JP2002265274A - Method for producing powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily produce ZrW2 O3 being a compound which has no generation of reaction by-products and has a clean, negative thermal expansion coefficient. SOLUTION: The average particle size of a powdery mixture obtained by blending ZrO2 and WO3 so as to produce ZrW2 O8 by dry blending or wet blending is controlled to 0.05 to 50 μm. Pressure of 0.001 to 500 ton/cm<2> is applied to the powdery mixture to be made into a mixture compact having the minimum dimensions of 0.2 to 100 mm. The compact is burnt at a burning temperature of 1,120 to 1,250 deg.C for >=1.5 hr, is rapidly cooled, and is thereafter pulverized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a powder having a negative coefficient of thermal expansion and having various uses for controlling thermal expansion of a substance. The present invention, more particularly,
Compositional formula (A _{1 -Z} D _Z ) (W _{1 -X} R _x ) ₂ O ₈ (A is Zr or Hf or a mixture thereof, D is an element capable of forming a solid solution in ZrO ₂ or HfO ₂ , Z is each element R is an element capable of forming a solid solution in WO ₃ , and X is a value not more than the maximum solid solution atomic ratio defined by each element). About.

[0002]

_2. Description of the Related Art AW ₂ O ₈ is a stable substance at 1105 to 1257 ° C. and 777 ° C. as a metastable substance.
It is known to exhibit isotropic negative thermal expansion in the following temperature range. As a manufacturing method thereof, USP551
In the specification of 4360, three methods are discussed and described.

As a first method, a method is described in which ZrO ₂ and WO ₃ powder are mixed, ZrW ₂ O ₈ is calcined in the stable temperature range, and quenched. Not synthesized.

As a second method, there is described a method in which tungstic acid is reacted with zirconium oxide and calcined, but it is not confirmed whether or not the produced substance is an intended substance.

[0005] As the third method, the stable WO ₃ at a temperature of about 1200 ° C. in a temperature region enclosing the mixed powder in a sealed platinum tube so as not to result in discrete gasified, WO ₃ A method is described in which gasification and separation are prevented. Based on the result, the US
P5514360 describes a method of reacting an aqueous solution of tungstate with an aqueous solution of zirconium oxyhalide (or the same hafnium) or zirconium nitrate (or the same hafnium), followed by drying and firing.

However, the above-mentioned method using a sealed platinum tube requires many steps for enclosing a raw material in the platinum tube and taking out a substance after firing,
In addition, there has been a problem that contamination of the platinum tube material to the powder occurs to lower the purity. Further, the method of US Pat. No. 5,514,360 has a problem as a production method since chlorine gas or nitrogen oxide is generated during firing, and further has a problem in purification of a product and the like.

[0007]

SUMMARY OF THE INVENTION According to the present invention, a powder represented by a composition formula (A _{1 -Z} D _Z ) (W _{1 -X} R _X ) ₂ O ₈ having a negative coefficient of thermal expansion is reduced in a small number of steps. Accordingly, it is an object of the present invention to provide a method for efficiently producing without causing harmful gas and reducing contamination by impurities.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, pressed the mixed powder of the raw materials and fired in the state of the obtained pressed compact of the mixture. (1) the molding pressure of the mixed powder,
It has been found that the object can be achieved by setting conditions such as (2) the particle size of the mixed powder, (3) the minimum size of the pressed compact of the mixture, and / or (4) the firing temperature to appropriate values. The present invention has been accomplished.

That is, the present invention comprises the following aspects. (1) The composition when fired is (A _{1 -Z} D _Z ) (W
_1-X R _x ) ₂ O ₈ (A is Zr or Hf or a mixture thereof, D is at least one element selected from elements capable of forming a solid solution in ZrO ₂ or HfO ₂ , and Z is limited by each element Values below the maximum solid solution atom ratio (including 0), R is WO
_At least one element selected from the elements capable of forming a solid solution in ₃ , X represents D and A in the AO ₂ and WO ₃ so as to have a value (including 0) or less of the maximum solid solution atomic ratio defined by each element. Or a simple substance or a compound of R is added, and a mixed powder obtained by dry or wet mixing is formed into a press-molded body, and then calcined, rapidly cooled, and then pulverized. Molding pressure 0.001 to 500 ton / cm ²
And pressurizing the mixture to form a mixture. (2) the composition when calcined _{(A 1-Z D Z)} (W
_1-X R _x ) ₂ O ₈ (A is Zr or Hf or a mixture thereof, D is at least one element selected from elements capable of forming a solid solution in ZrO ₂ or HfO ₂ , and Z is limited by each element Values below the maximum solid solution atom ratio (including 0), R is WO
_At least one element selected from the elements capable of forming a solid solution in ₃ , X represents D and A in the AO ₂ and WO ₃ so as to have a value (including 0) or less of the maximum solid solution atomic ratio defined by each element. Or a simple substance or compound of R is added, and a pressed body of a mixed powder obtained by dry or wet mixing is calcined, rapidly cooled, and then pulverized. The method for producing the powder, wherein the diameter is 0.05 to 50 μm. (3) The composition when fired is (A _{1 -Z} D _Z ) (W _{1 -X} R _X )
₂ O ₈ (A is Zr or Hf or a mixture thereof, D is Zr
Z is at least one element selected from elements capable of forming a solid solution in O ₂ or HfO ₂ , Z is a value (including 0) or less of a maximum solid solution atomic ratio defined by each element, and R is a solid solution in WO ₃ At least one element selected from the elements to be obtained, X is a single element of D and / or R in AO ₂ and WO ₃ so as to have a value (including 0) or less of the maximum solid solution atomic ratio limited by each element. A compound is added, and the mixed powder obtained by dry or wet mixing is formed into a pressed compact of a mixture, and then calcined, rapidly cooled, and then pulverized. Characterized in that the size of the powder is 0.2 to 100 mm. (4) The method for producing a powder according to the above (1), wherein the average particle diameter of the powder is 0.05 to 50 μm. (5) The minimum size of the pressed compact of the mixture is 0.2 to 100.
mm, the method for producing powder according to the above (1). (6) The minimum size of the pressed compact of the mixture is 0.2 to 100.
mm, wherein the powder is produced. (7) The method for producing a powder according to the above (1), wherein the average particle size of the powder is 0.05 to 50 μm, and the minimum size of the compact is 0.2 to 100 mm. (8) The firing temperature is in the range of 1120 to 1250 ° C. and D
Alternatively, the method for producing a powder according to any one of the above (1) to (7), wherein the melting point is not higher than the melting point of the oxide of R. (9) The method for producing a powder according to the above (8), wherein the firing time is 1.5 hours or more. (10) The method as described in any one of (1) to (9) above, wherein firing is performed by interposing ZrO ₂ or SnO ₂ between a firing furnace or a crucible material and a pressed compact of the mixed powder. Method for producing powder. (11) The method for producing a powder according to any one of (1) to (10), wherein the rapid cooling is performed by forced cooling. (12) The method for producing a powder according to any of (1) to (11) above, wherein the molar ratio of AO ₂ / WO ₃ is from 3/1 to 1/6. (13) The method for producing a powder according to the above (12), wherein the molar ratio of AO ₂ / WO ₃ is from か ら to ５. (14) The above (1) to (1), wherein A is Zr.
(13) The method for producing a powder according to any of (13).

Hereinafter, each embodiment of the present invention will be described. In the production method of the present invention, first, the mixture of the raw material powders has a composition (A _{1 -Z} D _Z ) (W
A single powder or a compound powder of D and / or R is added to the AO ₂ powder and the WO ₃ powder so as to satisfy _1-X R _x ) ₂ O ₈ , and the mixture is prepared by dry or wet mixing. Where A is Z
r or Hf or Zr and Hf. In addition, the above compounds of D and R include oxides, hydroxides, chlorides, nitrides,
Examples thereof include carbides, oxyacid salts such as nitrates and carbonates, and the like. Also, mixing is preferred because wet mixing often adds a greater effect of grinding.

D is an element capable of forming a solid solution in ZrO ₂ or HfO _2, and examples thereof include alkaline earth metals such as Ca and rare earth metals such as Y. Z is a value equal to or less than the maximum solid solution atom ratio determined by each solid solution element, specifically 0.2
And typically between 0 and 0.1.

R is an element which can be dissolved in WO ₃ ,
And elements from group VA or group VA or VIA.
The value of X is a value equal to or less than a maximum solid solution atom ratio determined by each element to be dissolved, specifically 0.25 or less, and typically 0 to 0.2. When Z = 0 and X = 0, it is preferable to produce a powder having a remarkable negative thermal expansion characteristic.

Next, this mixed powder is added in an amount of 0.001 to 500 t.
A molding pressure is applied by applying a molding pressure of on / cm ² to obtain a mixture compact. Next, after firing the mixture press-formed body, the mixture is rapidly cooled and pulverized. (Aspect of the above (1))

As described above, the mixed powder is molded by applying pressure, and is fired in the state of a pressed compact of the mixture, so that the temperature is raised from room temperature to the firing temperature. It is possible to prevent the unreacted WO ₃ from being separated by gasification, and it is possible to produce powder efficiently. When the pressure is lower than the above-mentioned pressure, the pressed body of the mixture becomes brittle, is broken by heating during handling or firing, and approaches a heating reaction of the powder, which is not preferable. On the other hand, if the pressure is higher than the above-mentioned pressure, it is difficult to alleviate the volume expansion that often occurs during the firing, and inexpensive production is hindered by the limitation of the hardware of the press machine.

The molding pressure is 0.002 to 200 to
n / cm ² is more preferable, and 0.005 to 50 ton /
cm ² is more preferable, 0.01 to 20 ton / cm ² is more preferable, 0.02 to 10 ton / cm ² is more preferable, and 0.05 to 5 ton / cm ² is more preferable.
0.1-2 ton / cm < ² > is more preferable.

Further, by limiting the average particle size of the mixed powder, the mixing of the mixed powder is made uniform, and the reaction time between AO ₂ and WO ₃ is shortened, whereby the unreacted WO ₃ is dispersed by gasification. Can be prevented, and efficient powder production can be achieved. That is, in another method for producing the powder of the present invention, the composition when calcined is (A _{1 -Z} D _Z ) (W
A single or compound of D and / or R is added to AO ₂ and WO ₃ so as to obtain _1-X R _X ) ₂ O _8, and the pressed compact of the mixed powder obtained by dry or wet mixing is fired. The method for producing a powder, wherein the average particle diameter of the mixed powder is 0.05 to 50 μm in the method for producing a powder which is rapidly cooled and then pulverized (aspect of the above (2)) ).

If the average particle size is smaller than this, the production cost increases. However, from the viewpoint of the reaction, the smaller the average particle size, the better. If the particle size is larger than this, the reaction speed in the solid-phase reaction is slowed, and the gasification of WO ₃ is easily dissipated, and the reaction time is undesirably long. A more preferable range of the average particle size is 0.1 to 10 μm, and further preferably 0.2 to 5 μm. For example, setting the average particle size to 1 μm is a preferable and practical example.

Furthermore, by limiting the minimum size of the pressed compact of the mixture, rapid and uniform cooling at the time of quenching from the firing temperature to a temperature equal to or lower than the non-equilibrium decomposition temperature becomes possible, and an efficient ZrW ₂ O ₈ can be produced. Here, the minimum dimension is, for example, the smaller of the length and the diameter of the cylindrical body, and the smaller of the thickness and the diameter of the disc body.

[0019] That is, still another method for producing a powder of the present invention, the composition when calcined _{(A 1- Z D Z) (} W 1-X R X)
Add a simple substance or a compound of D and / or R to AO ₂ and WO ₃ so as to obtain ₂ O ₈ , dry- or wet-mix and obtain a mixed powder, press-mold a mixed powder, fire, and rapidly cool. (3) The method for producing a powder according to the above (3), wherein the minimum size of the pressed compact of the mixture is 0.2 to 100 mm. ). If the minimum dimension is too small, the thickness of WO _{3 as} a gasification dissipation barrier is reduced, and the production efficiency is undesirably reduced. On the other hand, if the minimum dimension is too large, cooling is not performed rapidly to the inside, and decomposition products are often generated and remain inside, which is not preferable. The minimum dimension is more preferably 1 to 15 mm, still more preferably 2 to 10 mm, and still more preferably 3 to 7 mm.

The present invention is further improved by adopting a manufacturing condition having a plurality of requirements for the molding pressure of the mixed powder, the average particle size of the mixed powder, and the minimum size of the pressed compact of the mixture. Preferred results can be obtained (aspects of (4) to (7) above).

In the present invention, by forming the mixed powder into a compact, it is possible to react the substance in a stable temperature range to produce the powder. That is, in the present invention, the firing temperature is in the range of 1120 to 1250 ° C. and D
Alternatively, the melting point is not higher than the melting point of the oxide of R (aspect of the above (8)). More preferably, this temperature range is 1150 ° C. or higher.

For example, when V is added, their melting points may be 1200 ° C. or less, and when they are contained in a large amount, the firing temperature is in the range of 1150 to 1200 ° C. according to the value of X. It is desirable to perform in the temperature range.
More specifically, in a solid solution of these substances, the firing reaction temperature often moves to a lower temperature side. For example, when X = 0.2 using V as R, the optimal temperature of the firing reaction is about 1160 ° C., and melting occurs at 1170 ° C. Further, in the case of Ta, X forms a solid solution up to 0.05, but if it is larger than that, it does not form a solid solution.
In the case of Ta, the reaction can be performed at 1200 ° C. at this level. The optimum temperature conditions for the firing reaction of V will be described later with examples.

The firing time is required to be at least 1.5 hours (as described in the above (9)), and is preferably 2 to 4 hours. Even if the firing time is extended to 6 to 8 hours, the performance of the obtained powder is not significantly affected. As the firing atmosphere, an oxidizing atmosphere, an inert atmosphere, a reduced pressure atmosphere, or the like is appropriately selected according to the chemical forms of the D and R compounds used as the raw materials.

In the present invention, when performing the sintering,
Is WO in a temperature range of about 1200 ° C. _ThreeInduces reactions
Contact with alumina or silica-based material such as
Absent. Therefore, the firing is performed by a firing furnace or a crucible material and the firing
ZrO between the pressed compact of the mixture_TwoOr SnO_TwoIntervened
It is desirable to perform the process (embodiment (10) above).

For rapid cooling after baking the molded body of the mixture, forced cooling is required (as described in the above (11)). The forced cooling includes rapidly taking out the press-formed body, which has been heated to a high temperature by baking, into an atmosphere outside a furnace near room temperature and allowing it to cool naturally. In such natural cooling taken out of the furnace, decomposition products are often generated and remain, but some of the desired substances are generated, so that they can be used as powder. However, it is more preferable to forcibly cool by blowing a non-reactive gas. Furthermore,
Forcing the fired body into a large amount of non-reactive liquid as it is or dividing and putting it in, or using a non-reactive cooling gas, such as spraying from a cylinder such as nitrogen, or a non-reactive liquefied gas Cooling is preferred. As a liquid,
It is practical and preferable to use water, especially running water.

The molar ratio of AO ₂ / WO ₃ is from 3/1 to 1 /
By setting up to 6, a negative coefficient of thermal expansion is clearly exhibited (the aspect of (12) above). This molar ratio ranges from 1/2 to 1/5.
Is more preferable (as in the above (13)), and most preferably stoichiometrically from 1/2 to 1 / 2.1. The AO ₂ is ZrO ₂
Is easy to obtain and therefore practical for implementing the present invention (aspect (14)).

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the method for producing a powder of the present invention will be described with reference to examples, but the present invention is not limited to these examples.

ZrW ₂ O ₈ is a compound exhibiting negative thermal expansion, and the thermal expansion characteristics at that time are shown in FIG. First, the production rate of the raw material mixed powder was evaluated by changing the production conditions in various ways. Table 1 shows the results of evaluating the conditions of the manufacturing method.

[0029]

[Table 1]

FIG. 2 shows how the X-ray diffraction image changes depending on the cooling conditions. From FIG. 2, it can be seen that the state of decomposition changes with cooling conditions. Table 1 shows the molar ratio of ZrW ₂ O ₈ based on data calculated from the specific gravity of each compound. This value matches the result of the X-ray diffraction image. Conditions were selected based on such evaluation.

[0031]

Example 1 ZrO ₂ powder and WO ₃ powder ZrO ₂ / WO ₃
The mixed powder having a ratio of 1/2 was wet-mixed for 1 hour, dried at 90 ° C. for 5 hours, and then subjected to a uniaxial pressing machine to apply a pressure of 500 kgw / cm ² to 6 mm.
A pressure-molded body having a size of φ × 5 mm was used as a fired sample. This pressed body was placed in an alumina crucible covered with a zirconia sheet, and heated at a rate of 8 ° C. /
The temperature was raised to 1200 ° C. in min. Next, the fired body was rapidly cooled by water cooling and then pulverized. When the pressed compact is directly put into an alumina crucible without using a zirconia sheet, Al ₂ (W
O ₄ ) ₃ had occurred.

[0032]

Example 2 The ratio of ZrO ₂ / WO ₃ was changed from 2/1 to 1/11.
A powder was obtained in the same manner as in Example 1, except that the powder was changed to. Zr
FIG. 3 shows a situation in which the thermal expansion characteristic changes with the ratio of O ₂ / WO ₃ .
Shown in From this, it is understood that when the ratio of ZrO ₂ / WO ₃ is from 2/1 to 1/5, it can be sufficiently used as a negative thermal expansion material.

[0033]

Embodiment 3 ZrO ₂ and WO ₃ were mixed with V ₂ O ₅ and ZrO ₂ and WO ₃ were mixed.
_{X of} (W _{1 -X} V _X ) ₂ O ₃ is 0.00, 0.05, 0.1
A powder was obtained under the same conditions as in Example 1 except that the amounts were 0, 0.15, and 0.20. FIG. 4 shows the thermal expansion characteristics of the product obtained by the reaction. In addition, ZrO ₂
And WO ₃ mixed with Ta ₂ O ₅ , and Zr (W _{1 -X} Ta _X ) ₂ O ₈
Were obtained under the same conditions as in Example 1 except that X was adjusted to be 0.00, 0.02, 0.04, and 0.05. FIG. 5 shows the thermal expansion characteristics of the product obtained by the reaction.
Shown in According to these results, there is almost no elemental difference. In this case, 0.05 is the maximum solid solution amount of X, and even if added more, X does not become a uniform solid solution. However, 0.
Even if an amount of 2 or more is added, the mixture can be sufficiently applied as a negative thermal expansion material.

[0034]

Embodiment 4 MgO was mixed with ZrO ₂ and WO ₃ and (ZrO _2
1-Z Mg _Z ) _{Z of} W ₂ O ₈ is 0.00, 0.05, 0.0
A powder was obtained in the same manner as in Example 1 except that the amounts were 7, 0.10, and 0.15. FIG. 6 shows the thermal expansion characteristics of the product obtained by the reaction.

[0035]

Example 5 ZrO ₂ and WO ₃ were mixed with V ₂ O ₅ , and ZrO ₂ and WO ₃ were mixed.
_{X of} (W _{1 -X} V _X ) ₂ O ₃ is 0.00, 0.05, 0.1
Powders were obtained in the same manner as in Example 1 except that the amounts were 0, 0.15, 0.20, and 0.30, and the firing temperature was changed. Table 2 shows the reaction temperature range and the firing results when V was dissolved. The reaction temperature decreases as the solid solution atomic ratio X of V increases. If X is 0.3, it will melt even at 1110 ° C.

[0036]

[Table 2]

[0037]

According to the present invention, a compound having a negative coefficient of thermal expansion can be easily produced by a clean production process in which no reaction by-product is generated.

[0038]

[Brief description of the drawings]

FIG. 1 is a diagram showing the thermal expansion characteristics of ZrW ₂ O ₈ .

FIG. 2 is a view showing a difference in an X-ray diffraction image due to a difference in a cooling method of ZrW ₂ O ₈ .

FIG. 3 is a diagram showing a situation in which the thermal expansion characteristics change with the ratio of ZrO ₂ / WO ₃ .

FIG. 4 is a diagram showing a change in thermal expansion characteristics when X of Zr (W _{1 -X} V _X ) ₂ O ₈ is changed.

FIG. 5 is a diagram showing a change in thermal expansion characteristics when X of Zr (W _{1 -X} Ta _X ) ₂ O ₈ is changed.

FIG. 6 is a diagram showing a change in thermal expansion characteristic when Z of (Zr _{1 -Z} Mg _Z ) W ₂ O ₈ is changed.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Shin Nishiyama 1-20-22-303 Makuhari Hongo, Hanamigawa-ku, Chiba-shi, Chiba F-term (reference) 4G030 AA07 AA17 AA19 AA21 AA24 BA21 CA04 GA11 GA22 GA30 GA32 4G031 AA03 AA12 AA13 AA15 AA18 CA04 GA03 GA06 GA13 GA15

Claims (14)

[Claims] 1. The composition when fired is (A _{1 -Z} D _Z ) (W
_1-X R _x ) ₂ O ₈ (A is Zr or Hf or a mixture thereof, D is at least one element selected from elements capable of forming a solid solution in ZrO ₂ or HfO ₂ , and Z is limited by each element Values below the maximum solid solution atom ratio (including 0), R is WO
_At least one element selected from the elements capable of forming a solid solution in ₃ , X represents D and A in the AO ₂ and WO ₃ so as to have a value (including 0) or less of the maximum solid solution atomic ratio defined by each element. Or a simple substance or a compound of R is added, and a mixed powder obtained by dry or wet mixing is formed into a press-molded body, and then calcined, rapidly cooled, and then pulverized. Molding pressure 0.001 to 500 ton / cm ²
And pressurizing the mixture to form a mixture. 2. The composition when fired is (A _{1 -Z} D _Z ) (W
_1-X R _x ) ₂ O ₈ (A is Zr or Hf or a mixture thereof, D is at least one element selected from elements capable of forming a solid solution in ZrO ₂ or HfO ₂ , and Z is limited by each element Values below the maximum solid solution atom ratio (including 0), R is WO
_At least one element selected from the elements capable of forming a solid solution in ₃ , X represents D and A in the AO ₂ and WO ₃ so as to have a value (including 0) or less of the maximum solid solution atomic ratio defined by each element. Or a simple substance or a compound of R is added, and a pressed body of a mixed powder obtained by dry or wet mixing is calcined, rapidly cooled, and then pulverized, wherein the average particle size of the mixed powder is The method for producing the powder, wherein the diameter is 0.05 to 50 μm. 3. The composition when fired is (A _{1 -Z} D _Z ) (W
_1-X R _x ) ₂ O ₈ (A is Zr or Hf or a mixture thereof;
D (including 0) of at least one element selected from the elements that may be dissolved in ZrO ₂ or HfO _2, Z is maximum solid atomic ratio following values defined by each element, R represents a solid in WO ₃ At least one element selected from soluble elements, X
Is obtained by adding a simple substance or a compound of D and / or R to AO ₂ and WO ₃ so as to have a value (including 0) or less of the maximum solid solution atom ratio limited by each element, and performing dry or wet mixing. In a method for producing a powder, in which the obtained mixed powder is fired after being formed into a mixture press-molded body, rapidly cooled and then pulverized, the minimum size of the mixture press-molded body is set to 0.2 to 100 mm. A method for producing the powder, characterized in that: 4. The method according to claim 1, wherein the average particle diameter of the powder is 0.05 to 50 μm. 5. The minimum size of the pressed compact of the mixture is 0.2
The method for producing a powder according to claim 1, wherein the thickness is set to 100 mm. 6. The minimum size of the pressed compact of the mixture is 0.2
3. The method for producing a powder according to claim 2, wherein the thickness is set to 100 mm. 7. The method according to claim 1, wherein the average particle size of the powder is 0.05 to 50 μm, and the minimum size of the compact is 0.2 to 100 mm. 8. The method for producing a powder according to claim 1, wherein the sintering temperature is in the range of 1120 to 1250 ° C. and not higher than the melting point of the oxide of D or R. . 9. The method for producing a powder according to claim 8, wherein the sintering time is 1.5 hours or more. 10. The firing according to claim 1, wherein ZrO ₂ or SnO ₂ is interposed between a firing furnace or a crucible material and the pressed compact of the mixed powder. A method for producing the powder described in the above. 11. The method for producing a powder according to claim 1, wherein the rapid cooling is performed by forced cooling. 12. The method according to claim 1, wherein the molar ratio of AO ₂ / WO ₃ is from 3/1 to 1/6. 13. The method according to claim 12, wherein the molar ratio of AO ₂ / WO ₃ is from 1/2 to 1/5. 14. The method for producing a powder according to claim 1, wherein A is Zr.