JP7155812B2 - Oxide ion conductor and electrochemical device - Google Patents

Description

The present invention relates to oxide ion conductors and electrochemical devices, and more particularly, to an AMX ₂ O ₆ structure (where A is a monovalent metal element, M is a trivalent metal element, X is a tetravalent metal element ), and an electrochemical device using the same.

“Oxide ion conductor” refers to a material in which oxide ions (O ²⁻ ) preferentially diffuse in a solid. Oxide ion conductors are used as solid electrolytes in electrochemical devices such as solid oxide fuel cells (SOFC), oxygen gas sensors, and elements for purifying exhaust gas using electrochemical reactions. At present, yttria-stabilized zirconia (YSZ) to which about 8 mol % of Y ₂ O ₃ is added is widely used as an oxide ion conductor.

YSZ exhibits a high oxide ion conductivity of 10 ^-2 S/cm or higher at high temperatures (700°C or higher) (Non-Patent Document 1), and is used at operating temperatures around 1000°C. However, from the viewpoint of suppressing deterioration of peripheral members and improving energy utilization efficiency, it is required to lower the operating temperature of the electrochemical device. In particular, in the medium temperature range of 300°C to 500°C, materials with high oxide ion conductivity are desired.

Scandia (Sc ₂ O ₃ )-stabilized zirconia (ScSZ) has been investigated as an electrolyte that enables low-temperature operation of SOFCs. Lanthanum gallate (LaGaO ₃ )-based electrolytes are considerably inferior to zirconia in terms of strength, but are used in household fuel cells as electrolytes capable of significantly operating at low temperatures. Furthermore, the use of ceria-based electrolytes has also been attempted. However, electrolytes other than YSZ have problems in strength, oxidation-reduction resistance, price, etc., and have not been sufficiently successful.

P. J. Gellings and H. Bouwmeester, Handbook of solid state electrochemistry (CRC press, 1997), pp. 196

The problem to be solved by the present invention is to provide a novel oxide ion conductor that can be operated at low temperatures.
Another problem to be solved by the present invention is to provide an electrochemical device using such an oxide ion conductor.

In order to solve the above problems, the oxide ion conductor according to the present invention comprises:
An oxide having an AMX ₂ O ₆ structure (where A is a monovalent metal element, M is a trivalent metal element, and X is a tetravalent metal element),
It has a composition represented by the following formula (1).
(A1 _{- xBx} )(M1 _{- yCy} )(X1 _{-zDz ) 2O6 ±δ} (1)
however,
A is any one or more elements selected from the group consisting of Li, Na, K, Rb, and Cs;
The M is any one selected from the group consisting of Al, Ga, In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y one or more elements,
X is any one or more elements selected from the group consisting of Ge, Si, and Sn;
The B is an element different from the A, and is any one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Cu, and Ba;
The C is an element different from the M, and includes La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In, Sc, any one or more elements selected from the group consisting of Y, Ba, Sr, and Ca;
The D is an element different from the X and is any one or more elements selected from the group consisting of Ge, Si, Al, Ga, and Sn,
0≤x≤0.2, 0≤y≤0.2, 0≤z≤0.2,
δ is the value at which electroneutrality is maintained.

An electrochemical device according to the present invention uses the oxide ion conductor according to the present invention.

An oxide ion conductor having an AMX ₂ O ₆ structure has a relatively high ionic conductivity at 400 to 700° C., and depending on the composition, exhibits almost the same ionic conductivity as YSZ. Therefore, it can be applied to electrochemical devices such as SOFCs. In addition, the range of selection of materials used for peripheral members is widened. Therefore, improvement in performance, cost reduction, and durability improvement of electrochemical devices can be expected.

KEuGe ₂ O ₆ crystal structure.

An embodiment of the present invention will be described in detail below.
[1. oxide ion conductor]
The oxide ion conductor according to the present invention comprises an oxide having an AMX ₂ O ₆ structure (where A is a monovalent metal element, M is a trivalent metal element, and X is a tetravalent metal element). .

[1.1. Crystal structure]
FIG. 1 shows the crystal structure of KEuGe ₂ O ₆ , a kind of oxide with the AMX ₂ O ₆ structure. KEuGe ₂ O ₆ has a three-membered ring (Ge ₃ O ₉ ) ^6- . Ge ⁴⁺ is located at the center of the [GeO ₄ ] tetrahedron that constitutes the three-membered ring, and K ⁺ and Eu ³⁺ are located between the three-membered rings (see Reference 1). The three-membered rings are connected via the oxygen at the vertices, and it is thought that ion conduction is caused by conduction of oxide ions along the three-membered rings. Alkali metal ions such as K ⁺ are generally easily conductive ions, but KEuGe ₂ O ₆ has the characteristic of not conducting K ⁺ but conducting oxide ions. These points are the same for AMX ₂ O ₆ other than KEuGe ₂ O ₆ .
Various oxides having the AMX ₂ O ₆ structure are known, but it is not known that they function as oxide ion conductors. Moreover, it is not known that addition of appropriate dopants (elements B, C, and D) to oxides having the AMX ₂ O ₆ structure improves the ionic conductivity.
[Reference 1] Pei-Lin Chen et. al., Dalton Trans., 2008, 1721-1726

[1.2. Composition (1)]
An oxide ion conductor according to the present invention has a composition represented by the following formula (1).
(A1 _{- xBx} )(M1 _{- yCy} )(X1 _{-zDz ) 2O6 ±δ} (1)
however,
A is any one or more elements selected from the group consisting of Li, Na, K, Rb, and Cs;
The M is any one selected from the group consisting of Al, Ga, In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y one or more elements,
X is any one or more elements selected from the group consisting of Ge, Si, and Sn;
The B is an element different from the A, and is any one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Cu, and Ba;
The C is an element different from the M, and includes La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In, Sc, any one or more elements selected from the group consisting of Y, Ba, Sr, and Ca;
The D is an element different from the X and is any one or more elements selected from the group consisting of Ge, Si, Al, Ga, and Sn,
0≤x≤0.2, 0≤y≤0.2, 0≤z≤0.2,
δ is the value at which electroneutrality is maintained.

[1.2.1. Element A]
Element A is an element positioned at the site occupied by K in the KEuGe ₂ O ₆ structure shown in FIG. In the present invention, element A is a monovalent metal element consisting of Li, Na, K, Rb, or Cs. Any of these elements can occupy the site and is suitable as the element A. The oxide ion conductor may contain one of these elements A, or may contain two or more of them.
In the present invention, the term "metal element" includes semimetals such as Si and Ge.

[1.2.2. Element M]
Element M is an element positioned at the site occupied by Eu in the KEuGe ₂ O ₆ structure shown in FIG. In the present invention, the element M is a trivalent metal element, Al, Ga, In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu , Sc, or Y. Any of these elements can occupy the site and is suitable as the element M. The oxide ion conductor may contain one of these elements M, or may contain two or more of them.

[1.2.3. Element X]
Element X is an element positioned at the site occupied by Ge in the KEuGe ₂ O ₆ structure shown in FIG. In the present invention, the element X is a tetravalent metal element consisting of Ge, Si, or Sn. Any of these elements can occupy the site and is suitable as the element X. The oxide ion conductor may contain any one of these elements X, or may contain both of them.

[1.2.4. Element B]
Element B is a dopant that replaces element A. In the present invention, element B is an element different from element A and consists of Li, Na, K, Rb, Cs, Cu, or Ba. The oxide ion conductor may contain one of these elements B, or may contain two or more of them.
These elements B can substitute the element A because they all have ionic radii close to that of the element A. Further, when the valences of the element B and the element A are different, oxygen vacancies or interstitial oxygen are introduced into the crystal lattice, so that the ionic conductivity is higher than that of a material that does not contain the element B.

Element B preferably has an ionic radius difference of 15% or less from element A (Hume-Rothery's law). The ionic radius difference is preferably 12% or less, more preferably 10% or less.
Here, "the ionic radius difference (ΔR _BA ) between the element B and the element A" means a value represented by the following formula (2.1).
ΔR _BA (%)=|R _B −R _A |×100/R _A (2.1)
however,
RB is the ionic radius of element _B ;
RA is the ionic radius of element _A ;

[1.2.5. Element C]
Element C is a dopant that replaces element M. In the present invention, element C is an element different from element M and includes La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, Consists of In, Sc, Y, Ba, Sr, or Ca. The oxide ion conductor may contain one of these elements C, or may contain two or more of them.
These elements C have ionic radii close to that of the element M, so they can substitute for the element M. In addition, when the element C and the element M have different valences, oxygen vacancies or interstitial oxygen are introduced into the crystal lattice, so that the ionic conductivity is higher than that of a material that does not contain the element C.

Element C preferably has an ionic radius difference of 15% or less from element M (Hume-Lothery's law). The ionic radius difference is preferably 12% or less, more preferably 10% or less.
Here, "the ionic radius difference (ΔR _CM ) between the element C and the element M" means a value represented by the following formula (2.2).
ΔR _CM (%)=|R _C −R _M |×100/R _M (2.2)
however,
R _C is the ionic radius of element C;
RM is the ionic radius of element _M ;

[1.2.6. Element D]
Element D is a dopant that replaces element X. In the present invention, element D is an element different from element X and consists of Ge, Si, Al, Ga, or Sn. The oxide ion conductor may contain any one of these elements D, or may contain two or more of them.
These elements D have ionic radii close to that of the element X, and therefore can substitute for the element X. Also, when the valences of the element D and the element X are different, oxygen vacancies or interstitial oxygen are introduced into the crystal lattice, so that the ionic conductivity is higher than that of a material that does not contain the element C.

Element D preferably has an ionic radius difference of 15% or less from element X (Hume-Lothery's law). The ionic radius difference is preferably 12% or less, more preferably 10% or less.
Here, "the ionic radius difference (ΔR _DX ) between the element D and the element X" means a value represented by the following formula (2.3).
ΔR _DX (%)=|R _D −R _X |×100/R _X (2.3)
however,
_RD is the ionic radius of element D;
R _X is the ionic radius of element X;

[1.2.7. x]
x represents the amount of element B substituting element A; The oxide ion conductor according to the present invention exhibits relatively high ionic conductivity even when element B is not included. That is, x may be zero. In general, the larger x, the higher the ionic conductivity. In order to obtain such effects, x is preferably 0.01 or more. x is preferably 0.03 or more, more preferably 0.05 or more.
On the other hand, if x is too large, not all of the element B replaces the element A and may precipitate as a different phase. Therefore, x is preferably 0.2 or less. x is preferably 0.15 or less, more preferably 0.1 or less.

[1.2.8. y]
y represents the amount of element C substituting element M; The oxide ion conductor according to the present invention exhibits relatively high ionic conductivity even when element C is not included. That is, y may be zero. In general, the higher y, the higher the ionic conductivity. In order to obtain such effects, y is preferably 0.01 or more. y is preferably 0.03 or more, more preferably 0.05 or more.
On the other hand, if y is too large, not all of the element C replaces the element M and may precipitate as a different phase. Therefore, y is preferably 0.2 or less. y is preferably 0.15 or less, more preferably 0.1 or less.

[1.2.9. z]
z represents the amount of element D substituting element X; The oxide ion conductor according to the present invention exhibits relatively high ionic conductivity even when element D is not included. That is, z may be zero. In general, the higher z, the higher the ionic conductivity. In order to obtain such effects, z is preferably 0.01 or more. z is preferably 0.03 or more, more preferably 0.05 or more.
On the other hand, if z is too large, not all of the element D replaces the element X and may precipitate as a heterogeneous phase. Therefore, z is preferably 0.2 or less. z is preferably 0.15 or less, more preferably 0.1 or less.

[1.2.10. δ]
In the absence of dopants (elements B, C, D), δ is ideally zero. However, in practice, oxygen vacancies and interstitial oxygen may be generated even when no dopant is contained. Further, when dopants (elements B, C, D) having different valences from the main constituent elements (elements A, M, X) are added, oxygen vacancies or Interstitial oxygen is produced. When the valences of elements B, C, and D are b, c, and d, respectively, it can be expressed formally as 2δ=(b−1)x+(c−3)y+2(d−4)z.

[1.3. Composition (2)]
The oxide ion conductor according to the present invention particularly preferably has a composition represented by the following formula (1.1).
(K1 _{- xBx} )(Eu1 _{- yCy} )(Ge1 _{- zDz} ) _{2O6 ±δ} (1.1)
however,
B is any one or more elements selected from the group consisting of Li, Na, Rb, Cs, Cu, and Ba;
C is from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In, Sc, Y, Ba, Sr, and Ca any one or more elements selected,
D is any one or more elements selected from the group consisting of Si, Al, Ga, and Sn;
0≤x≤0.2, 0≤y≤0.2, 0≤z≤0.2,
δ is the value at which electroneutrality is maintained.

In formula (1.1), details of B, C, D, x, y, z, and δ are the same as in formula (1), so description thereof is omitted.
The composition represented by formula (1.1) is stable at high temperatures (up to 900° C.) in the atmosphere and exhibits high oxide ion conductivity. In particular, since it is composed mainly of elements such as K, Eu, and Ge, which are relatively difficult to move compared to oxide ions, there is no fear that the structure will be broken due to the movement of elements other than oxide ions.

[1.4. Application]
The oxide ion conductor according to the present invention can be used for solid electrolytes of various electrochemical devices. Electrochemical devices to which the present invention is applied include, for example,
(a) a solid oxide fuel cell (SOFC);
(b) an oxygen gas sensor;
(c) exhaust gas purifying element,
(d) a solid oxide electrolytic cell (SOEC);
and so on.

[2. Method for producing oxide ion conductor]
The oxide ion conductor according to the present invention is
(a) mixing raw materials so as to have a predetermined composition;
(b) calcining the raw material mixture under predetermined conditions;
(c) It can be produced by appropriately pulverizing the calcined powder and then molding and sintering the calcined powder.
The production conditions are not particularly limited, and it is preferable to select the optimum conditions according to the target composition.

[3. action]
YSZ is currently widely used as a solid electrolyte for electrochemical devices such as SOFC. However, YSZ
(a) the ionic conductivity is still insufficient,
(b) the conduction activation energy is relatively high (approximately 100 kJ/mol), and the ionic conductivity drops sharply at temperatures below 700°C;
(c) Since it is usually used at a high temperature of about 800 to 1000 ° C., it is necessary to use a material that has high heat resistance and does not react with YSZ in the operating temperature range for the peripheral members of YSZ.
(d) As an oxide, it has a high density, so the apparatus used is heavy.
There are problems such as

In order to solve this problem, methods of lowering the operating temperature of electrochemical devices have been proposed. For example, in the case of SOFC, as a method of lowering the operating temperature,
(a) a method of making the electrolyte into an extremely thin film;
(b) a method of reducing overvoltage using high performance electrodes;
(c) A method of developing a novel material with high ionic conductivity and the like have been proposed.

Among these, the method of thinning the electrolyte membrane and the method of searching for electrode materials are not essential solutions. As electrolytes other than YSZ, scandia (Sc ₂ O ₃ )-stabilized zirconia (ScSZ), lanthanum gallate (LaGaO ₃ )-based electrolytes, and ceria-based electrolytes have been proposed. There are problems with reduction resistance, price, etc., and we have not seen sufficient success.

On the other hand, oxide ion conductors having the AMX ₂ O ₆ structure have relatively high ionic conductivity at 400 to 700° C., and depending on the composition, exhibit almost the same ionic conductivity as YSZ. Specifically, the conductivity of YSZ at 700°C is 10 ^-2 to 10 ^-3 S/cm@700°C. On the other hand, the ion conductivity at 700°C of KEu _0.8 Ca _0.2 Ge ₂ O ₆ , which is a kind of oxide ion conductor according to the present invention, is 0.18 × 10 ^-2 S/cm @ 700°C.

In addition, the oxide ion conductor having the AMX ₂ O ₆ structure has a relatively high ion conductivity in a low temperature range of 400 to 500° C., and depending on the composition, shows almost the same ion conductivity as YSZ. Specifically, the conductivity of YSZ at 500° C. and 400° C. is 5.0×10 ⁻⁴ S/cm @ 500° C. and 3.0×10 ⁻⁵ S/cm @ 400° C., respectively. On the other hand, the conductivity at 500° C. and 400° C. of KEu _0.8 Ca _0.2 Ge ₂ O ₆ , which is a kind of oxide ion conductor according to the present invention, is 7.6×10 ⁻⁵ S/cm@500° C., and 1.8×10 ^-5 S/cm @ 400°C.
In addition, the oxide ion conductor having the AMX ₂ O ₆ structure has a more gradual change in ionic conductivity with temperature than YSZ, and exhibits good ionic conductivity even in a low temperature range of 400 to 500°C.

Therefore, it can be applied to electrochemical devices such as SOFCs. In addition, the range of selection of materials used for peripheral members is widened. Therefore, improvement in performance, cost reduction, and durability improvement of electrochemical devices can be expected.

An oxide having an AMX ₂ O ₆ structure, in which the elements A, M, and X are specific elements, exhibits high ionic conductivity. This is because the atomic arrangement, physical properties, and ion conduction path are similar to those of known oxide ion conductors. As a specific evaluation method,
(a) A method of evaluating conductivity by kernel ridge regression using smooth overlap of atomic positions (reference 2) as a descriptor for measuring the similarity of atomic sequences,
(b) A method of evaluating conductivity by partial least-squares regression using, as physical property values, density and melting point, which are considered to be important in correlation with conductivity, as descriptors,
(c) A method of evaluating conductivity with convolutional neural networks using reciprocal 3D voxel space (reference 3), which measures the similarity of ionic conductivity paths, as a descriptor,
and so on.
From the above three ionic conductivity regression results, it was shown that the oxide having the AMX ₂ O ₆ structure has the characteristic of exhibiting conductivity equivalent to that of existing oxide ionic conductors such as YSZ.
[Reference 2] S. De. AP Bartok, G. Csanyi, and M. Ceriotti, Phys. Chem. Chem. Phys. 18, 13754 (2016)
[Reference 3] Kajita, S., Ohba, N., Jinnouchi, R., & Asahi, R., (2017), Scientific reports, 7(1), 16991

(Examples 1 and 2, Comparative Example 1)
[1. Preparation of sample]
[1.1. Examples 1-2]
A sample was synthesized by a conventional solid-phase reaction method. As raw materials, commercial reagent oxides or carbonates were used. The charge composition is as follows. This time, it was synthesized with a stoichiometric composition and a composition that would be an oxygen interstitial type or an oxygen deficient type.
(a) _KEuGe2O6 (Example ₁ )
( _b ) _{KEu0.8Ca0.2Ge2O6} ( _{Example 2} )

A total of 10 g of ingredients was weighed into a 250 mL poly pot. Further, 80 mL of φ5 mm zirconia balls and 80 mL of ethanol as a solvent were added and mixed for 20 hours with a ball mill. After mixing, the slurry was recovered and the ethanol was evaporated using a rotary evaporator to recover the powder. The obtained powder was calcined in the air at 900° C. for 5 hours in a muffle furnace.

The calcined powder was pulverized in an alumina mortar until coarse powder disappeared (until the particle size became several μm or less). This powder was formed into a mold (approximately φ12×t2 mm), placed on an alumina boat, and sintered in a box furnace. The sintering atmosphere was oxygen. The sintering time was 1 hour, and the sintering temperature was 1000°C to 1100°C.

[1.2. Comparative Example 1]
Commercially available YSZ (8 mol % Y ₂ O ₃ ) was used for the test as it was.

[2. Test method]
[2.1. density]
The density of the sintered body was evaluated by bulk density.
[2.2. XRD]
The crystalline phases of the samples were identified by X-ray diffraction (XRD).
[2.3. Composition analysis]
The detailed composition was measured by the high frequency inductively coupled plasma atomic emission spectrometry (ICP) method. Simple compositional analysis was performed by X-ray fluorescence spectroscopy (XRF).

[2.4. Ionic conductivity]
Ionic conductivity was measured by the impedance method (Cole-Cole plot). The surface of the obtained sintered body was polished with #500 to #1000 sandpaper to remove the surface layer and smooth the surface. After that, a Pt electrode with a thickness of 500 nm was formed by sputtering. After forming the Pt film, heat treatment was performed at 950° C. for 1 hour in the air, and a Pt wire was attached with a Pt paste.

The sample was placed in a tube furnace in an air atmosphere. An LCR meter (manufactured by Hioki Electric Co., Ltd., LCR Hitester 3532-50) was connected to the electrode, and impedance was measured after raising the temperature to a predetermined temperature. The measurement frequencies were from 5 MHz to 40 Hz. The measurement temperatures were 400° C., 500° C., 600° C., and 700° C., and the measurement was carried out after holding at each temperature for about 15 minutes. Furthermore, the measurement was performed 16 times, and the average value was used as the measured value.

[3. result]
Table 1 shows the calcining temperature, sintering temperature, bulk density, and ionic conductivity. Table 1 shows the following.
(1) The activation energy of KEuGe ₂ O ₆ containing no dopant (Example 1) estimated from each temperature is 80 kJ/mol, which is smaller than the activation energy of YSZ (106 kJ/mol). Therefore, Example 1 has a feature that the change in conductivity with temperature is small. Further, since there is no difference between the conductivity in air and the conductivity in nitrogen flow (oxygen concentration: 10 ppm), the conductivity in Example 1 does not contribute to electron conduction.
(2) The activation energy of KEu _0.8 Ca _0.2 Ge ₂ O ₆ (Example 2) estimated from each temperature is 85 kJ/mol, which is smaller than the activation energy of YSZ (106 kJ/mol). Therefore, the conductivity of Example 2 exceeds that of YSZ at some operating temperatures. Further, since there is no difference between the conductivity in air and the conductivity in nitrogen flow (oxygen concentration: 10 ppm), the conductivity in Example 2 does not contribute to electron conduction.
(3) The density of the sintered body is 3.5 Mg/m ³ (Example 1) and 3.9 Mg/m ³ (Example 2), which is about 70% of YSZ, and the weight of the device can be reduced.
(4) It can be manufactured at a sintering temperature of 1000 to 1100° C., which is 500 to 400° C. lower than that of YSZ, and a reduction in manufacturing cost can be expected.

(Example 3)
[1. Investigation of base composition by quantum mechanical calculation]
The feasibility of synthesizing oxides with the AMX ₂ O ₆ structure was investigated by energy stability based on quantum mechanical calculations. Li, Na, K, Rb, Cs, and Cu were selected for the element A (+1 valence). Al, Ga, In, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu were selected for the element M (+3 valence). . Si, Ge, and Sn were selected for the element X (+4 valence).

Assuming the following formula (2) as a synthesis reaction, the heat of formation at that time was evaluated by formula (3) by quantum mechanical calculation.
1/2A2O+ ₁ / _{2M2O3 + 2XO2-} > _AMX2O6 ( ₂ )
Heat of Formation=E(AMX ₂ O ₆ )−1/2E(A ₂ O)−1/2E(M ₂ O ₃ )−2E(XO ₂ )
... (3)

[2. result]
This time, based on KEuGe ₂ O ₆ which was found to function as an oxide ion conductor, any one element of K, Eu and Ge was completely replaced with another element. Table 2 shows the heat of formation at that time. A negative heat of formation indicates that the oxide AMX ₂ O ₆ is stable and can be synthesized. That is, it indicates that the element capable of total substitution can be the main constituent elements (A, M, X) of the oxide AMX ₂ O ₆ . Table 2 shows the following.

(1) Regarding the element A (+1 valence), Li, Na, Rb, and Cs can totally substitute K, but Cu cannot totally substitute K.
(2) Regarding the element M (+3 valence), Al, Ga, In, Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are each , Eu can be totally substituted.
(3) Regarding the element X (+4 valence), Si and Sn can fully substitute for Ge.
(4) The heat of formation (formula (3)) when (1-s) moles of A=K and A=s moles of Cu are mixed is estimated to be -2.52(1-s)+0.14s. Therefore, when s≦0.95, the heat of formation is negative, so Cu can replace K if it is 95% or less.

(Examples 4-7)
[1. Preparation of sample]
A sample was prepared in the same manner as in Example 1. The charge composition is as follows.
_{KEu0.9Ca0.1Ge2O6 ( Example 4} )
_{KEu0.8Ca0.2Ge2O6 ( Example 5} )
_{KEu0.9Sr0.1Ge2O6 ( Example 6} )
_{KEu0.8Sr0.2Ge2O6 ( Example 7} )

[2. Test method]
[2.1. density]
The density of the sintered body was evaluated by the sintered body density D and the relative density Dr.
[2.2. Ionic conductivity]
The ionic conductivity was measured in the same manner as in Example 1. Also, the activation energy Ea and the O ² -transference number were obtained from the obtained ionic conductivity.

[3. result]
Table 3 shows the sintered body density D, relative density Dr, ionic conductivity, activation energy Ea, and O ² -transference number. Table 3 shows the following.
(1) The ionic conductivity of the samples obtained in Examples 4 to 7 was equal to or higher than that of KEuGeO ₆ containing no dopant (Example 1).
(2) The oxide ion (O ^2- ) transference number obtained from electromotive force measurement by an oxygen concentration cell is 90% or more, and the samples obtained in Examples 4 to 7 are almost pure oxide ion conductors. is the body.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The oxide ion conductor according to the present invention can be used as a solid electrolyte for solid oxide fuel cells (SOFC), oxygen gas sensors, exhaust gas purifying elements, and the like.

Claims (3)

An oxide having an AMX ₂ O ₆ structure (where A is a monovalent metal element, M is a trivalent metal element, and X is a tetravalent metal element) ,
K(Eu1 _{- yCy} ) Ge2O6 _± [ _delta ]
An oxide ion conductor having a composition represented by
however,
The C is Sr and/or Ca,
0≤y≤0.2,
δ is the value at which electroneutrality is maintained. 2. The oxide ion conductor according to claim 1, wherein the oxide contains a _three -membered ring ( _Ge3O9 ) ^6- in the unit cell. An electrochemical device using the oxide ion conductor according to claim 1 or 2 .

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