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C. R. Chimie 22 (2019) 363e368Contents lists avaiComptes Rendus Chimie
www.sciencedirect.comPreliminary communication/CommunicationFluorine and chlorine doping in oxygen-deficient perovskites:
A strategy for improving chemical stability
Nataliia Tarasova*, Irina Animitsa
Ural Federal University, Institute of Natural Sciences and Mathematics, Russiaa r t i c l e i n f o
Article history:
Received 21 March 2019
Accepted 2 May 2019
Available online 31 May 2019
Keywords:
Perovskite
Anionic doping
Fluorine doping
Chlorine doping
Proton conductors
Chemical stability* Corresponding author.
E-mail address: Natalia.Tarasova@urfu.ru (N. Tar
https://doi.org/10.1016/j.crci.2019.05.001
1631-0748/© 2019 Académie des sciences. Publishea b s t r a c t
The present work describes the effect of fluorine and chlorine doping on the chemical sta-
bility of proton conductors Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11 against carbon dioxide
and water steam. It was proved that both undoped and halide-doped compositions
demonstrate good chemical stability under H2O treatmentwithout degradation andwithout
any hydrolytic decomposition. The hydration process leads to the change in the crystal
structure only. The treatment in the CO2/air (1:1) atmosphere (500 C, 10 h) leads to the
decomposition of undoped samples only. Halide-doped samples retain their structure
without detectable products, that is, they are more chemically stable compared with
undoped compositions. Themethod of halide doping can be used as the promising technique
for obtaining the new perovskite-related materials with high level of chemical stability.
© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.1. Introduction
Perovskite-related complex oxides with high level of
ionic conductivity can be used as electrolytic materials of
solid oxide fuel cells. However, the operating temperatures
of these fuel cells (900e1000 C) are very high; and the
search of alternative materials with lower working tem-
peratures is very important. The use of proton-conducting
complex oxides has a several advantages [1e8], such as a
decrease in operating temperatures (500e700 C) and an
increase in the efficiency.
The poor chemical stability in the atmosphere of carbon
dioxide and water vapor is the limiting factor for applica-
tion of the protonic electrolytes as the components of solid
oxide fuel cells [9]. A compromise between high proton
conductivity and chemical stability must be found to create
long-term working electrochemical devices based on solid
oxide protonic conductors.
The novel strategy for simultaneously improving trans-
port properties and chemical stability is the method of theasova).
d by Elsevier Masson SAS. Aanionic doping of perovskite-related matrix. The halide-
doped (F, Cl, and Br) barium cerate BaCeO3 demon-
strates the good chemical stability against CO2 and H2O
steam [10,11]. At the same time, the introduction of small
amount of F and Cl ions in the oxygen sublattice of proton
conductors Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11 leads
to the increase in oxygen ion and proton conductivity
[12e15]. However, the investigation of chemical stability
against CO2 and H2O steam for these halide-doped com-
pounds was not carried out. Earlier it was proved that In-
contained compounds are not favorable for using for in-
dustrial applications because of the presence of In, reducing
stability of complex oxides [4]. In this work, the composi-
tions with different types of the structures Ba2In2O5,
Ba4In2Zr2O11, and Ba4Ca2Nb2O11 (brownmillerite, ordinary,
and double perovskites) were chosen to establish the
fundamental regularities of the effect of anion doping on
chemical stability of proton conductors.
2. Experimental section
The samples Ba2In2O5, Ba4In2Zr2O11, Ba4Ca2Nb2O11,
Ba2In2O4.95F0.1, Ba4In2Zr2O10.95F0.1, Ba4Ca2Nb2O10.95F0.1,ll rights reserved.
N. Tarasova, I. Animitsa / C. R. Chimie 22 (2019) 363e368364Ba2In2O4.95Cl0.1, Ba4In2Zr2O10.95Cl0.1, and Ba4Ca2N-
b2O10.95Cl0.1 were prepared by a solid state method from
preliminary dried stoichiometric amounts of high-purity
powders BaCO3, CaCO3, In2O3, ZrO2, Nb2O5, BaCl2. Before
weighing, the starting materials were dried. The stoichio-
metric mixtures were mixed in agate mortar and then
calcined (800, 900, 1000, 1100, 1200, and 1300 С) with
regrindings after each stage.
The X-ray powder diffraction (XRD) measurements
were made using a Bruker Advance D8 diffractometer with
Cu Ka radiation. The crystal structures of anhydrous and
hydrated samples were determined through Rietveld
refinement using FULLPROF software.
Thermogravimetric analysis was carried out using a
simultaneous thermal analyzer 409 PC analyzer (Netzsch)
coupled with a quadrupole mass spectrometer QMS 403 C
A€eolos (Netzsch). For the preparation of hydrated forms of
the specimens the powder samples were hydrated at slow
cooling from 900 to 200 C (1 C/min) under a flow of wet
nitrogen. The “wet” nitrogen was obtained by bubbling the
N2 at room temperature first through distilled water and
then through saturated solution of KBr ðpH2O ¼ 2
102 atmÞ. The cooling was performed to a temperature
not lower than 200 C to avoid the appearance of adsorbed
water.
The CO2 stability of samples was determined on the
anhydrous powders that were exposed to the CO2/air (1:1)
mixture at 500 C for 10 hours. The treated powders were
then examined by XRD.3. Results and discussions
3.1. X-ray characterization
All halide-doped samples are single phase and they are
isostructural to their parent compositions. Brownmillerite-
type phases Ba2In2O4.95X0.1 (hereinafter X ¼ F, Cl) are
characterized by the space group Ibm2; perovskites
Ba4In2Zr2O10.95X0.1 have the space groupPm3m; and double
perovskites Ba4Ca2Nb2O10.95X0.1 have the space group
Fm3m. It should be noted that the X-ray data for all inves-
tigated parent compositions, all fluorine-doped composi-
tions, and chlorine-doped sample Ba4Ca2Nb2O10.95Cl0.1 are
in good agreement with the previously reported data
[12e16]. The data for the samples Ba2In2O4.95Cl0.1 and
Ba4In2Zr2O10.95Cl0.1 are obtained for the first time in this
work.
As can be seen in Table 1, the introduction of ions with
smaller radius (F) in the oxygen sublattice leads to the
decrease in the cell parameters, and the introduction of
ions with larger radius (Cl) leads to the increase in the cell
parameters (rО2 , rF , and rCl ¼ 1:81A [17]).Table 1
Cell volume of anhydrous samples.
Sample V (Å3) Sample
Ba2In2O5 607.90 Ba4In2Zr2O11
Ba2In2O4.95F0.1 604.18 Ba4In2Zr2O10.95F0.1
Ba2In2O4.95Cl0.1 608.09 Ba4In2Zr2O10.95Cl0.13.2. Treatment under water vapor pressure ðpH2O ¼ 2
102 atmÞ
The capability for reversible water uptake from the gas
phase for undoped Ba2In2O5 [18], Ba4In2Zr2O11 [19],
Ba4Ca2Nb2O11 [20], all fluorine-doped [14,21], and
chlorine-doped Ba4Ca2Nb2O10.95Cl0.1 [13] samples was
described earlier. It was proved that anionic doping (F,
Cl) leads to the decrease in the degree of hydration with
the increase in the concentration of halide ions. Thermal
and mass spectrometric (MS) studies for Ba2In2O4.95Cl0.1
and Ba4In2Zr2O10.95Cl0.1 were carried out in the present
work (Fig. 1). As can be seen, the mass loss was observed at
the temperature range 300e400 C. This process was
accompanied by an intense endothermic effect on the dif-
ferential scanning calorimetry (DSC) curve. TheMS analysis
indicated that themass changes are the result of water loss;
no other volatile compounds (CO2, O2, or HCl) were
detected. It should be noted that the thermal and MS re-
sults for Ba2In2O4.95Cl0.1 and Ba4In2Zr2O10.95Cl0.1 were
similar to the results for other halide-doped compounds
based on Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11.
The hydration process leads to the changes in the crystal
structure of hydrated samples compared with anhydrous
samples. Halide-doped samples based on brownmillerite
Ba2In2O5 transform to the tetragonal structure similar to
Ba2In2O5$nH2O (space group P4/mmm). The hydrated
undoped Ba4In2Zr2O11$nH2O and halide-doped Ba4In2Z-
r2O10.95X0.1$nH2O samples belong to the P42/n space group.
The hydration of double perovskite Ba4Ca2Nb2O11 leads to
the appearance of monoclinic distortions, and hydrated
composition Ba4Ca2Nb2O11$nH2O belongs to P21/n space
group. The hydrated halide-doped Ba4Ca2N-
b2O10.95X0.1$nH2O samples belong to the P42/n space group.
It should be noted that values of cell volumes for hydrated
chlorine-doped samples were obtained for the first time.
The hydration of samples leads to the lattice expansion for
both undoped and doped samples in comparison with
anhydrous samples (Table 2). The X-ray data for sample
Ba4Ca2Nb2O10.95Cl0.1$nH2O are presented in Fig. 2.
Therefore, both undoped Ba2In2O5, Ba4In2Zr2O11, and
Ba4Ca2Nb2O11 and halide-doped Ba2In2O4.95X0.1, Ba4In2Z-
r2O10.95X0.1, and Ba4Ca2Nb2O10.95X0.1 samples demonstrate
good chemical stability under H2O treatment without
degradation and without any hydrolytic decomposition.
The hydration process leads to the change in the crystal
structure only.3.3. Treatment under carbon dioxide atmosphere
The investigation of the chemical stability of undoped
and halide-doped samples under CO2 treatment wasV (Å3) Sample V (Å3)
74.30 Ba4Ca2Nb2O11 602.28
74.25 Ba4Ca2Nb2O10.95F0.1 601.85
74.35 Ba4Ca2Nb2O10.95Cl0.1 603.15
Fig. 1. TG and MS data for the compositions Ba2In2O4.95Cl0.1$nH2O (a) and Ba4In2Zr2O10.95Cl0.1$nH2O (b). TG, thermogravimetric.
Table 2
Cell volume of hydrated samples.
Sample V (Å3) Sample V (Å3) Sample V (Å3)
Ba2In2O5 [16] 156.62 Ba4In2Zr2O11 [17] 75.50 Ba4Ca2Nb2O11 [18] 304.98
Ba2In2O4.95F0.1 156.51 Ba4In2Zr2O10.95F0.1 [14] 75.25 Ba4Ca2Nb2O10.95F0.1 [15] 615.68
Ba2In2O4.95Cl0.1 156.64 Ba4In2Zr2O10.95Cl0.1 75.69 Ba4Ca2Nb2O10.95Cl0.1 615.72
N. Tarasova, I. Animitsa / C. R. Chimie 22 (2019) 363e368 365
Fig. 2. XRD patterns of Ba4In2Zr2O10.95Cl0.1$nH2O. At the bottom of the figure, the pattern is the difference between the experimental and the calculated one after
refinement. Vertical bars show the Bragg angle positions.
N. Tarasova, I. Animitsa / C. R. Chimie 22 (2019) 363e368366carried out. The CO2/air (1:1) mixture at 500 C for 10 hours
was used. Fig. 3 shows the XRD patterns for powder sam-
ples after this treatment. As can be seen, no secondary
phases can be observed for Ba2In2O4.95X0.1, Ba4In2Z-
r2O10.95X0.1, and Ba4Ca2Nb2O10.95X0.1 samples. They retain
their structure without detectable impurities. On the con-
trary, undoped compounds Ba2In2O5, Ba4In2Zr2O11, and
Ba4Ca2Nb2O11 decompose in CO2/air atmosphere according
to the following chemical equations:Fig. 3. XRD patternsof (a) Ba2In2O5 (1), Ba2In2O4.95F0.1 (2), and Ba2In2O4.95Cl0.1 (3); (b
Ba4Ca2Nb2O11 (1), Ba4Ca2Nb2O10.95F0.1 (2), Ba4Ca2Nb2O10.95Cl0.1 and (3) after treatmBa2In2O5 þ2CO2/t BaCO3 þ In2O3 (1)
Ba4In2Zr2O11 þ2CO2/t 2 BaCO3 þ In2O3 þ 2BaZrO3 (2)) Ba4In2Zr2O11 (1), Ba4In2Zr2O10.95F0.1 (2), and Ba4In2Zr2O10.95Cl0.1 (3); and (c)
ent in a mixture of CO2/air 1:1 at 500 C for 10 hours
N. Tarasova, I. Animitsa / C. R. Chimie 22 (2019) 363e368 367Ba4Ca2Nb2O11 þ2CO2/t BaCO3 þ CaCO3 þ Ba3CaNb2O9
(3)On the basis of the obtained XRD patterns (peak height),
the approximate calculations (under the neglect of
corundum numbers) of the impurity content in decom-
posed samples were made. The total impurity content in
the samples Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11 after
CO2 treatment was 40%, 12%, and 8%, respectively.
It is clear that doped compositions are chemically more
stable than undoped Ba2In2O5, Ba4In2Zr2O11, and
Ba4Ca2Nb2O11; thus, the introduction of F and Cl ions in
the oxygen sublattice is a promising method for improving
the chemical stability of complex oxides. To explain this
fact, the thermodynamic stability of oxygen-deficient
complex oxides during anion doping was evaluated.
As it is well known, the evaluation criterion for the
possibility of happening of the chemical process is the
value of the change in Gibbs free energy DGp or the value of
the enthalpy DHp:
Ba2In2O50:5yXy þð20:5yÞCO2/t ð20:5yÞBaCO3
þIn2O3 þ 0:5y BaX2; DHp (4)
Ba4In2Zr2O110:5yXy þð20:5yÞCO2/t ð20:5yÞBaCO3
þIn2O3 þ 2BaZrO3 þ 0:5y BaX2; DHp (5)
Ba4Ca2Nb2O110:5yXy þð20:5yÞCO2/t ð10:5yÞBaCO3
þCaCO3 þBa3CaNb2O9 þ0:5y BaX2;DHp (6)
Because the value of DHp is defined as the difference
between of the enthalpies of formation DHf of the reaction
products and of DHf of the starting materials, it is obvious
that the smaller DHf of the first and the larger of the second
leads to the greater of the DHp, and, consequently, to the
decrease in the preference of this reaction.
The thermodynamic stability is associated with the
enthalpy of formation of the substance. The change in the
enthalpy of formation of solid solutions from the startingTable 3
Tolerance factors and average-weighted anion radius for halide-doped
solid solutions based on Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11.
Sample Average-
weighted
ionic radius (Å)
Tolerance
factor (t)
Ba2In2O5 1.40 [4] 0.96(745)
Ba2In2O4.95F0.1 1.3986 0.96(762)
Ba2In2O4.95Cl0.1 1.4081 0.96(648)
Ba4In2Zr2O11 1.40 [4] 0.98(536)
Ba4In2Zr2O10.95F0.1 1.3994 0.98(544)
Ba4In2Zr2O10.95Cl0.1 1.4037 0.98(489)
Ba4Ca2Nb2O11 1.40 [4] 0.99(829)
Ba4Ca2Nb2O10.95F0.1 1.3994 0.99(833)
Ba4Ca2Nb2O10.95Cl0.1 1.4037 0.99(784)materials (oxides and halides) can be calculated according
to the equations:
ð20:5yÞBaOþ In2O3 þ 0:5y BaX2/t Ba2In2O50:5yXy; DHp
(7)
ð40:5yÞBaOþ In2O3 þ ZrO2
þ 0:5yBaX2/t Ba4In2Zr2O110:5yXy; DHp (8)
ð40:5yÞBaOþ 2CaOþ Nb2O5
þ 0:5y BaX2/t Ba4Ca2Nb2O110:5yXy; DHp (9)
It is known, that for perovskites the enthalpy of for-
mation of a complex oxide from simple ones is related to
the tolerance factor (t) [22]. After the calculation of this
parameter, it is possible to estimate the changes in the
enthalpy of formation for the doped samples compared
with undoped samples. In general, for the perovskites ABO3
and perovskite-related structures (Ba2In2O5) the tolerance
factor is calculated as
t ¼ ðrO þ rAÞffiffiffi
2
p
ðrO þ rB0 Þ
(10)
In the case of the presence of two cations in the B-
sublattice of complex oxide with the perovskite structure
(Ba4In2Zr2O11), the tolerance factor is calculated as
t ¼ ðrO þ rAÞffiffiffi
2
p
ðrO þ 12rB0 þ 12rB00 Þ
(11)
For the complex oxides with a double perovskite
structure (Ba4Ca2Nb2O11) the tolerance factor can be
calculated as
t ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðrA þ rOÞ2
1
2ðrB0 þ rB0 þ 2rOÞ2
þ 1
4
ðrB0  rB0 Þ2
vuut (12)
The obtained values of the tolerance factor are repre-
sented in Table 3. For the calculations the weighted average
radius of the anion was used, which is also given in Table 3.
Because of the proximity of the values of the tolerance
factor for isostructural compositions it can be assumed that
the enthalpy DHp does not change with the introduction of
the dopant.
Then, we can transform the equations for the calculation
of DHp
DHp ¼ DHf ðBa2In2O50:5yXyÞ ð20:5yÞ$DHfBaO
DHf ðIn2O3Þ0:5y$DHf ðBaX2
 (13)
N. Tarasova, I. Animitsa / C. R. Chimie 22 (2019) 363e368368DHp ¼DHf ðBa4In2Zr2O110:5yXyÞ ð4 xÞ$DHf ðBaOÞ
DH ðIn O ÞDH ðZrO Þ0:5y$DH ðBaX  (14)f 2 3 f 2 f 2
DHp ¼ DHf ðBa4Ca2Nb2O110:5yXyÞ  ð4 xÞ ,DHf ðBaOÞ
2$DHf ðCaOÞDHf ðNb2O5Þ  0:5y$DHf ðBaX2

(15)
into the equations for the calculation of the enthalpy of
formation of solid solutions DHf:
DHf ðBa2In2O50:5yXxÞ ¼ DHp þð20:5yÞ$DHf ðBaOÞ
þDHf ðIn2O3Þþ0:5y$DHf ðBaX2

(16)
DHf ðBa4In2Zr2O110:5yXyÞ ¼ DHp þð40:5yÞ$DHf ðBaOÞ
þDHf ðIn2O3ÞþDHf ðZrO2Þþ0:5y$DHf ðBaX2Þ
(17)
DHf ðBa4Ca2Nb2O110:5yXyÞ ¼DHp þð40:5yÞ$DHf ðBaOÞ
þ2$DHf ðCaOÞþDHf ðNb2O5Þþ0:5y$DHf ðBaX2

(18)
As can be seen, the presence of the halide dopant affects
only on the terms DHf(BaO) and DHf(BaX2) (i.e. DHf(BaF2)
and DHf(BaCl2)), the amounts of which are 538, 1192,
860 kJ/mol, respectively. At the halide doping the
appearance of term DHf(BaX2) is more valuable than that of
the decrease of term DHf(BaO), which leads to the decrease
in the enthalpy of formation of complex oxides and to the
increase in their chemical stability.
4. Conclusions
The investigations of chemical stability of halogen-
substituted complex oxides based on proton conductors
Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11 to water vaporand carbon dioxide were carried out. It was proved that F-
and Cl-containing samples are more chemically stable
compared with undoped compositions. The enthalpy of
decomposition becomes less favorable (less exothermic)
with the increase in the halide concentration. The method
of halide doping can be used as the promising technique for
obtaining the new perovskite-related materials with high
chemical stability.Acknowledgments
This work was financially supported by the Russian
Science Foundation (project 18-73-00006).References
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