Method for producing perovskite metal oxynitride

Abstract

The method for producing a perovskite metal oxynitride of the present invention, comprises: a hydrogenation step (A) of forming a perovskite oxyhydride in which an oxide ion (O2) and a hydride ion (H) coexist, by reducing a perovskite oxide through a reductive oxygen elimination reaction using a metal hydride; and a nitriding step (B) of forming a perovskite oxynitride containing a nitride ion (N3) by heat-treating the perovskite oxyhydride in the presence of a nitrogen source substance at a temperature of 300 C. or higher and 600 C. or lower and exchanging the hydride ion (H) for a nitride ion (N3).

Claims

1. A method for producing a perovskite metal oxynitride, comprising: a hydrogenation step (A) of forming a perovskite oxyhydride in which an oxide ion (O.sup.2) and a hydride ion (H.sup.) coexist, by reducing a perovskite oxide through a reductive oxygen elimination reaction using a metal hydride; and a nitriding step (B) of forming a perovskite oxynitride containing a nitride ion (N.sup.3) by heat-treating the perovskite oxyhydride in the presence of a nitrogen source substance at a temperature of 300 C. or higher and 600 C. or lower and exchanging the hydride ion (H.sup.) for a nitride ion (N.sup.3).

2. The method for producing a perovskite metal oxynitride according to claim 1 wherein the nitrogen source substance in the nitriding step (B) is an ammonia gas flow.

3. The method for producing a perovskite metal oxynitride according to claim 1 wherein the nitrogen source substance in the nitriding step (B) is an ammonia gas-generating agent.

4. The method for producing a perovskite metal oxynitride according to claim 1 wherein the nitrogen source substance in the nitriding step (B) is a nitrogen gas flow.

5. The method for producing a perovskite metal oxynitride according to claim 1 wherein the perovskite oxide is a compound represented by the formula A.sub.n+1B.sub.nO.sub.3n+1 (in the formula, n is any one of 1, 2, 3 and , A is at least one of Ca, Ba, Sr, Pb and Mg, and B is at least one of Co, W, Mo, V, Ta, Zr, Nb, Ti and Hf), and the perovskite oxyhydride is a compound represented by the formula A.sub.n+1B.sub.n(O.sub.1-xH.sub.x).sub.3n+1 (in the formula, A and B are the same as those of the starting substance, H is a hydride ion (H.sup.) which has substituted an oxide ion, 0.01x0.2, and n is any one of 1, 2, 3 and ).

6. The method for producing a perovskite metal oxynitride according to claim 1 wherein the forms of the perovskite oxide, the perovskite oxyhydride and the perovskite metal oxynitride are powder or a thin film.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 The powder X-ray diffraction patterns of the nitride ion-containing barium titanates obtained using an ammonia gas flow method as the low-temperature ammonia treatment in Example 1. The temperature in the figure is the reaction temperature during the ammonia gas treatment.

(2) FIG. 2 A graph showing the results of the Rietveld structure analysis of the neutron diffraction pattern of the nitride ion-containing barium titanate (NH.sub.3 flow 500 C.) obtained in Example 1 (FIG. 2A) and a schematic view of the crystal structure obtained by Rietveld structure analysis (FIG. 2B).

(3) FIG. 3 The upper figure is a graph obtained by plotting the lattice constant (a-axis and c-axis) against the nitrogen content y of the BaTi(O.sub.3-zH.sub.xN.sub.y) (here, zx+y, and z-x-y indicates the amount of oxygen defects) obtained in Example 1. The lower figure is a graph showing the relation between the hydrogen content (x) and the nitrogen content (y) of the same samples.

(4) FIG. 4 A graph showing the results of the XPS measurement of the single crystal thin film after the nitriding treatment in Example 2.

(5) FIG. 5 The powder X-ray diffraction pattern of the nitride ion-containing barium titanate obtained in Example 3.

(6) FIG. 6 The powder X-ray diffraction pattern of the nitride ion-containing barium titanate obtained in Example 4.

DESCRIPTION OF EMBODIMENTS

Preparation of Oxyhydride

(7) A perovskite structure is one of the crystal structures of compounds represented by the chemical formula ABX.sub.3, and a perovskite metal oxide is represented by the chemical formula ABO.sub.3. The kind of the starting substance perovskite metal oxide is not limited to particular substances, but typical examples thereof are compounds represented by the formula ABO.sub.3, A.sub.2BO.sub.4, A.sub.3B.sub.2O.sub.7 or A.sub.4B.sub.3O.sub.10. The oxides are together represented by the following general formula.
A.sub.n+1B.sub.nO.sub.3n+1(Formula I)
(in the formula, n is any one of 1, 2, 3 and )

(8) In the case of n=1, the formula is A.sub.2BO.sub.4, and in the case of n=2, the formula is A.sub.3B.sub.2O.sub.7. In the case of n=3, the formula is A.sub.4B.sub.3O.sub.10, and in the case of n=, the formula is ABO.sub.3. In this regard, at least one element component of A, B and O may include up to 20 at. % of defects.

(9) Although A in the general formula above is typically at least one of Ca, Ba, Sr, Pb and Mg, A is not limited to these divalent cations and may be a solid solution containing a cation having a different valence, such as La and Na, or a defect. Also, B in the general formula above is at least one of Co, W, Mo, V, Ta, Zr, Nb, Ti and Hf.

(10) The forms of the starting substance perovskite metal oxide, the perovskite oxyhydride obtained and the perovskite metal oxynitride obtained are preferably powder or a thin film.

(11) A metal hydride such as CaH.sub.2, LiH, BaH.sub.2, SrH.sub.2 or MgH.sub.2 is used to extract some of the oxide ions contained in the starting substance and to substitute the oxide ions with hydride ions (H.sup.). Such reaction is called as reductive oxygen elimination reaction. It is believed that the oxide ions can be substituted with hydride ions because CaH.sub.2, LiH, BaH.sub.2, SrH.sub.2 or MgH.sub.2 exerts potent reducing power also at a low temperature of 600 C. or lower and has not only the capability of extracting oxygen from the oxide but also the capability of supplying hydride ions. Also, because the substitution reaction occurs at a relatively low temperature, deoxidation reaction and insertion reaction of a large amount of hydride ions can be topochemically achieved at the same time, without breaking the structural skeleton of the starting substance, and thus the production is easy.

(12) The perovskite oxide-hydride obtained by the above method is a substance in which up to about 20 at. % of oxygen in the oxygen sites has been substituted with hydrogen. Because a part of or all of the hydride ions can be exchanged for nitride ions, the amount of oxygen substituted may be 1 at. % or more, and 5 at. % or more, further preferably 10 at. % or more can be substituted to exchange for a large amount of nitride ions.

(13) That is, the perovskite oxide-hydride obtained by the above method can be represented by the following basic formula II.
A.sub.n+1B.sub.n(O.sub.1-xH.sub.x).sub.3n+1(Formula II)
(in the formula, A and B are the same as those of the starting substance, H is a hydride ion (H.sup.) which has substituted an oxide ion, 0.01x0.2, and n is any one of 1, 2, 3 and .)

(14) In this regard, at least one element component of A, B and O may include up to 20 at. % of defects.

(15) The hydrogen subjected to the substitution randomly (statistically) occupies oxygen sites. However, by controlling any of the factors determining the degree of hydridization, a gradient can be provided to the hydrogen concentration distribution from the surface of the powder or the thin film of the perovskite oxide-hydride obtained to its center.

Ion Exchange Process

(16) The exchange reaction of the hydride ions (H.sup.) contained in the metal oxyhydride A.sub.n+1B.sub.n(O.sub.1-xH.sub.x).sub.3n+1 obtained for nitride ions (N.sup.3) can be conducted by a heat treatment method using any of the following nitrogen source substances. (1) An ammonia gas flow. (2) An ammonia gas-generating agent, such as urea, which generates ammonia by thermal decomposition, or mixed powder of sodium amide (NaNH.sub.2) powder and ammonium chloride (NH.sub.4Cl) powder, which generates ammonia by heat reaction. (3) A nitrogen gas flow.

(17) Many reports on the synthesis of a nitride or an oxynitride using thermal decomposition of ammonia gas have been published, but reaction at a high temperature is required. In the invention, however, in the method of (1) above, powder or a thin film of the metal oxyhydride is exposed to an ammonia gas flow and heat-treated at 300 C. or higher and 600 C. or lower for a short time (generally about three hours). Thus, the ammonia gas is thermally decomposed into H and N, and a part of or all of the hydride ions in the oxyhydride are exchanged for nitride ions.

(18) The exchange reaction by heat treatment at such a low temperature is based on the phenomenon where hydride ions are weakly bound to B metal ions in the formula II in the metal oxyhydride and easily diffuse in the crystal. As the temperature of the heat treatment with ammonia gas becomes higher, the nitriding reaction of the metal oxyhydride advances faster. Also, the degree of exchange can be controlled also by extending the reaction period. That is, by adjusting the heat treatment temperature and the period, the amount of the nitride ions to be introduced to the metal oxyhydride can be controlled at will.

(19) In the method of (2) above, the ammonia gas-generating agent, such as NaNH.sub.2 and NH.sub.4Cl, causes the following chemical reaction when heated (200 C. or higher), generates ammonia gas and generates sodium chloride (NaCl).
NaNH.sub.2+NH.sub.4Cl.fwdarw.2NH.sub.3(gas)+NaCl(solid)

(20) Accordingly, when powder or a thin film of the metal oxyhydride is mixed with NaNH.sub.2 powder and NH.sub.4Cl powder and the mixture is heat-treated, ion exchange reaction of H for N through decomposition of ammonia gas can be caused as in the method of (1).

(21) With respect to the atmosphere containing the ammonia gas-generating agent, such as mixed powder of NaNH.sub.2 and NH.sub.4Cl, a mixed powder of NaNH.sub.2 and NH.sub.4Cl and powder or a thin film of the metal oxyhydride are preferably simultaneously vacuum-sealed in a heat and chemical resistant container such as a container of quartz glass. It is not necessary to bring NaNH.sub.2 and NH.sub.4Cl into contact with the metal oxyhydride. A low temperature of 300 C. or higher and 600 C. or lower is sufficient as the heat treatment temperature as in the method of (1), and the reaction period required is 12 hours to 24 hours. The atmosphere in the container may be an inert atmosphere such as argon or nitrogen gas.

(22) In the method of (3) above, nitrogen gas is used as the nitrogen source substance for the ion exchange reaction of H for N. By exposing powder or a thin film of the metal oxyhydride to a nitrogen gas flow and heat-treating the powder or the thin film at a low temperature of 300 C. or higher and 600 C. or lower for several hours, hydride ions are exchanged with nitride ions.

(23) A titanium-containing perovskite oxide which is represented by the above formula (I) in which B is Ti exhibits excellent electrical properties such as dielectric property, piezoelectric property and pyroelectric properties and thus has been studied for a long time in view of the application to various electronic materials. In addition to the properties, because titanium is a low cost material and has high biocompatibility, titanium is attractive as a constituent element of an oxynitride.

(24) The method for producing a barium titanate oxynitride using, as the starting material, barium titanate (BaTiO.sub.3), which is a typical example of the perovskite metal oxide represented by the formula ABO.sub.3, is explained specifically below using Examples. One skilled in the art would easily understand that the low-temperature ammonia treatment method of the invention can be applied in principle not only to barium titanate oxynitrides but also to all the metal oxynitrides with known compositions regardless of the difference in the difficulty of nitriding degree and that the creation of a novel metal oxynitride is also possible by applying the method of the invention.

Example 1

Hydrogenation Step (A)

(25) Barium titanate (BaTiO.sub.3) powder having a particle size of about 100 nm was dried at 100 C. in a vacuum atmosphere and mixed with three equivalents of calcium hydride (CaH.sub.2) powder in a glove box, and the mixture was pressed into pellets using a hand press. The pellets were put into a quartz tube having an internal volume of about 15 cm.sup.3, vacuum-sealed and then hydrogenated by heat-treating the pellets at 580 C. for 150 hours. The sample after the heat treatment was treated with a 0.1 M NH.sub.4Cl methanol solution to remove unreacted CaH.sub.2 and by-product CaO that were attached to the product.

(26) The powder obtained was dark blue close to black. It was found by powder X-ray diffraction and powder neutron diffraction that the perovskite crystal structure was maintained. It was confirmed by Rietveld analysis and thermal desorption spectroscopy (TDS) that the composition was BaTi(O.sub.0.8H.sub.0.2).sub.3.

Nitriding Step (B)

(27) The BaTi(O.sub.0.8H.sub.0.2).sub.3 powder was dried at 100 C. in a vacuum atmosphere and then pressed into pellets using a hand press. The pellets were put into a quartz tube having an internal diameter of about 3 cm and heat-treated in an ammonia gas flow (300 mL per minute) at 375 C., 400 C., 425 C. or 500 C. for three hours, and nitriding reaction was thus conducted at atmospheric pressure.

(28) The hydrogen amounts of the samples obtained were measured by thermal desorption spectroscopy (TDS), and the nitrogen amounts thereof were measured by trace element analysis, thereby determining the compositions. As a result, it was confirmed that the samples had the compositions containing hydride ions (H.sup.) and nitride ions (N.sup.3) as shown in Table 1 ( indicates a defect which was not occupied by any of O, H and N).

(29) TABLE-US-00001 TABLE 1 Sample Heat Treatment No. Temperature ( C. ) Composition a 375 BaTi(O.sub.0.8H.sub.0.13N.sub.0.03.sub.0.04).sub.3 b 400 BaTi(O.sub.0.8H.sub.0.06N.sub.0.08.sub.0.06).sub.3 c 425 BaTi(O.sub.0.8H.sub.0.01N.sub.0.12.sub.0.07).sub.3 d 500 BaTi(O.sub.0.8N.sub.0.13 .sub.0.07).sub.3

(30) A sample after the ammonia gas treatment is blue when the nitrogen amount is low, and the color becomes green as the nitrogen amount increases. It was found by powder X-ray diffraction and powder neutron diffraction that the samples obtained maintained the perovskite crystal structure.

(31) FIG. 1 shows the powder X-ray diffraction patterns of the nitride ion-containing barium titanates obtained at the respective heat treatment temperatures. FIG. 2 shows the crystal structure determined by Rietveld analysis of the powder neutron diffraction pattern of the sample No. d, which was heat-treated at 500 C. It was confirmed that in the sample after the heat treatment, all the hydride ions introduced were substituted with nitride ions. In this regard, it can be seen that N has exchanged at a lower ratio than H because BaTiO.sub.2.4N.sub.0.39 was generated from BaTiO.sub.2.4H.sub.0.6.

(32) The upper figure of FIG. 3 shows the lattice constant (a-axis and c-axis) plotted against the nitrogen content (y) of the nitride ion-containing barium titanates obtained at the respective heat treatment temperatures. In FIG. 3, because the compositions are represented by BaTi(O.sub.3-zH.sub.xN.sub.y), the values of x and y are three times as much as the values of the compositions shown in Table 1.

(33) The samples before nitriding had an ideal cubic perovskite structure, but the samples after nitriding changed into a distorted tetragonal structure due to the introduction of nitride ions. This is similar to the phenomenon where the ferroelectric substance BaTiO.sub.3 has a tetragonal structure at room temperature.

(34) The lower figure of FIG. 3 shows the relation between the hydrogen content (x) and the nitrogen content (y) of the same samples. It can be seen that a larger amount of nitride ions can be introduced as the heat treatment temperature becomes higher and that the nitride ion content can be controlled with the temperature. In addition, it can be seen that almost all the hydride ions can be exchanged for nitride ions also at a low temperature.

Example 2

(35) A BaTiO.sub.3 single crystal thin film having an area of 1 cm1 cm and a thickness of 100 nm was deposited as a sample on an LSAT [(La.sub.0.3Sr.sub.0.7)(Al.sub.0.65Ta.sub.0.35)O.sub.3] substrate by the PLD method as described below. BaTiO.sub.3 pellets were used as the target. The temperature of the substrate was 700 C., and the oxygen pressure during the deposition was 0.05 Pa. A KrF excimer laser pulse (wavelength=248 nm) was employed as an excitation light source.

Hydrogenation Step (A)

(36) In a glove box filled with nitrogen, the obtained single crystal thin film and 0.2 g of CaH.sub.2 powder were vacuum-sealed in a Pyrex (registered trademark) tube and heat-treated at a temperature of 530 C. for one day to conduct the hydrogenation reaction. Unreacted CaH.sub.2 and by-product CaO that were attached to the product were removed by ultrasonic cleaning with acetone.

Nitriding Step (B)

(37) The precursor single crystal thin film thus obtained was heat-treated in an ammonia gas flow as in Example 1 to conduct nitriding reaction, and a BaTi(O, N, ).sub.3 single crystal thin film was thus obtained.

(38) It was found by X-ray diffraction that the sample obtained was a single crystal thin film which maintained the perovskite crystal structure. The electrical resistance of the single crystal thin film after hydrogenation treatment exhibits metallic temperature dependence. However, as the ion exchange reaction of hydrogen for nitrogen advances, the electrical resistance increases gradually, and a completely nitrided thin film is an insulator.

(39) FIG. 4 shows the results of the XPS measurement of the single crystal thin film after the nitriding treatment with the surface partially removed out by Xe sputtering. Because there was a peak derived from nitrogen at the binding energy of 400 eV, the presence of a sufficiently large amount of nitrogen was confirmed in the thin film. Reflecting the non-centrosymmetry, which is a necessary condition of a ferroelectric substance, the thin film exhibits second harmonic generation (SHG).

Example 3

Hydrogenation Step (A)

(40) BaTi(O.sub.0.8H.sub.0.2).sub.3 powder was prepared by a similar procedure to that of Example 1.

Nitriding Step (B)

(41) The obtained BaTi(O.sub.0.8H.sub.0.2).sub.3 powder (about 0.1 g) was dried at 100 C. in a vacuum atmosphere and then pressed into pellets using a hand press. A mixture of equivalent moles of NaNH.sub.2 and NH.sub.4Cl powder (about 0.06 g) as an ammonia-generating agent was pressed into pellets using a hand press. The pellets were put into a quartz tube having an internal volume of about 15 cm.sup.3 and vacuum-sealed, and then heat treatment was conducted at 530 C. for 12 hours.

(42) The sample after the heat treatment is green with the ion exchange. FIG. 5 shows the powder X-ray diffraction pattern of the sample. The sample obtained maintained the perovskite crystal structure and changed from a cubic structure (a=b=c=4.01 ) before the reaction to a tetragonal structure (a=b=4.00 , c=4.02 ) due to the introduction of nitride ions.

Example 4

Hydrogenation Step (A)

(43) BaTi(O.sub.0.8H.sub.0.2).sub.3 powder was prepared by a similar procedure to that of Example 1.

Nitriding Step (B)

(44) The obtained BaTi(O.sub.0.8H.sub.0.2).sub.3 powder (about 0.1 g) was dehydrated at 100 C. in a vacuum atmosphere and then pressed into pellets using a hand press. The pellets were put into a quartz tube having an internal diameter of about 3 cm, heated to 600 C. at 5 C. per minute in a nitrogen gas flow (110 mL per minute), kept at 600 C. for five minutes and then cooled to room temperature at 10 C. per minute to conduct heat treatment, and nitriding reaction was thus conducted.

(45) The sample after the heat treatment is green with the ion exchange. FIG. 6 shows the powder X-ray diffraction pattern of the sample. The sample obtained maintained the perovskite crystal structure and changed from a cubic structure (a=b=c=4.01 ) before the reaction to a tetragonal structure (a=4.00 , c=4.02 ) due to the introduction of nitride ions.

INDUSTRIAL APPLICABILITY

(46) The method for producing a perovskite metal oxynitride of the invention is a process at a lower temperature in a shorter time than those of the conventional methods. Thus, the method has a lower environmental load and is a safe, and advantages such as a decrease in the production cost are also expected. Moreover, the method is means which allows the development of a novel oxynitride which could not be produced by the conventional methods, and it is believed that the method can contribute to the further development in the fields of electronic materials, electrical materials, photocatalysts, pigments and fluorescent materials.