JPH0597592A - Method for producing BaTiO3 single crystal - Google Patents

Abstract

(57) [Summary] [Objective] A BaTiO ₃ single crystal stably exhibiting a large photorefractive effect by controlling holes is produced. [Composition] A mixture of titanium dioxide and barium oxide or carbonate, and a transition metal composed of one or more elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni and Cu. A starting material is obtained by adding 5 ppm or more of the total content of the elements, and this mixture is heated to 1330 ° C. or more and melted to obtain a melt 6. Next, the oxygen partial pressure is 0.
BaTiO 3 was added to the melt 6 in a low oxygen partial pressure atmosphere of 02 atm or less.
After bringing the seed crystal 7 of ₃ into contact, the melt is gradually cooled to grow a single crystal on the surface of the seed crystal. After that, the grown single crystal is subjected to 600 ° C in an oxidizing atmosphere with an oxygen partial pressure of 0.1 atm or more.
Heat to above ℃.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in optical application equipment such as phase conjugate mirrors, laser resonators and optical image analysis equipment to produce barium titanate single crystal having high photorefractive properties. The present invention relates to a method for producing a barium acid single crystal.

[0002]

_2. Description of the Related Art A flux method using potassium fluoride (KF) or barium chloride (BaCl ₂ ) as a flux is known as a method for producing barium titanate (BaTiO ₃ ) single crystals (JP Remeika et al., J. . Am.
Chem. Soc. Vol. 76, 1954, p. 940). The barium titanate single crystal produced by this method is called a butterfly type crystal, and has a triangular shape with a maximum thickness of about 0.4 mm. In this method, it is impossible to obtain a large-sized and thick barium titanate crystal that can be used for optical application equipment.

After that, while gradually cooling the raw material melt having a composition containing titanium dioxide (TiO ₂ ) in excess, barium titanate is crystallized in the seed crystal to produce a barium titanate single crystal. Pulling up method (TSSG method; To
p Seeded Solution Growth method) was developed (A. Linz,
V, Belruss and CS Nailman, J. Electro. Chem. So
c. 60C, 1965, p. 112).

The barium titanate single crystal produced by this method can be formed into a desired bulk shape, and since impurities such as flux are less contaminated, it is better than the flux method. It has excellent optical characteristics. For this reason, research has started to use barium titanate single crystals as photorefractive crystals for optical application equipment (Kitayama, 95th Workshop, Crystal Engineering Subcommittee of the Japan Society of Applied Physics, 1991). Annual issue, page 13).

Recently, from the field of utilizing such a photorefractive crystal, it is required to further enhance the photorefractive property of a barium titanate single crystal. It is required to further enhance the photorefractive property of this barium titanate single crystal. The photorefractive property of this barium titanate single crystal is considered to depend on the oxygen deficiency in the single crystal and the content of the transition metal element existing as an impurity in the single crystal. Therefore, as a method for increasing the photorefractive property of a barium titanate single crystal, a method of heat-treating a grown barium titanate single crystal in a low oxygen partial pressure or a reducing atmosphere (PG Schunema
nn et al., J. Opt. Soc. Am. B, Vol. 5, 1988, No. 1702
Page) and a method for doping a transition metal element such as Fe and Cr into a barium titanate single crystal (D. Rytz et al., J. Opt. S.
oc. Am. B7, 1990, page 2234).

BaTiO 3 produced by these methods
₃ single crystal, the photo carrier density in the crystal is high,
It exhibits a relatively large photorefractive effect. This photo carrier is composed of holes and electrons. In this case, the BaTiO ₃ single crystal produced by the method of heat-treating the grown single crystal in a low oxygen partial pressure atmosphere has a large proportion of electrons in the photocarrier and exhibits electron-dominated photorefractive property. On the other hand, F
The BaTiO ₃ single crystal produced by the method of doping a transition metal element such as e and Co has a large proportion of holes in the photocarrier, and exhibits hole-dominated photorefractive properties.

[0007]

However, the magnitude of the photorefractive effect exhibited by the conventional BaTiO ₃ single crystal produced by the above-mentioned method is still insufficient, and
There are the following problems. First, in the method of doping a transition metal element into a BaTiO ₃ single crystal, usually several hundred ppm of the transition metal element is added, but this transition metal element segregates in the barium titanate melt during crystal growth or in the crystal after growth. It is difficult to stably produce a BaTiO ₃ single crystal that exhibits a large photorefractive effect.

On the other hand, the BaTiO ₃ single crystal produced by a method in which a crystal grown in air is heat-treated in a low oxygen partial pressure atmosphere or a reducing atmosphere, even if the crystal is not doped with a transition metal element, It exhibits a relatively large photorefractive effect. However, the BaTiO ₃ single crystal produced by this method has an electron-dominant photorefractive property. Therefore, in the method of heat-treating a crystal grown in air in a low oxygen partial pressure atmosphere or a reducing atmosphere, Ba that exhibits a hole-dominant photorefractive effect is obtained.
A TiO ₃ single crystal cannot be manufactured.

The present invention has been made in view of the above problems, and it is possible to produce BaTiO ₃ single crystal which stably exhibits a large photorefractive effect by controlling holes.
It is an object to provide a method for producing an iO ₃ single crystal.

[0010]

BaTiO 3 according to the present invention
_{3 The} method for producing a single crystal is a melting step in which a mixture of titanium dioxide and an oxide or carbonate of barium to which a transition metal element is added is used as a starting material, and the mixture is heated to 1330 ° C. or higher to melt it. BaTiO ₃ was added to the obtained melt.
The step of growing the single crystal on the surface of the seed crystal by gradually cooling the melt after bringing the seed crystal into contact with the seed crystal, and the heat treatment step of heat-treating the grown single crystal to 600 ° C. or higher. It is characterized by having.

In this case, the total content of the transition metal elements in the mixture of the starting materials is 5 ppm or more, and the transition metal elements are selected from the group consisting of V, Cr, Mn, Fe, Co, Ni and Cu. One or two or more kinds of elements are defined.

In the growing step, the oxygen partial pressure in the single crystal growing atmosphere is set to 0.02 atm or less.

Further, in the heat treatment step, the oxygen partial pressure in the heat treatment atmosphere is set to 0.1 atm or more.

[0014]

In order to produce a BaTiO ₃ single crystal exhibiting a large photorefractive effect, it is necessary to increase the density of photocarriers in the crystal. This photo carrier is composed of holes and electrons, and forms a space charge distribution upon incidence of light. Therefore, BaT showing a hole-dominant photorefractive effect
In order to manufacture an iO ₃ single crystal, the proportion of holes in the photocarrier in the crystal must be higher than the proportion of electrons.

From these facts, the inventors of the present application have paid attention to the following matters.

BaTiO ₃ single crystals grown in air exhibit an electron-dominated photorefractive effect when heated at high temperature in a reducing atmosphere (PG Schunemann et al., J. Opt. Soc. Am.
B5, 1988 1702). Therefore, a BaTiO ₃ single crystal grown in a low oxygen partial pressure atmosphere may exhibit a hole-dominated photorefractive effect if it is oxidized at a high temperature after the growth.

In the redox reaction described above, the transition metal element in the impurities contained in the BaTiO ₃ single crystal causes a valence change such as M ^{n +} = M ^{(n-1) +.} , BaTiO ₃ single crystal is considered to have a great influence on the photorefractive effect.

From these facts, the inventors of the present invention have considered that the total content of the transition metal elements in the starting material, which is considered to affect the photorefractive effect, the oxygen partial pressure during the growth, the heat treatment conditions after the growth, and the like. Experiments were conducted under various conditions.
As a result, using a starting material containing a predetermined amount or more of the transition metal element, a single crystal grown in an oxygen partial pressure atmosphere of a predetermined pressure or less, by heating to a certain temperature or more at an oxygen partial pressure of a predetermined pressure or more, , BaTiO3 showing desired photorefractive effect
It was found that ₃ single crystals could be obtained, and the present invention was completed.

[0019]

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

In this case, the total content of transition metal elements in the starting material is set to 5 ppm or more. The total content of transition metal elements
If less than 5 ppm, Ba produced using the melt
TiO ₃ single crystals do not exhibit the desired magnitude of photorefractive effect.

Examples of such transition metal elements include V and C.
Some are selected from the group consisting of r, Mn, Fe, Co, Ni and Cu. One or more such transition metal elements are added.

Then, a BaTiO ₃ seed crystal is brought into contact with the obtained melt, and then the melt is gradually cooled to grow a single crystal on the surface of the seed crystal. This growing step is performed in an atmosphere having an oxygen partial pressure of 0.02 atm or less. BaTiO3 grown in an atmosphere with an oxygen partial pressure exceeding 0.02 atm
₃ Single crystals do not show the desired large photorefractive effect.

Then, the grown single crystal is heated to 600 ° C. or higher in an atmosphere having an oxygen partial pressure of 0.1 atm or higher. In this case, when the heating temperature is less than 600 ° C. or the oxygen partial pressure is less than 0.1 atm, BaTiO ₃ after heat treatment is used.
Single crystals do not exhibit the desired magnitude of photorefractive effect.

As described above, the BaTiO ₃ single crystal grown in a low oxygen partial pressure atmosphere having an oxygen partial pressure of 0.02 atm or less undergoes a redox reaction in a high temperature oxidizing atmosphere in a heat treatment step after growth. In response, the hole-dominated photorefractive effect is exhibited. In this case, in this oxidation-reduction reaction, the transition metal element in the impurities contained in the BaTiO ₃ single crystal changes in valence. This enhances the photorefractive effect.

The BaTiO ₃ produced in this way
The single crystal becomes dominant in holes and exhibits an extremely large photorefractive effect.

Next, a growth apparatus for this BaTiO ₃ single crystal (barium titanate single crystal) will be described. FIG. 2 is a sectional view showing this growing device. The heat insulating material 3 is provided with a heating space penetrating from the center of the upper surface to the lower surface in the vertical direction, and the heater 2 is embedded in the heat insulating material 3 and disposed around the heating space. Insulation 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, and the growth state of the single crystal can be observed through this window 5. There is.

A stage 12 is arranged 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. The 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 blower port 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 muffle 10 is provided through the gas blower 4 and the gas outlet 14. The atmosphere for growing the single crystal inside 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. Further, a hole through which the seed rod 8 is inserted is provided in the center of the lid 11, and the lower half portion 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 in. The seed rod 8 is a double pipe consisting of an inner pipe and an outer pipe, and the upper end of the seed rod 8 is fixed to the ascending / descending head 9. A cooling gas is supplied to the seed rod 8 from the outer system into the inner pipe, and the cooling gas flows through the inner pipe, and then a gap between the outer pipe and the inner pipe is formed from the lower end thereof. The gas enters through the gap and is discharged from the gas exhaust port 8a provided in the upper portion of the outer tube. As a result, the seed rod 8 is cooled.

The ascending / descending head 9 is vertically driven by a driving device (not shown), and as the head 9 is moved up and down, the seed rod 8 is moved up or down. Seed stick 8
A seed crystal attachment portion is provided at the lower end of the seed crystal 7, and the seed crystal 7 is attached to the attachment portion by binding with a platinum wire or the like.

Next, the results of comparing the characteristics of the BaTiO ₃ single crystals produced according to the examples and comparative examples of the present invention using the growing apparatus constructed as described above will be described. Table 1 below shows the content of the transition metal element in the starting materials of each Example and Comparative Example.

Example 1 First, TiO ₂ powder and BaCO ₃ powder were mixed in a molar ratio of 65:35, and this mixed powder was added with the amount of the transition metal element shown in the above-mentioned Example 1 column of Table 1. The resulting mixed powder was added 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 melt 6 was obtained by heating the raw material powder by heating with the heater 2. The raw material melt 6 was heated by the heater 2 and maintained at a temperature of 1400 ° C.

Next, a seed crystal 7 of barium titanate (BaTiO ₃ ) was attached to the lower end of the seed rod 8 with a platinum wire.

[0033]

[Table 1]

Next, the raising / lowering 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 drop rate of the melt 6 is changed.
The temperature was changed to 0.3 ° C./hour, and the seed rod 8 was raised at a speed of 0.4 mm / hour.

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

Then, the BaTiO ₃ single crystal after the heat treatment was cut into hexahedrons along the (100) plane or the (001) plane, and all the surfaces were mirror-polished. After that, by subjecting it to single-domain processing, cubic BaTi with 3.5 mm on each side
O ₃ single crystal was obtained.

Example 2 As shown in Table 1, a BaTiO ₃ single crystal was obtained in the same manner as in Example 1 except that starting material powder having a total content of transition metal elements of 5 ppm was used.

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 growing atmosphere was 0.02 atm.

Example 4 In the same manner as in Example 1, except that in the heat treatment step after growth, 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. To produce BaTiO ₃ .

Example 5 In the heat treatment step after growing, the heating temperature is 600 ° C.
BaTiO ₃ was produced in the same manner as in Example 1 except that the above was adopted.

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

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

Comparative Example 3 The same as Example 1 except that the oxygen partial pressure of the growing atmosphere was 0.1 atm.

Comparative Example 4 The same as Example 4 except that the oxygen partial pressure in the heating atmosphere after growing was 0.01 atm.

Comparative Example 5 In the heat treatment step after growing, the heating temperature is 500 ° C.
The same as Example 1 except that

Next, the photorefractive effect of the BaTiO ₃ single crystal of each Example and Comparative Example was measured by the 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 is arranged with the direction of arrow 26.
It is coherent and has a wavelength of 514.
The two laser beams 22 and 23 of 5 nm are incident on the BaTiO ₃ single crystal 21 at an angle of θ with respect to the direction perpendicular to the polarization direction. The laser beams 24 and 25 are BaTiO 3.
_This is the 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 and 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, and the measured value and the single crystal 21 are measured. The amplification coefficient Γ shown in the following formula 1 was obtained from the length L of

[0048]

## EQU1 ## Γ = ln (I ₂₄ / I _'24 ) / L

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

Table 2 below shows the measurement results of the amplification coefficient Γ by the two-wave mixing experiment. In addition, Examples 1 to 5 and Comparative Examples 1 to 3 and 5 exhibited hole-dominant photorefractive effect. On the other hand, Comparative Example 4 showed an electron-dominated photorefractive effect.

[0051]

[Table 2] As is clear from Table 2, in each of the comparative examples, the amplification factor is small, whereas in the example of the present invention, the amplification factor is large and the hole-controlled BaT showing a large photorefractive effect.
iO ₃ could be obtained.

[0052]

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

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view showing a single crystal growth apparatus used in this example.

FIG. 3 is a schematic diagram showing a two-light 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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoji Ajimura 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Electric Line Co., Ltd. (72) Haruo Tominaga 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Electric Wire Co., Ltd.

Claims (1)

[Claims] 1. A mixture of titanium dioxide and barium oxide or carbonate containing V, Cr, Mn, Fe, Co and Ni.
And a mixture of transition metal elements consisting of one or more elements selected from the group consisting of Cu and Cu in a total content of 5 ppm or more as a starting material,
BaT was added to the obtained melt in a melting step of heating to ℃ or more to melt and in an atmosphere with an oxygen partial pressure of 0.02 atm or less.
A step of growing the single crystal on the surface of the seed crystal by gradually cooling the melt after contacting the iO ₃ seed crystal, and the grown single crystal having an oxygen partial pressure of 0.1 atm or more. 6 in the atmosphere
BaTiO ₃ growth method of a single crystal and having a heat treatment step of heat treatment to 00 ° C. or higher.