO ₃ single crystal was produced in the same manner as in Example 1 except that the heat treatment after growth was not performed.

Comparative Example 1 A BaTiO ₃ single crystal was produced in the same manner as in Example 2 except that the growth atmosphere was air.

Comparative Example 2 A BaTiO ₃ single crystal was produced in the same manner as in Example 1 except that the growth atmosphere was air.

Comparative Example 3 A BaTiO ₃ single crystal was produced in the same manner as in Example 1 except that the oxygen partial pressure of the growing atmosphere was changed to 0.1 atm.

Comparative Example 4 A BaTiO ₃ single crystal was produced in the same manner as in Example 4 except that the oxygen partial pressure in the heating atmosphere after the growth was set to 0.01 atm.

Next, the photorefractive effect of the BaTiO ₃ single crystals of each of the examples and comparative examples was measured by a two-wave mixing experiment described below. FIG. 3 is a schematic diagram showing a two-wave mixing experiment. The barium titanate single crystal 21 having a length L is
The polarization direction is arranged in the direction of arrow 26.
And it is coherent (coherence) and the wavelength is 514.
The two laser beams 22 and 23 of 5 nm are incident on the BaTiO ₃ single crystal 21 at an angle θ with respect to a direction perpendicular to the direction of polarization. The laser beams 24 and 25 are BaTiO
₃ Light that has passed through the single crystal 21. Here, laser light 2
The intensity of 3 is, for example, about 100 times the intensity of the laser beam 22, and the laser beams 22, 23 are S-polarized.

Then, the intensity I ₂₄ of the laser beam 24 when the laser beam 23 is irradiated and the intensity I ′ ₂₄ of the laser beam 24 when the irradiation of the laser beam 23 is stopped are measured. From the length L, an amplification coefficient に shown in the following equation 1 was obtained.

[0041]

Γ = In (I ₂₄ / I ′ ₂₄ ) / L

The incident angle θ of the laser beams 22 and 23 is fixed at 31 °, and the lattice spacing Δg is 0.5 μm (Δg = λ / 2).
The amplification coefficient と き when sin θ = 514.5 nm / 2 sin 31 °) was obtained. The larger the value of the amplification coefficient Γ, the greater the photorefractive effect.

Table 2 below shows the measurement results of the amplification coefficient Γ by the two-wave mixing experiment. Further, Examples 1 to 6 and Comparative Examples 1 to 3 showed a hole-dominated photorefractive effect. On the other hand, Comparative Example 4 showed an electron dominated photorefractive effect.

[0044]

[Table 2]

As can be seen from Table 2, the following can be confirmed from the two-wave mixing experiment. First, a comparison between Example 1 and Comparative Example 2 shows that B grown in the air as in the prior art was used.
The BaTiO ₃ single crystal grown under a reducing atmosphere by the method of the present invention has a larger photorefractive effect than the aTiO ₃ single crystal. Also, from the comparison between Example 1 and Example 6, and Example 4 and Comparative Example 4, if the oxygen partial pressure condition of the heating atmosphere after the growth is set as defined in claim 3 of the present invention, the hole is obtained. The dominant photorefractive effect increases. Further, from the comparison between Example 2 and Comparative Example 2, if the content of the transition metal element is 2 ppm or more, the effect of the method of the present invention can be obtained.

[0046]

According to the present invention, it is possible to manufacture a BaTiO ₃ single crystal exhibiting a photorefractive effect in which holes are dominant.

[Brief description of the drawings]

FIG. 1 is an equilibrium diagram of a BaO—TiO ₂ system.

FIG. 2 is a sectional view showing a single crystal growing apparatus used in the present embodiment.

FIG. 3 is a schematic diagram showing a two-wavelength mixing experiment.

[Explanation of symbols]

1; crucible 2; heater 3; heat insulating material 4; gas blowing port 5; observation window 6; melt 7; seed crystal 10; muffle 11; BaTiO ₃

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01S 3/18 H01S 3/08 (72) Inventor Shoji Ajimura 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Electric Wire Co., Ltd. (72) Inventor Haruo Tominaga 1-5-1, Kiba, Koto-ku, Tokyo Inside Fujikura Electric Wire Co., Ltd. (56) References JP-A-5-97592 (JP, A) JP-A-4-325498 (JP, A) JP-A-4-305097 (JP, A) JP-A-4-300297 (JP, A) JP-B-39-4979 (JP, B1) (58) Fields investigated (Int. Cl. ⁶ , DB name) C30B 1/00-35/00

Claims (3)

(57) [Claims] 1. A melting step in which a mixture of titanium dioxide and an oxide or carbonate of barium is used as a starting material, and the mixture is heated and melted.
A step of growing a single crystal on the surface of the seed crystal by gradually cooling the melt after bringing the seed crystal of BaTiO ₃ into contact with the seed crystal in a growth atmosphere at a pressure of not more than the atmospheric pressure, and heating to heat-treat the single crystal after the growth. And a process for producing a barium titanate single crystal. 2. The barium titanate according to claim 1, wherein in the heat treatment step, an oxygen partial pressure in a heat treatment atmosphere is higher than an oxygen partial pressure in a single crystal growth atmosphere in the growth step. Single crystal production method. 3. The method according to claim 1, wherein the melt is V, Cr, Mn, Fe, C
one or two selected from the group consisting of o, Ni and Cu
The method for producing a barium titanate single crystal according to claim 1 or 2, wherein the total content of the transition metal element is 2 ppm or more.