JP7681986B2 - Solid electrolyte assembly and method for producing same - Google Patents

Description

The present invention relates to a solid electrolyte junction and a method for producing the same.

As a conventional technique relating to a solid electrolyte joint having an oxide ion conductor, for example, the technique described in Patent Document 1 is known.
Patent Document 1 describes that the electrode layer of a solid electrolyte junction used as a solid oxide electrochemical cell is made of a mixed phase of cerium oxide doped with samarium, gadolinium, or yttrium, and alumina carrying nickel particles, and that this electrode layer suppresses the growth and movement of nickel particles due to sintering.

The present applicant has also previously proposed solid electrolyte assemblies described in Patent Documents 2 and 3.
Patent Document 2 describes that an intermediate layer located between a solid electrolyte and an anode or a cathode in a solid electrolyte junction is made of cerium oxide containing lanthanum and a rare earth metal (excluding lanthanum and cerium). This solid electrolyte junction has improved oxide ion conductivity.
Patent Document 3 also describes a technique of disposing an intermediate layer between a solid electrolyte and an anode or cathode in a solid electrolyte junction. The intermediate layer is made of cerium oxide containing one or more elements selected from the group consisting of samarium, yttrium, gadolinium, and lanthanum. Alternatively, the intermediate layer is made of bismuth oxide or a composite oxide of bismuth and lanthanum, gadolinium, or yttrium. The use of this intermediate layer allows smooth exchange of oxide ions between the solid electrolyte and the electrodes.

JP 2015-5527 A International Publication No. 2019/203219 International Publication No. 2019/235383

In order to smoothly transfer oxide ions between the solid electrolyte and the electrodes in a solid electrolyte joint, it is necessary to increase the oxide ion conductivity of the intermediate layer. From this viewpoint, there is a demand for further improvement in the oxide ion conductivity of the intermediate layer. There is also a demand for further improvement in the oxide ion conductivity of the device as a whole.
Therefore, an object of the present invention is to further improve the oxide ion conductivity of a device having a solid electrolyte.

The present invention provides a solid electrolyte assembly having an anode, a cathode, and a solid electrolyte located therebetween,
an intermediate layer between at least one of the anode and the cathode and the solid electrolyte;
the solid electrolyte comprises a metal oxide,
the intermediate layer contains cerium oxide containing one or more rare earth metals (excluding cerium) and does not contain a transition metal (excluding rare earth metals) or contains a transition metal (excluding rare earth metals) in an amount of 7.0 at% or less relative to cerium;
The above-mentioned problems are solved by providing a solid electrolyte joint in which the average equivalent circle diameter of the crystal grains in the intermediate layer is 0.75 μm or more and 10 μm or less.

The present invention provides a method for producing a solid electrolyte joint, the method comprising the steps of: firing an object to be fired, the object being made of a solid electrolyte made of an oxide containing lanthanum; and an intermediate layer which contains cerium oxide containing one or more rare earth metals (excluding cerium) and which does not contain a transition metal (excluding rare earth metals) or which contains a transition metal (excluding rare earth metals) in an amount of 7.0 at % or less relative to cerium;
The present invention provides a method for producing a solid electrolyte joint, which comprises increasing the temperature at a rate of 1° C./hr to 2000° C./hr, maintaining the temperature in a range of 700° C. to 2000° C. for 1 minute to 36 hours, and then cooling at a rate of 1° C./hr to 500° C./hr.

The present invention improves the oxide ion conductivity of devices equipped with a solid electrolyte and also improves the adhesion between the electrolyte and the intermediate layer.

FIG. 1 is a schematic diagram of a cross section along the thickness direction showing one embodiment of a solid electrolyte joined body of the present invention. FIG. 2 is a graph showing a Cole-Cole plot measured for the solid electrolyte assembly obtained in Example 1. As shown in FIG. FIG. 3 is a diagram showing one component of an equivalent circuit obtained by the relaxation time distribution method used to obtain the impedance of the solid electrolyte junctions obtained in the examples and comparative examples. FIG. 4 is a diagram showing an equivalent circuit obtained by the complex nonlinear least squares method used for determining the impedance of the solid electrolyte junctions obtained in the examples and comparative examples.

The present invention will be described below based on preferred embodiments with reference to the drawings. Fig. 1 shows one embodiment of a solid electrolyte joint of the present invention. A solid electrolyte joint 10 shown in the figure includes a solid electrolyte 11 (hereinafter also referred to as "solid electrolyte layer 11") having a layered form. The solid electrolyte layer 11 is made of a material having ion conductivity, preferably oxide ion conductivity, at a predetermined temperature or higher.
The solid electrolyte layer 11 is located between two electrodes, i.e., a cathode 12 and an anode 13. That is, the cathode 12 and the anode 13 are disposed on different sides of the solid electrolyte layer 11. The cathode 12 can be electrically connected to a negative electrode of a DC power supply (not shown). On the other hand, the anode 13 can be electrically connected to a positive electrode of a DC power supply (not shown). Therefore, a DC voltage is applied between the cathode 12 and the anode 13.

A cathode-side intermediate layer 15 is disposed between the cathode 12 and the solid electrolyte layer 11. On the other hand, an anode-side intermediate layer 16 is disposed between the anode 13 and the solid electrolyte layer 11.
1, the cathode 12 and the cathode-side intermediate layer 15 are shown to have different sizes, but the size relationship between the two is not limited to this, and for example, the cathode 12 and the cathode-side intermediate layer 15 may be the same size. The same applies to the anode 13 and the anode-side intermediate layer 16, and they may be the same size, or, for example, the anode-side intermediate layer 16 may be larger than the anode 13.
1, the size of the cathode-side intermediate layer 15 and the size of the solid electrolyte layer 11 are shown to be the same, but the size relationship between the two is not limited to this, and for example, the size of the solid electrolyte layer 11 and the size of the cathode-side intermediate layer 15 may be different. The same applies to the anode 13 side.

As shown in FIG. 1, the cathode-side intermediate layer 15 is in direct contact with the cathode 12 and the solid electrolyte layer 11. Therefore, no layer is interposed between the cathode-side intermediate layer 15 and the cathode 12. Also, no layer is interposed between the cathode-side intermediate layer 15 and the solid electrolyte layer 11. The same is true for the anode 13 side, where the intermediate layer 16 is in direct contact with the solid electrolyte layer 11 and the anode 13.

The cathode-side intermediate layer 15 and the anode-side intermediate layer 16 (hereinafter, for convenience, both may be collectively referred to simply as "intermediate layers") are used for the purpose of improving the ion conductivity, preferably the oxide conductivity, between the solid electrolyte layer 11 and the cathode 12 and/or the anode 13 in the solid electrolyte junction 10. In order to reduce the electrical resistance in the solid electrolyte junction 10, it is important to increase the oxide ion conductivity of the solid electrolyte layer 11. However, even if the solid electrolyte layer 11 is constructed using a material with high oxide ion conductivity, if the oxide ion conductivity between the solid electrolyte layer 11 and the anode 13 and/or the cathode 12 is low, there is a limit to increasing the oxide ion conductivity of the solid electrolyte junction 10 as a whole. As a result of the inventor's investigations, it was found that the oxide ion conductivity of the solid electrolyte junction 10 as a whole is increased by using a specific solid electrolyte layer 11 having oxide ion conductivity and disposing an intermediate layer of an oxide made of a specific material and having a specific crystal grain size between the solid electrolyte layer 11 and the cathode 12 and/or the anode 13. Additionally, it was discovered that the adhesion between the solid electrolyte layer 11 and the intermediate layer is improved.

Specifically, it was found that the oxide ion conductivity of the solid electrolyte junction 10 as a whole is improved by using a metal oxide for the solid electrolyte layer 11 in the solid electrolyte junction 10, using cerium oxide doped with a rare earth metal for the intermediate layer, and setting the average circle equivalent diameter of the crystal grains in the intermediate layer to 0.75 μm or more and 10 μm or less. The solid electrolyte layer 11, the cathode side intermediate layer 15, and the anode side intermediate layer 16 are described below.

The solid electrolyte layer 11, which contains a metal oxide, is a conductor in which oxide ions serve as carriers. A single crystal or polycrystalline material is used as the solid electrolyte that constitutes the solid electrolyte layer 11. In particular, it is preferable to use an oxide of a rare earth metal as the material that constitutes the solid electrolyte layer 11, since this further increases the oxide ion conductivity.

From the viewpoint of further increasing the oxide ion conductivity, it is preferable to use an oxide of lanthanum as the rare earth metal oxide contained in the solid electrolyte layer 11. Examples of the oxide of lanthanum include a composite oxide containing lanthanum and gallium, a composite oxide in which strontium, magnesium, cobalt, or the like is added to the composite oxide, and a composite oxide containing lanthanum and molybdenum.
In particular, it is preferable to use an oxide ion conductive material made of a composite oxide of lanthanum and silicon because of its high oxide ion conductivity.

Examples of the composite oxide of lanthanum and silicon include apatite-type composite oxides containing lanthanum and silicon. As the apatite-type composite oxide, one containing lanthanum, which is a trivalent element, silicon, which is a tetravalent element, and O, and having a composition represented by La _x Si ₆ O _1.5x+12 (X represents a number of 8 or more and 10 or less), is preferred from the viewpoint of high oxide ion conductivity. When this apatite-type composite oxide is used as the solid electrolyte layer 11, it is preferable to make the c-axis coincide with the thickness direction of the solid electrolyte layer 11. The most preferred composition of this apatite-type composite oxide is La _9.33 Si ₆ O _26. This composite oxide can be produced, for example, according to the method described in JP-A-2013-51101.

Another example of the material constituting the solid electrolyte layer 11 is a composite oxide represented by the general formula: A _9.33+x [T _6.00-y M _y ]O _26.00+z . This composite oxide also has an apatite structure. In the formula, A is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. In the formula, T is an element containing Si or Ge, or both. In the formula, M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B (boron), Ge, Zn, Sn, W, and Mo. From the viewpoint of enhancing the c-axis orientation, M is preferably one or more elements selected from the group consisting of B (boron), Ge and Zn.

In the above formula, x is preferably a number of −1.33 or more and 1.50 or less, more preferably 0.00 or more and 0.70 or less, and even more preferably 0.45 or more and 0.65 or less, from the viewpoint of increasing the degree of orientation and oxide ion conductivity.
In the formula, y is preferably a number of 0.00 or more and 3.00 or less, more preferably 0.40 or more and 2.00 or less, and even more preferably 0.40 or more and 1.00 or less, from the viewpoint of filling the T element position in the apatite-type crystal lattice.
In the formula, z is preferably a number of −5.00 or more and 5.20 or less, more preferably −2.00 or more and 1.50 or less, and even more preferably −1.00 or more and 1.00 or less, from the viewpoint of maintaining electrical neutrality in the apatite-type crystal lattice.

In the above formula, the ratio of the number of moles of A to the number of moles of T, in other words, (9.33+x)/(6.00-y) in the above formula, is preferably 1.33 or more and 3.61 or less, more preferably 1.40 or more and 3.00 or less, and even more preferably 1.50 or more and 2.00 or less, from the viewpoint of maintaining the spatial occupancy rate in the apatite-type crystal lattice. Note that, in the general formula: A9.33 _+x [T6.00 _{-yMy ] O26.00+z} , when both T and M contain Ge, in the above formula (9.33+x)/(6.00-y), y=0.

Among the composite oxides represented by the above formula, it is preferable to use a composite oxide in which A is lanthanum, that is, a composite oxide represented by La _9.33+x [T _6.00-y M _y ]O _26.00+z , from the viewpoint of further increasing the oxide ion conductivity. Specific examples of the composite oxide represented by La9.33 _+x [T6.00 _{-yMy ] O26.00+z} include La9.33 _{+x(Si4.70B1.30)O26.00+z, La9.33+x ( Si4.70Ge1.30)O26.00+z , La9.33 +x} ( _Si4.70Zn1.30 )O26.00 _{+ z} , La9.33 _{+ x ( Si4.70W1.30 ) O26.00+z} , _{La9.33+x ( Si4.70Sn1.30} ) _O26.00+x , and La9.33 _+x ( _{Ge 4.70} B _1.30 ) O _26.00+z , etc. The composite oxide represented by the above formula can be produced, for example, according to the method described in International Publication WO2016/111110.

From the viewpoint of further increasing the oxide ion conductivity of the solid electrolyte assembly 10, the orientation degree of the solid electrolyte layer 11 measured by the Lotgering method, i.e., the Lotgering orientation degree, is preferably 0.60 or more, more preferably 0.80 or more, even more preferably 0.90 or more, and even more preferably 0.97 or more. In order to make the Lotgering orientation degree of the solid electrolyte layer 11 this value or more, the solid electrolyte layer 11 may be made, for example, single-phase and high-density.

From the viewpoint of effectively reducing the electrical resistance of the solid electrolyte assembly 10, the thickness of the solid electrolyte layer 11 is preferably 10 nm or more and 1000 μm or less, more preferably 50 nm or more and 700 μm or less, and even more preferably 100 nm or more and 500 μm or less. The thickness of the solid electrolyte layer 11 can be measured, for example, using a stylus step gauge or an electron microscope.

The intermediate layer is preferably composed of cerium oxide containing one or more rare earth metals (hereinafter also referred to as "LnDC"). However, the "rare earth metal" does not include cerium. In LnDC, rare earth metals other than cerium are contained in a solid solution (doped) form in the base material cerium oxide ( _CeO2 ). The rare earth metal, which is a doping element, is usually present at a site in the crystal lattice of cerium oxide by substituting the site where cerium is located.

From the viewpoint of further increasing the oxide ion conductivity of the solid electrolyte junction 10, the intermediate layer is preferably composed of cerium oxide containing lanthanum and a rare earth metal (excluding lanthanum and cerium) (hereinafter also referred to as "La-LnDC"). In La-LnDC, lanthanum may be present in the crystal lattice of cerium oxide in the form of a substitution for a site where cerium is located, or may be present at the grain boundaries of crystal grains of cerium oxide doped with a rare earth metal.

From the viewpoint of further increasing the oxide ion conductivity of the solid electrolyte joint 10, the intermediate layer is preferably composed of cerium oxide containing lanthanum and at least one selected from the group consisting of samarium, gadolinium, yttrium, erbium, ytterbium, and dysprosium.
In particular, it is preferable that the intermediate layer contains cerium oxide containing lanthanum and either samarium or gadolinium, since this can further increase the oxide ion conductivity of the entire solid electrolyte assembly 10. The La-LnDC constituting both intermediate layers 15, 16 may be the same or different. Alternatively, one of the cathode side intermediate layer 15 and the anode side intermediate layer 16 may be composed of La-LnDC, and the other may be composed of another substance.

In La-LnDC, the ratio of rare earth metals (excluding lanthanum and cerium) doped into cerium oxide is preferably 0.05 at% to 0.5 at%, more preferably 0.1 at% to 0.4 at%, and even more preferably 0.2 at% to 0.3 at%, expressed as Ln/Ce, which is the atomic ratio of rare earth metals (Ln) to cerium. By setting the degree of doping of the rare earth metal within this range, the oxide ion conductivity between the solid electrolyte layer 11 and the cathode 12 and/or anode 13 is improved.
The fact that the rare earth metal is dissolved in the cerium oxide as a solid solution can be confirmed by X-ray diffraction.

In the La-LnDC constituting the intermediate layer, lanthanum is contained for the purpose of improving the oxide ion conductivity of the solid electrolyte assembly 10. For this purpose, it is preferable that the value of La/Ce (at%), which is the atomic ratio of lanthanum to cerium in La-LnDC, is 0.3 or more. In addition, if there is too much lanthanum, the oxide ion conductivity decreases, so it is preferable that the value of La/Ce (at%) is 1.2 or less. It is more preferable that the value of La/Ce (at%) is 0.4 or more and 1.1 or less, and even more preferable that it is 0.5 or more and 1.0 or less.

The total amount of rare earth metals doped in the cerium oxide constituting the intermediate layer, i.e., the sum _LnT of the amount of lanthanum and the amount of rare earth metals other than lanthanum, is preferably expressed as an atomic ratio to cerium, i.e., _LnT /Ce (at%), of 0.3 to 1.5 in terms of improving the oxide ion conductivity of the solid electrolyte joint 10. To make this advantage more prominent, the value of _LnT /Ce (at%) is more preferably 0.4 to 1.4, and even more preferably 0.5 to 1.3. A detailed method for measuring _LnT /Ce (at%) will be described in the examples below.

It is preferable that the intermediate layer contains as little transition metal (excluding rare earth metal) as possible from the viewpoint of improving the oxide ion conductivity of the entire solid electrolyte joint 10. When the intermediate layer unavoidably contains a transition metal, the amount of the transition metal is preferably 7.0 at % or less relative to cerium from the viewpoint of improving the oxide ion conductivity of the entire solid electrolyte joint 10. From this viewpoint, when the intermediate layer unavoidably contains a transition metal, the amount of the transition metal is preferably 3.0 at % or less relative to cerium, more preferably 1.0 at % or less, and most preferably the intermediate layer does not contain a transition metal. It is most preferable that both the cathode side intermediate layer 15 and the anode side intermediate layer 16 are intermediate layers that do not contain a transition metal or have a transition metal content as low as possible, but even if either one of the cathode side intermediate layer 15 and the anode side intermediate layer 16 does not contain a transition metal or has a transition metal content as low as possible, it is possible to improve the oxide ion conductivity of the entire solid electrolyte joint 10.
The present inventors believe that the transition metal may be derived from a heating element used when forming an intermediate layer by firing in the process for producing a solid electrolyte joint, which will be described later.

The inclusion of a transition metal (excluding rare earth metals) in the intermediate layer hinders the improvement of the oxide ion conductivity of the solid electrolyte joint 10, and as a result of the investigations by the present inventors, it has been found that the transition metal that most hinders the improvement of the oxide ion conductivity is molybdenum. Therefore, it is preferable that the intermediate layer does not contain molybdenum, or if it contains molybdenum, the content of molybdenum is 7.0 at % or less relative to cerium.
Since molybdenum is widely used as a resistance heating element in the state of molybdenum disilicide, the present inventors believe that there is a high possibility that molybdenum is mixed into the intermediate layer from a heating element used when forming the intermediate layer by firing in the production process of a solid electrolyte joint described below.
The method for measuring the amount of transition metals (excluding rare earth metals) contained in the intermediate layer will be explained in the examples described later.

It is desirable that the intermediate layer has large crystal grains of the oxide constituting the intermediate layer. When the crystal grain size is large, there are fewer grain boundaries between the crystal grains than when the crystal grain size is small. As a result, the conduction of oxide ions in the intermediate layer is less likely to be hindered, which improves the oxide ion conductivity of the solid electrolyte joint 10. From the viewpoint of making this advantage more prominent, the size of the crystal grains of the oxide constituting the intermediate layer, expressed as the average circle equivalent diameter, is 0.75 μm or more, preferably 0.80 μm or more, and more preferably 1.0 μm or more.
The larger the crystal grain size, the more advantageous it is for reducing grain boundaries. However, if the average value is as large as about 10 μm, the oxide ion conductivity of the solid electrolyte assembly 10 is sufficiently improved.
From the above viewpoints, the size of the crystal grains of the oxide constituting the intermediate layer, expressed as the average equivalent circle diameter, is 0.75 μm or more and 10 μm or less, preferably 0.80 μm or more and 9.0 μm or less, and more preferably 1.0 μm or more and 8.0 μm or less.
It is most preferable that the above-mentioned crystal grain size is satisfied in both the cathode-side intermediate layer 15 and the anode-side intermediate layer 16. However, if the size is satisfied in either one of the cathode-side intermediate layer 15 or the anode-side intermediate layer 16, the oxide ion conductivity of the entire solid electrolyte assembly 10 can be improved.
In order to set the crystal grain size of the oxide constituting the intermediate layer within the above range, the intermediate layer may be formed, for example, according to the manufacturing method described below.
The crystal grain size of the oxide constituting the intermediate layer is measured by the method described in the examples below.

The inventors' investigations have revealed that the intermediate layer can effectively improve the oxide ion conductivity between the solid electrolyte layer 11 and the cathode 12 and/or anode 13 if the intermediate layer has a certain thickness or more. In detail, the thickness of the intermediate layer is preferably 1 nm or more and 1000 nm or less, and more preferably 10 nm or more and 700 nm or less, on the cathode 12 side and the anode 13 side, respectively. The thickness of this intermediate layer can be measured using a stylus step gauge or an electron microscope. The thickness of the cathode side intermediate layer 15 and the anode side intermediate layer 16 may be the same or different.

The cathode 12 and anode 13, which are disposed in direct contact with the intermediate layer, can each be independently made of, for example, a metal material or an oxide having oxide ion conductivity. When the cathode 12 and anode 13 are made of a metal material, the metal material is preferably made of a platinum group element, since it has advantages such as high catalytic activity. Examples of platinum group elements include platinum, ruthenium, rhodium, palladium, osmium, and iridium. These elements can be used alone or in combination of two or more. The anode 13 and cathode 12 can also each be independently made of a cermet containing a platinum group element.

On the other hand, when either the cathode 12 or the anode 13 is composed of an oxide having oxide ion conductivity, an oxide having a perovskite structure represented by ABO _3-δ is preferably used as the oxide. In the formula, A represents an alkaline earth metal element. B represents a transition metal element, such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Ta, and W. δ is a fraction resulting from the valence and amount of A, B, and O. Various oxides having a perovskite structure represented by _{ABO 3-δ} are known, and such oxides are known to have various crystal systems, such as cubic, tetragonal, rhombohedral, and orthorhombic crystals. Of these crystal systems, it is preferable to use an ABO _3-δ type oxide having a cubic perovskite structure as the cathode 12 and/or the anode 13. By directly bonding the cathode 12 and/or the anode 13 made of such an oxide to the intermediate layer made of the above-mentioned material to form the solid electrolyte joint 10, the oxide ion conductivity of the joint 10 as a whole can be further increased.

From the viewpoint of increasing the oxide ion conductivity of the solid electrolyte joint 10 as a whole, it is advantageous for the oxide represented by ABO _3-δ to contain lanthanum in a part of the A site. Hereinafter, this oxide will also be referred to as "oxide a". The content of lanthanum in oxide a is preferably 0.01 or more and 0.80 or less, more preferably 0.05 or more and 0.80 or less, even more preferably 0.10 or more and 0.70 or less, even more preferably 0.15 or more and 0.70 or less, and most preferably 0.15 or more and 0.60 or less, expressed as the atomic ratio of lanthanum to all elements located at the A site.

Whether or not lanthanum is located in a part of the A site in the oxide represented by ABO _3-δ can be confirmed by X-ray diffraction. The ratio of lanthanum to all elements located in the A site can be measured by energy dispersive X-ray spectroscopy (EDS), electron probe microanalyzer (EPMA), or ICP optical emission spectroscopy.

Similarly, from the viewpoint of increasing the oxide ion conductivity of the solid electrolyte joint 10 as a whole, it is advantageous that the oxide represented by ABO _3-δ has iron as part of the transition metal elements at the B site. Hereinafter, this oxide will also be referred to as "oxide b." The content of iron in oxide b, expressed as the atomic ratio of iron to all transition metal elements located at the B site, is preferably 0.1 or more and 1.0 or less, more preferably 0.2 or more and 1.0 or less, and even more preferably 0.3 or more and 1.0 or less.

Whether or not iron is located at the B site in the oxide represented by ABO _3-δ can be confirmed by X-ray diffraction. The proportion of iron in all elements located at the B site can be measured by energy dispersive X-ray spectroscopy (EDS), electron probe microanalyzer (EPMA), or ICP optical emission spectrometry.

When the above-mentioned oxide a is used as the material constituting the cathode 12, it is preferable that the alkaline earth metal element occupying the A site is one or more elements selected from the group consisting of barium and strontium, from the viewpoint of increasing the oxide ion conductivity of the solid electrolyte assembly 10 as a whole. In other words, it is preferable that at least lanthanum and, in addition, one or more elements selected from the group consisting of barium and strontium are located at the A site of the oxide a.

On the other hand, it is preferable that a portion of the transition metal element occupying the B site of oxide a contains at least one of the elements belonging to the fourth and fifth periods of the periodic table. In particular, it is preferable that the transition metal element located at the B site contains at least one of the elements selected from the group consisting of iron, cobalt, nickel, copper, titanium, zirconium, and niobium, and furthermore, it is even more preferable that at least a portion of the transition metal element is iron, from the viewpoint of increasing the oxide ion conductivity of the solid electrolyte joint 10 as a whole.

When iron is located at the B site of oxide a, from the viewpoint of increasing the oxide ion conductivity of the solid electrolyte assembly 10 as a whole and not affecting the crystal system of oxide a, the atomic ratio of iron to all elements located at the B site is preferably 0.05 to 0.95, more preferably 0.10 to 0.90, and even more preferably 0.10 to 0.80. The value of the atomic ratio can be measured by energy dispersive X-ray spectroscopy (EDS), electron probe microanalyzer (EPMA), or ICP optical emission spectroscopy.

On the other hand, it is preferable that the element occupying the A site of oxide b is at least one of the alkaline earth metal elements, particularly barium and strontium, from the viewpoint of increasing the oxide ion conductivity of the solid electrolyte joint 10 as a whole. From the same viewpoint, lanthanum may be contained in a part of the A site of oxide b. In particular, it is preferable that lanthanum and one of barium and strontium are located in the A site of oxide b.

When lanthanum is located in part of the A site of oxide b, the atomic ratio of lanthanum to all elements located in the A site is preferably 0.01 to 0.80, more preferably 0.05 to 0.80, even more preferably 0.05 to 0.70, even more preferably 0.10 to 0.70, even more preferably 0.10 to 0.60, and most preferably 0.15 to 0.60. This atomic ratio can be measured by energy dispersive X-ray spectroscopy (EDS), electron probe microanalyzer (EPMA), or ICP atomic emission spectroscopy.

Preferred oxides as oxide a and oxide b are oxides represented by the general formula: La1 _{- xAxBO3 -δ} (wherein A is an element containing Ba or Sr or both. B is one or more elements selected from Co, Ni, Fe, Cu, Ti, Zr and Nb. In particular, B is preferably one or more elements selected from Co, Ni, Fe, Cu and Zr. x is a number of 0.01 or more and 0.80 or less).

If the cathode 12 and the anode 13 have a predetermined thickness, the oxide ion conductivity of the solid electrolyte assembly 10 can be more effectively increased. In particular, the thickness of the cathode 12 and the anode 13 bonded to the intermediate layer is preferably 100 nm or more, more preferably 500 nm or more, and even more preferably 1000 nm or more and 30000 nm or less. The thickness of the cathode 12 and the anode 13 can be measured by a stylus step gauge or an electron microscope.

The solid electrolyte junction 10 of the embodiment shown in FIG. 1 can be suitably manufactured, for example, by the method described below. First, the solid electrolyte layer 11 is manufactured by a known method. For the manufacturing, for example, the methods described in JP 2013-51101 A and WO 2016/111110 can be adopted. The solid electrolyte layer is preferably composed of an oxide containing lanthanum.

Next, the cathode side intermediate layer 15 and the anode side intermediate layer 16 are formed on the two main surfaces of the solid electrolyte layer 11, respectively. For example, sputtering can be used to form each intermediate layer 15, 16. The target used for sputtering can be manufactured, for example, by the following method. That is, a powder of an oxide of a rare earth metal (excluding cerium) and a powder of cerium oxide are mixed using a stirrer such as a mortar or a ball mill, and sintered in an oxygen-containing atmosphere to obtain a raw material powder. This raw material powder is molded into the shape of a target and hot-press sintered. The sintering conditions can be a temperature of 1000°C to 1400°C, a pressure of 20 MPa to 35 MPa, and a time of 60 minutes to 180 minutes. The atmosphere can be an inert gas atmosphere such as nitrogen gas or rare gas. The sputtering target obtained in this manner is composed of LnDC. Note that the manufacturing method of the sputtering target is not limited to this manufacturing method, and for example, a molded body in the shape of a target may be sintered in the air atmosphere or in an oxygen-containing atmosphere.

The LnDC target thus obtained is used to form a sputtering layer on each surface of the solid electrolyte layer 11, for example by radio-frequency sputtering. The temperature of the substrate may be raised in advance to within the range of 300 to 500°C, and sputtering may be performed while maintaining this temperature. The sputtering layer is composed of LnDC.

After the sputtering is completed, the intermediate layer made of LnDC is sintered by heat treatment. This sintering process grows the crystal grains of LnDC that constitute the intermediate layer. This sintering process also improves the adhesion between the intermediate layer and the solid electrolyte layer.
In this manufacturing method, a sintering object in which LnDC is disposed on a solid electrolyte made of an oxide containing lanthanum is sintered under predetermined conditions to grow crystal grains of LnDC. As for the sintering conditions, it is advantageous from the viewpoint of crystal grain growth to control (i) the heating rate during heating, (ii) the holding temperature, (iii) the holding time, and (iv) the cooling rate during cooling.

The heating rate is preferably set to 1° C./hr or more and 2000° C./hr or less, more preferably 5° C./hr or more and 1500° C./hr or less, and even more preferably 10° C./hr or more and 1000° C./hr or less.
The holding temperature is preferably set to 700°C or higher and 2000°C or lower, more preferably 1000°C or higher and 1800°C or lower, and even more preferably 1300°C or higher and 1700°C or lower.
With regard to the holding time, provided that the holding temperature is within the above-mentioned range, it is preferably set to 1 minute or more and 36 hours or less, more preferably 5 minutes or more and 24 hours or less, and even more preferably 10 minutes or more and 12 hours or less.
The temperature drop rate is preferably set to 1° C./hr or more and 500° C./hr or less, more preferably 5° C./hr or more and 450° C./hr or less, and even more preferably 10° C./hr or more and 400° C./hr or less.

As for the firing atmosphere, an oxygen-containing atmosphere such as air or an inert gas atmosphere such as nitrogen or argon can be used.

When the solid electrolyte layer is made of an oxide containing lanthanum, depending on the firing conditions of the object to be fired, the lanthanum contained in the solid electrolyte layer may thermally diffuse into the intermediate layer made of LnDC. In this case, the intermediate layer after firing will be made of LnDC containing lanthanum (i.e., the La-LnDC mentioned above).

The firing of the object to be fired is preferably performed in a state in which the object to be fired is placed in a crucible consisting of a main body having an opening and a lid that closes the opening of the main body, from the viewpoint of preventing unintended contamination of impurities. In particular, firing the object to be fired in a state in which a rare earth metal oxide or rare earth metal hydroxide is placed in the gap between the opening of the main body of the crucible and the lid is preferable from the viewpoint of effectively preventing contamination of impurities, particularly transition metals such as molybdenum (excluding rare earth metals). By placing a rare earth metal oxide or rare earth metal hydroxide in the gap, the transition metal contained in the heating element used during firing is captured by the oxide or hydroxide. This prevents the transition metal from flowing into the crucible, and as a result, contamination of the object to be fired is prevented.
From the viewpoint of more effectively preventing the inclusion of impurities, particularly transition metals (but excluding rare earth metals), the rare earth metal oxide is preferably an oxide of lanthanum or neodymium. From the same viewpoint, the rare earth metal hydroxide is preferably a hydroxide of lanthanum or neodymium.

The rare earth metal oxide or rare earth metal hydroxide may be filled into the gap to the extent that gas can flow between the inside of the crucible and the outside world, but it is more preferable that it is arranged so as to completely seal the gap between the main body and the lid of the crucible.

The above-mentioned firing process results in an intermediate layer in which the crystal grains of LnDC have grown sufficiently. Next, the anode 13 and the cathode 12 are formed on the surface of each intermediate layer. When the cathode 12 and/or the anode 13 are metal electrodes, a paste containing particles of a platinum group metal, for example, can be used to form the metal electrodes. The paste is applied to the surface of the intermediate layer to form a coating film, and the coating film is fired to form a metal electrode made of a porous body. The firing conditions can be a temperature of 600°C to 900°C and a time of 30 minutes to 120 minutes. The atmosphere can be an oxygen-containing atmosphere such as air.

On the other hand, when the cathode 12 and/or the anode 13 are made of an oxide having oxide ion conductivity, for example, an oxide having a cubic perovskite structure represented by ABO3 _-δ, a method can be adopted in which a slurry containing a powder of the oxide is applied to the surface of the intermediate layer to form a coating film, and the coating film is then fired. The slurry can be obtained, for example, by adding a powder of the oxide to a binder in which ethyl cellulose is dissolved in α-terpineol, and adjusting the concentration.
The concentration of the oxide powder in the slurry can be, for example, 10% by mass or more and 70% by mass or less. The firing conditions for the coating film formed by applying this slurry can be, for example, an oxygen-containing atmosphere such as air or an inert gas atmosphere such as nitrogen or argon. The firing temperature is preferably 700° C. or more and 1200° C. or less, more preferably 800° C. or more and 1100° C. or less, and even more preferably 900° C. or more and 1000° C. or less. The firing time is preferably 1 hour or more and 10 hours or less, more preferably 3 hours or more and 8 hours or less, and even more preferably 5 hours or more and 7 hours or less. The cathode side intermediate layer 15 and the anode side intermediate layer 16 may be formed simultaneously or successively.

The above method can provide the desired solid electrolyte joint 10. The solid electrolyte joint 10 thus obtained has high oxide ion conductivity. Specifically, the solid electrolyte joint 10 exhibits a high current density of preferably 170 mA/ ^cm2 or more, more preferably 200 mA/ ^cm2 or more, and even more preferably 250 mA/cm2 or more, measured under conditions of an applied voltage of 0.5 V, a temperature of 600° ^C , and an air atmosphere.

The solid electrolyte joint 10 has a low resistance such that the sum (R b ) of the internal resistances of the solid electrolyte constituting the solid electrolyte joint, the crystal grains in the intermediate layer, and the crystal grain boundaries, etc., is preferably 1.4 _Ωcm2 or ^less , more preferably 1.2 Ωcm2 or ^less , and even more preferably 1.0 Ωcm2 or ^less , at a temperature of 600° C. in an air atmosphere.

The solid electrolyte joint 10, taking advantage of its high oxide ion conductivity, is suitably used as, for example, an oxygen permeation element, a gas sensor, a solid electrolyte fuel cell, etc. Whatever the application of the solid electrolyte joint 10, it is advantageous to use La-LnDC as the intermediate layer 15 on the side of the cathode 12, which is the electrode where the reduction reaction of oxygen gas occurs.
For example, when the solid electrolyte junction 10 is used as an oxygen permeable element, the cathode 12 is connected to the negative electrode of a DC power supply, and the anode 13 is connected to the positive electrode of the DC power supply, and a predetermined DC voltage is applied between the cathode 12 and the anode 13. As a result, oxygen receives electrons on the cathode 12 side to generate oxide ions. The generated oxide ions move through the solid electrolyte layer 11 and reach the anode 13. The oxide ions that reach the anode 13 release electrons and become oxygen gas. By such a reaction, the solid electrolyte layer 11 is capable of transmitting oxygen gas contained in the atmosphere on the cathode 12 side through the solid electrolyte layer 11 to the anode 13 side. If necessary, a current collecting layer made of a conductive material such as platinum may be further formed on at least one of the surfaces of the cathode 12 and the anode 13.

When the solid electrolyte junction 10 is also used as a limiting current type oxygen sensor, a current is generated due to the oxide ions generated on the cathode 12 side migrating to the anode 13 side via the solid electrolyte layer 11. Since the current value depends on the oxygen gas concentration on the cathode 12 side, the oxygen gas concentration on the cathode 12 side can be measured by measuring the current value.

Although the present invention has been described above based on its preferred embodiment, the present invention is not limited to the above embodiment. For example, in the above embodiment, an intermediate layer is disposed between both the cathode 12 and the solid electrolyte layer 11 and between the anode 13 and the solid electrolyte layer 11. Alternatively, an intermediate layer may be disposed only between the cathode 12 and the solid electrolyte layer 11, or only between the anode 13 and the solid electrolyte layer 11. When an intermediate layer is disposed only on one of the cathode 12 or the anode 13, it is preferable to dispose the intermediate layer only between the cathode 12 and the solid electrolyte layer 11 from the viewpoint of effectively improving the oxide ion conductivity of the entire solid electrolyte junction 10.

The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples.

In this example, the solid electrolyte assembly 10 shown in FIG. 1 was manufactured according to the following steps (1) to (4).
(1) Manufacturing of solid electrolyte layer _{11 La2O3} powder and _SiO2 powder were blended in a molar ratio of 1:1, and ethanol was added and mixed in a ball mill. This mixture was dried, crushed in a mortar, and fired in an air atmosphere at 1650 ° C for 3 hours using a platinum crucible. Ethanol was added to this fired product, and it was crushed in a planetary ball mill to obtain a fired powder. This fired powder was placed in a 20 mmφ molding machine and pressed from one direction to perform uniaxial molding. Furthermore, cold isostatic pressing (CIP) was performed at 600 MPa for 1 minute to form a pellet. This pellet-shaped molded body was heated in air at 1600 ° C for 3 hours to obtain a pellet-shaped sintered body. When this sintered body was subjected to X-ray diffraction measurement and chemical analysis, it was confirmed that it had a structure of _{La2SiO5 .}

800 mg of the pellets obtained and 140 mg of B ₂ O ₃ powder were placed in a sagger with a lid and heated in air at 1550°C (furnace atmosphere temperature) for 50 hours using an electric furnace. This heating generated B ₂ O ₃ vapor in the sagger and reacted the B ₂ O ₃ vapor with the pellets to obtain the desired solid electrolyte layer 11. This solid electrolyte layer 11 had a structure of La _9.33+x [Si _6.00-y B _y ]O _26.0+z , where x = 0.50, y = 1.17, and z = 0.16, and the molar ratio of La to B was 8.43 (hereinafter, this compound is abbreviated as "LSBO"). The oxide ion conductivity at 600°C was 6.3 x 10 ^-2 S/cm. The thickness of the solid electrolyte layer 11 was 350 μm.

(2) Manufacturing of the cathode-side intermediate layer 15 and the anode-side intermediate layer 16 The powder of _Sm0.2Ce1.8O2 was placed in a molding machine _with a diameter of 50 mm _, pressed from one direction to perform uniaxial molding, and then hot press sintering was performed. The sintering conditions were a nitrogen gas atmosphere, a pressure of 30 MPa, a temperature of 1200°C, and 3 hours. In this way, a target for sputtering was obtained. Using this target, sputtering was performed on each surface of the solid electrolyte layer 11 made of LSBO by a high-frequency sputtering method, and a sputtering layer of samarium-doped cerium oxide (hereinafter also referred to as "SDC"). The sputtering conditions were an RF output of 400 W and an argon gas pressure of 0.5 Pa.
After sputtering, the sputtered layer was fired in an air atmosphere under the conditions shown in Table 1 below. The firing was performed in a crucible consisting of a body and a lid. _La2O3 powder was filled into the gap between the opening of the body in the _crucible and the lid, and the body and the lid were sealed. In this way, a cathode-side intermediate layer 15 and an anode-side intermediate layer 16 made of SDC were produced. The thickness of each of the intermediate layers 15 and 16 was 300 nm.

(3) Manufacturing of the Cathode 12 and the Anode 13 As the oxide constituting the cathode 12 and the anode 13, a powder of La _0.6 Sr _0.4 Co _0.8 Ni _0.02 Fe _0.18 O _3-δ (hereinafter also referred to as "LSCNF") having a perovskite structure was used. This oxide was obtained by the following method. First, lanthanum nitrate, strontium nitrate, cobalt nitrate, iron nitrate, nickel nitrate, and DL-malic acid were dissolved in ion-exchanged water, and ammonia water was added while stirring to adjust the pH to about 5.0 to 6.0. Next, the solution was evaporated at about 350°C to obtain a powder (build-up method). The obtained powder was pulverized in a mortar. The powder obtained in this manner was pre-calcined in air at 900°C for 5 hours to produce the desired LSCNF powder. From the X-ray diffraction peak, it was confirmed that this LSCNF was a single phase of a perovskite structure represented by ABO _3-δ .
A 25% by mass paste was prepared by dispersing LSCNF powder in a binder prepared by dissolving ethyl cellulose in α-terpineol. This paste was applied to the surfaces of the cathode-side intermediate layer 15 and the anode-side intermediate layer 16 to form coating films. These coating films were fired at 900° C. for 5 hours in an air atmosphere to obtain the cathode 12 and anode 13 made of a porous body. The thickness of each of the cathode 12 and anode 13 was 20 μm.

(4) Manufacturing of the current collecting layer Platinum paste was applied to the surfaces of the cathode 12 and the anode 13 to form a coating film. These coating films were fired at 700° C. for 1 hour in the air to obtain a current collecting layer. In this manner, the solid electrolyte joint 10 was manufactured.

[Examples 2 and 3]
The firing conditions for the sputtering layer made of SDC were changed as shown in Table 1. A solid electrolyte layer 11 was obtained in the same manner as in Example 1 except for this.

Comparative Examples 1 to 3
The firing conditions of the sputtering layer made of SDC were changed as shown in Table 1. In addition, when firing the sputtering layer made of SDC, the gap between the main body and the lid in the crucible was not sealed with _{La2O3 .} A solid electrolyte layer was obtained in the same manner as in Example 1 except for these.

〔evaluation〕
For the solid electrolyte assemblies obtained in the examples and comparative examples, the average value of the equivalent circle diameter of the crystal grains in the intermediate layer was determined by the method described below.
In addition, the Ln _T /Ce atomic ratio in the intermediate layer was measured by the method described below.
Furthermore, the current density of the solid electrolyte joint was measured under conditions of an applied voltage of 0.5 V, a temperature of 600° C., and an air atmosphere by the method described below.
Furthermore, the impedance characteristics of the solid electrolyte joint at a temperature of 600° C. in an air atmosphere, i.e., the sum (R _b ) of internal resistances of the solid electrolyte constituting the solid electrolyte joint, the grain interior and grain boundaries of the intermediate layer, and the like, and the polarization resistance (R _p ) caused by the anode and cathode constituting the solid electrolyte joint, were measured by the method described below.
The results are shown in Table 1 below.

[Average circle equivalent diameter of crystal grains]
An arbitrary region of the surface of the intermediate layer facing the anode or cathode was observed with a scanning electron microscope. The observation magnification was 2,000 to 10,000 times, and the average area per crystal grain (S _ave ) of 20 crystal grains in one observed field of view was obtained using image analysis software Image-Pro. Based on the obtained S _ave value, the average circle equivalent diameter (D _ave ) of the crystal grains was calculated using the following formula (1).
D _ave = (4×S _ave /π) ^1/2 ... (1)

[Ln _T /Ce atomic ratio and Mo/Ce atomic ratio]
The Ln _T /Ce ratio and the Mo/Ce ratio were calculated from the analysis results obtained by measuring the surface of the intermediate layer facing the anode or cathode by energy dispersive X-ray spectroscopy (EDS) and quantitatively analyzing each element.

[Current density]
The measurement was carried out at 600° C. A direct current of 0.5 V was applied between the current collectors of the solid electrolyte joint in the air, and the current density was measured.

[Impedance characteristics]
An AC voltage of 10 mV was applied between the current collectors of the solid electrolyte joint in the air at 600° C., and the impedance was measured in the frequency range of 0.1 Hz to 200 kHz, to obtain a Cole-Cole plot as shown in FIG. 2. The Cole-Cole plot was subjected to equivalent circuit fitting by the complex nonlinear least squares (CNLS) method. In the CNLS equivalent circuit fitting, the number of elements and the value of each component of the R-CPE circuit as shown in FIG. 3, which consists of resistance (R) and CPE (Constant Phase Element), were calculated using the distributed relaxation time (DRT) method. The CNLS method was performed using R, relaxation time (t), and CPE power (p) obtained by the DRT method as initial values, and an equivalent circuit as shown in FIG. 4 was obtained.
4 is composed of R ₀ and multiple R-CPE (R ₁ -CPE ₁ , R ₂ -CPE ₂ , ... R _n -CPE _n ) circuits. _{R 0} represents the sum (R _b ) of internal resistances of the solid electrolyte constituting the solid electrolyte assembly, the grains in the intermediate layer, and the grain boundaries, and R ₁ +R ₂ + ... R _n represents the polarization resistance (R _p ) caused by the anode and cathode constituting the solid electrolyte assembly.

As is clear from the results shown in Table 1, the solid electrolyte assemblies obtained in the examples have a higher current density and a lower total internal resistance of the crystal grains and crystal grain boundaries of the solid electrolyte and intermediate layer constituting the solid electrolyte assemblies, compared to the solid electrolyte assemblies of the comparative examples. In other words, it is found that the solid electrolyte assemblies obtained in the examples have high oxide ion conductivity as a whole, and the electrical resistance between each portion of the solid electrolyte assemblies is reduced.
Although not shown in Table 1, no transition metals (excluding rare earth metals) other than molybdenum were detected in the intermediate layer of the solid electrolyte junction obtained in the Examples.

REFERENCE SIGNS LIST 10 solid electrolyte assembly 11 solid electrolyte layer 12 cathode 13 anode 15 cathode-side intermediate layer 16 anode-side intermediate layer

Claims (12)

A solid electrolyte assembly having an anode, a cathode, and a solid electrolyte disposed therebetween,
an intermediate layer between at least one of the anode and the cathode and the solid electrolyte;
the solid electrolyte comprises an apatite-type composite oxide containing lanthanum and silicon ,
the intermediate layer contains cerium oxide containing lanthanum and at least one selected from the group consisting of samarium, gadolinium, yttrium, erbium, ytterbium, and dysprosium , and does not contain a transition metal (excluding rare earth metals) or contains a transition metal (excluding rare earth metals) in an amount of 7.0 at % or less relative to cerium;
A solid electrolyte joint, wherein the average equivalent circle diameter of crystal grains in the intermediate layer is 1.0 μm or more and 10 μm or less. The solid electrolyte junction according to claim 1, wherein the atomic ratio of rare earth metal to cerium in the intermediate layer is 0.3 or more and 1.5 or less. 3. The solid electrolyte joint according to claim 1, which has a current density of 170 mA/ ^cm2 or more under conditions of an applied voltage of 0.5 V, a temperature of 600°C, and an air atmosphere. 4. The solid electrolyte joint according to claim 1, wherein the sum of internal resistances ( _Rb ) is 1.4 ^Ωcm2 or less under conditions of a temperature of 600°C and an air atmosphere. The thickness of the solid electrolyte is 10 nm or more and 1000 μm or less,
5. The solid electrolyte joint according to claim 1, wherein the intermediate layer has a thickness of 1 nm or more and 1000 nm or less. The solid electrolyte has a formula A _9.33+x [T _6.00-y M _y ]O _26.00+z (wherein A is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T is an element containing Si or Ge or both. M is Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, S and n is one or more elements selected from the group consisting of W and Mo. In the formula, x is a number from −1.33 to 1.50, y is a number from 0.00 to 3.00, z is a number from −5.00 to 5.20, and a ratio of the number of moles of A to the number of moles of T (A/T) is 1.33 to 3.61 . 7. The solid electrolyte joint according to claim 6 , wherein the degree of orientation of the solid electrolyte measured by a Lotgering method is 0.60 or more. A method for producing a solid electrolyte joint, comprising: a step of firing an object to be fired, the object being made of a solid electrolyte made of an oxide containing lanthanum, and an intermediate layer which contains cerium oxide containing one or more rare earth metals (excluding cerium) and which does not contain a transition metal (excluding rare earth metals) or which contains a transition metal (excluding rare earth metals) in an amount of 7.0 at % or less relative to cerium, the intermediate layer being disposed on the solid electrolyte made of an oxide containing lanthanum,
The object to be fired is placed in a crucible having a main body having an opening and a lid for closing the opening of the main body, and a rare earth metal oxide or a rare earth metal hydroxide is placed in a gap between the opening of the main body and the lid, and the object to be fired is fired;
A method for producing a solid electrolyte joint, comprising: increasing the temperature at a rate of 1°C/hr to 2000°C/hr; holding the temperature in the range of 700°C to 2000°C for 1 minute to 36 hours; and cooling at a rate of 1°C/hr to 500°C/hr. The method according to claim 8 , wherein the rare earth metal oxide or the rare earth metal hydroxide is an oxide of lanthanum or neodymium or a hydroxide of lanthanum or neodymium. 10. The method according to claim 8 , wherein the rare earth metal is one selected from the group consisting of lanthanum, samarium, gadolinium, yttrium, erbium, ytterbium and dysprosium. The method according to claim 8 , wherein the solid electrolyte contains a composite oxide of lanthanum and silicon. The solid electrolyte has a formula _A9.33+x [T6 _{-yMy ] O26.00+z} (wherein A is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T is an element containing Si or Ge, or both. M is Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, 12. The method according to claim 8, wherein x is a number equal to or greater than -1.33 and equal to or less than 1.50, y is a number equal to or greater than 0.00 and equal to or less than 3.00, z is a number equal to or greater than -5.00 and equal to or less than 5.20, and a ratio of the number of moles of A to the number of moles of T (A/T) is 1.33 or greater and equal to or less than 3.61 .

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