WO2007145110A1 - Method for producing lithium tantalate single crystal - Google Patents

Abstract

A method for producing a lithium tantalite single-crystal comprises the steps of: forming a growing crystal (17) from a material melt (15) in a first ambient gas including an inert gas as a main component and oxygen of 0.04% or more but not more than 0.08% by volume; cooling the growing crystal (17) in a second ambient gas including an inert gas as a main component and oxygen of 0.01% or less by volume; heating the cooled growing crystal (17) to the Curie temperature or higher; and cooling the growing crystal (17) while permitting a current to flow through the growing crystal (17) after the heating. In the method, the ratio of the mole number of lithium oxide to a sum of the mole numbers of lithium oxide and tantalum oxide in the growing crystal (17) is 0.49 or higher but not more than 0.50.

Description

 Specification

 Method for producing lithium tantalate single crystal

 Technical field

 [0001] The present invention relates to a method for producing a lithium tantalate single crystal, and more particularly to a method for producing a lithium tantalate single crystal that can suppress yellow coloring and improve the transmittance of visible light.

 Background art

 Conventionally, lithium tantalate single crystals have been used as materials for optical elements, piezoelectric elements, surface acoustic wave filters, and the like for modulating laser light.

 [0003] As a method for producing such a lithium tantalate single crystal, for example, Patent Document 1 (Japanese Patent Laid-Open No. 2001-316195) discloses a step of melting a raw material in a crucible placed inside a growth tank, The step of introducing an inert gas into the growth tank so that the oxygen concentration in the growth tank is 0.1 volume% or more and 1.2 volume% or less, and by pulling up while rotating the lifting shaft, And a step of growing a single crystal at the end of the substrate (see, for example, claim 6, paragraphs [0051] to [0071] of Patent Document 1)

) o

 [0004] Patent Document 2 (Japanese Patent Laid-Open No. 6-234597) discloses a method in which a pulling atmosphere of a lithium tantalate single crystal is an inert gas atmosphere containing 0.2 to 1.5% by volume of oxygen. (See, for example, claim 5 of Patent Document 2).

 Patent Document 1: Japanese Patent Laid-Open No. 2001-316195

 Patent Document 2: JP-A-6-234597

 Disclosure of the invention

 Problems to be solved by the invention

However, when a lithium tantalate single crystal is produced by the method disclosed in Patent Document 1 and Patent Document 2, there is a problem that the lithium tantalate single crystal is colored yellow.

[0006] When a yellow-colored lithium tantalate single crystal is used for a surface acoustic wave filter, However, yellow coloring means that the transmittance of visible light of blue with a short wavelength is reduced, so lithium tantalate single crystals are used for optical applications such as optical elements. In the case of using for the above, the reduction of the transmittance may be a problem.

 [0007] Accordingly, an object of the present invention is to provide a method for producing a lithium tantalate single crystal capable of improving visible light transmittance by suppressing yellow coloring.

 Means for solving the problem

[0008] The present invention includes a step of forming a grown crystal from a raw material melt in a first atmosphere gas containing an inert gas as a main component and oxygen in an amount of 0.04% by volume or more and 0.08% by volume or less. Cooling the grown crystal in a second atmosphere gas containing an inert gas as a main component and having an oxygen concentration of 0.01% by volume or less; and heating the grown crystal after cooling to a Curie temperature or higher; A step of cooling the grown crystal while passing an electric current through the grown crystal after heating, and (the number of moles of lithium oxide) Z (number of moles of lithium oxide + number of moles of tantalum oxide) in the grown crystal is 0. This is a method for producing a lithium tantalate single crystal of 49 or more and 0.50 or less.

 [0009] Here, in the present invention, the "main component" means 50% by volume or more of the entire gas. That is, in the present invention, 50% by volume or more of the first atmosphere gas is composed of an inert gas, and 50% by volume or more of the second atmosphere gas is composed of an inert gas. In the present invention, the oxygen concentration of the second atmospheric gas may be 0% by volume.

 [0010] In the method for producing a lithium tantalate single crystal of the present invention, the raw material melt is preferably prepared by adding 1 mol% or more and 5 mol% or less of magnesium oxide. .

 [0011] In the method for producing a lithium tantalate single crystal of the present invention, the raw material melt is preferably prepared by adding 0.1 mol% or more and 1 mol% or less of scandium oxide. .

 [0012] In the method for producing a lithium tantalate single crystal of the present invention, the raw material melt is preferably prepared by adding 0.5 to 3 mol% of zinc oxide.ヽ

[0013] Further, in the method for producing a lithium tantalate single crystal of the present invention, the raw material melt is an acid solution. Preferably, scandium is added in an amount of 0.1 mol% to 1 mol% and zinc oxide is added in an amount of 1 mol% to 3 mol%.

 The invention's effect

 [0014] According to the present invention, it is possible to provide a method for producing a lithium tantalate single crystal capable of suppressing yellow coloring and improving the transmittance of visible light.

 Brief Description of Drawings

 FIG. 1 is a diagram showing a schematic configuration of an example of an apparatus used in the method for producing a lithium tantalate single crystal of the present invention.

 FIG. 2 is a schematic cross-sectional view illustrating an example of a first step in the present invention.

 FIG. 3 is a schematic cross-sectional view illustrating an example of a second step in the present invention.

 FIG. 4 is a schematic cross-sectional view illustrating an example of a third step and an example of a fourth step in the present invention.

 Explanation of symbols

 [0016] 1 Growth device body, 2 Raw material supply device, 3 Load cell, 4 Vacuum pump, 5 Flow meter, 6 Mass flow controller, 7 Oxygen concentration meter, 8 Oxygen gas supply device, 9 Nitrogen gas supply device, 10 Lid 11 Incubation furnace material, 12 High frequency coil, 13 Lifting shaft, 14 Crucible, 15 Raw material melt, 16 seed crystal, 17 Growing crystal, 18 Heating device, 19, 20 Electrode.

 BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described. In the drawings of the present invention, the same reference numerals represent the same or corresponding parts.

FIG. 1 shows a schematic configuration of an example of an apparatus used in the method for producing a lithium tantalate single crystal of the present invention. This equipment consists of a growth device body 1, raw material supply device 2, load cell 3, vacuum pump 4, flow meter 5, flow crystal growth of raw material melt and lithium tantalate single crystal by liquid phase growth. A mass flow controller 6, an oxygen concentration meter 7, an oxygen gas supply device 8 and a nitrogen gas supply device 9 are provided.

[0019] A raw material supply device 2, a load cell 3, a vacuum pump 4, and an oxygen concentration meter 7 are attached to the growth apparatus main body 1. [0020] Further, oxygen supplied from the oxygen gas supply device 8 and nitrogen as an inert gas supplied from the nitrogen gas supply device 9 are controlled in flow rate by the mass flow controller 6 and then mixed. Then, the mixed gas of oxygen and nitrogen is introduced into the growth apparatus main body 1, the raw material supply apparatus 2 and the load cell 3 through gas pipes, respectively.

 Then, the mixed gas introduced into the raw material supply device 2 and the load cell 3 flows into the growth apparatus main body 1 and merges with the mixed gas directly introduced into the growth apparatus main body 1 in the growth apparatus main body 1.

 Thereafter, the mixed gas in the growth apparatus main body 1 flows from the exhaust port provided in the growth apparatus main body 1 to the exhaust pipe. The oxygen concentration meter 7 attached to the exhaust pipe causes the mixed gas in the growth apparatus main body 1 to flow. The oxygen concentration in the atmospheric gas is measured.

 FIG. 2 shows a schematic cross section of an example of the growing apparatus main body 1 shown in FIG. Here, in the growth apparatus main body 1, the crucible 14 is installed at the lower part of the heat insulation furnace material 11, and the lid body 10 is installed at the upper part of the heat insulation furnace material 11. A high-frequency coil 12 is installed outside the crucible 14, and a pulling shaft 13 having a seed crystal 16 at the end is installed through the center of the lid 10.

 In the growth apparatus main body 1 having such a configuration, a raw material melt 15 serving as a raw material of a lithium tantalate single crystal is accommodated in a crucible 14. Then, the seed crystal 16 provided on the pulling shaft 13 is immersed in the raw material melt 15 and pulled upward while rotating the pulling shaft 13, so that the lithium tantalate single crystal is formed on the surface of the seed crystal 16. The grown crystal 17 is gradually formed.

[0025] Here, in the present invention, the grown crystal is formed from the raw material melt in the first atmosphere gas containing an inert gas as a main component and oxygen in an amount of 0.04 volume% or more and 0.08 volume% or less. The first step is performed. This process will be described with reference to FIG. 2 as an example. In the present invention, the atmosphere inside the growth apparatus main body 1 contains an inert gas as a main component and oxygen is contained at 0.04 vol% or more. A first atmosphere gas atmosphere containing less than or equal to volume% is formed, and a grown crystal 17 that becomes a lithium tantalate single crystal is formed from the raw material melt 15 in the atmosphere of the first atmosphere gas. Here, the total pressure of the atmosphere inside the growth apparatus main body 1 can be set to 1 atmosphere. [0026] Subsequently, in the present invention, a step (second step) of cooling the grown crystal is performed in the second atmosphere gas having an oxygen concentration of 0.01% by volume or less. This process will be described with reference to FIG. 3, for example. The atmosphere inside the growth apparatus main body 1 is the second atmosphere having an oxygen concentration of 0.01 vol% or less immediately from the atmosphere of the first atmosphere gas. The atmosphere is changed to the atmosphere of the atmosphere gas, and the grown crystal 17 is cooled while being separated from the raw material melt 15 in the atmosphere of the second atmosphere gas. Again, the total pressure of the atmosphere inside the growth apparatus body 1 can be set to 1 atmosphere.

 [0027] Thereafter, in the present invention, a step (third step) of heating the grown crystal cooled in the second step above the Curie temperature is performed. For example, FIG. 4 is used to explain this process. The grown crystal 17 after cooling is sandwiched between the electrodes 19 and 20 placed on the surface perpendicular to the direction for the single domain treatment. For example, it is installed in a heating device 18 such as a resistance heating device. Then, after the grown crystal 17 is heated to the Curie temperature or higher in the heating device 18, an electric current is passed through the grown crystal 17 in the direction for performing the single domain treatment through the electrodes 19 and 20.

 [0028] Here, the Curie temperature refers to a temperature at which a ferroelectric material transitions to a paraelectric material. For example, the Curie temperature of a lithium tantalate single crystal is a force that varies depending on the molar ratio of lithium and tantalum that make up the lithium tantalate single crystal. Generally, the temperature is either 600 ° C to 700 ° C. Temperature.

 [0029] Subsequently, in the present invention, a step (fourth step) is performed in which the grown crystal is cooled while an electric current is applied to the grown crystal heated in the third step. This process will be described with reference to FIG. 4, for example. The growth crystal 17 heated to a temperature above the Curie temperature is passed through the electrodes 19 and 20 while passing a current in the direction for single domain treatment. Cool down.

Here, from the viewpoint of reducing the yellow coloration of the lithium tantalate single crystal, it is preferable to pass a direct current of 0.05 mAZcm ² or less to the grown crystal 17. Further, the lower limit of the current density of the direct current flowing through the grown crystal 17 can be set to, for example, 0. OlmAZcm ² or more.

[0031] From the viewpoint of reducing the yellow coloration of the lithium tantalate single crystal, the growth The formed crystal 17 is preferably cooled at a cooling rate of 50 ° C. Zh or less while a direct current is passed. In the present specification, the cooling rate of 50 ° CZh or lower means the rate at which the temperature decreases by 50 ° C per hour.

[0032] With the grown crystal obtained through the first to fourth steps, it is possible to obtain a lithium tantalate single crystal in which yellow coloring is suppressed and visible light transmittance is improved.

[0033] The method for producing a lithium tantalate single crystal of the present invention including the first to fourth steps described above has been found by the present inventors from the following viewpoints.

 [0034] In general, the composition of the lithium tantalate single crystal, including the one described in Patent Document 1, is a congruent melt composition. This is not a stoichiometric composition. In other words, the growth crystal (number of moles of lithium oxide) Z (number of moles of lithium oxide + number of moles of tantalum oxide) is around 0.485, and there are many tantalum ions with fewer lithium ions. In other words, in the crystal structure of the lithium tantalate single crystal, tantalum ion occupies part of the site that should be occupied by lithium ions. As a result, vacancies are generated to compensate for the charges. This vacancy is considered to cause light absorption.

 [0035] We have a composition range in which lithium is in excess of the coincident melting composition generally used in the production of lithium tantalate single crystals, ie, (number of moles of lithium oxide) Z (number of moles of lithium oxide + oxidation in the grown crystal) We found the possibility of improving this problem by setting the non-matching melting composition range (mol of tantalum) to 0.49 or more and 0.50 or less.

 [0036] However, as a further problem, it was proved that the reproducibility of high transmittance (no yellowing) was not obtained. As a result of investigating the cause, when a lithium tantalate single crystal was grown in a conventional atmospheric gas with an oxygen concentration of 0.1% by volume or more, LiO, LiO,

 2

 It was also confirmed that a particularly large amount of the vapor of thium oxide tends to scatter.

[0037] Therefore, as a result of intensive studies by the inventor, the raw material melt force in the first atmosphere gas containing the inert gas as a main component and oxygen in the range of 0.04% by volume to 0.08% by volume. The number of moles of lithium oxide) Z (number of moles of lithium oxide + number of moles of tantalum oxide) is grown from the raw material melt by liquid phase growth of grown crystals with a ratio of 0.49 to 0.50 (first step). Lithium oxide such as Li 2 O and LiO can be prevented from splashing vapor, and the above first Subsequently, the second to fourth steps described above were performed in this order, so that the lithium tantalate single crystal was less likely to be colored yellow, and the visible light transmittance could be improved. The present invention has been completed.

 [0038] Furthermore, the present inventor, when introducing a different ion such as magnesium ion, zinc ion or scandium ion as an additive, these ions are incorporated into the lithium tantalate single crystal. We also found that tantalum ion occupies the site that should occupy! /, And as a result, the vacancies causing light absorption are reduced, and the transmittance of visible light is improved.

 [0039] In the present invention, the raw material melt is not particularly limited as long as it can produce a lithium tantalate single crystal, and the lithium tantalate single crystal is yellow even if it can be used. From the viewpoint of reducing coloring, the raw material melt may be lithium oxide (L

It is preferable to use a raw material melt containing 1 O) and tantalum oxide (Ta 2 O 3).

 2 2 5

 [0040] The raw material melt is more preferably prepared by adding 1 mol% or more and 5 mol% or less of magnesium oxide (MgO). In this case, it tends to be possible to further reduce the yellow coloration of the lithium tantalate single crystal.

 [0041] Here, when MgO is added in an amount of 1 mol% or more and 5 mol% or less, MgO is added so as to satisfy the following formula (1).

 1≤ 100 X (Mole number of MgO added) / (Mole number of the whole mixture (including MgO added) before melting of raw material melt) ≤ 5

 In addition, as a raw material melt, scandium oxide (Sc 2 O 3) is 0.1 mol% or more and 1 mol% or less.

 twenty three

 More preferably, it is prepared by adding it underneath. Also in this case, it tends to be possible to further reduce the yellow coloration of the lithium tantalate single crystal.

Here, the addition of 0.1 mol% or more and 1 mol% or less of Sc 2 O satisfies the following formula (2).

 twenty three

 This means that ScO is added.

 twenty three

 0. 1≤ 100 X (number of moles of Sc added) / (total mixture before melting of raw material melt (added)

 twenty three

 The number of moles of Sc (including Sc O)) ≤ 1 · '· (2)

 twenty three

The raw material melt is more preferably prepared by adding 0.5 to 3 mol% of zinc oxide (ΖηΟ). Also in this case, the lithium tantalate single crystal is yellow It tends to be possible to further reduce the coloring.

 [0043] Here, ZnO force of 0.5 mol% or more and 3 mol% or less added means that ZnO is added so as to satisfy the following formula (3)!

 0. 5≤ 100 X (number of moles of ZnO added) / (number of moles of the entire mixture (including ZnO to be added) before melting) ≤ 3--(3)

 In addition, as a raw material melt, ScO is added in an amount of 0.1 mol% to 1 mol%.

 twenty three

 More preferably, ZnO is added at 1 mol% or more and 3 mol% or less. Also in this case, it tends to be possible to further reduce the yellow coloration of the lithium tantalate single crystal.

[0044] Here, with Sc O is added 0.1 mole _^0/0 or 1 mole _^0/0 or less, ZnO is 1 mole ^_0/0

 twenty three

 More than 3 mol% is added when ScO is added to satisfy the following formula (4).

 twenty three

 In both cases, ZnO is added so as to satisfy the following formula (5).

 0. 1≤ 100 X (number of moles of ScO added) / (total mixture before melting of raw material melt (added)

 twenty three

 Number of moles (including ScO and ZnO)) ≤ 1 · · · (4)

 twenty three

 1≤ 100 X (number of moles of ZnO added) / (number of moles of the entire mixture (including ScO and ZnO added) before melting of the raw material melt) ≤ 3 (5)

 twenty three

 Further, in the above description, the force described in the case of growing a lithium tantalate single crystal mainly using the Tyoklalsky (CZ) method. In the present invention, in addition to the CZ method, a continuous raw material supply type double crucible is used. A growth method such as a method, a solution method (Top-Seeded Solution Growth method) or a Bridgeman method can also be used.

[0045] Further, in the above description, the force described for the case where nitrogen is used as the inert gas that is the main component of the first atmosphere gas and the second atmosphere gas. In the present invention, in addition to nitrogen, a rare gas such as argon is used. Gas can also be used as an inert gas. Example

[0046] Raw material melt force having the structure shown in Table 1 and Table 2 Grown crystals of sample numbers 1 to 34 were grown by the CZ method, and each of the grown crystals of sample numbers 1 to 34 was grown at the time of crystal growth of the grown crystals. The state, internal transmittance for visible light (blue) and crystal coloration were evaluated. Specifically, the growth crystal was formed and evaluated as follows. First, the molar ratio of lithium carbonate (Li 2 CO 3) to tantalum oxide (Ta 2 O 3) is Li CO: Ta 2 O =

 2 3 2 5 2 3 2 5

57:43 Weighed 70.8 g of Li 2 CO and weighed 39.2 g of Ta 2 O. Pop these

 2 3 2 5

 The mixture was stirred and mixed for 1 hour while paying attention to moisture absorption. Then, it was calcined by heating at 1300 ° C for 8 hours.

 [0048] Next, the calcined product is placed in an iridium crucible while paying attention to moisture absorption, and is homogenized for 1 hour while being heated to prepare a raw material melt. Using this raw material melt, tantalum acid is obtained by the CZ method. Growing lithium single crystal (number of moles of Li 2 O)

 2 Z (number of moles of Li 2 O + number of moles of Ta 2 O) is 0

 2 2 5

 A grown crystal to be 497 was formed (Sample Nos. 11 to 15).

[0049] In the same manner as described above, (number of moles of Li 2 O) number of moles + number of moles of Ta 2 O)

 2 Z (Li O

 2 2 5

 Growing crystals were formed to become 0.490 (sample numbers 1 to 5), 0.493 (sample numbers 6 to: L0), and 0.500 (sample numbers 16 to 20), respectively.

[0050] Further, the Li CO is adjusted so that the molar ratio of Li CO to Ta O is Li CO: Ta O = 55:45.

 2 3 2 5 2 3 2 5 2 3

. 2g, pop M this person is to 323. 8g样量the Ta O, further, added Caro amount force of MgO 1 Monore _^0/0, 3

 twenty five

. 7 Monore _^0/0, 5 Monore _^0/0 become manner, 0. MgO respectively 66 g, 2. 74 g, was added 3. 45 g to the pot. After calcining this, it is homogenized to produce a raw material melt, and using this raw material melt, a lithium tantalate single crystal is grown by the CZ method (number of moles of Li 2 O).

 2

 / Grown crystals with (number of moles of Li O + number of moles of Ta O) being 0.495 (sample number

2 2 5

 No. 21-23).

 [0051] Further, Li CO is adjusted so that the molar ratio of Li CO to Ta O is Li CO: Ta O = 55:45.

 2 3 2 5 2 3 2 5 2 3

. 2 g, placed in a pot and 323. 8 g weighed Ta O, further, Sc O of added Caro weight force 0.1 Monore ^_0/0

 2 5 2 3

, So that 0.35 Monore _^0/0, respectively 0. 23 g of Sc O, then added Caro a 0. 79 g to the pot

 twenty three

 It was. Then, after calcining this, homogenized to produce a raw material melt, and using this raw material melt, a lithium tantalate single crystal was grown by the CZ method (number of moles of Li 2 O)

 2 Z (Li O

 A grown crystal having a number of 2 + the number of moles of TaO was 0.495 (sample numbers 24 to 25).

 twenty five

 [0052] In addition, Li CO is adjusted so that the molar ratio of Li CO to Ta O is Li CO: Ta O = 57:43.

 2 3 2 5 2 3 2 5 2 3

• 8g, put in a pot with 319 · 2g样量the Ta O, further, added Caro amount mosquito Sc O 0 · 5 Monore ^_0/0

 2 5 2 3

, 1 Monore _^0/0 become so were added Caro Sc O each 1 · 16g, 2. 34g to its pot.

 twenty three

After calcining this, it is homogenized to produce a raw material melt, and using this raw material melt, C Growing lithium tantalate single crystal by Z method, (number of moles of Li 2 O) Z (number of moles of Li 2 O

 twenty two

 A grown crystal having a + Ta 2 O mole number of 0.497 was formed (Sample Nos. 26 to 27).

 twenty five

 [0053] Further, Li CO is adjusted so that the molar ratio of Li CO to Ta O is Li CO: Ta O = 57:43.

 2 3 2 5 2 3 2 5 2 3

. 8 g, human being in the pot and 319. 2 g样量a Ta O, further, added Caro amount force of ZnO 0. 5 Monore ^_0/0,

 twenty five

1 Monore _^0/0, 3 Monore _^0/0 become manner, 0. ZnO respectively 69 g, 1. 38 g, was added to 4. 23 g on the pots. Then, after calcining this, it is homogenized to prepare a raw material melt, and using this raw material melt, a lithium tantalate single crystal is grown by the CZ method, and the (number of moles of Li 2 O) Z (

 2

 A grown crystal was formed in which the number of moles of Li 2 O + the number of moles of Ta 2 O was 0.497 (Sample No. 28

2 2 5

 ~ 30).

 [0054] In addition, Li CO is adjusted so that the molar ratio of Li CO to Ta O is Li CO: Ta O = 57:43.

 2 3 2 5 2 3 2 5 2 3

8g, 319. 2g sample of Ta O and put it in the pot.

 2 5 2 3

%, 0.5 Monore _^0/0, 0.5 Monore _^0/0, 1 Monore _^0/0 so that, respectively 0. 58 g of Sc O, 1. 1

 twenty three

6 g, 1. 16g, 2. 34g and added Caro its pot, added Caro amount force of ZnO Monore _^0/0, 1 Monore _^0/0, 2 Monore _^0/0, 1 Monore _^0/0 so as to, ZnO 1.38 g, 1.38 g, 2.79 g and 1.38 g, respectively, were added to the pot. After calcining this, it is homogenized to produce a raw material melt, and using this raw material melt, a lithium tantalate single crystal is grown by the CZ method, and the (Li 2 O mol) is grown.

 2) Growing crystals with Z (number of moles of Li 2 O + number of moles of Ta 2 O) of 0.497 were formed (sample

 2 2 5

 Numbers 31-34).

 [0055] Here, the growth conditions of the growth crystals of sample numbers 1 to 34 are as follows: crystal growth direction is Z-axis direction, pull-up shaft rotation speed is 6rpm, bow I lift-up shaft bow | At a constant. Under these conditions, the grown crystals of Sample Nos. 1 to 34 having a diameter of about 25 mm and a thickness of about 20 mm were formed on the seed crystal installed at the end of the pulling shaft.

[0056] In addition, the oxygen concentration is 0.03 vol _^0/0 as the first atmosphere gas at the time of forming the grown crystal of Sample No. 1-34, 0.04 volume _^0/0, 0.06 volume _^0/0, 0.08 volume _^0/0 or 0. nitrogen containing oxygen is 1 volume _^0/0 was used. The oxygen concentration (% by volume) was measured using an oxygen concentration meter (LC--Oxygen concentration meter (LC—) manufactured by Toray Engineering Co., Ltd. 700) measured by). The total pressure of the first atmosphere gas during the growth of the growth crystal was set to 1 atmosphere. [0057] It should be noted that the above composition is not a coincidence melt composition, and therefore, growth crystals of sample numbers 1 to 34 were formed at 10% by mass of the raw material melt without the influence of compositional deviation. If the raw material melt is 10% by mass or less, it is not necessary to supply the raw material for correcting the composition deviation. However, if it exceeds 10% by mass of the raw material melt, in order to correct the composition deviation, for example, It is necessary to supply the raw material from the raw material supply device 2 shown in FIG.

 [0058] Next, after the growth crystal was formed, the growth crystal was separated from the raw material melt, the supply of oxygen to the oxygen supply device was stopped, and only nitrogen was introduced into the growth device main body. Then, cooling of the grown crystal starts when the above oxygen concentration meter shows 0.01 volume% (that is, the second oxygen concentration is not more than 0.01 volume% and the remainder other than oxygen is nitrogen. (Cooled in atmospheric gas). This operation was performed for each of the grown crystals of sample numbers 1 to 34. The time from when the supply of oxygen was stopped until the oxygen concentration meter showed 0.01% by volume was within 5 hours. The total pressure of the second atmosphere gas during cooling of the grown crystal was set to 1 atmosphere.

[0059] After the above cooling, the grown crystals of sample numbers 1 to 34 were blackened. Then, the blackened growth crystals of Sample Nos. 1 to 34 were placed in a resistance heating furnace while being sandwiched between Pt plates so as to face each other in the Z-axis direction. After that, these were heated to 750 ° C in the atmosphere and sufficiently maintained at that temperature, and then 20 ° while flowing a direct current with a current density of 0.02 mAZcm ² through the growth crystal using the Pt plate as an electrode. The grown crystal was cooled to room temperature at the cooling rate of CZh.

 [0060] Then, for each grown crystal of sample numbers 1 to 34 after cooling, a plate sample was cut out in a direction perpendicular to the Z-axis direction, both main surfaces of the plate sample were mirror-polished, and the final finish thickness Parallel plate samples with sample numbers 1 to 34 with lengths of 10 mm and 3 mm were prepared.

 [0061] Then, transmittance was measured using a spectrophotometer (UV-2500 manufactured by Shimadzu Corp.) for each of the parallel plate samples of sample numbers 1 to 34 having a thickness of 10 mm and a thickness of 3 mm. Here, the transmittance was measured using an integrating sphere and arranged so that the light beam was incident perpendicularly to the surface of the parallel plate sample.

[0062] The measured transmittance (linear transmittance) for parallel plate samples with thicknesses of 10mm and 3mm was calculated using the following equation (6) for each wavelength of light. 10mm thick parallel plate using It was converted to the internal transmittance of the sample.

 log τ =-10 X (logT -logT) / Ad

 1 2

 In the above equation (6), τ represents the internal transmittance of a 10 mm thick parallel plate sample, Δ d represents the thickness difference between a 10 mm thick parallel plate sample and a 3 mm thick parallel plate sample, and T is

 1 Shows the linear transmittance including reflection loss of a parallel plate sample with a thickness of 3 mm.

 2

 Show the linear transmittance including reflection loss of parallel plate samples!

[0063] Tables 1 and 2 show the composition of the raw material melt of the grown crystals of sample numbers 1 to 34 and the number of moles of Li 2 O in the grown crystals of sample numbers 1 to 34

 2 Z (number of moles of Li 2 O + number of moles of Ta 2 O) and

 2 2 5

 The oxygen concentration of the 1st atmosphere gas at the time of formation of the growth crystal of sample numbers 1-34 is shown. Table 1 and Table 2 show that the grown crystals of Sample Nos. 1 to 34 were visually observed, and the results for the parallel plate samples of Sample Nos. 1 to 34 with respect to visible light (blue) with a wavelength of 400 nm are shown. The values of the internal transmittance and the results of visual observation of the coloration of the crystals immediately before cutting out the grown crystals of sample numbers 1 to 34 are shown.

[0064] In the column of "Li 2 O / (Li 2 O + Ta 2 O 3)" of the raw material melts in Table 1 and Table 2, the sample number

 2 2 2 5

 (Li 2 O mol in each raw material melt used for the formation of growth crystals 1 to 34

 2 Number) The value of Z (number of moles of Li 2 O + number of moles of Ta 2 O) is shown.

 2 2 5

 [0065] In the column of “MgO (mol%)” in Tables 1 and 2, 100 X (number of moles of MgO added) in each of sample numbers 1 to 34 Z (MgO-added charge) Li CO, Ta O and MgO

 The value of the number of moles of the mixture consisting of 2 3 2 5 is shown.

 [0066] In the column of "Sc 2 O (mol%)" in Tables 1 and 2, 1 for Sample Nos. 1 to 30

 twenty three

 00 X (number of moles of Sc O added) Z (from Li CO, Ta O and Sc O after Sc O addition

 2 3 2 3 2 3 2 5 2 3) is shown. For sample numbers 31-34, 100 X (number of moles of scO 2 added) Z (Sc O and ZnO Li CO, Ta O, Sc O and Z

 2 3 2 3 2 3 2 5 2 3 The value of ηθ and the number of moles of the force mixture is shown!

[0067] In addition, the column of "ZnO (mol _^0/0)" in Table 1 and Table 2, (the number of moles of添Ka卩been ZnO) 10 0 X for sample No. 1 to 30 Z (ZnO added Powered by Li CO, Ta O and ZnO

 2 3 2 5

The number of moles of the mixture is shown. For sample numbers 31-34, 100 X (number of moles of ZnO added) Z (Sc O and LiO, TaO and Sc after addition of ZnO) O and ZnO The value of the number of moles of the mixture consisting of

 [0068] In the column of "Li 0 / (Li 2 O + Ta 2 O 3)" of the grown crystals in Tables 1 and 2, the sample number

 2 2 2 5

 In the grown crystals of 1 to 34 (number of moles of Li 2 O) Z (number of moles of Li 2 O + number of moles of Ta 2 O)

 2 2 2 5

 Values are shown. Note that (number of moles of Li 2 O) in the grown crystals of sample numbers 1 to 34

 2 Z (

The value of the number of moles of Li 2 O + the number of moles of Ta 2 O is the value of the grown crystal (number of moles of Li 2 O)

 5 2 / (Li O

2 2 2 Number of moles + number of moles of Ta O) and the Curie temperature of the grown crystal are proportional to each other

 twenty five

 In other words, the Curie temperature of the grown crystal was measured by differential thermal analysis, and the measured temperature force was calculated.

 [0069] In addition, in the column of "Oxygen concentration of first atmosphere gas (volume%)" in Tables 1 and 2, 100 X (first atmosphere gas) at the time of formation of each grown crystal of sample numbers 1 to 34 is shown. The value of (volume of oxygen in) / (volume of the entire first atmosphere gas composed of oxygen and nitrogen) is indicated.

 [0070] Also, in Tables 1 and 2, "occurrence of separation from the melt" means that the grown crystal is separated from the raw material melt during the growth of the crystal and the crystal cannot be grown. Means state.

[0071] [Table 1]

满 Hakoto 嫘 tίψ / ^ ¾ίψ / S 04ο ± ορ 08ο

The lithium tantalate single crystals of Sample Nos. 2 to 4 formed in the lower range are formed with the raw material melt force of the same composition, but the oxygen concentration of the first atmosphere gas is 0.04% by volume or more. Compared to the lithium tantalate single crystals of Sample No. 1 and Sample No. 5 that are not in the volume% or less range,

 The yellow coloration of the crystals is suppressed, and the internal transmittance of visible light (blue) with a wavelength of 400 nm is improved.

 [0074] Similarly, the lithium tantalate single crystals of Sample Nos. 7 to 9 formed in the range where the oxygen concentration of the first atmosphere gas is 0.04% by volume or more and 0.08% by volume or less have the same composition. Compared with the lithium tantalate single crystals of Sample No. 6 and Sample No. 10 where the raw material melt is also formed, but the oxygen concentration of the first atmosphere gas is not in the range of 0.04 volume% or more and 0.08 volume% or less. As a result, the yellow coloration of the crystal is suppressed and the internal transmittance of visible light (blue) having a wavelength of 400 nm is improved.

 [0075] Similarly, the lithium tantalate single crystals of Sample Nos. 12 to 14 formed in the range where the oxygen concentration of the first atmosphere gas is 0.04 vol% or more and 0.08 vol% or less have the same composition. Raw material melt strength Compared to the lithium tantalate single crystals of Sample No. 11 and Sample No. 15 where the oxygen concentration of the first atmosphere gas is not in the range of 0.04% by volume or more and 0.08% by volume or less. Thus, the yellow coloration of the crystal is suppressed, and the internal transmittance of visible light (blue) with a wavelength of 400 nm is improved.

 [0076] Similarly, the lithium tantalate single crystals of Sample Nos. 17 to 19 formed in the range where the oxygen concentration of the first atmosphere gas is 0.04 vol% or more and 0.08 vol% or less have the same composition. Raw material melt strength Compared to the lithium tantalate single crystals of Sample No. 16 and Sample No. 20, which are formed but the oxygen concentration of the first atmosphere gas is not in the range of 0.04% by volume or more and 0.08% by volume or less. Thus, the yellow coloration of the crystal is suppressed, and the internal transmittance of visible light (blue) with a wavelength of 400 nm is improved.

 [0077] Further, as shown in Table 1 and Table 2, at least one of MgO, ScO and ZnO is added.

 twenty three

The lithium tantalate single crystals of Sample Nos. 21 to 34 formed from the calcined raw material melt tend to suppress the yellow coloration of the crystals and transmit visible light (blue) with a wavelength of 400 nm. The rate tended to improve. [0078] The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

 Industrial applicability

[0079] According to the present invention, it is possible to provide a method for producing a lithium tantalate single crystal capable of suppressing yellow coloring and improving visible light transmittance.

Claims

The scope of the claims  [1] A step of forming a raw material melt (15) force-grown crystal (17) in a first atmosphere gas containing an inert gas as a main component and oxygen in the range of 0.04 vol% to 0.08 vol% Cooling the grown crystal (17) in a second atmosphere gas containing an inert gas as a main component and having an oxygen concentration of 0.01% by volume or less;  Heating the grown crystal (17) after cooling to a Curie temperature or higher;  Cooling the grown crystal (17) while passing current through the grown crystal (17) after heating,  The method for producing a lithium tantalate single crystal, wherein (number of moles of lithium oxide) Z (number of moles of lithium oxide + number of moles of tantalum oxide) in the grown crystal (17) is from 0.49 to 0.50.  [2] The lithium tantalate according to claim 1, wherein the raw material melt (15) is prepared by adding 1 mol% to 5 mol% of magnesium oxide. Crystal manufacturing method.  [3] The lithium tantalate according to claim 1, wherein the raw material melt (15) is prepared by adding scandium oxide in an amount of 0.1 mol% to 1 mol%. A method for producing a crystal.  [4] The lithium tantalate according to claim 1, wherein the raw material melt (15) is prepared by adding 0.5 to 3 mol% of zinc oxide. Crystal manufacturing method.  [5] The raw material melt (15) was prepared by adding scandium oxide in an amount of 0.1 mol% to 1 mol% and adding zinc oxide 1 mol% to 3 mol%. The method for producing a lithium tantalate single crystal according to claim 1, characterized in that: