Ishihara T. (Ed.) Perovskite Oxide for Solid Oxide Fuel Cells (Fuel Cells and Hydrogen Energy)

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Fig. 6.7 (a) Refined crystal structure of double perovskite-type La

0.64

(Ti

0.92

0.08

2.99

at 1631 K,

and (b, c, d) isosurfaces of nuclear density distribution at –0.1 fm A

3

(light blue)and+0.1fmA

3

(yellow) and nuclear density on the (100) and (001) planes in La

0.64

(Ti

0.92

0.08

2.99

at 1631 K (b),

1281 K (c), and 769 K (d) [11]. Since the Ti atom has negative scattering length, the Ti,Nb site is

drawn with light blue equi-density surface at –0.1 fm A

3

. Oxygen atoms at the O3 site have a large

spatial distribution to the <101> directions shown by the line with an arrow (B). The O3 atoms do

not move along the straight line shown by the dotted line with arrows but along the curved solid line

with arrows (A in (a)and(b))

128 M. Yashima

drawn with the refined crystallographic parameters [11]. This is the high-

temperature form of La

0.64

(Ti

0.92

0.08

2.99

, which has an A-site-deficient

perovskite-type structure with double perovskite ABO

3–d

units along the

c axis (number of chemical formula in a unit cell: Z = 2), where A=La

0.64

and B = (Ti

0.92

0.08

). The occupancy factors for La at the La1 and La2 sites

are g(La1) = 1.00 and g (La2) = 0.271 [11]. The dissimilarity of these values

reflects the chemical ordering of La-occupied La1-O1 and La-defective La2-O2

layers [Fig. 6.7(a)]. All the refined atomic displacement parameters increase

with temperature (Table 6.2). The equivalent isotropic atomic displacement

parameters of the oxygen atoms are larger than those of the cations, suggesting

a larger diffusion coefficient for the oxide ions (Table 6.2). The oxygen atoms

also display large anisotropy in terms of atomic displacement parameters,

suggesting directionality in the movement of oxide ions around the stable

positions. Similar large and anisotropic thermal motions of oxide ions were

observed for the cubic perovskite-type oxide ion conductor (La

0.8

0.2

(Ga

0.8

0.15

0.05

2.8

(Fig. 6.5) [10].

MEM analysis was conducted using diffraction data in the range 2y ¼4.08–1408,

corresponding to d > 1.0 A

(d, spacing of lattice planes), with the structure

factors obtained by Rietveld analysis. A total of 59 structure factors were

obtained for all data measured at three different temperatures. The 001 reflec-

tion appearing at the lowest 2y position (138) was included, as this peak

provides information on the disordered arrangement of the oxide ions. MEM

calculations were performed with the unit cell divided into 64 64 128 pixels.

Use of the REMEDY cycle resulted in significant improvement in the R factors

based on the Bragg intensities (R

) and structure factors (R

) (Table 6.2). Figure

6.7(b, c, d) shows the isosurface of nuclear density and the nuclear density

distributions on the (100) and (001) planes obtained afte r the REMEDY cycle.

Figure 6.8 shows the temperature dependence of the nuclear density contour

map at z = 0.2 on the ab plane. Figures 6.7(b, c, d) and 6.8 provide much

information on the positional disorder and diffusion paths of mobile oxide

ions compared to the simple atomistic model (Fig. 6.7(a)).

At 769 K, the O3 atoms are localized near the stable 4i site (1/2, 0, 0.234). The

O3 atoms display small bulges in the <101> direction (B in Fig. 6.7(d)), which

become larger at 1281 and 1631 K (Fig. 6.7(c,b)). The probability density of

each O3 atom is connected with that of its nearest neighbor O3 atoms, indicat-

ing diffusion paths (A in Fig. 6.7(a, b)). The diffusion path is along the [100] or

[010] direction near the stable O3 positions, and along the [110] or ½1

10

direction around the center of the paths. The O3 atoms migrate to the nearest

neighbor 4i site through a triangle formed by adjacent La1, La2, and (Ti,Nb)

atoms. The spatial distribution of the O3 atoms becomes larger with increasing

temperature (Figs. 6.7 and 6.8). Such an increase in the density of oxide ions

with increasing temperature is consistent with the higher conductivity observed

at higher temperatures [21]. The O3 atom migr ates following a curved route to

maintain a relatively constant distance from the (Ti,Nb) atoms (solid curves

A in Figs. 6.7(a, b) and 6.8(a)), rather than a direct linear path along the <110>

6 Perovskite-Type Oxides and Related Materials 129

direction (straight dotted lines with arrows between regular positions in

Figs. 6.7(a, b) and 6.8(a)). Similar curved migration pathways were found in

the nuclear density distribution of an ideal cubic perovskite-type compound,

(La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

[10]. Computer simulations [40, 54] for

perovskite-structured LaBO

(B = Co, Mn, Ga, Cr, and Fe) comp ounds also

revealed deviations of the migration pathway from the direct path.

The oxide ion conductor (La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

, which has an

ideal perovskite-type structure, exhibits diff usion paths along the [110], ½1

10,

[011], ½01

1, [101] and ½101 directions to form a three-dimensional network

of equivalent diffusion pathways (Fig. 6.5(b)) [10]. In contrast, in the present

double pe rovskite-type material La

0.64

(Ti

0.92

0.08

2.99

, a two-dimensional

diffusion pathway, by which O3 atoms migrate along the [110] and ½ 1

10

directions (Fig. 6.7(b)), is present. This two-dimensional feat ure is attributable

to the layered structure of the material, which consists of La-occupied La1-O1,

Fig. 6.8 Nuclear density distribution in the ab plane at z = 0.2 (0 < x, y < 2 ) of double

perovskite-type P4/mmm La

0.64

(Ti

0.92

0.08

2.99

at (a) 1631 K, (b) 1281 K, and (c) 769 K

[11]. Contours are in the range 0.05–0.35 fm A

3

with steps of 0.05 fm A

3

. The solid line in (a)

denotes the curved diffusion path of the oxide ions, and the dotted line denotes the direct path

between ideal positions. At low temperature (769 K), oxide ions are localized near the

equilibrium position (see (c)); at high temperature (1631 K), the oxide ions are dispersed

over a wide area between the regular positions (see (a))

130 M. Yashima

(Ti,Nb)-O, and La-deficient La2-O2 layers (Fig. 6.7(a)). Two-dimensional

lithium catio n conduction has also been reported in the orthorhombic layered

perovskite-type compound La

0.62

0.16

TiO

[55], in which the Li cation exists

and migrates only near the La-deficient La2-O2 layer. This work has thus

revealed that oxide ion diffusion in an ionic conductor with a double perovskite

structure is two dimens ional.

6.6 Crystal Structure and Structural Disorder of Oxide Ions in

Cathode Materials, La

0.6

0.4

CoO

3–d

and

0.6

0.4

0.8

0.2

3–d

, with a Cubic Perovskite-Type

Structure [12, 13]

6.6.1 Introduction

The lanthanum strontium cobaltites La

1–x

CoO

3–d

and La

1–x

1–y

3–d

which ha ve a perovskite-type structure, are promising cathode materials for

use in conjunction with doped lanthanum gallate electrolyte in SOFCs [56–58].

The crystal structure of trigonal R

3c La

1–x

CoO

3–d

has been the subject of

a number of investigations [59–65]. However, far less attention has been

paid to the high-temperature cubic phase of La

1–x

CoO

3–d

and La

1–x

1–y

3–d

, which is important for application in SOFCs. In this section,

we describe the crystal structure and structural disorder of cubic Pm

perovskite-type oxides La

1–x

CoO

3–d

at 1531 K [12] and La

0.6

0.4

0.8

0.2

3–d

at 1533 K [13].

6.6.2 Experiments and Data Processing

0.6

0.4

CoO

3–d

and La

0.6

0.4

0.8

0.2

3–d

specimens were prepared by a

solid-state reaction method. Neutron powder diffraction data of

0.6

0.4

CoO

3–d

were collected in air using the HERMES diffractometer [25]

at room temperatur e and at 1531 K. Neutr on diffraction data of

0.6

0.4

0.8

0.2

3–d

were measured in air using the HERMES at 667 K

and at 1533 K. The powder patterns were measured in the range 2y ¼58–1558.

The wavelength of the incident neutrons was 1.8207 A

. The sample temperature

was kept constant during each data collection, using the furnace with MoSi

heaters [24]. The diffraction data of La

0.6

0.4

CoO

3–d

at 1531 K and of

0.6

0.4

0.8

0.2

3–d

at 667 K and 1533 K were analyzed by the Rietveld

method and MPF analysis with the RIETAN-2000 program [27] and the MEM

analysis with the PRIMA [29].

6 Perovskite-Type Oxides and Related Materials 131

6.6.3 Results and Discussion

6.6.3.1 Crystal Structure and Disorder of La

0.6

0.4

CoO

3–d

The neutron diffraction data of La

0.6

0.4

CoO

3–d

at room temperature indi-

cated that the specimen consisted of a single phase of trigonal R



0.6

0.4

CoO

3–d

. All the peaks in the neutron diffraction pattern of

0.6

0.4

CoO

3–d

at 1531 K were indexed by a cubic perovskite-type structure

with Pm

3m symmetry (Fig. 6.9(a)), indicating phase transformation from a

low-temperature trigonal to a high-temperature cubic phase. Rietveld analysis

was performed using the diffraction data in the range 2y ¼208–1538, based on a

cubic perovskite-type structure (Fig. 6.9( a)). The La and Sr atoms were placed

at the special positions 1b 1/2, 1/2, 1/2 in the Pm

3m symmetry. The Co and O

atoms were placed at the 1a 0, 0, 0 and 3d 1/2, 0, 0 sites, respectively. Isotropic

and anisotropic atomic displacement parameters were used for cations and

anions, respectively (Table 6.3). The refined crystallo graphic parameters and

reliability fact ors are shown in Table 6.3. The atomic displacement parameters

of the O atom exhibited a large anisotropy (Fig. 6.9(a) and Table 6.3). The

occupancy factor of O atoms at the 3d site was estimated to be 0.886(6),

indicating an oxygen deficiency of d ¼0.34(2) in the La

0.6

0.4

CoO

3–d

1531 K. The averaged valence of the Co cations was estimated to be 2.72 at

1531 K, which is consistent with the calculated bond valence sum of 2.8. Here,

the average value of the bond valence parameters, 1.698, was used for the

calculation [66].

MPF analysis was conducted using diffraction data in range 2y ¼208–1538,

corresponding to d > 1.07 A

, based on the structure factors obtaine d by

Fig. 6.9 (a) Refined crystal structure and (b) isosurface of nuclear density at 2 fm A

–3

for

0.6

0.4

CoO

3–d

at 1531 K [12]. The arrows denote possible diffusion paths of oxide ions. The

dashed straight line is the edge of the CoO

octahedron

132 M. Yashima

Rietveld analysis. A total of 16 structure factors were obtained. The 100

reflection appearing at the lowest 2y position (26.78) was included, as this

peak provides information on the disorder of the oxide ions. MEM calcula-

tions were performed with the unit cell divided into 64 64 64 pixe ls. The R

factor based on the Bragg intensities (R

) was improved from 2.33% (Rietveld

analysis) to 1.71% (MPF), and that based on the structure factors (R

)was

improved from 1.72% to 1.25%. The MEM nuclear density distribution map

for the (100) plane is shown in Fig. 6.10. The map reveals that the oxide ions in

cubic Pm



3m La

0.6

0.4

CoO

3–d

exhibit a large thermal motion perpendicular to

the Co–O bond, corresponding to the large anisotropy in the atomic displace-

ment parameters (Figs. 6.9 and 6.10). The arrows in Fig. 6.9 and dotted circles

in Fig. 6.10 indicate possible diffusion paths of oxide ions in La

0.6

0.4

CoO

3–d

Table 6.3 Refined crystallographic parameters and reliability factors obtained from Rietveld

and MPF analysis for La

0.6

0.4

CoO

3–d

at 1531.4 K (d ¼0.34(2)) [12]

Site and atoms Wyckoff position gxyzU(A

)

0.6

0.4

1b 1.0 1/2 1/2 1/2 0.0425(9)

Co 1a 1.0 0 0 0 0.025(2)

O3d 0.886(6) 1/2 0 0 0.066*

Note: Cubic space group Pm

3m (No. 221). Number of formula units of La

0.6

0.4

CoO

3–d

in a

unit cell: Z ¼1. Unit-cell parameters: a ¼b ¼c ¼3.9496 (3) A

, a ¼b ¼g ¼908; unit-cell volume:

61.612(9) A

; g, occupancy; x, y, z, fractional coordinates; U, isotropic atomic displacement

parameters; *equivalent isotropic atomic displacement parameters; anisotropic atomic displa-

cement parameters of O atom: U

¼0.027(2) A

, U

¼U

¼0.085(1) A

¼U

¼0A

Reliability factors from Rietveld analysis: R

¼3.73%, R

¼2.68%, R

¼1.86%, R

¼2.00,

¼2.33%, R

¼1.72%. Reliability factors from first MPF analysis: R

¼1.71%, R

¼1.25%.

Fig. 6.10 Nuclear density

distribution in the (100)

plane for La

0.6

0.4

CoO

3–d

measured at 1531 K, with

black contours in the range

from 2 to 10 fm A

–3

(2 fm

–3

steps) [12]. The color

scale of 100% corresponds

to the maximum density of

46.4 fm A

–3

. The dotted

circles indicate possible

oxide-ion diffusion paths.

The dashed straight line

indicates the edge of the

CoO

octahedron. The solid

straight lines indicate the

unit cell. The figure shows

four unit cells

6 Perovskite-Type Oxides and Related Materials 133

The diffusion path does not follow the edge of the CoO

octahedron (shown as

straight dashed lines between the ideal O1 and O2 positions in Figs. 6.9 and

6.10), but displays an arc shape (curved solid arrows in Fig. 6.9 and dotted

circles in Fig. 6.10), avoiding the Co cation. This possible diffusion path is

similar to that observed for (La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

[10]. Compu-

ter simulations have also indicated a similar curved path f or oxide ion migra-

tion [40].

6.6.3.2 Crystal Structure and Disorder of La

0.6

0.4

0.8

0.2

3–d

Neutron diffraction data for La

0.6

0.4

0.8

0.2

3–d

(LSCF6482) at 667 K

indicated that the specimen consisted of a single trigonal R

3c phase. All the

peaks in the neutron diffraction pattern of LSCF6482 at 1533 K were indexed

by the cubic perovskite-type structure with Pm

3m symmetry, indicating a phase

transformation from the low-temperature trigonal to high-temperature cubic

phase between 667 and 1533 K, which is consistent with the literature [67].

Rietveld analysis of LSCF6482 was performed using the neutron diffraction

data taken at 667 K in the 2y range of 208–1538 by a trigonal R

3c perovskite-

type structure. La and Sr atoms were placed at the special position 6a 0, 0, 1/4 of

the R

3c symmetry. Co and Fe atoms were put at the 6b 0, 0, 0 site. O atom was

placed at the 18ex, 0, 1/4. In a preliminary analysis, the refined occupancy

factor of O atoms at the 18e site g(O) was unity within the estimated standard

deviation in the Rietveld analysis Thus, the g(O) was fixed to be unity in the

final refinement. Isotropic and anisotropic atomic displacement parameters

were used for the cations and anions, respectively. The calculated profile agreed

well with the observed one [13]. The refined crystal parameters and reliabil ity

factors are shown in Table 6.4 [13]. The averaged valence of the Co and Fe

cations was estimated to be 3.4 from the occupancy factor at 667 K, which is

consistent with the calculated bond valence sum (BVS) value of 3.3. Here the

average value of the bond valence parameter of 1.7118 was used for the

Table 6.4 Refined crystal parameters and reliability factors in Rietveld and MPF analyses for

0.6

0.4

0.8

0.2

3–d

at 667 K (d ¼0) [13]

Site and atoms Wyckoff position gx yzU(A

)

0.6

0.4

6a 1.0 0 0 1/4 0.0100(6)

0.8

0.2

6b 1.0 0 0 0 0.0061(9)

O18e 1.0 0.5202(3) 0 1/4 0.0228*

Note: Trigonal space group R

3c (hexagonal setting); number of formula units of

0.6

0.4

0.8

0.2

3–d

in a unit cell: Z ¼6. Unit-cell parameters: a ¼b ¼5.46974(18) A

c ¼13.3693 (5) A

, a ¼b ¼908, g ¼1208; unit-cell volume: 346.40(2) A

; g, occupancy; x, y, z,

fractional coordinates; U, isotropic atomic displacement parameters; *equivalent isotropic

atomic displacement parameter.

Reliability factors in the Rietveld analysis: R

¼7 .64%, R

¼6.00%, R

¼4.75 %,

¼1.26, R

¼3.76%, R

¼2.49%. Reliability factors in the first MPF analysis:

¼1.04%, R

¼0.82%.

134 M. Yashima

calculation [66]. Although the BVS is usually applied to the crystal structure at

room temperature, we can use it at high temperatures because of small change

of unit-cell parameters between different temperatures.

MPF analysis of R



3c LSCF6482 was conducted using diffraction data in the

2y range from 208 to 1538 with the structure factors obtained from Rietveld

analysis. The R factors for the Bragg intensities, R

, and for the structure factors,

, were improved from 3.76% in the Rietveld analysis to 1.04% in the MPF,

and from 2.49% to 0.82%, respectively. The resultant nuclear density indicated

that the oxide ions are localized near the stable positions (Fig. 6.11(a)).

Rietveld analysis was performed using the diffraction data of LSCF6482

taken at 1533 K in the 2y range of 208–1538 by a cubic Pm

3m perovskite-type

structure (Table 6.5). La and Sr atoms were placed at the special position 1b 1/2,

1/2, 1/2 of the Pm

3m symmetry. Co and Fe atoms were put at the 1a 0, 0, 0 site,

whereas the O atom was placed at the 3d 1/2, 0, 0 posit ion. The atomic

displacement parameters of the O atom exhibited large anisotropy

(Fig. 6.11(b) and Table 6.5), which reflects the rotational motion of O atoms

in the rigid (Co,Fe)O

octahedron. Similar anisotropy has been observed in

other cubic perovskite-type compounds [10, 12, 68, 69]. The atomic displace-

ment parameters at 1533 K were higher than those at 667 K. The equivalent

isotropic displacement parameter of O atom is larger than those of cations

(Tables 6.4 and 6.5), suggesting the higher diffusivity of O atoms. The occu-

pancy factor of the O atom at the 3d site was estimated to be 0.904(6), indicating

an oxygen deficiency of d ¼0.288(15) in La

0.6

0.4

0.8

0.2

3–d

at 1533 K. The

change of oxygen deficiency from d ¼0 at 667 K to 0.288 at 1533 K, which was

obtained in the Rietveld analyses, is reasonably consistent with the weigh t loss

Fig. 6.11 (a) Nuclear

density distribution on the

(012) plane of trigonal

0.6

0.4

0.8

0.2

3-d

measured at 667 K. (b)

Nuclear density distribution

on the (100) plane of cubic

0.6

0.4

0.8

0.2

3–d

measured at 1533 K. Black

contours in the range from 3

to 13 fm A

–3

(3 fm A

–3

step).

The dotted circle indicates

the possible oxide-ion

diffusion path. The straight

solid line indicates the unit

cell

6 Perovskite-Type Oxides and Related Materials 135

observed in the TG curve (Fig. 6.12). The average d valence of the B-site Co and

Fe cations was estimated to be 2.8 from the refined occupancy of O atoms at

1533 K, which is consistent with the calculated bond valence sum value of 2.9.

Here, the average value of the bond valence parameter of 1.7118 was used for

the calculation [66].

MPF analysis of LSCF6482 was conducted using diffraction data taken at

1533 K in the 2y range from 208 to 1538, with the structure factors obtained

from Rietveld analysis. The R factors for the structure factors, R

was improved

from 3.20% in the Rietveld analysis to 2.32% in the MPF. To visualize the

structural disorder, the MEM nuclear density distribution map on the (100)

Table 6.5 Refined crystal parameters and reliability factors in Rietveld and MPF analyses for

0.6

0.4

0.8

0.2

3–d

at 1533 K (d ¼0.288(15))

Site and atoms Wyckoff position gxyzU(A

)

0.6

0.4

1b 1.0 1/2 1/2 1/2 0.0416(9)

0.8

0.2

1a 1.0 0 0 0 0.0294(11)

O3d 0.904(6) 1/2 0 0 0.071*

Note: Cubic space group Pm

3m, number of formula units of La

0.6

0.4

0.8

0.2

3–d

in a unit

cell; Z ¼1. Unit-cell parameters: a ¼b ¼c =3.9540(3) A

, a ¼b ¼g ¼908; unit-cell volume:

61.815(8) A

; g, occupancy; x, y, z, fractional coordinates; U, isotropic atomic displacement

parameters. *Equivalent isotropic atomic displacement parameters, anisotropic atomic displa-

cement parameters of O atom: U

¼0.0290(13) A

; U

¼U

¼0.0921(12) A

;

¼U

¼0A

. Reliability factors in the Rietveld analysis: R

¼4.82%,

¼3.50%, R

¼1.70%, R

¼2.83, R

¼3.56%, R

¼3.20%. Reliability factor in the

first MPF analysis: R

¼2.32%.

Fig. 6.12 Temperature

dependence of 3–d in

0.6

0.4

0.8

0.2

3–d

where d is the concentration

of oxygen vacancy. Solid

line was obtained using

the weight loss from the

TG data of

0.6

0.4

0.8

0.2

3–d

where no oxygen vacancy is

assumed at room

temperature. Circle was

calculated from the refined

occupancy at the oxygen site

in the Rietveld analyses of

high-temperature neutron

diffraction data

136 M. Yashima

plane in LSCF6482 is shown in Fig. 6.11(b). The nuclear density map reveals that

the oxide ions in the cubic Pm



3m LSCF6482 exhibit a large thermal motion

perpendicular to the (Co,Fe)-O bond, corresponding to large anisotropy of the

atomic displacement parameters (Table 6.5). The dotted circle in Fig. 6.11(b)

indicates possible diffusion paths of the oxide ions in LSCF6482. The diffusion

path does not follow the edge of the (Co,Fe)O

octahedron, but displays an arc

shape away from the Co,Fe cation. The nuclear density of O atoms in LSCF6482

did not connect with that of nearest neighbor O atoms (Fig. 6.11). On the contrary,

the (La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

perovskite exhibited connected diffusion

paths (Figs. 6.5 and 6.6) [10]. This finding strongly suggests that the diffusivity of

oxide ions in LSCF6482 is lower than that in (La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

Similarly, the nuclear density around an O site in La

0.6

0.4

CoO

3–d

did not connect

with that around the nearest neighbor O site (Figs. 6.9 and 6.10) [12].

It has been commonly assumed that the migr ating anion in the AB O

3–d

perovskite-type structure takes a direct linear path along the <110> edge of

the BO

octahedron. However, based on the results of this work, we suggest a

curved diffusion path of the oxide ions in the electrode material LSCF6482 at

1533 K. The oxide ions migrate in the <100> directions near the stable 3d

position, while they move along the <110> directions around the center of the

diffusion path. Similar diffusion paths have been observed in the MEM nuclear

density maps of (La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

(Figs. 6.5 and 6.6), of

0.64

(Ti

0.92

0.08

2.99

(Figs. 6.7 and 6.8), and of La

0.6

0.4

CoO

3–d

(Figs. 6.9

and 6.10) [10–12]. Computer simulations have also indicated a similar curved

path for the oxide ion migration in perovskite-type compounds [40]. The diffu-

sion paths of the cubic perovskite-type LSCF6482, La

0.6

0.4

CoO

3–d

,and

(La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

form a three-dimensional network. As

described previously, Ali et al. [11] reported a similar but two-dimensional curved

diffusion path of oxide ions in an oxide ion conductor, La

0.64

(Ti

0.92

0.08

2.99

with a double perovskite-type structure. All four ABO

3–d

perovskite-type com-

pounds LSCF6482, La

0.6

0.4

CoO

3–d

,(La

0.8

0.2

)(Ga

0.8

0.15

0.05

2.8

,and

0.64

(Ti

0.92

0.08

2.99

exhibit a curved migration path of the mobile oxide

ions, keeping the B–O distance constant to some degree. Thus, this curved feature

should be common in perovskite-type ionic and mixed conductors.

6.7 Structural Disorder and Diffusion Path of Oxide Ions in a

Doped Pr

NiO

-Based Mixed Ionic-Electronic Conductor

(Pr

0.9

0.1

)

(Ni

0.74

0.21

0.05

4+d

with a K

NiF

-Type

Structure [15]

6.7.1 Introduction

-based oxides with K

NiF

-type structure have extensively been stu-

died as new mixed ionic-electronic conductors [23, 70–80], where A and B are

6 Perovskite-Type Oxides and Related Materials 137