JP2682458B2 - Method for growing rare earth vanadate single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing a rare earth vanadate single crystal for optics.

[0002]

2. Description of the Related Art Rare earth vanadate single crystals are useful as crystals for optical devices such as polarizers and solid-state lasers.
It is mainly grown by the CZ method (Czochralski method, pulling method). Rare earth oxides (R ₂ O ₃ ,
R: rare earth element) and vanadium pentoxide (V ₂ O ₅ ) are used. Usually, both are mixed and fired to produce vanadate (RVO ₄ ) by a solid-phase reaction, which is melted and used as a growth material. There is. In addition, since the melting point is high, the crucible is made of iridium exclusively, and from the viewpoint of protecting the crucible, crystal growth is performed in a neutral or slightly oxidizing atmosphere.

In the industrial production of crystals, a method has been adopted in which after the crystal growth, a new raw material corresponding to the weight consumed is replenished (additional charge) to continue the crystal growth from the second time.

[0004]

In the rare earth vanadate crystal growth by the CZ method, since the raw materials are gradually decomposed and scattered during the crystal growth, the composition as a whole deviates from the stoichiometry (stoichiometric ratio). . And due to the deviation, the crystals are colored and cannot be used for optics.
Inclusion (inclusion) in the grown crystal in extreme cases
It was a cause of cracks. Usually, when a new high-purity raw material is used, such an inconvenience is unlikely to occur in the first growth, but as the additional charge growth is repeated, such a problem always occurs in the number of times. . Therefore, it is necessary to stop the supplementary charge and renew the raw material at some point, but it is difficult to predict how many supplemental charges can be made because it is greatly affected not only by the type of rare earth crystal but also by a slight difference in the growth conditions. there were.
Further, even if macroscopic defects such as cracks and inclusions do not occur in the grown crystal, the crystal is useless if coloring, that is, unnecessary absorption as an optical element is more than the allowable value.

Therefore, conventionally, there is no way to judge the effectiveness of the remaining raw material after the growth, so that the safety is estimated to be 2-3.
It has been practiced to uniformly update the raw material after the additional charge has been cultivated. However, from the viewpoint of the effective use of the raw material and the labor for updating the raw material, it has been desired to continue the charge growth to the extent that the performance of the device using the crystal is affected.

An object of the present invention is to provide a method for growing a rare earth vanadate single crystal in which the renewal time of the remaining raw material can be clarified by a simple treatment after growing the crystal.

[0007]

The effectiveness of the residual raw material according to the first invention is confirmed by the following procedure. After the growth is completed, the crystal is separated from the melt, and the crucible is gradually cooled at a temperature decrease rate of 200 ° C./h or less. After cooling, the crucible was taken out and the solidified state was observed. As shown in FIG. 1, a transparent or translucent layer was visible on the upper layer 2. There is a black layer 3 in the lower part, but it may not be confirmed when the transparency of the upper layer 2 is low. What is needed here is the thickness of the upper layer 2, but in order to measure this, as shown in FIG. 4, the crushing tool 4 made of a hard material should be used to impact the surface to break a corner.
As shown in (a) and (b), a thin sample rod 6 is hollowed out by the ultrasonic rotary machine 5 and the state is examined. If the transparency of the upper layer 2 is good, it is possible to measure the thickness by an optical method. Thus, if the thickness t ₁ of the upper layer 2 is known, the ratio of t ₁ to the total thickness (t ₁ + t ₂ ) can be known. If t ₁ is 30% or more of the whole, this residual raw material is effective, and if it is recharged, a good crystal can be obtained next time. If the value is 30% or less, it is necessary to update all raw materials.

[0008]

[0009]

According to the present invention, the residual raw material melt in the crucible after the growth of the rare earth vanadate crystal is selected by the cooling condition.
It is based on the fact that it was found to separate into layers and solidify. Figure 1 shows that after growing rare earth vanadate crystals,
The grown crystal is separated from the raw material melt, and the crucible is gradually cooled to solidify the raw material melt, which schematically shows two layers having different properties. In terms of composition, the upper layer 2 is almost RVO ₄ , and the lower layer 3 is RVO _4-x, which is on average close to a solid solution of RVO ₄ and RVO ₃ . Also, these two layers have different colors, for example, YVO
_In the example of ₄ growth, the upper layer has a pale yellow or brown transparent or translucent crystal form, and the lower layer has a black metallic luster form. Then, it was found that the thickness of the upper layer 2 decreased and the thickness of the lower layer 3 increased as the number of growing times increased. further,
Macroscopic defects such as cracks in grown crystals, microscopic defects, and coloring of crystals caused by impurities are related to the ratio of the thickness of these two layers, and as a result of experiments, the crystal quality as an optical element is secured. Therefore, it was found that the thickness of the upper layer 2 needs to be 30% or more of the entire thickness in order to achieve the above. Therefore, this numerical value can be used as a criterion for determining whether the residual raw material is still effective or the raw material needs to be renewed.

[0010]

[0011]

Next, an embodiment of the present invention will be described. FIG.
FIG. 1 is a sectional view of a crucible for explaining a first embodiment of the present invention.

YVO ₄ was grown using a high frequency induction heating type oxide single crystal pulling apparatus. As shown in FIG. 1, the crucible was made of iridium having a diameter of 50 mm and a depth of 50 mm, and YVO ₄ powder having a purity of 99.999% was used as a growth raw material. The growth rate was 1 mm / h, and the growth atmosphere was nitrogen containing 0.1% oxygen. After growing a single crystal having a diameter of about 25 mm and a length of about 40 mm by using a c-axis seed, the same weight of raw material as that of the grown crystal was added and the same crystal growth was performed twice. After the crucible was gradually cooled at a temperature decrease rate of 200 ° C./h or less to form the upper layer 2 and the lower layer 3, FIG.
The sample rod 6 having a diameter of 2 mm was cut out from the solidified raw material in the crucible by the method shown in (b), and the thicknesses t ₁ and t ₂ of the upper layer 2 and the lower layer 3 were measured, respectively, t ₁ = 14 mm,
At t ₂ = 29 mm, the thickness t ₁ of the upper layer was 32.6% of the whole. Since the ratio of t ₁ / (t ₁ + t ₂ ) was 30% or more, when a crystal was grown by charging the residual raw material, a high quality crystal without cracks or inclusions was grown and the transmission characteristics were improved. The absorption coefficient at a wavelength of 500 nm was 0.16 cm ⁻¹ .

When confirming the effectiveness of the present invention, the follow-up charge growth was continued and the situation after the sixth growth was examined. The ratio of t ₁ was 30% or less, 26.9%. It was a value. As a precaution, when the crystal was further grown once, large cracks and inclusions were observed in the grown crystal. Also, coloring of the crystals was observed, and the absorption coefficient at a wavelength of 500 nm increased to 0.20 cm ^-1 .

For reference, the following residual raw materials were used.
A method for growing a single crystal is shown. Figure 2 shows the temperature program at that time.
FIG. Residual raw material conventionally taken from crucibles
The fee was considered non-renewable and was discarded. Difficult to reproduce
The reason why it was thought to be
Degradation will RVO _4-x is simply reduction RVO ₄ only
But various vanadiumizations indicated by RV _y O _z
Since the compound scatters, the leftover is also complicated
This is because it was considered to be a mixture of the murine compounds. Only
Then, the residue was heat treated in oxygen, powder X-ray diffraction
In RVO _4, the main peak is and other peaks remain slightly
Restored to a degree. Subsequent heat treatment in air is almost the same.
Although it is not the best, it is acceptable to use for crystal growth
It turned out to recover within a few degrees.

First, using a high-frequency induction heating type oxide single crystal pulling apparatus, YVO ₄ single crystal growth was carried out 5 times continuously by additional charge using an iridium crucible having a diameter of 50 mm and a depth of 50 mm. After the crystal growth was completed, the remaining raw material was extracted again using the seed crystal. The pulling speed was 30 mm / h on average, and the total amount of residual raw material was 3 in about 5 hours.
40 g was recovered. The recovered product had a blackish gray color, and powder X-ray diffraction confirmed YVO ₄ and YVO ₃ peaks. The residual raw material was transferred to an electric furnace and heat-treated at 1000 ° C. for 8 hours in air according to the temperature program shown in FIG. After the heat treatment, the sample became pale yellowish green, and the powder X-ray diffraction showed a dominant YVO ₄ peak and a few unidentifiable peaks. A total of 280 g of this treated product and 100 g of the new YVO ₄ powder raw material was combined to obtain a diameter of 5 g.
When a crystal was grown by filling it in a crucible with a depth of 0 mm and a depth of 50 mm, it was a good quality YV with no cracks or inclusions.
An O ₄ single crystal was grown.

In the above examples, the case where Y was used as the rare earth element was explained, but the same result was obtained even when Gd or Yb was used.

[0017]

As described above, according to the present invention, after the single crystal growth is completed, the raw material is separated into two layers and solidified, and the thicknesses of these two layers are compared to increase the additional charge growth of rare earth vanadate crystals. The effect is that the timing of renewing the raw material can be specified. Therefore, not only the raw material utilization efficiency is increased, but also the number of man-hours can be reduced by reducing the number of times of renewing the raw material.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a crucible for explaining a first embodiment of the present invention.

FIG. 2 is a diagram showing a temperature program for explaining a reference example .

FIG. 3 is a cross-sectional view of a crucible for explaining a sampling method for measuring the thickness of a layer of residual raw material.

FIG. 4 is a cross-sectional view of a crucible for explaining another sampling method for measuring the thickness of a layer of residual raw material.

[Explanation of symbols]

 1 Iridium crucible 2 Upper layer 3 Lower layer 4 Crushing tool 5 Ultrasonic rotary processing machine 6 Sample rod

Claims (2)

(57) [Claims] 1. Rare earth vanadate (Chemical formula RV
In the method for growing a single crystal (O ₄ , R: rare earth element), after the completion of the crystal growth, the molten raw material in the crucible is gradually cooled at a predetermined temperature drop rate and separated into two layers having different physical and chemical properties. A rare earth vanadate single crystal characterized by being solidified and using this remaining raw material as a criterion for the next growth, which is carried out next time with the numerical value of the thickness ratio of the upper and lower layers. How to grow. 2. The method for growing a rare earth vanadate single crystal according to claim 1, wherein the remaining raw material is used as it is when the thickness of the upper layer is 30% or more of the solidified raw material layer.