Titanium oxide crystal, method for producing same, and use thereof

Abstract

The present invention provides: a titanium oxide crystal in which defects such as oxygen defects are suppressed; and a method for producing the same. A rutile titanium oxide crystal according to an embodiment of the present invention contains at least titanium (Ti), oxygen (O), and hydrogen (H), and has a crystal structure substantially identical to that of a rutile TiO2 crystal. A method for producing a rutile titanium oxide crystal according to the present embodiment includes growing a raw material containing each element contained in the crystal by a hydrothermal synthesis method in the presence of: a mineralizer containing an alkali metal and / or an alkaline earth metal; or a mineralizer of an acid, or a mixed mineralizer which is a mixture of said mineralizers. Optionally, the method, further includes heat-treating the grown crystal.

Description

Titanium oxide crystals, their production method and uses

[0001] The present invention relates to titanium oxide crystals, a method for producing the same, and uses thereof.

[0002] Titanium oxide (TiO ２ It is known that TiO 2 has several types of crystal structures, and among these, rutile single crystals are used as materials for prisms and polarizers. In particular, with the recent advancement and spread of optical communication technology, rutile TiO 2 , which is useful as an optical component, has become available. ２ Demand for rutile crystals (hereinafter also referred to as rutile crystals) is increasing, and there is a demand for larger diameter and higher quality crystals.

[0003] Techniques for growing rutile crystals include the Verneuil method (V method) and the floating zone method (FZ method) (see Patent Document 1). However, these methods have the problem that the size of the grown crystal is limited, making it difficult to increase its size. In addition, due to the principle of crystal growth, a steep temperature gradient (e.g., 100°C / cm or more) occurs, which makes it easy for defects to occur in the grown crystal, making it difficult to obtain high-quality crystals.

[0004] Therefore, as an alternative growth technique, attempts have been made to grow crystals from a raw material melt in a crucible using the Czochralski method (CZ method) or the EFG (Edge-defined Film-fed Growth) method (see Patent Documents 2 and 3). In principle, these methods allow the size of the grown crystal to be adjusted by changing the size of the crucible, which is expected to enable the growth of larger crystals. However, in the CZ method, particularly when growing rutile crystals by the rotational pulling method using high-frequency induction heating, Ti ４＋It has been pointed out that there is a problem in that the reduction of titanium dioxide occurs, resulting in a decrease in resistivity, and the growing crystal is subjected to high-frequency induction heating. This heating makes it difficult to dissipate heat from the crystal, making it difficult to obtain high-quality crystals. Furthermore, in terms of the melting point and reactivity of titanium oxide, growth using the CZ method requires the use of iridium (Ir) as a crucible material. Therefore, in practical applications, it is difficult to reduce manufacturing costs, and growth is required under conditions where the oxygen partial pressure is controlled low to prevent oxidation of the Ir crucible. Furthermore, it has been pointed out that a significant spiral (bending) phenomenon occurs during the crystal growth process, making it extremely difficult to obtain practically useful rutile crystals. According to one study, the growing rutile crystal is colored from dark blue to black, and absorbs light over a wide wavelength range. This explains why thermal radiation increases the temperature from the growth interface throughout the growing crystal, making heat dissipation from the crystal even more difficult, further destabilizing the growth interface and making the spiral phenomenon more likely to occur. Furthermore, although there have been cases in which the spiral phenomenon described above has been avoided in previous studies using the EFG method (see Non-Patent Documents 1 and 2), no manufacturing method has yet been established to replace the V method or the FZ method. Note that, like the CZ method, the EFG method also requires the use of Ir as a material for the crucible and die, making it difficult to reduce manufacturing costs in practical use.

[0005] JP 10-231195 JP 09-235195 JP 2001-181091

[0006] H. Machida, et al., Journal of Crystal Growth, 1993, 128, 829-833.H. Machida, et al., Journal of Crystal Growth, 1994, 137, 516-520.

[0007] The present invention was made in consideration of these circumstances, and aims to provide titanium oxide crystals in which defects such as oxygen defects are suppressed, and in particular to provide large, high-quality titanium oxide crystals. Another aim of the present invention is to provide a method for producing the above-mentioned titanium oxide crystals under conditions of high oxygen partial pressure without using conventional crucibles, dies, or other components that use Ir. Another aim of the present invention is to provide uses for the above-mentioned titanium oxide crystals.

[0008] The rutile titanium oxide crystals according to the present invention contain at least titanium (Ti), oxygen (O) and hydrogen (H), and are rutile TiO ２ The rutile-type titanium oxide crystal of the present invention has a crystal composition represented by the general formula TiO ２－ｘ (wherein, −0.5≦x≦0.5). In the rutile-type titanium oxide crystal of the present invention, the hydrogen concentration is 1×10 １５ atoms / cm ３ 1x10 or more ２２ atoms / cm ３ The rutile-type titanium oxide crystal of the present invention may further contain an alkali metal and / or alkaline earth metal. The concentration of the alkali metal and / or alkaline earth metal may be in the range of 1×10 １２ atoms / cm ３ 1x10 or more ２０ atoms / cm ３ The rutile-type titanium oxide crystal of the present invention may further contain at least one halogen element selected from the group consisting of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The halogen element may include at least F. The halogen element may also be F. The concentration of the halogen element may be in the range of 1×10 １５ atoms / cm ３ 1x10 or more ２３ atoms / cm ３ The following ranges may also be used:

[0009] The method for producing the above-mentioned rutile-type titanium oxide crystals according to the present invention comprises growing crystals by hydrothermal synthesis from raw materials containing the elements to be contained in the crystals in the presence of a mineralizer containing an alkali metal and / or alkaline earth metal, or an acid mineralizer, or a mixed mineralizer of both, and optionally further heat-treating the grown crystals, thereby solving the above-mentioned problems. The concentration of the mineralizer may be in the range of 1 M to 50 M. In growing the crystals by hydrothermal synthesis, the temperature may be in the range of 400°C to 800°C, and the maximum pressure may be in the range of 25 MPa to 250 MPa. A seed crystal may be used in growing the crystals by hydrothermal synthesis.

[0010] The optical component according to the present invention uses the above-mentioned rutile titanium dioxide crystal, thereby solving the above-mentioned problems. The above-mentioned optical component may be a prism, a birefringent crystal, or a polarizer. The optical isolator according to the present invention includes a birefringent crystal or a polarizer made of the above-mentioned rutile titanium dioxide crystal, thereby solving the above-mentioned problems. The optical isolator may be polarization-independent. The optical communication device, optical communication line, or laser processing machine according to the present invention includes the above-mentioned optical isolator, thereby solving the above-mentioned problems.

[0011] According to the present invention, rutile-type titanium oxide crystals are provided in which defects such as oxygen defects are suppressed. The defect-controlled rutile-type titanium oxide crystals of the present invention can be provided as large, high-quality single crystals, and are suitable for use in various optical components.

[0012] The rutile titanium dioxide crystals of the present invention are produced by hydrothermal synthesis, which allows for growth at temperatures significantly lower than those achieved by the conventional methods. Furthermore, bulk single crystals can be grown under conditions of high oxygen partial pressure (in an oxygen-rich atmosphere). As a result, the oxygen defects and other defects described above are suppressed in the resulting crystals, resulting in defect-controlled crystals. Furthermore, the present invention eliminates the need for conventional Ir-based components such as crucibles and dies, thereby reducing the cost of producing the desired crystals. Furthermore, the use of hydrothermal synthesis makes it relatively easy to grow larger crystals compared to conventional methods, and seed crystals can also be used. The rutile titanium dioxide crystals of the present invention produced in this manner can be provided as large, high-quality single crystals, and are suitable for use as materials for prisms and polarizers, among other applications, and are useful as materials for various optical components.

[0013] 1 is a schematic diagram showing one embodiment of an optical isolator using rutile-type titanium oxide crystals according to the present invention; (a) a micrograph of the crystal of Example 2, (b) a micrograph of the crystal of Example 5, and (c) a micrograph of the crystal of Example 6; (a) a micrograph of the crystal of Example 2, (b) a micrograph of the crystal of Example 2A (after 24 hours of heat treatment), and (c) a micrograph of the crystal of Example 2A (after 48 hours of heat treatment); and (b) a micrograph of the crystal of Example 2 and the starting material TiO. ２ FIG. 1 is a diagram showing the results of powder X-ray diffraction measurement of the crystal of Example 1 and the simulation result of the diffraction chart of the rutile crystal. FIG. 2 is a diagram showing the results of powder X-ray diffraction measurement of the crystal of Example 2 and the simulation result of the diffraction chart of the rutile crystal. FIG. 3 is a diagram showing the results of powder X-ray diffraction measurement of the crystal of Example 5 and the simulation result of the diffraction chart of the rutile crystal. FIG. 4 is a diagram showing the results of powder X-ray diffraction measurement of the crystal of Example 6 and the simulation result of the diffraction chart of the rutile crystal. FIG. 5 is a diagram showing the concentrations of hydrogen, fluorine and cesium in the secondary ion mass spectrometry results of the crystal of Example 2.

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0015] [Titanium oxide crystal] The titanium oxide crystal according to one embodiment of the present invention has a rutile crystal structure. Hereinafter, the rutile titanium oxide crystal according to this embodiment (also simply referred to as "rutile titanium oxide crystal") will be described.

[0016] Rutile-type titanium oxide crystals contain at least titanium (Ti), oxygen (O), and hydrogen (H) as constituent elements. In one embodiment, the rutile-type titanium oxide crystals are represented by the general formula TiO ２－ｘ It is preferable that the composition be expressed as (wherein, −0.5≦x≦0.5).

[0017] Rutile-type titanium oxide crystals may have a number of oxygen atoms greater or less than 2 due to defects in the crystal, but by satisfying the condition of x in the general formula, -0.5≦x≦0.5, deterioration of the properties as a material for optical components is sufficiently suppressed. In other words, it is preferable that the value of x in the general formula is close to zero. Most preferably, x in the general formula is zero, and in this case, the rutile-type titanium oxide crystals are TiO ２ It has a composition represented by the formula:

[0018] In one embodiment, the concentration of hydrogen is 1×10 １５ atoms / cm ３ 1x10 or more ２２ atoms / cm ３ The range is preferably 1×10 １６ atoms / cm ３ 1x10 or more ２１ atoms / cm ３ The range is preferably 1×10 １７ atoms / cm ３ 1x10 or more ２０ atoms / cm ３ The range is as follows:

[0019] The method for measuring the hydrogen concentration in rutile-type titanium dioxide crystals is not particularly limited, and can be confirmed by conventional methods, such as secondary ion mass spectrometry (SIMS). The same applies to the concentrations of elements other than hydrogen (for example, the concentrations of optional constituent elements (alkali metals and / or alkaline earth metals, and halogen elements) described below). Here, when SIMS is used, the hydrogen concentration is the value at a depth of 3 μm, and the same applies to the concentrations of elements other than hydrogen. An example of SIMS measurement will be described in the Examples section.

[0020] <Optional Constituent Elements> The rutile-type titanium oxide crystals may further contain optional constituent elements other than Ti, O, and H described above.

[0021] In one embodiment, the rutile titanium dioxide crystals further contain an alkali metal and / or alkaline earth metal as an optional constituent element. In one exemplary embodiment, the alkali metal is selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), with K, Rb, and Cs being preferred. The alkaline earth metal is selected from the group consisting of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), with Ca, Sr, and Ba being preferred. The alkali metal and alkaline earth metal may be used alone or in combination of two or more. The inclusion of an alkali metal and / or alkaline earth metal in the rutile titanium dioxide crystals is expected to passivate defects within the crystals.

[0022] In an embodiment in which the rutile-type titanium oxide crystal contains an alkali metal and / or an alkaline earth metal, the concentration of the alkali metal and / or the alkaline earth metal is 1×10 １２ atoms / cm ３ 1x10 or more ２０ atoms / cm ３ The range is preferably 1×10 １３ atoms / cm３ 1x10 or more １９ atoms / cm ３ More preferably, it is in the range of 1×10 １４ atoms / cm ３ 1x10 or more １８ atoms / cm ３ The range is as follows:

[0023] In one embodiment, the rutile titanium oxide crystal further contains a halogen element as an optional constituent element. In one exemplary embodiment, the halogen element is selected from the group consisting of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The halogen element may be a single element or a combination of two or more elements. In a preferred embodiment, the halogen element includes at least F. In a more preferred embodiment, the halogen element is F. It is expected that the halogen element contained in the rutile titanium oxide crystal will passivate defects within the crystal. The concentration of the halogen element is preferably 1×10 １５ atoms / cm ３ 1x10 or more ２３ atoms / cm ３ More preferably, it is in the range of 1×10 １６ atoms / cm ３ 1x10 or more ２２ atoms / cm ３ The range is more preferably 1 × 10 １７ atoms / cm ３ 1x10 or more ２１ atoms / cm ３ The following ranges may be satisfied:

[0024] <Crystal Structure> The rutile titanium oxide crystal is the above-mentioned rutile crystal (rutile TiO ２ Specifically, rutile-type titanium oxide crystals belong to the tetragonal crystal system, and P4 ２ / mnm (space group 136 of the International Tables for Crystallography (also simply referred to as International Table(s))), and has the crystal parameters and atomic coordinate positions shown in Table 1.

[0025]

[0026] The lattice constant of rutile titanium oxide crystals changes when its constituent elements are replaced by other elements or when optional constituent elements are dissolved in solid solution. However, the atomic positions given by the crystal structure, the sites occupied by the atoms, and their coordinates do not change so much that the chemical bonds between the skeletal atoms are broken.

[0027] [Method for Producing Crystalline Titanium Oxide] Next, a method for producing the above-mentioned rutile-type titanium oxide crystal will be described.

[0028] A method for producing rutile-type titanium oxide crystals according to one embodiment of the present invention involves growing crystals by hydrothermal synthesis from a raw material containing at least titanium (Ti) in the presence of a mineralizer containing an alkali metal and / or alkaline earth metal, or an acid mineralizer, or a mixed mineralizer of both. Here, the term "hydrothermal synthesis" refers to an embodiment in which water is used as a solvent in a "solvothermal method." An example of this production method is described below.

[0029] <Step S110: Step of Preparing Raw Materials> In step S110, raw materials necessary for obtaining a compound that satisfies the composition of the target crystal are prepared.

[0030] Specifically, a raw material containing at least Ti is prepared. The raw material is preferably an oxide. For convenience, in this specification, the raw materials containing each constituent element prepared in step S110 will also be referred to as the raw materials of each constituent element. For example, "a raw material containing Ti" and "a raw material of Ti" are synonymous.

[0031] As each raw material, a simple substance of the constituent element, an oxide, a hydroxide, a halide, an inorganic salt (sulfate, nitrate, carbonate, etc.), an organic salt (acetate, etc.), etc. can be used. Each compound other than the simple substance may be an anhydride or a hydrate. Below, non-limiting examples of usable raw materials for each constituent element that can be contained in the rutile-type titanium oxide crystal of the present invention are shown.

[0032] Examples of raw materials for Ti include TiO ２ As the raw material for the optional constituent elements, as described above, the elemental substance, oxide, halide, etc. of the constituent element may be used, or the element may be used as a mineralizer, which will be described later. Furthermore, the raw material may contain two or more of each of the elements exemplified above.

[0033] In one embodiment, the raw material is TiO ２ Here, if necessary, a raw material mixture containing each raw material may be prepared.

[0034] <Step S120: Step of growing crystals from raw materials by hydrothermal synthesis in the presence of a mineralizer> In step S120, the raw materials prepared in step S110 (or a raw material mixture prepared by mixing the respective raw materials) are grown into crystals by hydrothermal synthesis in the presence of a mineralizer.

[0035] As the mineralizer, a mineralizer containing an alkali metal and / or alkaline earth metal, an acid mineralizer, or a mixture of both can be used. The mineralizer containing an alkali metal and / or alkaline earth metal is a compound containing an alkali metal and / or alkaline earth metal, and for example, halides (fluorides, chlorides, bromides, iodides, etc.) and inorganic salts (carbonates, etc.) of alkali metals and / or alkaline earth metals can be used. Specific examples of the mineralizer include KF, K ２ CO ３ , RbF, Rb ２ CO ３ , CsF, Cs ２ CO ３ , CaF ２ , CaCO ３ , SrF２ , SrCO ３ , BaF ２ , BaCO ３ and the like, and it is preferable to select at least one from the group consisting of these compounds, but the present invention is not limited to these. For example, at least one from the group consisting of the above compounds may be selected as the main mineralizer, and an alkali metal and / or alkaline earth metal hydroxide (e.g., KOH) or the like may be used in combination as a compound (auxiliary mineralizer) that complements the action of the mineralizer. The alkali metal and / or alkaline earth metal and / or halogen element contained in the mineralizer may be contained in the finally obtained rutile-type titanium dioxide crystals. Examples of acid mineralizers include, but are not limited to, hydrochloric acid, nitric acid, sulfuric acid, formic acid, and phosphoric acid. The elements contained in the acid mineralizer may be contained in the finally obtained rutile-type titanium dioxide crystals.

[0036] The method for preparing the solution (reaction solution) used for crystal growth by hydrothermal synthesis is not particularly limited. For example, when the raw material mixture is prepared in the above-mentioned step S110, a solution of a mineralizer may be added to the raw material mixture, and further mixing may be performed as needed. Alternatively, a mineralizer (preferably in powder or tablet form) may be added to the raw material mixture, and further mixing may be performed as needed. Alternatively, the above-mentioned raw materials may be added to an aqueous solution of the mineralizer and appropriately mixed. Here, the concentration of the mineralizer in the finally prepared solution is preferably in the range of 1 M to 50 M, more preferably in the range of 2 M to 45 M, even more preferably in the range of 3 M to 40 M, and particularly preferably in the range of 5 M to 30 M. Within the above concentration range, the upper limit may be less than 30 M. In exemplary embodiments, the concentration of the mineralizer may range from 1M to less than 30M, from 1M to 25M, from 1M to 20M, from 1M to 15M, from 2M to 15M, or from 2M to 12M.

[0037] The crystal growth conditions (specifically, temperature conditions) for hydrothermal synthesis can be adjusted depending on the type of alkali metal and / or alkaline earth metal contained in the mineralizer used and the concentration of the mineralizer. Although a certain degree of care is required when handling a highly alkaline solution, the desired solution can be prepared by appropriately adjusting the concentration of the mineralizer. It is also preferable to select the type of mineralizer depending on the raw materials used and / or the types of constituent elements of the target crystal.

[0038] The conditions for crystal growth using hydrothermal synthesis are not particularly limited and can be set depending on the size of the hydrothermal synthesis vessel (reaction vessel) used, etc. In an exemplary embodiment, the temperature condition is preferably set in the range of 400°C to 800°C. This ensures the production of the desired crystals. If the temperature is below 400°C, the desired crystals may not be produced, and if the temperature is above 800°C, the temperature may exceed the heat-resistant temperature of the reaction vessel. The pressure condition is preferably set in the range of 25 MPa to 250 MPa. In this context, the pressure condition means that the maximum pressure achieved during crystal growth is within the above range. The pressure condition varies depending on the amount of water contained in the reaction vessel, the size of the ampoule (sealed vessel) containing the reaction solution, the temperature conditions, etc. In an exemplary embodiment, the maximum pressure may be set in the range of 30 MPa to 250 MPa, 50 MPa to 225 MPa, or 75 MPa to 200 MPa.

[0039] The hydrothermal synthesis time can be adjusted appropriately from the viewpoint of completing crystal growth depending on the type and amount of raw materials used. Two or more temperature conditions may be set within the above-mentioned temperature range to set a predetermined temperature profile. Such a temperature profile can be designed taking into consideration the viewpoints of further improving the homogeneity and stability of the solution, and more efficiently producing the target crystals. An example of a specific temperature profile will be shown in the examples below.

[0040] In the manufacturing method of the present invention, crystals are grown by hydrothermal synthesis from raw materials (or raw material mixture) under temperature conditions that are significantly lower than those of the conventional methods described above. For example, in the CZ method, a phenomenon occurs in which a specific component evaporates from the (interface of) the melt during the crystal growth process (crystal growth process) due to the melting point of the target crystal (TiO ２ In the case of hydrothermal synthesis, decomposition and evaporation are likely to occur, and such decomposition and evaporation may also occur in the grown crystal, which may result in defects in the grown crystal. However, the manufacturing method of the present invention is less likely to cause such decomposition and evaporation, and therefore it is possible to suppress defects that may occur in the grown crystal. Furthermore, in the case of hydrothermal synthesis, the obtained crystal is generally obtained with the most stable composition, and although there is a possibility that this composition may deviate slightly from the target composition, even if such a deviation occurs, it is considered that the difference from the target composition is very small and can be adjusted.

[0041] Furthermore, in the production method of the present invention, seed crystals may be used during crystal growth by hydrothermal synthesis. Specifically, for example, seed crystals having a specific crystal orientation may be prepared and placed so that the seed crystals are immersed in the reaction solution described above, and crystal growth may be carried out. This allows rutile-type titanium dioxide crystals having the desired crystal orientation to be grown efficiently.

[0042] <Step S130: Heat-treating the crystal> The manufacturing method according to this embodiment may further include a step of heat-treating (annealing) the crystal obtained in step S120, as needed. This may make it possible to control defects that may inevitably occur during the crystal growth process. It may also make it possible to improve the properties (optical properties, etc.) of the rutile-type titanium dioxide crystal that are suitable for the intended use.

[0043] In other words, when rutile-type titanium dioxide crystals are used as materials for optical components such as prisms and polarizers, it may be preferable to subject them to heat treatment, but heat treatment is not an essential step in the manufacturing method of the crystals themselves.

[0044] The atmosphere during the heat treatment may be an inert gas atmosphere or an air atmosphere. In particular, in the latter case, the presence of oxygen in the atmosphere can eliminate or reduce oxygen defects that inevitably occur during the crystal growth process. Therefore, instead of an air atmosphere, an oxygen-containing atmosphere containing a gas adjusted to a certain oxygen content (oxygen partial pressure) may be used. In the former case, the inert gas may be nitrogen gas, helium gas, neon gas, argon gas, or the like. Generally, inert gases are used as gases with a low content of reactive gases such as oxygen and carbon dioxide (or no reactive gases). However, when a significant amount of oxygen remains in the inert gas, or when there is a possibility of trace oxygen leaking from materials used in components of the heat treatment apparatus (e.g., alumina ceramics constituting the furnace tube of a heat treatment furnace), the inert gas may be used to eliminate or reduce oxygen defects that inevitably occur during the crystal growth process, similar to the air atmosphere.

[0045] The heat treatment temperature is set to a range of 100°C or higher and lower than the melting point of the target rutile-type titanium oxide crystal. If the heat treatment temperature is lower than 100°C, the effects of removing or reducing oxygen defects described above may not be sufficiently achieved. Furthermore, if the heat treatment temperature exceeds the melting point of the crystal, unintended defects may occur in the crystal structure. From the viewpoint of more reliably achieving the above-mentioned effects, the heat treatment temperature is preferably 500°C or higher, more preferably 750°C or higher, even more preferably 800°C or higher, even more preferably 900°C or higher, and most preferably 1000°C or higher. As for the upper limit of the heat treatment temperature, a temperature about 10°C to about 20°C lower than the melting point of the target crystal can more reliably avoid the unintended effects on the crystal structure described above. In practice, taking into account crystal productivity, etc., the heat treatment temperature is preferably 1800°C or lower, more preferably 1700°C or lower, and even more preferably 1600°C or lower.

[0046] The heat treatment time is preferably 0.5 hours or more. If the above-mentioned atmosphere and temperature conditions are met, it is expected that the above-mentioned effects of removing or reducing oxygen defects can be obtained by heat treatment for 0.5 hours or more. The heat treatment time may be 1 hour or more, 5 hours or more, 10 hours or more, or 30 hours or more. On the other hand, even if heat treatment is performed for a longer period than necessary, it is difficult to obtain an effect that correlates only with the time, so the upper limit of the heat treatment time is set at 150 hours or less as a rough guideline.

[0047] [Uses of Crystals] Next, uses of the rutile-type titanium oxide crystals of the present invention will be described.

[0048] The use of the rutile-type titanium dioxide crystal of the present invention is not particularly limited, but it is suitable for use as a material for optical components such as the above-mentioned prisms and polarizers. Here, in the context of its use as a polarizer material, the rutile-type titanium dioxide crystal of the present invention is intended to be a birefringent crystal. That is, in one aspect of the technical field, a distinction is sometimes made between a polarizer for an optical component and a birefringent crystal, which means that the crystal used as an optical component has birefringence. However, it should be noted that in this specification, the term "polarizer" in a broader sense can also include cases where the material constituting the polarizer is a birefringent crystal.

[0049] In one exemplary embodiment, the rutile titanium oxide crystal of the present invention can be used as a birefringent crystal as a polarizer to form an optical isolator.

[0050] <Optical Isolator> FIG. 1 is a schematic diagram showing one embodiment of an optical isolator according to the present invention. As shown in FIG. 1, the optical isolator 100 includes polarizers 110 (110a, 110b), a Faraday rotator 120, and a half-wave plate 130. The optical isolator 100 according to this embodiment is a polarization-independent optical isolator, characterized by its ability to be used regardless of the polarization state of incident light. Here, the polarizers 110a, 110b are made of birefringent crystals made of rutile-type titanium dioxide crystals according to the present invention. However, the configuration of the optical isolator is not limited to the embodiment shown in FIG. 1, and various design modifications are possible, such as a configuration without a wave plate.

[0051] In the optical isolator 100 shown in FIG. 1, when light L indicated by a right-pointing arrow enters the polarizer 110a, the incident light is separated into ordinary light and extraordinary light whose polarization planes differ by 90° (birefringence). These polarized light beams are rotated 45° by the Faraday rotator 120 and then another 45° by the half-wave plate 130 (i.e., a total of 90° rotation), and the ordinary light beam becomes extraordinary light, and the extraordinary light beam becomes ordinary light. These polarized light beams then enter the polarizer 110b. As a result, the light beams are combined by the polarizer 110b and output as a single beam. Meanwhile, the light beams traveling in the opposite direction are separated into ordinary light and extraordinary light by the polarizer 110b and pass through the half-wave plate 130 and the Faraday rotator 120, just as in the case described above. However, unlike the case described above, the polarization planes remain unchanged, and the light beams are output without being combined by the polarizer 110a. The combination of the rotation angle generated by the Faraday rotator and the wave plate is an example, and is not limited to the above embodiment.

[0052] The optical isolator 100 having such a configuration can be suitably used, for example, in optical communication devices or optical communication lines. Examples of optical communication devices include, but are not limited to, optical amplifiers (optical amplifiers) that constitute repeaters. Examples of optical communication lines include, but are not limited to, optical submarine cables and optical cables for terrestrial communication. Furthermore, the applications of the optical isolator of the present invention are not limited to the above-mentioned optical communication devices and optical communication lines, and it can also be used in the field of laser technology, such as laser processing machines (also called optical processing machines).

[0053] The present invention will be explained in more detail below based on examples, but the present invention is not limited to these examples.

[0054] [Production of Crystals] <Example 1> Commercially available TiO ２ (powder, manufactured by Furuuchi Chemical Co., Ltd.) was prepared. ２CsF (powder equivalent to a 2M aqueous solution) as a mineralizer and pure water (0.7 mL) were placed in a silver ampoule (diameter 5 mm x length 10 cm) and sealed. The sealed ampoule was then placed in a reaction vessel containing pure water for hydrothermal synthesis. The hydrothermal synthesis conditions were as follows: the temperature was raised to 550°C over 12 hours, and then further raised to 700°C over 150 hours without a holding time, completing the crystal growth. The maximum pressure reached during this period was 182 MPa. The temperature reduction program from 700°C to room temperature at the end of crystal growth was set to 1 hour, followed by natural cooling. This yielded the crystals of Example 1.

[0055] Example 2 The crystals of Example 2 were obtained under the same conditions as in Example 1, except that the amount of CsF used as a mineralizer was changed to an amount equivalent to a 5 M aqueous solution. The maximum pressure reached during crystal growth by hydrothermal synthesis was 158 MPa.

[0056] Example 3 The crystals of Example 3 were obtained under the same conditions as in Example 1, except that the amount of CsF used as a mineralizer was changed to an amount equivalent to a 10 M aqueous solution. The maximum pressure reached during crystal growth by hydrothermal synthesis was 132 MPa.

[0057] Example 4 The crystals of Example 4 were obtained under the same conditions as in Example 1, except that the amount of CsF used as a mineralizer was changed to an amount equivalent to a 20 M aqueous solution. The maximum pressure reached during crystal growth by hydrothermal synthesis was 177 MPa.

[0058] Example 5: Instead of CsF, RbF (powder in an amount equivalent to a 10 M aqueous solution) was used as the mineralizer, and the hydrothermal synthesis conditions were a temperature profile in which the temperature was raised to 730°C over 12 hours, maintained at that temperature for 100 hours, and then crystal growth was completed. The maximum pressure reached during this period was 178 MPa. The temperature was lowered from 730°C to room temperature at the end of crystal growth over a 12-hour period, and the crystal was then allowed to cool naturally. The remaining conditions were the same as in Example 1, and the crystals of Example 5 were obtained.

[0059] <Example 6> KF (powder in an amount equivalent to a 2M aqueous solution) was used instead of CsF as the mineralizer, and the hydrothermal synthesis conditions were a temperature profile in which the temperature was raised to 730°C over 12 hours, maintained at that temperature for 100 hours, and then crystal growth was completed. The maximum pressure reached in this case was 185 MPa. The temperature reduction program from 730°C to room temperature at the end of crystal growth was set to 12 hours, and then natural cooling was allowed. The other conditions were the same as in Example 1, and the crystals of Example 6 were obtained.

[0060] Example 7 A thin plate-like crystal having a roughly rectangular shape and a (110) plane (one cross-sectional surface of which is a (001) plane) was prepared as a seed crystal, and the seed crystal was placed in the reaction solution in an ampoule using a silver wire so that it was immersed in the solution. The remaining conditions were the same as in Example 5, and the crystal of Example 7 was obtained. The maximum pressure reached during hydrothermal synthesis was 127 MPa.

[0061] Tables 2-1 and 2-2 show the production conditions for the crystals of Examples 1 to 4 and Examples 5 to 7, respectively.

[0062]

[0063] [Measurement and Evaluation] <Microscopic Observation> The crystals obtained in Examples 1 to 7 above were observed under an optical microscope. Figures 2(a) to 2(c) show microscopic images of the crystals of Examples 2, 5, and 6 as representative examples. The scale bar in each figure is 1.0 mm. Figure 3 shows a microscopic image of the crystals of Example 7. The scale bar in the figure is 2.0 mm.

[0064] 2(a) to (c) and 3, it was confirmed that bulk crystals could be obtained by hydrothermal synthesis using the method of the present invention. Furthermore, it was confirmed from 3 that seed crystals could be used in growing crystals by hydrothermal synthesis in the method of the present invention. Although not shown, bulk crystals were also confirmed in microscope images of the crystals of Examples 1, 3, and 4.

[0065] Here, a sample was prepared by subjecting the crystal of Example 2 to heat treatment (air atmosphere, 1500°C, 24 to 48 hours), and the sample was observed under an optical microscope. The results are shown in Figure 4. For convenience, the heat-treated sample will be referred to as "crystal of Example 2A" below, and will be identified by adding the letter A after the number.

[0066] 4, (a) is the crystal of Example 2 before heat treatment, (b) is the crystal of Example 2A obtained by 24 hours of heat treatment, and (c) is the crystal of Example 2A obtained by 48 hours of heat treatment. The scale bar in the figure is 0.5 mm for (a) and 1.0 mm for (b) and (c).

[0067] As shown in Figure 4, the crystals before heat treatment had a slightly bluish appearance, but the transparency improved after heat treatment. More specifically, after 24 hours of heat treatment, the right-hand sample of the three crystals shown in (b) became almost transparent, while the left-hand and middle samples only exhibited a slight brownish color near the center of the crystal. Furthermore, after 48 hours of heat treatment, the three crystals shown in (c) were almost transparent throughout. Although the orientation of the three crystals in (b) and (c) differs, they are all the same sample. These results indicate that heat treatment of crystals grown by hydrothermal synthesis using the method of the present invention in an oxygen-containing atmosphere under specified temperature conditions can at least eliminate or reduce oxygen defects that inevitably occur during the crystal growth process. This suggests that this effect can be achieved more efficiently by adjusting the heat treatment time depending on the crystal size, etc.

[0068] <X-ray Diffraction Measurement> Powder samples were prepared from the crystals obtained in Examples 1 to 7 above, and powder X-ray diffraction measurements were carried out.

[0069] The upper part of Fig. 5 shows the results obtained for the crystals of Example 2 as a representative example. The middle part of Fig. 5 shows the results obtained for the raw material TiO ２ The graph shows the measurement results of the diffraction chart of rutile crystal (tetragonal system) in the lower part.

[0070] According to FIG. 5, the diffraction chart of the crystal of Example 2 is ２ The pattern was completely different from that of Example 1, and it was found that the peak positions (angle 2θ) and intensities (normalized intensity) matched extremely well with the diffraction chart obtained by the simulation. For ease of understanding, the powder X-ray diffraction measurement results for the crystals of Example 2 and the simulation results for the diffraction chart of the rutile crystal are shown in the upper and lower rows of Figure 6, respectively. Furthermore, the upper rows of Figures 7 and 8 show the results obtained for the crystals of Examples 5 and 6, respectively, and the lower rows of each figure show the simulation results for the diffraction chart of the rutile crystal. From these results, it can be seen that the obtained crystals are tetragonal TiO ２ Although not shown, the crystals of Examples 1, 3, 4 and 7 also had a tetragonal TiO ２ It was confirmed that the crystalline phase was

[0071] <Secondary Ion Mass Spectrometry> The hydrogen (H), fluorine (F), and cesium (Cs) concentrations of the crystals of Example 2 were measured using a secondary ion mass spectrometer (IMS-6F, manufactured by Cameca) and a time-of-flight secondary ion mass spectrometer (TOF-SIMS, manufactured by IonTOF), respectively. The results are shown in FIG.

[0072] According to FIG. 9, the hydrogen concentration in the obtained crystal was about 1×10 at a depth of 3 μm. １８ atoms / cm ３ and 1×10 １９ atoms / cm ３ The following is satisfied, and the fluorine concentration is about 2×10 １８ atoms / cm ３ and 1×10 １９ atoms / cm ３ The following is satisfied, and the cesium concentration is 1 x 10 at a depth of 3 μm. １５ atoms / cm ３ is less than 1×10 １６ atoms / cm ３It was found that the following was satisfied. It was also confirmed that the oxygen concentration ratio was nearly identical to the theoretical composition. Although not shown, similar component analysis was also performed on the crystals of Example 1 and Examples 3 to 7, and results similar to those of the crystals of Example 2 were obtained (however, in Examples 5 to 7, the cesium concentration should be read as the rubidium or potassium concentration).

[0073] As described above, the present invention can provide rutile-type titanium oxide crystals in which defects such as oxygen defects are suppressed and which are suitable as materials for optical components such as prisms and polarizers. In particular, the rutile-type titanium oxide crystals can be provided as large single crystals. The present invention also provides a method for producing the above-mentioned rutile-type titanium oxide crystals under milder conditions than conventional methods, particularly at lower temperatures. Furthermore, since the production method of the present invention can use a seed crystal, it is possible not only to grow larger crystals but also to improve their quality by controlling the crystal orientation, etc. The present invention also provides optical components using the above-mentioned rutile-type titanium oxide crystals, which can be suitably used in various devices, etc.

[0074] 100 Optical isolator 110 (110a, 110b) Polarizer (birefringent crystal) 120 Faraday rotator 130 1 / 2 wavelength plate L Light

Claims

1. Rutile TiO containing at least titanium (Ti), oxygen (O) and hydrogen (H) ２ Rutile-type titanium oxide crystals having substantially the same crystal structure as the rutile-type titanium oxide crystals.

2. The crystal composition is of the general formula TiO ２－ｘ (wherein, -0.5≦x≦0.5).

3. The concentration of hydrogen is 1 x 10 １５ atoms / cm ３ 1x10 or more ２２ atoms / cm ３ The crystal according to claim 1 or 2, which has the following range:

4. The crystal according to any one of claims 1 to 3, further containing an alkali metal and / or an alkaline earth metal.

5. The concentration of the alkali metal and / or alkaline earth metal is 1×10 １２ atoms / cm ３ 1x10 or more ２０ atoms / cm ３ The crystal according to claim 4, wherein the crystal has a molecular weight of 1000 or more and a molecular weight of 1000 or more.

6. The crystal according to any one of claims 1 to 5, further containing at least one halogen element selected from the group consisting of fluorine (F), chlorine (Cl), bromine (Br) and iodine (I).

7. The crystal according to claim 6, wherein the halogen element includes at least F.

8. The crystal according to claim 7, wherein the halogen element is F.

9. The concentration of the halogen element is 1 x 10 １５ atoms / cm ３ 1x10 or more ２３ atoms / cm ３ The crystal according to any one of claims 6 to 8, wherein the crystal has the following range:

10. A method for producing the crystals according to any one of claims 1 to 9, comprising growing crystals by hydrothermal synthesis from raw materials containing the elements to be contained in the crystals in the presence of a mineralizer containing an alkali metal and / or an alkaline earth metal, or an acid mineralizer, or a mixed mineralizer of both, and optionally further comprising heat-treating the grown crystals.

11. The method of claim 10, wherein the concentration of the mineralizer ranges from 1M to 50M.

12. The method according to claim 10 or 11, wherein in growing the crystal by hydrothermal synthesis, the temperature is in the range of 400°C to 800°C, and the maximum pressure is in the range of 25 MPa to 250 MPa.

13. The method according to any one of claims 10 to 12, wherein a seed crystal is used in growing the crystal by the hydrothermal synthesis method.

14. An optical component using the crystal according to any one of claims 1 to 9.

15. The optical component according to claim 14, which is a prism, a birefringent crystal or a polarizer.

16. An optical isolator comprising a birefringent crystal or polarizer made of the crystal according to any one of claims 1 to 9.

17. The optical isolator of claim 16, which is polarization independent.

18. An optical communication device, an optical communication line, or a laser processing machine, comprising the optical isolator according to claim 16 or 17.

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