Improved structural stability, electron transport and defect formation in PrBaCo2–xAlxO6–δ

Article abstract of DOI:10.1016/j.jallcom.2018.07.089

The double perovskite-like solid solutions PrBaCo_2–xAl_xO_6–δ are obtained via combustion of glycerol-nitrate organo-metallic precursors. The aluminum doping occurs favorable for mitigation of thermal expansion and stabilization of the tetragonal structure in a wide range of temperature changes. The variations of equilibrium oxygen content in PrBaCo_1.9Al_0.1O_6–δ are measured with a coulometric titration technique, and analyzed in terms of defect chemistry. It is shown that aluminum incorporation is accompanied by formation of rigid AlO₆ octahedra in the crystal structure and enhanced disproportionation of Co³⁺ cations. The developed defect model is successfully applied in order to explain the data for conductivity and thermopower. The structural stability, moderate thermal expansion and high conductivity represent an advantageous properties combination for the using of PrBaCo_1.9Al_0.1O_6–δ cobaltite in various high-temperature solid state electrochemical devices.

Full text of DOI:10.1016/j.jallcom.2018.07.089

Journal of Alloys and Compounds 767 (2018) 1041e1047
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Improved structural stability, electron transport and defect formation
in PrBaCo2exAl O
x 6e
d
a
a
a,
*
a
b
S.N. Marshenya , B.V. Politov , A. Yu Suntsov , I.A. Leonidov , S.A. Petrova ,
a
a
M.V. Patrakeev , V.L. Kozhevnikov
a
Institute of Solid State Chemistry UB RAS, 620990, Yekaterinburg, Russia
Institute of Metallurgy UB RAS, 620016, Yekaterinburg, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 February 2018
Received in revised form
The double perovskite-like solid solutions PrBaCo2exAl O
nitrate organo-metallic precursors. The aluminum doping occurs favorable for mitigation of thermal
expansion and stabilization of the tetragonal structure in a wide range of temperature changes. The
6ed
are obtained via combustion of glycerol-
x
26 June 2018
6ed
variations of equilibrium oxygen content in PrBaCo1.9Al0.1O are measured with a coulometric titration
Accepted 8 July 2018
Available online 12 July 2018
technique, and analyzed in terms of defect chemistry. It is shown that aluminum incorporation is
6
accompanied by formation of rigid AlO octahedra in the crystal structure and enhanced dispropor-
tionation of Co3 cations. The developed defect model is successfully applied in order to explain the data
þ
Keywords:
for conductivity and thermopower. The structural stability, moderate thermal expansion and high con-
ductivity represent an advantageous properties combination for the using of PrBaCo1.9Al0.1
in various high-temperature solid state electrochemical devices.
Oxide materials
Electrical transport
Point defects
6ed
O cobaltite
Thermal expansion
Vacancy formation
© 2018 Elsevier B.V. All rights reserved.
1
. Introduction
partial replacement of cobalt with metals having substantially
stronger bonding with oxygen is expected to more expressly affect
oxygen retention and structural robustness. Considering the ne-
cessity for the efficient dopants to be close in the size and charge to
the regular cations for ensured substitution, we selected aluminum
as a doping element in this work because of the excellent stability
of the aluminum oxide at heating and low pressures of oxygen, i.e.,
very strong chemical bonds of aluminum with oxygen, and near
The layered double perovskite-like cobaltites RBaCo
2 6ed
O ,
where R is a rareeearth metal, have attracted considerable interest
as materials for the using as cathodes in intermediate temperature
SOFCs [1,2], oxygen semi-permeable membranes [3,4], catalysts [5],
oxygen sensors [6], etc. The potential of their application is related
with the capability of the cobaltites to accommodate a large
amount of oxygen vacancies favorable for fast oxygen transport and
high level of electron conductivity provided by charge dispropor-
3
þ
3þ
ionic radii (r VA Il
¼ 0.535 and rVI
equality of Al
and Co
3
þ
3þ
Co
¼
0.545) [10].
tionation of Co3 cations. Still, such drawbacks as large thermal
expansion coefficient (TEC) and insufficient stability toward
reduction greatly impair the utility of the cobaltites. The efficient
means of circumventing the noted deficiencies can be associated
with the properties tuning via partial replacement of cobalt in the
original crystalline lattice with other metals. For instance, the
introduction of nickel and iron in place of cobalt is shown to favour
a decrease of TEC values [7,8]. In this regard it should be noted
additionally that a considerable part of the overall TEC in cobaltites
occurs due to the oxygen depletion at heating [9]. Therefore, the
þ
It is known that aluminum can readily replace B-site cations
thus rendering considerable impact on functional properties of
cubic perovskite-like oxides. For instance, Mori et al. observed
improved structural stability and the TEC decrease of La0.9Sr0.1CrO
3
at introduction of a small amount of aluminum in place of chro-
mium [11]. Authors [12,13] also found that moderate aluminum
substitution is favorable for mitigation of TEC in SrFe1ꢀxAl
La0.5Sr0.5Fe1exAl . At the same time, the effects of aluminum
incorporation in the double perovskite-like cobaltites have not
been considered yet. Therefore, the most conducting PrBaCo
x
O
3ꢀ
d
and
x 3ed
O
2 6ed
O
cobaltite is selected as a parent oxide for aluminum doping in this
work [14]. We studied aluminum solubility, structural features and
*
Corresponding author.
E-mail address: suntsov@ihim.uran.ru (A.Y. Suntsov).
defect
equilibrium
in
the
obtained
solid
solutions
https://doi.org/10.1016/j.jallcom.2018.07.089
925-8388/© 2018 Elsevier B.V. All rights reserved.
0

1042
S.N. Marshenya et al. / Journal of Alloys and Compounds 767 (2018) 1041e1047
ꢁ
PrBaCo2exAl
x
O
6e
d
. Special attention was given to the analysis of the
temperature gradient near 20 С. Another wired specimen for
conductivity measurements was set in a cross-wise orientation in
order to ensure strict equality of temperature values at voltage
probes. The measurements were carried out in an isothermal
temperature and oxygen pressure driven changes in thermal
expansion, electrical conductivity and thermopower.
ꢁ
ꢁ
measuring mode at 700e950 С with the temperature step 50 C in
ꢀ
6
2
. Experimental
the oxygen partial pressure range 3$10 e 0.7 atm. The oxygen
pressure during the measurements was maintained and controlled
with the help of a zirconia pump and a sensor, respectively. Addi-
tional experimental details can be found in work [19].
x 6ed
In order to synthesize PrBaCo2exAl O we used high purity
(
99.99%) cobalt and aluminum powder metals, barium carbonate
and praseodymium oxide Pr 11. The starting materials were
6
O
weighed in desirable proportions, placed in a quartz beaker and
ꢁ
dissolved in nitric acid at 200 C. Then glycerol was added in the
3. Results and discussion
solution in equimolar ratio to the sum of metal cations. The ob-
tained mixture of organo-metallic complexes was subjected to
evaporation at 200 C till self-ignition of the residue occurred. The
XRD patterns in Fig. 1 for as-synthesized oxides PrBaCo2-xAl
, where x ¼ 0, 0.05, 0.10 and 0.15, give evidence to the aluminum
solubility limit within 0.1 < x < 0.15 as precipitation of barium
aluminate BaAl
x 6-
O
ꢁ
d
powder-like combustion product was carefully grinded and addi-
ꢁ
tionally fired at 900 C to remove trace organics, pelletized at 2 kBar
2
O
4
[20] can be observed at x ¼ 0.15 while the
ꢁ
of uniaxial pressure, sintered at 1100e1200 C for 20 h in the air,
sample x ¼ 0.10 is phase pure. The single phase aluminum doped
and cooled down to room temperature in the furnace.
materials crystallize with a tetragonal structure (S.G. P4/mmm) in
X-ray powder diffraction (XRD) at room temperature was
applied for phase purity control of the obtained materials with a
2 6ed
difference with the orthorhombic parent cobaltite PrBaCo O
[21]. The tetragonal unit cell parameters a ¼ b ¼ 3.905 and
Shimadzu XRD 7000 diffractometer (Cu-K
Advance diffractometer equipped with
a
radiation). A D8
high temperature
c ¼ 7.633 Å do not virtually depend on aluminum content, which is
3
þ
3þ
a
consistent with the replacement of Co by Al cations. The XRD
HTK1200 N chamber was used for XRD measurements at heating
pattern for the fully reduced sample PrBaCo1.9Al0.1
collected in the air, Fig. 2. The reduced material is seen to contain
Pr(OH) and BaCO . These forms are believed to appear in the result
of the interaction of the primary reduction products Pr and BaO,
respectively, with water vapors and carbon dioxide in the air.
6ed
O has been
ꢁ
from room temperature to 1000 C. The XRD spectra were collected
ꢁ
with the temperature step 5 C and the acquisition time 545 s at
3
3
each temperature. The processing of the diffraction data was car-
ried out with the help of a DIFFRACplus: EVA calculation package
2 3
O
[
15] and ICDD PDF4 data base [16]. The unit cell parameters were
XRD data in Fig. 3 for PrBaCo1.9Al0.1
O
6e
d
at temperature increase
ꢁ
calculated by making use of the least squares in a Celref calculation
media [17].
to 1000 С show always tetragonal structure and the absence of
phase admixtures, i.e. rather good structural stability of the
aluminum doped derivative at heating in the air. The coincidence of
crystal lattice parameters in the heating and cooling modes in Fig. 4
suggests thermal equilibration of the sample during the measure-
The changes of oxygen content (6ed) in aseprepared samples at
heating in the air and reducing gas mixtures were determined with
a Setaram Setsys Evolution e 18 thermoanalyzer. The treatment in
ꢁ
ꢁ
5
% H
PrBaCo2exAl
and cobalt metal. The respective mass decrease (
2
e 95% Ar atmosphere at 900 C resulted in the reduction of
ments. The small kinks near 300 С on the plots in Fig. 4 reflect the
x
O
6e
d
to the mixture of BaAl
2
O
4
, Pr
2
O
3
and BaO oxides,
incipient oxygen exchange between the sample and ambient gas
phase. It should be stressed, however, that in difference with the
Dm) was used for
calculations of oxygen content in the aseprepared samples. The
expansion of the materials at heating in the air was measured by a
Linseis L75 dilatometer with the using of dense 3 ꢂ 3 ꢂ 10 mm
ceramic samples. The heating rate at the expansion measurements
ꢁ
was 3 C.
The changes of equilibrium oxygen content in the samples at
variations of temperature (T) and oxygen partial pressure pO2 in the
ambient gas phase were obtained by a coulometric titration tech-
nique in the isothermal mode of measurements with the temper-
ꢁ
ature step of 50 C. The titration cell was made of cubically
stabilized zirconia oxygen solid electrolyte. Two pairs of platinum
electrodes were deposited on the inner and outer sides of the cell,
and served as an oxygen pump and a sensor. The wired measuring
cell was placed in a similar larger cell where oxygen pressure was
maintained nearly equal to the pressure inside the measuring cell.
The using of such a ‘double-cell technique’ allowed us to achieve a
high accuracy of oxygen content measurements with the uncer-
tainty that did not exceed Dd ¼ ± 0.002. More experimental details
can be found elsewhere [18]. The thermal analysis results obtained
at heating in the air log pO2 ¼ ꢀ0.678 were used as reference for the
coulometric titration data. The numerical computations were car-
ried out in a Maple calculation media.
The rectangular ceramic samples 2 ꢂ 2 ꢂ 15 mm with the den-
sity near 98% of theoretical one were prepared for thermopower
and four-probe d.c. conductivity measurements. The specimen for
thermopower experimental determination was equipped with butt
electrodes and thermocouples, and placed in the measuring cell
along the external tubular heater which ensured the lengthwise
Fig. 1. The powder XRD spectra for PrBaCo2exAl
x
O
6e
d
, where x ¼ 0, 0.05, 0.10 and 0.15.
ꢁ
The small peak near 28 is indicative of BaAl
2
O
4
phase admixture in the sample with
x ¼ 0.15.

S.N. Marshenya et al. / Journal of Alloys and Compounds 767 (2018) 1041e1047
1043
Fig. 2. XRD pattern for fully reduced PrBaCo1.9Al0.1
O
6ed
cobaltite.
Fig. 4. The temperature dependent plots for the unit cell parameters in
PrBaCo1.9Al0.1 at heating (red) and cooling (blue). (For interpretation of the ref-
O
6ed
erences to colour in this figure legend, the reader is referred to the Web version of this
article.)
2 6ed
parent cobaltite PrBaCo O where this singularity signals also a
gradual transition from orthorhombic to tetragonal structure [21],
the doped sample permanently retains the tetragonal structure.
x 6ed
The heating of the solid solution PrBaCo2exAl O results in
oxygen depletion as it takes place in other double perovskite co-
baltites [2,14]. The effect of aluminum doping is demonstrated in
Fig. 5 where thermogravimetric data show that heating of the
samples with larger aluminum content is accompanied with
smaller weight loss. The oxygen content (6e
d
) ¼ 5.83 in as-
prepared sample x ¼ 0.1 occurs larger compared to x ¼ 0 as ther-
mogravimetric reduction experiments reveal.
On the other hand, the substituted samples show decreased
thermal expansion while the slopes of the relative elongation plots
do not reveal any singular behaviour peculiar to PrBaCo O , Fig. 6.
2 6ed
These results clearly show that replacement of cobalt for aluminum
is beneficial for enhanced retention of oxygen in the crystalline
structure, i.e., for the increase of the average metal e oxygen
bonding energy in the cobaltites.
The experimental isothermal pressure dependent plots of oxy-
gen content are shown in Fig. 7 for PrBaCo1.9Al0.1O as an
6ed
example. In order to explain the observed changes in their shape
Fig. 5. The weight loss of PrBaCo2exAl O
6ed
samples at heating in the air.
x
we use the approach developed earlier [22e24]. With the cubic
3
perovskite PrCoO as a reference crystal. We assume on the basis of
better retention of oxygen in the aluminium doped samples
compared to the pristine cobaltite in heating.
It has been shown already [22e24] that defect equilibration in
doubleeperovskite cobaltites takes place as a result of the simul-
taneous reactions
the structural data that the aluminum substitution of quasi-neutral
ꢂ
cobalt sites Co in PrBaCo
2
O
6e
d
results in appearance of quasi-
Co
ꢂ
neutral defects Al in octahedral oxygen coordination that does
Co
not change with variations of T and pO2 . As a result, oxygen posi-
tions around aluminum cations occur excluded from oxygen ex-
change in accord with the thermal analysis data that evidence
6ed
Fig. 3. The changes in positions of Xeray reflections for PrBaCo1.9Al0.1O at heating and cooling.

1044
S.N. Marshenya et al. / Journal of Alloys and Compounds 767 (2018) 1041e1047
(
4)
while the structure conservation requirement must be presented as
h
i
g þ p þ n ¼ 2 ꢀ Alꢂ ¼ 2 ꢀ x
(5)
Co
where ½Alꢂ ꢃ stands for the concentration of Alꢂ defects, and
Co
Co
symbols n, p and g designate concentrations of localized electrons
=
ꢂ
Co , holes
and regular sites Co , respectively. The combi-
Co
Co
nation of the equilibrium constant for the intrinsic reaction (2)
(
6)
with relations (4) and (5) results in the following expression
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ꢀ x ꢀ
D
þ D
cdð0Þ cdðxÞ
Fig. 6. The relative linear elongation of PrBaCo2exAl
The average thermal expansion coefficient values ($10
are given in the inset table.
x
O
6e
d
samples at heating in the air.
3
x
2
6
ꢀ1
K
) at selected temperatures
p ¼ 2
ꢀ
d
ꢀ
þ
(7)
2ð4K ꢀ 1Þ
cd
where
2
D
¼ ð2
d
ꢀ 1Þ þ 4Kcdð3 ꢀ 2
d
Þð1 þ 2
d
Þ ꢀ 16Kcd
(8)
(9)
cdð0Þ
2
D
¼ ꢀ4Kcdðx ꢀ 2Þ
cdðxÞ
It follows from (7) that the replacement of electroactive cobalt
by aluminum cations is generally favorable for a decrease in the
concentration of holes and, consequently, in the exclusion of the
respective amount of holes from electric transport.
In order to calculate the oxygen vacancy concentrations in
different crystallographic positions we have to use the equilibrium
constant for reaction (3)
(
10)
Additionally, we have to take into account oxygen positions
supposedly excluded from gas exchange by aluminum. The
respective amount can be easily calculated from binomial distri-
bution under assumption of random spreading of aluminum dop-
6ed
Fig. 7. The pressure dependent plots for oxygen content in PrBaCo1.9Al0.1O at
different temperatures. Symbols represent experimental data; dotted and solid lines in
the inset show results as calculated with the help of Eq. (20) at x ¼ 0 and 0.1,
respectively.
x 6ed
ants over the crystalline lattice of PrBaCo2exAl O as
ꢀ
ꢁ
h
i
X
ꢂ
2
z
z
2ꢀz
i
O
¼
ðx$m Þ ð1 ꢀ x$m Þ
(11)
Oi
i
ðexclÞ
z
where z is the amount of aluminum ions near i'th oxygen position,
and mi is the amount of oxygen ions in i'th position per B-site in the
elementary unit. Considering the crystalline lattice of the double-
(1)
perovskite we can see that m
positions, respectively. Therefore, the concentrations of O2 and O3
positions excluded from gas exchange follow as
2
¼ 2 and m ¼ 1/2 for O2 and O3
3
(
(
2)
3)
h
h
i
i
ꢂ
O
¼ 4xð1 ꢀ xÞ
(12)
(13)
O2
ðexclÞ
ꢂ
ꢂ
=
Co
where
,
, O , O
,
and Co designate vacant and filled
O2
O3
O2 and O3 positions, and electron holes and electrons localized on
cobalt atoms, respectively. As far as aluminum doping does not
change electrical equilibrium in the crystalline lattice, the electrical
xð4 ꢀ xÞ
ꢂ
O
¼
O3
ðexclÞ
4
neutrality condition for PrBaCo2exAl
x 6ed
O remains the same as for
while the concentrations of O2 and O3 positions available for gas
exchange can be found from
PrBaCo
2 6ed
O
(
14)

S.N. Marshenya et al. / Journal of Alloys and Compounds 767 (2018) 1041e1047
1045
6ed
PrBaCo1.9Al0.1O , which may serve as a measure of phase stability
at low pressures of oxygen [23], must be about the same as in
PrBaCo1.8Ni0.2O6ed so that the both derivatives should exhibit the
same capability to withstand reducing atmospheres. One can notice
also that nickel occurs about twice less efficient a dopant for
enhanced structural robustness of the cobaltite than aluminum.
This observation is in accord with stronger aluminum e oxygen
bonding.
(
(
15)
16)
The total vacancy concentration is simply
The set of equations (10) and (14)e(16) can be used in order to
The plots in Fig. 8 show variations of electrical conductivity in
find
PrBaCo2exAl
x
O
6e
d
, where x ¼ 0 and 0.1, with temperature and
(
17)
equilibrium pressure of oxygen in the gas phase. It is seen rather
unexpectedly that incorporation of electrically inactive aluminum
in the structure is accompanied with some increase of the con-
ductivity. In order to understand such a behaviour it is instructive to
consider changes in the concentration of holes related to the unit
where
n
ꢂ
ꢃ
o
2
od
2
2
2
D
¼ 16 K
ðd
ꢀ 4Þ ꢀ 2K
d
ꢀ 5
d
ꢀ 4 þ ð
d
ꢀ 1Þ
odð0Þ
od
cell volume VT
.
cell
(18)
(
22)
3
D
¼ ð16Kod þ 1Þx ðð16K þ 1Þðx ꢀ 4Þ ꢀ 4 Þ þ 8xð4Kod þ 1Þð
odðxÞ
od
ꢂ ð3x ꢀ 4Þð8Kod þ 1Þ ꢀ 4 Þ þ 8x
ꢂ ð16Kod ꢀ 1Þ ꢀ 3 Þ
d
ðKod ꢀ 1Þðð1 ꢀ xÞ
where N ¼ 2 is the number of formula units in the refereed
perovskite Pr Ba CoO . The oxygen partial pressure and
0
.5 0.5
3ed/2
(
19)
temperature driven variations in concentrations of electron defects
and regular sites can be calculated from (4), (5) and (7) while
changes of the unit cell with temperature are shown in Fig. 4. The
respectively calculated data demonstrate that the concentration of
holes at oxygen pressure variations in isothermal conditions ap-
pears to be larger in the aluminum doped sample as it may be
deduced from the conductivity in Fig. 8. At the same time, the
thermopower measurements unambiguously show p-type domi-
nating charge carriers, Fig. 9. Considering that the high-
temperature thermopower S for localized positive charge carriers
Finally, the equilibrium pressure of oxygen over
PrBaCo2exAl follows from the equilibrium constants KOx and
x 6ed
O
K
cd for reactions (1) and (2) as
(
20)
where variables (4), (7), (15) and (17) are to be used. The calculated
values of the pressure may change with temperature because of the
equilibrium constants KOx and Kcd that depend on temperature as
is proportional to logð1=C Þ [25], we find that the increasing ther-
h
mopower signals the hole concentration decreasing with pressure
in apparent agreement with the conductivity data.
ꢁ
!
ꢁ
!
Nevertheless, the pressure dependent plots in Figs. 8 and 9 are
insufficient to unambiguously interpreter the apparent increase of
conductivity. Let's consider the electrical conductivity in the system
of localized charge carriers. In the thermodynamic treatment above
D
Sj
ꢀ
D
Hj
Kj ¼ exp
$exp
(21)
R
RðT þ 273Þ
ꢁ
j
ꢁ
j
where
D
H and
D
S denote changes in standard enthalpy and en-
tropy for the respective j'th defect formation reaction R is the gas
constant. Respectively, the independent variations of enthalpies
ꢁ
ꢁ
D
Hj and entropies
D
Sj for reactions (1), (2) and (3) are to be
employed in order to simulate the experimental results for
log pO2 ¼ fðT; ; xÞ in Fig. 7. It is distinctly seen in the figure inset
d
that the disregard of the exclusion effects by setting x ¼ 0 does not
allow obtaining satisfactory fitting, particularly in the low-pressure
range. The calculated thermodynamic parameters for reactions (1),
(
2), and (3) in PrBaCo1.9Al0.1
the respective values in PrBaCo1.8Ni0.2
24] for comparison. It can be seen that the standard enthalpy and
entropy changes for defect formation reactions are virtually inde-
O
6e
d
are shown in Table 1 together with
O
6e
d
[23] and PrBaCo
2 6ed
O
[
2
þ
3þ
pendent on the content of either Ni or Al . Consequently, the
ꢁ
for reduction of Co3
þ
to Co
2þ
in
free energy
D
G
Red
Table 1
The standard entropy and enthalpy values for defect formation reactions (1)e(3).
ꢁ
j
ꢁ
Reaction
D
S , J/mol$K
D
Hj kJ/mol
(
1)
(2)
(3)
(1)
(2)
(3)
x 6ed
Fig. 8. The data for equilibrium isothermal total conductivity in PrBaCo2exAl O ,
PrBaCo1.9Al0.1
O
O
6e
d
ꢀ100 ± 5 15 ± 9
ꢀ103 ± 4 21 ± 4
ꢀ20 ± 6 ꢀ152 ± 5 53 ± 10 61 ± 7
where x ¼ 0 and 0.1, at variations of oxygen partial pressure. The variations in con-
PrBaCo1.8Ni0.2
PrBaCo
6e
6e
d
ꢀ20 ± 3 ꢀ152 ± 5 59 ± 3
63 ± 5
centrations of electron holes are shown in the inset as calculated with the help of Eq.
2
O
d
ꢀ60 ± 5
ꢀ13 ± 2 ꢀ40 ± 4 ꢀ116 ± 7 31 ± 3
127 ± 8
ꢁ ꢁ ꢁ
22) at 700 С (dotted lines), 800 С (dashed lines) and 900 С (solid lines).
(

1046
S.N. Marshenya et al. / Journal of Alloys and Compounds 767 (2018) 1041e1047
Fig. 9. Isothermal dependencies of thermopower in PrBaCo1.9Al0.1
O
6e
d
vs log ðpO Þ.
2
it is proportional to the product of the charge carrier concentration
p ≡ [Co4 ] by the number of positions [Co ] available for their
þ
3þ
3
þ
4þ
jumps, i.e.,
s
ðT; ; xÞ ~ [Co ]$[Co ]. Hence, interrelations (4), (5)
d
and (7) can be used for the calculations. The respective results are
shown in Fig. 10a where changes in concentrations of different
cobalt species are shown at variations of oxygen content. One can
observe that introduction of aluminum is accompanied with some
increase in the concentration of electron holes. At the same time,
the decrease in the concentration of available positions is more
considerable so that the product [Co3 ]$[Co ] and the conduc-
þ
4þ
tivity
content at least at
s
ðT;
d
; xÞ both tend to decrease with the increase of aluminum
0
d
s within (6 e
d
) < 5.5. In order to compare the
calculation results with the experiment we have to combine the
electrical conductivity in Fig. 8 with the equilibrium oxygen con-
tent data in Fig. 7 so that to find variations of conductivity with
oxygen content as shown in Fig. 10b. It is clearly seen that oxygen
non-stoichiometry most strongly affects the conductivity, while
temperature variations at permanent oxygen content are so small
that different isotherms in Fig. 10b practically merge on the figure
scale. Moreover, when considered at equal oxygen content,
aluminum doping indeed, as expected, results in a decline of con-
ductivity, which is confirmed with the calculated concentration of
electron defects in Fig. 10a.
Fig. 10. The calculated oxygen content dependent changes in concentrations of
different cobalt species (a) and measured total conductivity (b) in PrBaCo2exAl
where x ¼ 0 and 0.1. The dashed and solid lines show results at 700 and 950 С,
respectively. The experimental data for x ¼ 0 are taken from Ref. [24].
x
O
6e
ꢁ
d
,
features for the using of PrBaCo1.9Al0.1O6ed in electrochemical de-
vices designed to work at elevated temperatures.
Acknowledgements
4
. Conclusions
This work is supported by the Russian Foundation for Basic
Research under grant N 16e33e60202.
ꢁ
The solid solutions PrBaCo2exAl
.15, are obtained via combustion of glycerol-nitrate precursors.
x
O
6e
d
, where x ¼ 0, 0.05, 0.10 and
0
References
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