The origin of triple conductivity and water uptake in layered double perovskites: A case study on lanthanum-substituted GdBaCo2O6?δ

Article abstract of DOI:10.1016/j.jallcom.2020.156309

Some layered double perovskite cobaltites have been shown recently to absorb water and exhibit increasing electrical conductivity in humid atmospheres. However, the assumptions that their crystal lattice is capable of proton uptake, and that these oxides really possess triple (oxide ion, protonic and electronic) conductivity, have already been brought into question. We investigated in detail the crystal structure and phase composition of various lanthanum-substituted GdBaCo₂O_6?δ and came to several important conclusions. Firstly, in oxidative conditions (e.g. in air), the substitution of La for either only Gd or only Ba in GdBaCo₂O_6?δ results in formation of multiphase materials. For example, BaCo_1–xGd_xO_3?δ exsolves from Gd_1–xLa_xBaCo₂O_6?δ due to the redistribution of La between Gd and Ba sites in Gd_1–xLa_xBaCo₂O_6?δ lattice. Secondly, a single-phase double perovskite can be synthesized in air only by simultaneously substituting, within certain limits, both Gd and Ba in GdBaCo₂O_6?δ with La. Finally, using Gd_0.8La_0.2Ba_0.95La_0.05Co₂O_6?δ and BaCo_0.8Gd_0.2O_3?δ as examples, we demonstrated that while single-phase double perovskite does not hydrate, in the same conditions, the cubic perovskite BaCo_0.8Gd_0.2O_3?δ (BaCo_1–xGd_xO_3?δ is encountered in Gd_1–xLa_xBaCo₂O_6?δ as an impurity) absorbs significant amount of water. Thus, the water uptake by lanthanum-substituted GdBaCo₂O_6?δ is most likely to occur due to the impurities, and not the main double perovskite phase.

Full text of DOI:10.1016/j.jallcom.2020.156309

Journal of Alloys and Compounds 845 (2020) 156309
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
The origin of triple conductivity and water uptake in layered double
perovskites: A case study on lanthanum-substituted GdBaCo2O6ꢀ
d
,
b
,
, b
Dmitry Malyshkin a *, Andrey Novikov a, Ivan Ivanov a, Vladimir Sereda a
,
Dmitry Tsvetkov a, Andrey Zuev a
a Institute of Natural Sciences and Mathematics, Ural Federal University, 620002, 19 Mira St., Ekaterinburg, Russia
b Institute of High-Temperature Electrochemistry UB RAS, 620137, 20 Akademicheskaya St., Ekaterinburg, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Some layered double perovskite cobaltites have been shown recently to absorb water and exhibit
increasing electrical conductivity in humid atmospheres. However, the assumptions that their crystal
lattice is capable of proton uptake, and that these oxides really possess triple (oxide ion, protonic and
electronic) conductivity, have already been brought into question. We investigated in detail the crystal
Received 20 April 2020
Received in revised form
3 July 2020
Accepted 4 July 2020
Available online 8 July 2020
structure and phase composition of various lanthanum-substituted GdBaCo2O6ꢀ and came to several
d
important conclusions. Firstly, in oxidative conditions (e.g. in air), the substitution of La for either only Gd
or only Ba in GdBaCo2O6ꢀ results in formation of multiphase materials. For example, BaCo1exGdxO3ꢀ
d
d
Keywords:
exsolves from Gd1exLaxBaCo2O6ꢀ due to the redistribution of La between Gd and Ba sites in Gd1
d
Double perovskites
Layered perovskites
Triple-conducting phenomena
Proton conducting oxides
exLaxBaCo2O6ꢀ lattice. Secondly, a single-phase double perovskite can be synthesized in air only by
d
simultaneously substituting, within certain limits, both Gd and Ba in GdBaCo2O6ꢀd with La. Finally, using
Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ and BaCo0.8Gd0.2O3ꢀ as examples, we demonstrated that while single-
d
d
phase double perovskite does not hydrate, in the same conditions, the cubic perovskite
BaCo0.8Gd0.2O3ꢀ (BaCo1exGdxO3ꢀ is encountered in Gd1exLaxBaCo2O6ꢀ as an impurity) absorbs sig-
d
d
d
nificant amount of water. Thus, the water uptake by lanthanum-substituted GdBaCo2O6ꢀ is most likely
d
to occur due to the impurities, and not the main double perovskite phase.
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
otherwise it proceeds only on the triple-phase boundary. In other
words, it would be quite beneficial for the performance of HeSOFC
At least since the relatively recent discovery of fast oxide ion
transport in A-site-ordered layered double perovskite cobaltites
REBaCo2O6ꢀd (RE ¼ rare-earth element) [1], these oxides have been
receiving constant attention as suitable materials for various elec-
trochemical devices. High mixed oxide ion and electronic conduc-
tivity coupled with the catalytic activity towards the oxygen
if its cathode possessed triple (oxide ion, protonic and electronic)
conductivity.
In this respect, the recent studies where possible proton con-
ductivity in REBaCo2O6ꢀ was pointed out, only reinforced the in-
d
terest towards REBaCo2O6ꢀ [6e10]. In particular, Grimaud et al.
d
[6,7], using a combination of thermogravimetric analysis (TGA) and
reduction reaction made REBaCo2O6ꢀ look promising as cathodes
electrochemical studies, found out that PrBaCo2O6ꢀ shows some
d
d
for both oxide ion- and proton-conducting solid oxide fuel cells
(OeSOFC and HeSOFC, respectively) [2e5]. The latter are generally
considered to be better suited for practical applications due to
lower operating temperatures and the absence of fuel contamina-
tion. In HeSOFC, the kinetics of the cathodic reaction can be greatly
enhanced by “extending” the proton transfer into the cathode bulk,
proton conductivity and can be considered as a triple-conducting
oxide. These conclusions are consistent with those of Goupil et al.
[8], who interpreted their data on TGA coupled with mass spec-
trometry as evidence for the reversible hydration of REBaCo2O6ꢀ
d
(RE ¼ Gd, Sm, Pr, Nd).
However, in the work of Goupil et al. [8], the amount of water in
REBaCo2O6ꢀ (RE ¼ Gd, Sm, Pr, Nd) decreased upon cooling and
d
reached zero at room temperature, whereas the opposite trend is
expected of proton-conducting oxides. Moreover, the H2O signal
measured by mass spectrometry during TGA experiments indicated
* Corresponding author. Institute of Natural Sciences and Mathematics, Ural
Federal University, 620002, 19 Mira St., Ekaterinburg, Russia.
E-mail address: dmitry.malyshkin@urfu.ru (D. Malyshkin).
https://doi.org/10.1016/j.jallcom.2020.156309
0925-8388/© 2020 Elsevier B.V. All rights reserved.

2
D. Malyshkin et al. / Journal of Alloys and Compounds 845 (2020) 156309
water condensation in the thermobalance [8]. And even if the ex-
periments are free from the methodological errors, both the con-
ductivity measurements and the TGA are, perhaps, necessary, but
not sufficient to prove that a certain oxide phase possesses proton
conductivity. The other factors, such as changes in structure or
composition of a material, may be responsible for the observed
conductivity or mass changes. For example, while some, though
rather low, water content was detected in PrBaCo2O6ꢀd with the aid
of Karl Fischer titration [11] and TGA [9], X-ray diffraction (XRD)
analysis performed on a hydrated PrBaCo2O6ꢀd sample revealed the
formation of a secondary phase, which decomposes during TGA
experiments under dry atmosphere [6]. Later, secondary ion mass
Germany). The scanning electron microscopy (SEM) and energy-
dispersive X-ray (EDX) studies into the microstructure and the
chemical composition of the samples were made on an AURIGA
CrossBeam (FIB-SEM) Workstation (Carl Zeiss, Germany).
Thermogravimetric experiments were performed using a ther-
mogravimetric setup based on CI Precision (UK) microbalance. Dry
atmosphere in the thermobalance was obtained by passing the
inlet gas through the zeolite-filled column, and the humid atmo-
sphere was controlled by flowing the gas through a small Peltier-
controlled thermostat. The actual humidity during the TGA exper-
iments was measured with factory-calibrated SHT31 (Sensirion,
Switzerland) relative humidity sensors.
spectrometry (SIMS) studies showed that water in PrBaCo2O6ꢀ is
d
likely to be present only in form of hydrated barium compound
layer on the surface (the interior pore walls) of this double perov-
skite [5]. As for some other substituted REBaCo2O6ꢀd, the uptake of
3. Results and discussion
3.1. Phase composition and crystal structure of Gd1exLaxBaCo2O6ꢀ
d
water by GdBaCo1.8Fe0.2O6ꢀd, NdBaCo2exMnxO6ꢀ (x ¼ 0, 0.5) and
d
La0.5Ba0.5CoO3ꢀ could not be clearly established from TGA [9], in
d
X-ray diffraction patterns of the as-prepared single-phase dou-
ble perovskites with nominal compositions Gd1exLaxBaCo2O6ꢀ
situ high-temperature neutron powder diffraction [12] and SIMS
[10] measurements, respectively.
d
(x ¼ 0.2e0.6) are shown in Fig. 1a. They were indexed using
P4=mmm space group. It should be emphasized that, although the
single-phase (at least as indicated by XRD) samples were obtained,
it was rather hard to prepare them. The synthesis procedure
employed quite often resulted in the two-phase powders, as veri-
fied by XRD analysis. This resembles the behavior of a solid solution
with the chemical composition being very close to the solubility
limit, where even a tiny variation of the conditions destabilizes
such a solution, and the secondary phase exsolves. As will be
described in detail below, even the samples that, according to XRD,
can be regarded as single-phase, in fact, contain a small amount of
impurities, whose composition strongly depends on the samples’
history.
Therefore, the water uptake capability of the bulk of
REBaCo2O6ꢀ double perovskites and the origins of proton con-
d
ductivity in them still remain a moot point.
For this reason the present work is focused on
Gd1exLaxBaCo2O6ꢀ double perovskites, at least one of which,
d
Gd0.8La0.2BaCo2O6ꢀd, is, reportedly, a triple-conductive oxide [9,10].
Its protonation at temperature between 300 ꢁC and 400 ꢁC was
confirmed by TGA [9] and SIMS [10]. At the same time, it was
demonstrated that in the deuterated Gd0.8La0.2BaCo2O6ꢀ the con-
d
centration of D decreases over the first z300 nm, possibly due to
deuterium-containing species adsorbed on the surface of this ma-
terial [10]. We suspected that, similarly to PrBaCo2O6ꢀ [5,6], the
d
nature of proton conductivity in Gd0.8La0.2BaCo2O6ꢀ may be not
d
As for the Gd1exLaxBaCo2O6ꢀ samples with x > 0.6, as seen in
d
quite straightforward e and, indeed, our suspicions were confirmed
by the phase composition studies on Gd1exLaxBaCo2O6ꢀd, the results
of which are presented below.
Fig. 1a, the pattern of the as-prepared slowly cooled
Gd0.2La0.8BaCo2O6ꢀ (GL80BC SC in Fig. 1a) exhibits anomalous in-
d
tensities of some diffraction lines, e.g., those at 2
q
¼ 22:75ꢁ; 32:55ꢁ
; 46:6ꢁ and 58ꢁ. It is known that the endpoint composition LaB-
aCo2O6ꢀd (Gd1exLaxBaCo2O6ꢀd with x ¼ 1.0) with a layered structure
can be prepared only in a reducing atmosphere, and that it can be
synthesized as a mixture of ordered (double perovskite) and
2. Experimental
The powder samples of Gd1exLaxBa1eyLayCo2O6ꢀ
and
d
Ba1eyLayCo1exGdxO3ꢀ were synthesized from high-purity La(N-
d
disordered (cubic perovskite) phases, depending on T and pO [14].
2
O3)3$6H2O, Gd2O3, BaCO3 and Co precursors via a glycerol-nitrate
route. Metallic cobalt was obtained by reducing Co3O4 in
hydrogen at 600 ꢁC. Gadolinium oxide was pre-annealed at 1100 ꢁC
to decompose the gadolinium hydrates and carbonates. The exact
amount of water in La(NO3)3$6H2O was estimated using TGA.
Stoichiometric quantities of precursors were dissolved in concen-
trated HNO3, followed by addition of glycerol, which acted as a
complexing agent and a fuel, and heated on a hot plate until the
water evaporation and pyrolysis of the precursor mixture was
complete. The resulting ash was calcined once at 900 ꢁC and thrice
at 1100 ꢁC, with ca 10 h calcination dwell time and intermediate
regrindings in an agate mortar. After the last annealing,
Gd1exLaxBa1eyLayCo2O6ꢀd samples were slowly (100 ꢁC,hꢀ1) cooled
to room temperature, while those of Ba1eyLayCo1exGdxO3ꢀd e were
For this reason, the XRD pattern of GL80BC SC was analyzed and
refined as a two-phase mixture of ordered (P4=mmm) and disor-
dered (Pm3m) perovskites, and the fraction of the cubic (disor-
dered) phase was found to be (42
1) wt %. In turn,
Gd0.2La0.8BaCo2O6ꢀd annealed for 24 h at 1100 ꢁC and p ¼ 10ꢀ3:0
O2
atm and then quenched to room temperature (GL80BC Q in Fig. 1a),
has double perovskite structure.
The refined lattice parameters and cell volume of double pe-
rovskites Gd1exLaxBaCo2O6ꢀd with x ¼ 0.2e0.6 are shown in Fig. 1b
as functions of lanthanum content at room temperature in air. The
cell parameters are in good agreement with those reported previ-
ously for Gd0.8La0.2BaCo2O6ꢀ [15]. The a parameter is almost in-
d
dependent of the lanthanum content, whereas the c parameter
significantly increases with lanthanum substitution. For the sake of
comparison, Fig. 1b also shows the lattice parameters and cell
volume of undoped cobaltites GdBaCo2O6ꢀ [16] and LaBaCo2O6ꢀ
quenched to room temperature, similarly to BaCo1exYxO3ꢀ [13].
d
The quenching experiments were carried out in an original setup
described elsewhere [14].
d
d
The phase composition of all the samples prepared accordingly
was investigated by means of room-temperature XRD. The XRD
patterns were obtained with 7000S diffractometer (Shimadzu,
[14]. As follows, the lattice volume monotonically increases with
the lanthanum content because of the larger crystal ionic radius of
La as compared to that of Gd [17].
Japan) using Cu K
XRD) experiments were implemented using Cu K
DRON 6 diffractometer (JSC Bourevestnik, Russia) equipped with a
high-temperature chamber HDK 1200 (Edmund Buehler GmbH,
a
radiation. In situ high-temperature XRD (HT
radiation on a
In situ XRD patterns obtained in the temperature range
a
(25e900) ꢁC in air for the Gd0.8La0.2BaCo2O6ꢀ oxide are shown in
d
Fig. 2a. As seen, the patterns do not show significant changes with
temperature, indicating that a P4=mmm structure is retained at all

D. Malyshkin et al. / Journal of Alloys and Compounds 845 (2020) 156309
3
Fig. 1. (a) XRD pattern of Gd1exLaxBaCo2O6ꢀ (x
Gd0.2La0.8BaCo2O6ꢀd (GL80BC Q). (b) The refined lattice parameters and cell volume of Gd1exLaxBaCo2O6ꢀd as functions of lanthanum content at room temperature. Lines are guides
¼
0.2e0.8) samples slowly (100 ꢁC,hꢀ1 cooled from 1100 ꢁC to room temperature in air, and quenched sample of
)
d
to the eye only.
Fig. 2. (a) In situ HT XRD patterns of Gd0.8La0.2BaCo2O6ꢀd in the temperature range between 25 ꢁC and 900 ꢁC in air (
e Pt sample holder). (b) The refined lattice parameters and
;
cell volume of Gd0.8La0.2BaCo2O6ꢀ vs temperature in air.
d
the temperatures investigated. The refined lattice parameters and
the unit cell volume of Gd0.8La0.2BaCo2O6ꢀd are presented in Fig. 2b
as functions of temperature in air.
nanoparticles does not allow determining their chemical compo-
sition using EDX analysis. However, the presence of a secondary
phase in the as-prepared Gd1exLaxBaCo2O6ꢀ samples with
d
x ¼ 0.2e0.6 can often be detected even by conventional XRD
analysis. The reason why the secondary phase is not always
discernible on the XRD pattern, while its nanoparticles seem to be
always present, lies, most probably, in the size of these particles and
their nature, as will be explained below.
3.2. Exsolution of secondary phase from lanthanum-substituted
GdBaCo2O6ꢀ
d
As mentioned above, it is very difficult to reproducibly prepare
the single-phase samples of Gd 1exLaxBaCo2O6ꢀd with x ¼ 0.2e0.6.
In fact, even seemingly single-phase, according to XRD,
Fig. 3b shows the XRD patterns of the as-prepared
Gd0.8La0.2BaCo2O6ꢀ (GL20BC 2ph), which contained peaks (;)
d
related to the secondary phase. Indexing these peaks can be per-
formed within the cubic symmetry. After careful evaluation, we
Gd0.8La0.2BaCo2O6ꢀ samples were found to be dotted with the
d
nanoparticles covering the grain surfaces of the main phase, as
shown in Fig. 3a. Unfortunately, the very small size of the
excluded undoped BaCoO3ꢀ from consideration as a possible
d
Fig. 3. (a) SEM micrograph of Gd0.8La0.2BaCo2O6ꢀd slowly (100 ꢁC,hꢀ1) cooled from 1100 ꢁC to room temperature in air. (b) XRD patterns of two-phase Gd0.8La0.2BaCo2O6ꢀd samples
quenched from 1100 ꢁC to room temperature in air (
e BaCo1exGdxO3ꢀ phase).
;
d

4
D. Malyshkin et al. / Journal of Alloys and Compounds 845 (2020) 156309
impurity because it is not cubic under the experimental conditions
(in air) [18], and even if it were, its peak positions would not
correspond to the XRD pattern of GL20BC 2ph. Nor do the peak
positions of hexagonal BaCoO3ꢀd match those seen in Fig. 3b. In that
our results do not support those of Vøllestad et al. [15], who
temperature with 30% relative humidity (see Fig. 3b). Such change
in the intensities, being probably associated with the decomposi-
tion of BaCo1exGdxO3ꢀ in humid atmosphere [13], was also
d
observed in two-phase Gd0.8La0.2BaCo2O6ꢀ (GL20BC 2ph). There-
d
fore, it is most likely that Gd 1exLaxBaCo2O6ꢀ oxides with
d
claimed that hexagonal BaCoO3ꢀ is one of the products of
x ¼ 0.2e0.6, prepared in air, always contain BaCo1exGdxO3ꢀ
d
d
Gd0.8La0.2BaCo2O6ꢀ decomposition after stability test at 1.5 bar of
cobaltite (or its decomposition products, depending on the syn-
thesis route and storage conditions) even if no impurity phase is
detected with XRD.
d
water vapor and 600 ꢁC. Note, however, that inconsistencies in the
work of Vøllestad et al. [15] do not allow checking their assertions:
two XRD patterns of partially decomposed Gd0.8La0.2BaCo2O6ꢀ
,
It should be noted that Rivas-Murias et al. [20] synthesized in air
d
that presented for the sake of comparison with the XRD pattern of
as-sintered sample and that used for the Rietveld refinement (see
pages 3 and 4 in the Supplementary to Ref. [15]) differ significantly
from each other, and both contain peaks that are hard to account for
within the framework of the decomposition scheme proposed by
Vøllestad et al. [15].
the GdBaCo2O6ꢀ sample with 50% of Gd replaced with Pr
d
(Gd0.5Pr0.5BaCo2O6ꢀd), whose size is close to that of La. Interest-
ingly, the XRD pattern of this sample clearly shows additional
reflection at 2q a radiation), which was not indexed in
y33:5ꢁ (Cu K
P4=mmm space group [20]. Moreover, the XRD patterns of double
perovskites Pr0.5RE0.5BaCo2O6ꢀd, prepared in air, also show this
additional reflection in the case of RE ¼ EueDy [20] and Y [21],
whereas the compounds with RE ¼ Nd, Sm [20] were found to be
single phase. Therefore, it can be expected that layered double
perovskites with different A-site lanthanide cations in the rare-
earth layers can be prepared as single-phase samples in air only if
these cations have close sizes.
In fact, the positions of the impurity peaks in the GL20BC 2ph
XRD pattern (see Fig. 3b) are almost identical to those of cubic
BaCo0.75Y0.25O2.55 perovskite (PDF-2 entry #00-057-0194). No in-
formation concerning the existence of Gd-substituted barium
cobaltite, isostructural with BaCo0.75Y0.25O2.55, had been found in
the literature. However, it is known that, in contrast to Nd [19] and
larger RE, not only Y [13] but also Sm [19] and, possibly, other
smaller RE can substitute Co ions in BaCoO3ꢀd. Thus, it seems likely
that the impurity phase in Gd0.8La0.2BaCo2O6ꢀd, the amount of
which was about 2 wt % according to Rietveld refinement, may have
It is noteworthy that
a single-phase double perovskite
Gd0.8La0.2BaCo2O6ꢀ can be obtained by annealing two-phase
d
Gd0.8La0.2BaCo2O6ꢀ material (GL20BC 2ph) under reducing con-
d
ditions, e.g. pO ¼ 10ꢀ5:3 atm at 1000 ꢁC for 160 h as it follows from
2
a composition represented by the general formula BaCo1exGdxO3ꢀ
.
Fig. 5a. For the sake of comparison, the XRD pattern of two-phase
d
In order to support this assumption, we prepared the
Gd0.8La0.2BaCo2O6ꢀ sample (GL20BC 2ph), calcined under less
d
Gd0.8La0.2BaCo2O6ꢀd oxide with 10 mol % excess of BaCo0.8Gd0.2O3ꢀ
reducing conditions e 1000 ꢁC and pO ¼ 10ꢀ3:2 atm for 160 h e is
d
2
(GL20BC þ BCG20). The XRD pattern of this material was found to
correspond fairly well to that of the two-phase double perovskite
Gd0.8La0.2BaCo2O6ꢀd (GL20BC 2ph), except for increased intensities
of the secondary phase diffraction lines (;), as seen in Fig. 3b.
In addition, to exclude the possible substitution of La for Ba in
also given in Fig. 5a. If the secondary phase in GL20BC 2ph simply
decomposed at pO ¼ 10ꢀ5:3 atm and 1000 ꢁC, then some evidence
2
of the products of its decomposition would be seen in the XRD
pattern or the SEM micrograph. However, the XRD pattern and SEM
micrograph of the as-annealed GL20BC 2ph sample, quenched to
room temperature, showed no impurity phase peaks and the uni-
formity of the grain surfaces, as seen in Fig. 5a and 5b, respectively.
Hence, it can be concluded that BaCo1exGdxO3ꢀd, being unstable
BaCo1exGdxO3ꢀd, the Ba 1eyLayCo0.8Gd0.2O3ꢀ samples with y ¼ 0,
d
0.1 and x ¼ 0.2 were prepared at 1100 ꢁC in air. As a result, whereas
a single-phase cubic perovskite BaCo0.8Gd0.2O3ꢀ (Pm3m space
d
under reducing conditions, should dissolve in Gd0.8La0.2BaCo2O6ꢀ
d
group) was obtained, even the sample with y ¼ 0.1 already con-
bulk, making the material single phase. In air, accordingly, an
exsolution of BaCo1exGdxO3ꢀ from Gd0.8La0.2BaCo2O6ꢀ occurs.
tained a mixture of BaCo1exGdxO3ꢀ and Ba3exLaxCo2O8e (;), as
d
d
d
d
seen in Fig. 4.
We should briefly mention here that partial substitution of La
Furthermore, it was found that the intensity of the diffraction
for Ba in the GdBaCo2O6ꢀ double perovskite also leads to the for-
d
lines corresponding to BaCo1exGdxO3ꢀ in the XRD pattern of the
d
mation of multiphase materials. Thus, the XRD patterns for the as-
two-phase sample GL20BC þ BCG20 decreases significantly, and
the width of the respective peaks e simultaneously increases after
storage under ambient conditions, e.g., for 1 week at room
prepared GdBa1eyLayCo2O6ꢀ with y ¼ 0.05 and 0.1 contain peaks
d
(;) related to the Gd1exLaxCoO3ꢀ perovskite secondary phase, as
d
seen in Fig. 6. Although Pietosa et al. [22] reported the successful
preparation of NdBa1eyLayCo2O6ꢀd with a partial substitution of La
for Ba (y ¼ 0.06, 0.1), their findings should not be seen as contra-
dicting ours: the stability of Nd-containing compounds may differ
from that of Gd-containing ones. In addition, the authors [22] did
not provide XRD patterns for these samples to corroborate their
statement of the samples’ phase purity.
3.3. A genuinely single-phase lanthanum-substituted GdBaCo2O6ꢀ
d
Logically, our next assumption was that if the cobaltite
BaCo1exGdxO3ꢀ is formed as a result of exsolution from the
d
Gd1exLaxBaCo2O6ꢀd lattice, then some deficit of Ba and Co cations in
the latter may promote formation of a truly single-phase double
perovskite. In order to check whether it is the case the
Gd0.8La0.2BaCo2O6ꢀd samples with 5 mol % and 10 mol % deficiency
of BaCoO3ꢀ (GL20BC-5%BaCoO3 and GL20BC-10%BaCoO3, respec-
d
tively), i.e. with the simultaneous deficiency of Ba and Co cations,
were prepared. Fig. 7 shows that GL20BC-5%BaCoO3 can be suc-
cessfully synthesized in air at 1100 ꢁC as a single phase, as
Fig. 4. XRD patterns of Ba1eyLayCo0.8Gd0.2O3ꢀ cobaltites prepared at 1100 ꢁC in air
d
followed by quenching to room temperature ( e Ba3exLaxCo2O8e phase).
;
d

D. Malyshkin et al. / Journal of Alloys and Compounds 845 (2020) 156309
5
Fig. 5. (a) XRD patterns of the samples obtained by annealing of two-phase Gd0.8La0.2BaCo2O6ꢀd (GL20BC 2ph) at 1000 ꢁC and different pO for 160 h, followed by quenching to room
2
temperature (
e BaCo1exGdxO3ꢀd phase). (b) SEM micrograph of the sample obtained by annealing of two-phase Gd0.8La0.2BaCo2O6ꢀd (GL20BC 2ph) at 1000 ꢁC and pO ¼ 10ꢀ5:3
;
2
atm for 160 h, followed by quenching to room temperature.
composition of the single-phase GL20BC-5%BaCoO3 can be
assuming the
rewritten as Gd0.821La0.179Ba0.974La0.026Co2O6ꢀ
,
d
possible redistribution of lanthanum between gadolinium and
barium layers [15]. To sum up, the double perovskites
Gd1exLaxBa1eyLayCo2O6ꢀ can be prepared in air as a single phase
d
only by simultaneous substitution of La for Gd and Ba. Conse-
quently, the abovementioned exsolution of BaCo1exGdxO3ꢀ from
d
Gd0.8La0.2BaCo2O6ꢀd may be viewed as a result of the redistribution
of La cations between rare-earth and barium sites.
Indeed, by trial and error we found that, for Gd1exLaxBa1e
yLayCo2O6ꢀ with x ¼ 0.2, the Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ double
d
d
perovskite can be successfully and reproducibly synthesized in air at
1100 ꢁC as a single-phase material. The corresponding XRD pattern
and SEM micrograph of the slowly (100 ꢁC,hꢀ1) cooled sample
showed noindicationforthepresenceofasecondaryphase, asseenin
Fig. 8a and 8b, respectively. However, further increase in the
lanthanum content in Ba-sublattice leads to formation of a mixture of
Fig. 6. XRD patterns of the double perovskites GdBa1eyLayCo2O6ꢀd slowly (100 ꢁC,hꢀ1
)
the Gd1exLaxBa1eyLayCo2O6ꢀ
double perovskite and the
Gd1exLaxCoO3ꢀ perovskite phase. Similar results were reported for
d
cooled from 1100 ꢁC to room temperature in air (
e Gd1exLaxCoO3ꢀ phase).
;
d
d
the Gd0.8La0.2Ba1eyLayCo2O6ꢀd samples with y ¼ 0.2, 0.4 [15]: though
Vøllestad et al. [15] did not focus on it specifically, the XRD patterns of
these cobaltites also showed additional reflection at 2q
z34ꢁ, sug-
gesting that these materials are not single phase. Thus, there is a
narrowrangeofthepossibleratiosoflanthanumcationsinGdandBa-
sublattices of Gd1exLaxBa1eyLayCo2O6ꢀd, within which a single-phase
double perovskite can be obtained.
In this regard, we would like to comment once again on the recent
studyof Vøllestadetal. [15], who found thatthe partial substitution of
La for Ba in the double perovskite Gd0.8La0.2Ba1eyLayCo2O6ꢀ in-
d
creases its chemical stability in high water steam pressures. They
reported that stability tests at 600 ꢁC under 1.5 bar of steam for 72 h
showed a separation of Gd0.8La0.2Ba1eyLayCo2O6ꢀd with y ¼ 0, 0.2, 0.4
into two phases, whereas at y ¼ 0.5 there was no sign of a second
phase formation [15]. However, it seems that Vøllestad et al. [15]
exaggerated the maximum possible amount of lanthanum in barium
layers of Gd0.8La0.2Ba1eyLayCo2O6ꢀd, at which the sample remains a
single-phase double perovskite. As mentioned above, the unac-
counted reflection at 2q
z34ꢁ in the XRD patterns of as-prepared
Fig. 7. XRD patterns of the Gd0.8La0.2BaCo2O6ꢀ with simultaneous Ba and Co defi-
d
Gd0.8La0.2Ba1eyLayCo2O6ꢀ with y ¼ 0.2, 0.4 casts doubt on the au-
ciency, prepared at 1100 ꢁC in air followed by slow (100 ꢁC,hꢀ1) cooling to room
d
thors’ [15] assertions about the phase purity of these oxides. What is
temperature.
more, the XRD pattern of as-prepared Gd0.8La0.2Ba0.5La0.5Co2O6ꢀ
d
looks similar to that of cubic perovskite, and, probably, not exactly
single phase at that, for several small diffuse peaks, e.g. at 2q
z32:5ꢁ;
confirmed by XRD analysis of the slowly cooled sample. However, a
larger deficiency leads to formation of Gd2O3 impurity, as seen in
the inset in Fig. 7.
Taking into account that high amount of cation vacancies in the
double perovskite cobaltites is unlikely [23], the nominal
46:7ꢁ; 58:3ꢁ are clearly seen [15]. It is true that the XRD patterns of
Gd0.8La0.2Ba0.5La0.5Co2O6ꢀ before and after the pressurized water
d
steam treatment, being close to each other, indicate the stabilization
of the structure and phase composition of this material, as compared

6
D. Malyshkin et al. / Journal of Alloys and Compounds 845 (2020) 156309
Fig. 8. (a) XRD patterns of the double perovskites Gd0.8La0.2Ba1eyLayCo2O6ꢀ prepared at 1100 ꢁC in air followed by slow (100 ꢁC,hꢀ1) cooling to room temperature. (b) SEM
d
micrograph of Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ prepared at 1100 ꢁC in air followed by slow (100 ꢁC,hꢀ1) cooling to room temperature.
d
to Gd0.8La0.2Ba1eyLayCo2O6ꢀ with y ꢂ 0.4. Nonetheless, in case of
exsolution on the electrochemical properties of Gd 1exLaxBaCo2O6ꢀ
d
d
Gd0.8La0.2Ba0.5La0.5Co2O6ꢀd, it is not exactly the double perovskite
structure that is stabilized.
oxides is of key importance for successful development of novel
triple-conducting materials for practical applications. Such work is in
progress now.
3.4. The hydration of lanthanum-substituted GdBaCo2O6ꢀ
d
4. Conclusions
Fig. 9 presents the temperature dependences of the oxygen
In this work, we investigated the crystal structure of double pe-
content in dry (pH O ¼ 10ꢀ3:97 atm) synthetic air for
2
rovskites with nominal compositions Gd1exLaxBaCo2O6ꢀ
d
Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ and BaCo0.8Gd0.2O3ꢀd, respectively. In
d
(x ¼ 0.2e0.8). By sintering in air, it was possible to obtain the layered
contrast with the Gd0.8La0.2BaCo2O6ꢀ material [9], for the single-
d
double perovskite structure (P4=mmm space group) for the samples
phase double perovskite Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ the differ-
ence between the weight change in humid (pH O ¼ 10ꢀ1d:88 atm) and
with x up to 0.6. Annealed in air Gd 1exLaxBaCo2O6ꢀ with x ¼ 0.8
d
2
dry atmospheres of both synthetic air and nitrogen (pO ¼ 10ꢀ4:2
comprised a mixture of ordered (P4=mmm) and disordered (Pm3m)
perovskite phases, which transformed into a double perovskite
(P4=mmm) after annealing in mildly reducing conditions (1100 ꢁC
2
atm) is insignificant, as shown in the inset of Fig. 9a. Likewise, there
is still no convincing evidence of water absorption in A-sublattice-
undoped GdBaCo1.8Fe0.2O6ꢀd [9] and La0.5Ba0.5CoO3ꢀd [10] cobaltites.
At the same time, the inset in Fig. 9b demonstrates significant weight
and pO ¼ 10ꢀ3:0 atm). The lattice volume of Gd1exLaxBaCo2O6ꢀ
d
2
(x ¼ 0.2e0.8) was shown to increase upon increasing lanthanum
increase in humid (pH O ¼ 10ꢀ2:05 atm) synthetic air for the
content x. In situ HT XRD demonstrated that Gd0.8La0.2BaCo2O6ꢀ
d
2
BaCo0.8Gd0.2O3ꢀd cubic perovskite. Hence, we can conclude that the
sample possesses double perovskite structure (P4=mmm) in the
temperature range of (25e900) ꢁC with monotonically increasing
unit cell parameters a, c, and unit cell volume.
Our subsequent study concerned the nature of a secondary
phase, the nanoparticles of which were found by SEM to cover the
surface of Gd0.8La0.2BaCo2O6ꢀd. Though it proved hard to pinpoint
the actual composition of this impurity phase, most likely, it cor-
responds to the gadolinium-substituted barium cobaltite
protonation observed earlier in Gd0.8La0.2BaCo2O6ꢀ [9,10] is due to
d
the impurities, such as BaCo1exGdxO3ꢀd, that are always present in
this material. Moreover, as compared to the other double perovskite
cathodes without La-doping, Gd0.8La0.2BaCo2O6ꢀ cathode was
d
shown earlier [9] to possess the lowest polarization resistance in
contact with proton-conducting electrolyte. It may well be that the
exsolution of BaCo1exGdxO3ꢀ cubic perovskite promotes the elec-
d
trochemical activity of Gd0.8La0.2BaCo2O6ꢀd. Thus, a detailed study of
water absorption and the possible proton conductivity in the cubic
perovskites BaCo1exRExO3ꢀd, as well as the influence of their
BaCo1exGdxO3ꢀd. The identification of BaCo1exGdxO3ꢀ as an im-
d
purity in Gd1exLaxBaCo2O6ꢀd oxides is hampered by the instability
of BaCo1exGdxO3ꢀd in ambient humid air; the intensity of its peaks
Fig. 9. Main plots e temperature dependences of the oxygen content in Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ (a) and BaCo0.8Gd0.2O3ꢀ (b) in dry (p
show relative weight differences between TGA curves in wet and dry atmospheres for Gd0.8La0.2Ba0.95La0.05Co2O6ꢀd (a) and BaCo0.8Gd0.2O3ꢀd. (b) The solid lines and points represent
the data obtained in dynamic and static mode, respectively.
¼ 10ꢀ3:97 atm) synthetic air. Insets
d
d
H2
O

D. Malyshkin et al. / Journal of Alloys and Compounds 845 (2020) 156309
PrBaCo2O6ꢀd-Ce0.8Sm0.2O1.9
7
composite
cathodes
for
intermediate-
on the XRD patterns depends strongly on the sintering conditions
and decreases drastically even upon storage. In addition, genuinely
single-phase double perovskites, which are stable in ambient air,
cannot be produced by substitution of La for either just Gd or just
Ba in GdBaCo2O6ꢀd. Only simultaneous replacement, within certain
limits, of both Gd and Ba in GdBaCo2O6ꢀd with La can yield single-
phase, according to both SEM and XRD, A-site ordered double
perovskite. As an example of such Gd1exLaxBa1eyLayCo2O6ꢀd oxide,
Gd0.8La0.2Ba0.95La0.05Co2O6ꢀd was synthesized.
temperature solid oxide fuel cells: stability and cation interdiffusion, En-
ergies 12 (2019) 417, https://doi.org/10.3390/en12030417.
ꢀ
[5] H. Tellez Lozano, J. Druce, S.J. Cooper, J.A. Kilner, Double perovskite cathodes
for proton-conducting ceramic fuel cells: are they triple mixed ionic electronic
conductors? Sci. Technol. Adv. Mater. 18 (2017) 977e986, https://doi.org/
10.1080/14686996.2017.1402661.
[6] A. Grimaud, F. Mauvy, J.M. Bassat, S. Fourcade, L. Rocheron, M. Marrony,
J.C. Grenier, Hydration properties and rate determining steps of the oxygen
reduction reaction of perovskite-related oxides as Hþ-SOFC cathodes,
J. Electrochem. Soc. 159 (2012) B683eB694, https://doi.org/10.1149/
2.101205jes.
Finally, the hydration experiments demonstrated that, while
[7] A. Grimaud, J.M. Bassat, F. Mauvy, M. Pollet, A. Wattiaux, M. Marrony,
J.C. Grenier, Oxygen reduction reaction of PrBaCo2ꢀxFexO5þ compounds as
d
truly single-phase layered Gd0.8La0.2Ba0.95La0.05Co2O6ꢀ does not
d
Hþ-SOFC cathodes: correlation with physical properties, J. Mater. Chem. 2
(2014) 3594e3604, https://doi.org/10.1039/C3TA13956E.
show any water uptake, the cubic perovskite BaCo0.8Gd0.2O3ꢀ
d
significantly increases its weight in wet atmospheres. Our data
[8] G. Goupil, T. Delahaye, B. Sala, F. Lefebvre Joud, G. Gauthier, Selection and
study of basic layered cobaltites as mixed ioniceelectronic conductors for
proton conducting fuel cells, Solid State Ionics 263 (2014) 15e22, https://
doi.org/10.1016/j.ssi.2014.04.022.
[9] R. Strandbakke, V.A. Cherepanov, A.Y. Zuev, D.S. Tsvetkov, C. Argirusis,
G. Sourkouni, S. Prünte, T. Norby, Gd- and Pr-based double perovskite co-
baltites as oxygen electrodes for proton ceramic fuel cells and electrolyser
cells, Solid State Ionics 278 (2015) 120e132, https://doi.org/10.1016/
j.ssi.2015.05.014.
[10] K. Leonard, J. Druce, V. Thoreton, J.A. Kilner, H. Matsumoto, Exploring mixed
proton/electron conducting air electrode materials in protonic electrolysis
cell, Solid State Ionics 319 (2018) 218e222, https://doi.org/10.1016/
j.ssi.2018.02.016.
[11] D. Hashimoto, D. Han, T. Uda, Dependence of lattice constant of Ba, Co-
contained perovskite oxides on atmosphere, and measurements of water
content, Solid State Ionics 262 (2014) 687e690, https://doi.org/10.1016/
j.ssi.2013.12.029.
indicate that, similarly to what was found recently about so-called
triple conductivity in PrBaCo2O6ꢀ double perovskite [5,6], the
d
water absorption by La-substituted GdBaCo2O6ꢀ is not due to the
d
bulk double perovskite phase, but rather due to the impurities
covering the surface of these materials. From the practical stand-
point, it probably does not make much difference whether the
superior electrochemical performance of a material, such as
Gd0.8La0.2BaCo2O6ꢀ [9], is because of the main phase or the sec-
d
ondary phase. However, in our opinion, the focus of the further
development of better HeSOFC cathodes should be shifted to either
investigating these impurity phases as possible triple conductors or
studying the double-perovskite-based composite materials, where
the beneficial properties of in situ exsolved secondary phase
nanoparticles may complement those of the main phase.
[12] M. Bahout, S.S. Pramana, J.M. Hanlon, V. Dorcet, R.I. Smith, S. Paofai,
S.J. Skinner, Stability of NdBaCo2ꢀxMnxO5þ (x ¼ 0, 0.5) layered perovskites
d
under humid conditions investigated by high-temperature in situ neutron
powder diffraction, J. Mater. Chem. 3 (2015) 15420e15431, https://doi.org/
10.1039/C5TA02776D.
CRediT authorship contribution statement
[13] M.V. Lomakov, S.Y. Istomin, A.M. Abakumov, G. Van Tendeloo, E.V. Antipov,
Synthesis and characterization of oxygen-deficient oxides BaCo1ꢀxYxO3ꢀy
,
Dmitry Malyshkin: Conceptualization, Investigation, Writing -
original draft, Writing - review & editing, Funding acquisition.
Andrey Novikov: Investigation. Ivan Ivanov: Investigation, Writing
- original draft. Vladimir Sereda: Validation, Writing - original
draft, Writing - review & editing. Dmitry Tsvetkov: Conceptuali-
zation, Project administration, Writing - original draft, Writing -
review & editing. Andrey Zuev: Supervision, Writing - original
draft, Writing - review & editing.
x¼0.15, 0.25 and 0.33, with the perovskite structure, Solid State Ionics 179
(2008) 1885e1889, https://doi.org/10.1016/j.ssi.2008.05.004.
[14] D.A. Malyshkin, A.Y. Novikov, V.V. Sereda, I.L. Ivanov, D.S. Tsvetkov, A.Y. Zuev,
In situ and ex situ study of cubic La0.5Ba0.5CoO3ꢀ to double perovskite LaB-
d
aCo2O6ꢀ transition and formation of domain textured phases with fast oxy-
d
gen exchange capability, Inorg. Chem. 57 (2018) 12409e12416, https://
doi.org/10.1021/acs.inorgchem.8b02167.
[15] E. Vøllestad, R. Strandbakke, M. Tarach, D. Catalan-Martínez, M.-L. Fontaine,
ꢀ
D. Beeaff, D.R. Clark, J.M. Serra, T. Norby, Mixed proton and electron con-
ducting double perovskite anodes for stable and efficient tubular proton
ceramic electrolysers, Nat. Mater. 18 (2019) 752e759, https://doi.org/
10.1038/s41563-019-0388-2.
Declaration of competing interest
[16] D.S. Tsvetkov, I.L. Ivanov, A.Y. Zuev, Crystal structure and oxygen content of
the double perovskites GdBaCo2ꢀxFexO6ꢀd, J. Solid State Chem. 199 (2013)
154e159, https://doi.org/10.1016/j.jssc.2012.12.013.
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
[17] R.D. Shannon, Revised effective ionic radii and systematic studies of inter-
atomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976)
751e767, https://doi.org/10.1107/S0567739476001551.
ꢀ
ꢀ
[18] C. de la Calle, A. Alonso Jose, T. Fernandez-Díaz Maria, Polymorphism of
Ba1exSrxCoO3e (0ꢂxꢂ 1) perovskites: a thermal and structural study by
d
Acknowledgments
neutron diffraction, Z. Naturforsch. B Chem. Sci. 63 (2008) 647e654, https://
doi.org/10.1515/znb-2008-0609.
This work was supported by Council on grants of the President
of the Russian Federation (grant N MK-800.2020.3). We are also
grateful to the Ural Center for Shared Use “Modern Nanotech-
nology” (Ural Federal University, Ekaterinburg, Russia) whose
equipment was used in this work.
[19] L.Y. Gavrilova, T.V. Aksenova, N.E. Volkova, A.S. Podzorova, V.A. Cherepanov,
Phase equilibria and crystal structure of the complex oxides in the
LneBaeCoeO (Ln¼Nd, Sm) systems, J. Solid State Chem. 184 (2011)
2083e2087, https://doi.org/10.1016/j.jssc.2011.06.006.
ꢁ
~
[20] B. Rivas-Murias, J. Rivas, M.A. Senarís-Rodríguez, Synthesis, characterization
and transport properties of Pr0.50Ln0.50BaCo2O5þ (Ln: Pr, Nd, Sm, Eu, Gd, Tb
d
and Dy), J. Alloys Compd. 516 (2012) 113e118, https://doi.org/10.1016/
j.jallcom.2011.11.146.
[21] H. Zhao, Y. Zheng, C. Yang, Y. Shen, Z. Du, K. Swierczek, Electrochemical
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