Phase equilibria, crystal structure at 1373 K and properties of complex oxides in the Nd–Co–Fe–O system

Article abstract of DOI:10.1134/S0036023617080022

Phase equilibria in the Nd–Co–Fe–O system were systematically studied at 1373 K in air. The homogeneity range and crystal structure of solid solution NdCo_1–xFe_xO₃ (0.0 ≤ x ≤ 1.0) have been studied by the X-ray powder diffraction method. The structural parameters of complex oxides have been refined by the full-profile Rietveld method. It was shown that all oxides reveal practically stoichiometric oxygen composition within the entire temperature range under investigation. The values of thermal expansion coefficients for the cobaltites NdCo_1–xFe_xO₃ (x = 0.3, 0.7) have been calculated within the wide temperature range in air. Chemical stability of NdCo_1–xFe_xO₃ (x = 0.3, 0.7) in respect to the solid electrolyte materials (Ce_0.8Sm_0.2O_2–δ and La_0.88Sr_0.12Ga_0.82Mg_0.18O3-δ) was examined. Electrical conductivity of NdCo_1–xFe_xO₃ (x = 0.3, 0.7) was measured as a function of temperature within the range 300–1373 K in air. It was shown that substitution of cobalt for iron leads to the decrease of conductivity. The isothermal-isobaric cross-section of the phase diagram for the Nd–Co–Fe–O system at 1373 K in air has been presented.

Full text of DOI:10.1134/S0036023617080022

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2017, Vol. 62, No. 8, pp. 1090–1098. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © T.V. Aksenova, Sh.I. Elkalashy, A.S. Urusova, V.A. Cherepanov, 2017, published in Zhurnal Neorganicheskoi Khimii, 2017, Vol. 62, No. 8, pp. 1092–1101.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Phase Equilibria, Crystal Structure at 1373 K
and Properties of Complex Oxides in the Nd–Co–Fe–O System
T. V. Aksenova*, Sh. I. Elkalashy, A. S. Urusova, and V. A. Cherepanov
Ural Federal University, Yekaterinburg, 620000 Russia
*e-mail: TV.Aksenova@urfu.ru
Received July 21, 2016
Abstract—Phase equilibria in the Nd–Co–Fe–O system were systematically studied at 1373 K in air. The
homogeneity range and crystal structure of solid solution NdCo1 – xFexO3 (0.0 ≤ x ≤ 1.0) have been studied
by the X-ray powder diffraction method. The structural parameters of complex oxides have been refined by
the full-profile Rietveld method. It was shown that all oxides reveal practically stoichiometric oxygen com-
position within the entire temperature range under investigation. The values of thermal expansion coeffi-
cients for the cobaltites NdCo1 – xFexO3 (x = 0.3, 0.7) have been calculated within the wide temperature range
in air. Chemical stability of NdCo1 – xFexO3 (x = 0.3, 0.7) in respect to the solid electrolyte materials
(Ce0.8Sm0.2O2 – δ and La0.88Sr0.12Ga0.82Mg0.18O3-δ) was examined. Electrical conductivity of NdCo1 – xFexO3
(x = 0.3, 0.7) was measured as a function of temperature within the range 300–1373 K in air. It was shown
that substitution of cobalt for iron leads to the decrease of conductivity. The isothermal-isobaric cross-section
of the phase diagram for the Nd–Co–Fe–O system at 1373 K in air has been presented.
DOI: 10.1134/S0036023617080022
Perovskite-related manganites, ferrites and cobal- [8]; the value of total conductivity at room tempera-
ture is equal to ≈7 × 10–3 S/cm [7].
tites of LnMO3
and Ln2MO4 δ, compositions,
where Ln = La–Y,δM = Mn, Fe, Co, find a variety of
practical applications as materials for fuel cell elec-
trodes, oxygen membranes, cathodes of CO2 lasers,
catalysts [1–6]. Varying the composition by a partial
replacing of components in both sublattices allows
the creation of materials with a certain set of proper-
ties necessary for practical use. High-temperature
The only complex oxide NdFeO3 has formed in the
Nd–Fe–O system [18, 19]. Neodymium ferrite NdFeO3
possesses the orthorhombically distorted perovskite
structure with the unit cell parameters: a = 5.576 Å, b =
7.756 Å, c = 5.447 Å (sp. gr. Pnma) [20]; it is a semicon-
ductor which exhibit paramagnetic behavior with the
chemistry and thermodynamics of neodymium fer- Néel temperature TN = 687 K [20, 21].
rites and neodymium cobaltites is the subject of
CoFe2O4 is known to exist in the Co–Fe–O system
many publications. The phase equilibria and the
crystal structure of oxides formed in the Nd–М–O
(M = Fe, Co) and Co–Fe–O quasi-binary systems
are the most studied items up to date.
[22–29]. Cobalt ferrite CoFe2O4 was prepared by the
solid state synthesis [22], by using the citrate tech-
nique [23], by the thermal decomposition of hydrox-
ides [24] or polymer composites [23], by the co-pre-
cipitation method [25] and by the hydrothermal
method [26]. CoFe2O4 possesses the reversed spinel
structure [22, 24]. Based on CoFe2O4 a series of solid
solutions with the general formula CoxFe3 – xO4 have
been prepared. The homogeneity range of CoxFe3 – xO4
at 1373 K in air was reported as 0.6 ≤ x ≤ 1.2 [28], but
according to [27] the composition of the both bound-
aries are enriched by cobalt 1.0 ≤ x ≤ 1.35. The study of
phase equilibria at 1373 K in air for the CoO–Fe2O3
system [29] has revealed the formation of three types
of solid solutions: Co1 – yFeyO (0.0 ≤ y ≤ 0.15) with the
NaCl-type structure (sp. gr. Fm3m), Fe2 – uCouO3
within the range 0.0 ≤ u ≤ 0.06 (sp. gr. R3c) and
Three ternary oxides: NdCoO3, Nd4Co3O10 and
Nd2CoO4 are known to exist in the Nd–Co–O system
[5–17]. Only NdCoO3 is stable at 1373 K in air; cobal-
tite Nd4Co3O10 had been prepared within the tempera-
ture range 1373–1423 K in the atmosphere with lower
oxygen pressure 10–2.6–10–4.5 atm [14–16], and oxide
Nd2CoO4 was synthesized at 1473 K in the atmosphere
of purified nitrogen [17].
Neodymium cobaltite NdCoO3 is crystallized in
the cubic cell with the parameter a = 3.76 Å [8, 9] or
in the orthorhombic cell with the parameters: a =
5.3316 Å, b = 7.5457 Å, c = 5.3434 Å [6], which is in
good agreement with the data reported in [7, 10].
NdCoO3 exhibit semiconducting p-type conductivity
1090

PHASE EQUILIBRIA, CRYSTAL STRUCTURE AT 1373 K
1091
Fe3 – zCozO4 с 0.84 ≤ z ≤ 1.38 with the spinel structure 7000 with Cu radiation (λ = 1.5418 Å) using a mono-
chromator made from the pyrolytic graphite. XRD
measurements were performed within the angle range
20° ≤ 2θ ≤ 90° with the rate 0.02°–0.05° per minute
with the exposition time from 1 to 10 s.
The unit cell parameters were calculated using the
Сelref 3 software and refined by the Rietveld analysis
using Fullprof 2008 package [31, 32].
The measurements of the relative linear thermal
expansion were performed using a dilatometer
DIL 402 C (Netzsch GmBH) in air within the tem-
perature range 298–1373 K with the heating and cool-
ing rate of 5 K/min.
The oxygen content in the oxides and its changes
with temperature have been determined by the ther-
mogravimetric analysis (STA 409 PC, Netzsch
GmBH) followed by the samples reducing in the
hydrogen flow at 1373 K inside the TG cell and also by
the iodometric titration of the samples slowly cooled
from 1373 K to room temperature with the rate 1 K/min,
reported elsewhere [33].
(sp. gr. Fd3m).
The phase relations in the quasi-ternary Nd–Co–
Fe–O system are poorly studied [7, 30]. It was reported
that Co-substituted complex oxides NdFe1 – xCoxO3
with the orthorhombic structure (sp. gr. Pbnm) have
formed within the range 0.0 ≤ x ≤ 0.5 [30]. The solid
solutions NdFe1 – xCoxO3 with x < 0.3 in air exhibit
semiconducting p-type conductivity within the tem-
perature range 298–673 K, while the oxides with x > 0.3
exhibit n-type conductivity [7].
The present work aimed to the study of phase equi-
libria, crystal structure and physicochemical proper-
ties of complex oxides formed in the quasi-ternary
Nd–Co–Fe–O system.
EXPERIMENTAL
The samples for investigations were synthesized by
the glycerol-nitrate technique. Nd2O3 (NO-L qualifi-
cation), preliminary calcined for the desorption of the
adsorbed water and gases, iron oxalate FeC2O4 · 2Н2О
(pure for analysis grade) and metallic Co were used as
starting materials. Metallic Co was prepared by the
reducing of cobalt oxide Сo3O4 (pure for analysis
grade) in the hydrogen flow at 673–873 K. Samarium
oxide Sm2O3 (SmO-L qualification), lanthanum
oxide (LaO-D qualification), magnesium oxide MgO
(pure for analysis grade), cerium nitrate Ce(NO3)3 ·
6Н2О (pure for analysis grade), strontium carbonate
SrCO3 (special purity grade), and metallic gallium
were used for the synthesis of Ce0.8Sm0.2O2 – δ and
Since the studied oxides are considered as potential
cathode materials for the solid oxide fuel cells the infor-
mation about their chemical indifference with respect
to electrolyte materials become an issue of vital impor-
tance. Chemical stability of substituted cobaltites with
respect to electrolyte materials was studied by annealing
of powder mixtures of NdCo1 – xFexO3 (x = 0.3, 0.7)
with the solid electrolytes (Ce0.8Sm0.2O2-δ or
La0.88Sr0.12Ga0.82Mg0.18
O3 – δ) taken in mass ratio 1 : 1 at
1573 K in air.
Electrical conductivity of ceramic complex oxides
sintered in a form of rectangular bars with the sizes 4 ×
4 × 25 mm measured by the 4-probe technique in air
as a function of temperature within the range 300–
1373 K.
La0.88Sr0.12Ga0.82Mg0.18O3 – δ
.
The weights of the starting components in the
required stoichiometric amounts were dissolved in
dilute nitric acid HNO3 (special purity grade) while
heating. Then an equimolar amount of glycerol
C3H8O3 (pure for analysis grade), required for the
complete reduction of nitrate ions, was added. The
resulting solution was heated in order to evaporate the
solvent. Dry residual was slowly heated within the
temperature range 298–1273 K. Final annealing was
performed in air during 100–120 h with intermediate
grindings in the ethanol media after each 10–12 h. The
samples were quenched from 1373 K to room tempera-
ture (cooling rate of about ~400–500 K/min).
A chemical indifference test for mixed cobaltite-
ferrite of neodymium with respect to solid electrolyte
materials was carried out using Sm-stabilized ceria
and (Sr, Mg)-doped lanthanum gallate, as the exam-
ples. In order to prepare Ce0.8Sm0.2O2 – δ the final
annealing was performed at 1373 K in air during 36 h,
while La0.88Sr0.12Ga0.82Mg0.18O3 – δ was annealed at
1573 K during 12 h.
RESULTS AND DISCUSSION
In order to study the phase equilibria in the Nd–
Co–Fe–O system at 1373 K in air 26 samples with var-
ious ratios of metallic components were prepared
using the glycerol-nitrate technique.
NdCo1 – xFexO3 Solid Solutions
In order to determine the homogeneity range and
the crystal structure of solid solutions with the general
formula NdCo1 – xFexO3 the samples with overall com-
positions х = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0 have been
prepared. According to the XRD results all samples
quenched from 1373 K to room temperature were singe
phases. XRD patterns for the cobaltites NdCo1 – xFexO3
with 0.1 ≤ x ≤ 1.0 are shown in Fig. 1. The differences
in the mutual solubility of cobaltite and ferrite in Nd-
and La-containing systems (unlimited solubility in the
NdCoO3–NdFeO3 system and limited solubility from
the both cobaltite and ferrite sides in the LaCoO3–
Phase identification of the studied oxides was per-
formed using diffractometers Inel Equinox 3000 with
Fe radiation (λ = 1.93609 Å) and Shimadzu XRD
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 62 No. 8 2017

1092
I/I0
AKSENOVA et al.
the orthorhombic distortions of the perovskite lattice.
Similarly to undoped NdMO3 (M = Fe, Co), the XRD
patterns for the NdCo1 – xFexO3 complex oxides with
0.1 ≤ x ≤ 0.9 have been indexed within the orthorhom-
x = 1.0
x = 0.9
x = 0.7
x = 0.5
bic cell
(sp. gr. Pbnm), where ap – is
2ap × 2ap × 2ap
the unit cell parameter for the cubic perovskite, which is
in good agreement with the data reported in [30]. Figure
2 demonstrates XRD patterns, refined by the full-pro-
file Rietveld method for the NdCo1 – xFexO3 solid
solutions with x = 0.3 and 0.7, as an example. How-
ever, the ratio of the unit cell parameters a, b,
x = 0.3
x = 0.1
c
2
and the values of parameter that characterized the ort-
horhombic distortions D that can be calculated
according to the formula:
20
30
40
50
60
70
2θ, deg
3
αi − α
α
1
3
(1)
D =
×100%,
∑
i=1
Fig. 1. The XRD patterns for the single phase
NdCo
Fe O solid solutions (0.1 ≤ x ≤ 1.0).
x 3
1 ‒ x
where α = a, α = b, α =
and = (a × b ×
)1/3), indicate the appearance of two types of ort-
с
2
α
1
2
3
I
c
2
horhombic structure: O-type that characterized by the
parameter’s ratio or , and
а ≤ с 2 ≤ b
O'-type with the parameter’s ratio
b ≤ с 2 ≤ a
2 ≤ b ≤ a
.
с
NdCo1 – xFexO3 solid solutions with 0.0 ≤ x < 0.1 and
0.1 < x ≤ 1.0 possess O-type orthorhombic cell, while
the oxide with x = 0.1—O'-type. The values of unit cell
parameters, unit cell volume and parameter of orthor-
hombic distortions for NdCo1 – xFexO3 with 0.0 ≤ x ≤ 1.0
are listed in Table 1. One can see that the value of
D parameter is minimal for NdCo0.9Fe0.1O3 and it is sig-
nificantly increases with the raise of iron concentration.
Thus, while the dopant (Fe) content is increasing, an
orthorhombic structure of NdCo1 – xFexO3 at 1373 K in
air is undergoes the phase transformation: O-type →
O'-type within the range x = 0.0–0.1 and the reverse
transformation O'-type → O-type within the range x =
0.1–0.2. Often the structural changes in perovskite-like
oxides are associated with a change in the oxygen con-
tent (oxygen nonstoichiometry). However, according to
the available data for neodymium ferrite and neodym-
ium cobaltite [13, 14, 18, 34] and according to the
results of thermogravimetric measurements and iodo-
metric titration for the NdCo1 – xFexO3 solid solutions
(x = 0.3, 0.7), obtained in the present work, all oxides
have stoichiometric oxygen composition and oxygen
content remained practically unchanged with the vari-
ations of sample composition and temperature. The
existence of an O'-type structure at small values of x in
NdCo1 – xFexO3 is apparently associated with a fine
interaction of isolated iron ions via oxygen bridges
with the cobalt ions surrounding them. The increase in
the iron content and the greater probability of the iron
ions neighborhood eliminate this effect, and the struc-
ture again returns to the O-type.
x = 0.7
x = 0.3
1
2
3
1
2
3
20
30
40
50
60
70
2θ, deg
Fig. 2. The XRD patterns for the NdCo
Fe O (x = 0.3,
x 3
1 – x
0.7), refined by the Rietveld method. Points are experi-
mental results; (1) theoretical profile; (2) peak positions
with the allowed Miller indices (hkl); (3) the difference
between experimental data and theoretical profile.
LaFeO3 system [29]) are explained by the difference in
crystal structure distortions. Lanthanum cobaltite
LaCoO3 is the rhombohedrally distorted perovskite
while NdCoO3, as well as lanthanum ferrite and neo-
dymium ferrite LnFeO3 (Ln = La, Nd) possess the
orthorhombically distorted unit cell.
The splitting of the diffraction peaks observed at
the scattering angles 31°–33°, 39°–41° and 57°–60°
increases with the raise of iron replacing cobalt con-
tent in NdCo1 – xFexO3, which indicates an increase of
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PHASE EQUILIBRIA, CRYSTAL STRUCTURE AT 1373 K
1093
Table 1. The unit cell parameters, type of the orthorhombic structure, and orthorhombic distortion parameter for the
NdCo1 – xFexO3 solid solutions (0.0 ≤ x ≤ 1.0), quenched from 1373 K in air
Parameters
relation
Type
of structure
V, Å3
x
a, Å
b, Å
c, Å
D, %
, Å
2
c
0.0
0.1
0.3
0.5
0.7
0.9
1.0
5.345(1)
5.354(1)
5.375(1)
5.396(1)
5.419(1)
5.440(1)
5.453(1)
5.331(1)
5.348(1)
5.394(1)
5.444(1)
5.498(1)
5.553(1)
5.581(1)
7.549(1)
7.561(2)
215.13(1)
216.51(1)
5.33795
5.34643
5.37825
5.40866
5.44119
5.47159
5.48927
O
O'
O
O
O
O
O
0.087
0.056
0.143
0.341
0.552
0.785
0.885
b ≤ c 2 < a
c
2 < b < a
7.606(1) 220.54(1)
7.649(1) 224.71(3)
7.695(1) 229.28(3)
7.738(1) 233.78(2)
7.763(2) 236.27(3)
a < c 2 < b
a ≤ c 2 < b
a < c 2 < b
a ≤ c 2 < b
a < c 2 < b
Table 2. Atomic coordinates in the unit cells of NdCo1 – xFexO3 solid solutions (0.0 ≤ x ≤ 1.0), quenched from 1373 K in air
Sp. gr. Pbnm: Co/Fe (0.5, 0, 0); Nd (x, y, 0.25); O1 (x, y, 0.25); O2 (x, y, z)
x
0.0
0.1
0.3
0.5
0.7
0.9
1.0
x(Nd)
y(Nd)
x(O1)
y(O1)
x(O2)
y(O2)
z(O2)
RBr, %
Rf, %
Rp, %
–0.0094(9) –0.007(1)
–0.0082(1)
0.0372(1)
0.070(2)
0.486(1)
–0.285(2)
0.289(2)
0.034(1)
2.08
–0.007(1)
0.0408(6)
0.066(4)
0.481(6)
0.292(7)
0.260(8)
0.048(5)
5.80
–0.008(1)
0.0435(6)
0.088(9)
0.485(6)
0.288(6)
0.278(6)
0.043(4)
4.61
–0.0089(9)
0.0473(6)
0.099(8)
0.465(6)
0.285(6)
0.284(5)
0.045(4)
6.49
–0.0079(7)
0.0480(4)
0.092(6)
0.477(4)
0.290(4)
0.276(4)
0.051(3)
5.67
0.0334(5)
0.095(6)
0.493(4)
–0.241(6)
0.180(3)
–0.015(4)
4.50
0.0347(6)
0.073(8)
0.495(6)
–0.282(1)
0.286(1)
0.029(9)
2.80
6.47
3.23
2.97
7.05
7.25
8.14
7.90
4.77
8.80
5.71
11.02
11.10
12.4
11.01
The atomic coordinates for the unit cells of were used for the conductivity measurements. The rel-
NdCo1 – xFexO3 solid solutions (0.0 ≤ x ≤ 1.0) refined ative density of cobaltites NdCo1 – xFexO3 was ranged
by the full-profile Rietveld method are listed in Table 2. as 90–93% from theoretical value. Temperature
dependences of the relative linear expansion of
NdCo1 – xFexO3 (x = 0.3, 0.7) within the range 298–
1373 K in air, obtained in the heating and cooling
modes, completely coincide and are shown in Fig. 4.
Isobaric dependencies ΔL/L0 = ƒ(T) are non-linear.
In order to describe the obtained ΔL/L0 = ƒ(T) curves
the equation of third-degree polynomial was used:
Within the homogeneity range, an increase of the
concentration of iron replacing cobalt in NdCo1-xFexO3
leads to a gradual raise of the unit cell parameters and
unit cell volume, which is related to the size effect: the
3+
radius of cobalt ions ( = 0.75 Å, CN = 6) is less than
rCo
3+
the radius of iron ions (
(Fig. 3).
= 0.785 Å, CN = 6) [35]
rFe
2
3
ΔL
L0
(2)
= a + b1T + b2T + b3T ,
Thermal Properties of the NdCo1 – xFexO3 Oxides
where T is absolute temperature (K); a, b1, b2 and b3 are
constants.
In order to determine the linear thermal expansion
coefficients (LTEC) by means of high-temperature
dilatometry the powder samples of NdCo1 – xFexO3
oxides (x = 0.3, 0.7) were pressed into bars with the
sizes 4 × 4 × 25 mm and sintered at 1632 K in air
during 24 h, followed by slow cooling to room tem-
perature with the rate of 1 K/min. Same ceramic bars
Isobaric linear thermal expansion coefficients (αp)
were calculated using following equation:
1 dΔL
L0 dT
(3)
αP =
= b1 + 2b2T + 3b3T 2
.
(
)
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1094
AKSENOVA et al.
The coefficients in the equation (2) and the values
V, Å3
a, b, c, Å
7.8
of LTEC within the temperature range 298–1373 K in
air, calculated by means of equation (3) are listed in
Table 3. The value of LTEC noticeably decreases with
the raise of content of iron substituting cobalt in the
neodymium cobaltite NdCo1 – xFexO3 from 21.8 × 10–6
240
7.7
V
232
224
216
208
c
7.6
down to 16.1 × 10–6 K–1.
7.5
5.5
5.4
5.3
Significant increase of the slope for the linear thermal
expansion dependencies with temperature that observed
for the related cobaltites-ferrites of rare earth elements
and barium LnBaCo2 – xFexO5 + δ (Ln = Nd–Y) caused
by the high temperature oxygen release from the crys-
tal lattice and, as a consequence, by the decrease of
average oxidation state of 3d metal in oxide [36, 37].
This contribution to the relative expansion of oxides is
called as “chemical expansion” [38–40]. The tem-
perature dependencies of thermal expansion for
NdCo1 – xFexO3 (x = 0.3, 0.7) also exhibit noticeable
increase of slope at temperature higher than 600 K.
However, as it was mentioned above, it was found that
the oxygen content in NdCo1 – xFexO3 (x = 0.3, 0.7)
has not changed within the temperature range 298–
1373 K in air according to the results of high tempera-
ture thermogravimetry. In this case the non-linear
shape of ΔL/L0 = ƒ(T) plot can be explained by the
a
a, b
b
c
V
0
0.2
0.4
0.6
0.8
1.0
x in NdCo1–xFexO3
Fig. 3. The unit cell parameters and the unit cell volumes
versus iron content for the NdCo
Fe O solid solutions
x 3
1 – x
(0.0 ≤ x ≤ 1.0).
ΔL/L0
0.024
0.016
0.008
0
1
2
transition of Co3+ from the low spin (LS) to the high
spin (HS) state ( 3+(LS) = 0.685 Å, 3+(HS) = 0.75 Å
rCo
rCo
[35]); its possibility for the Co3+ ions was reported in
NdCoO3 [41] and also in the other related complex
oxides [42].
In order to study the chemical stability of the cobal-
tites NdCo1 – xFexO3 (x = 0.3, 0.7) in the contact with
the electrolyte material the mixtures of NdCo1 – xFexO3
with Ce0.8Sm0.2O2 – δ or La0.88Sr0.12Ga0.82Mg0.18O3 – δ in
the 1 : 1 mass ratio were annealed during 1 h at 1573 K
in air. It was found that NdCo1 – xFexO3 (x = 0.3, 0.7)
has not react with Ce0.8Sm0.2O2 – δ (Fig. 5a).
400
600
800 1000 1200 1400
T, K
Fig. 4. The relative linear thermal expansion versus tem-
perature for NdCo Fe O (x = 0.3 , 0.7) in air.
1 – x
x 3
Electrical Conductivity of NdCo1 – xFexO3
Chemical reaction occurs when NdCo1 – xFexO3 (x =
Isobaric dependencies of electrical conductivity for
NdCo1 – xFexO3 (x = 0.3, 0.7) versus temperature are
presented in Fig. 6. One can observe the raise of con-
ductivity with increasing temperature for the studied
oxides that indicates a semiconducting nature of con-
0.3, 0.7) is contacted with La0.88Sr0.12Ga0.82Mg0.18O3 – δ
;
the solid solution based on neodymium gallate NdGaO3
has been detected as the main product of this interaction
(Fig. 5b).
Table 3. The coefficients in the equations corresponding to the isobaric ΔL/L0 = ƒ(T) dependencies and the average values
of LTEC for NdCo1 – xFexO3 (x = 0.3, 0.7) in air
2
= a + b1T + b2T + b3T 3
ΔL
αp × 106, K–1,
298–1373
L0
b1
6.74934 × 10–4 –1.54932 × 10–5 4.84008 × 10–8 –1.82986 × 10–11
3.53385 × 10–4 –8.49185 × 10–6 2.7599 × 10–8 –9.05099 × 10–12
x
R^2
b2
b3
a
0.3
0.7
21.8
16.1
0.99984
0.99992
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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PHASE EQUILIBRIA, CRYSTAL STRUCTURE AT 1373 K
(a)
1095
I
2500
2000
1500
1000
500
0
NdCo0.3Fe0.7O3
Ce0.8Sm0.2O2
–500
20
30
40
50
60
70
80
90
2θ, deg
(b)
I
4000
3000
2000
1000
0
NdCo0.7Fe0.3O3
LSGM
NdGaO3
20
30
40
50
60
70
80
2θ, deg
Fig. 5. The XRD patterns for the mixtures: NdCo Fe
O
+ Ce Sm
O
(a) and NdCo Fe
O
+
0.3 0.7
3
0.8
0.2 2 – δ
0.7 0.3
3
La Sr Ga Mg
O
(b), after high temperature treatment at 1573 K in air refined by the Rietveld method.
0.88 0.12
0.82
0.18 3 – δ
ductivity in the doped neodymium cobaltites. Intro- log(σT) = ƒ(1/T) dependences’ slope (see the inset in
duction of iron into the cobalt sites leads to a signifi-
cant decrease of electrical conductivity of the parent
NdCoO3 at temperature higher than 600 K, which is in
good agreement with the data reported in [7]. The val-
ues of the conductivity activation energies, listed in
Fig. 6). The activation energies of conductivity for the
undoped neodymium cobaltite and neodymium fer-
rite [43, 44] are presented in the same table for com-
parison. The values of conductivity activation energy
for all oxides are typical for the hopping polaron
Table 4, were calculated from the tangent of the mechanism of charge transfer. Since the stoichiomet-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 62 No. 8 2017

1096
AKSENOVA et al.
σ, S/cm
x = 0 [43]
x = 1.0 [44]
x = 0.3
log (σT)
6
4
600
400
200
0
x = 0.7
2
0
–2
1.0 1.5 2.0 2.5 3.0 3.5
103/T, K–1
200
400
600
800
T, K
1000
1200
1400
Fig. 6. The isobaric dependencies of electrical conductivity versus temperature for NdCoO according to data from [43], NdFeO
3
3
3
according to data from [44] and NdCo
Fe O (x = 0.3, 0.7)—present work. The insert illustrates log(σT) = ƒ(10 /T) depen-
1 – x
x 3
dencies for NdCo
Fe O .
x 3
1 – x
ric oxygen composition remains unchanged for all
The information for the phase equilibria on the
oxides the appearance of charge carriers can be associ- CoO–1/2Fe2O3 side at 1373 K in air has been taken
ated with the disproportionation process, which is
characteristic for this type of oxides:
from [29]. Based on the XRD results for 26 samples
quenched to the room temperature, the phase diagram
for the Nd–Co–Fe–O system at 1373 K in air has
been divided into six phase fields (Fig. 7).
2M3+ = M4+ + M2+.
(4)
Significant raise of conductivity at temperature
higher 600 K for NdCoO3 and NdCo0.7Fe0.3O3 and
slight increase of conductivity for NdCo0.3Fe0.7O3
allow assuming that mainly cobalt ions are dispropor-
tionate predominantly while iron ions involved in this
process to much lesser extent.
According to the XRD data, the samples from the
phase fields 3 and 5 were three-phase. In accordance
with the Gibbs rule, coexistence of three phases in the
quasi-ternary system under consideration at a fixed
Table 4. The values of activation energy of conductivity for
the NdCo1 – xFexO3 complex oxides
Phase Diagram for the Nd–Co–Fe–O System at 1373 K
Temperature
range, K
Еa, eV
Composition
NdCoO3
Reference
[43]
The phase diagram of the four-component system
in isobaric-isothermal conditions should be repre-
sented as a three-dimensional tetrahedron. However,
the presentation of such systems on the plane in the
form of a triangle, where the sample composition are
expressed in terms of the molar fraction of metal com-
ponents, and the vertices of the triangle correspond to
the simple oxides 1/2Nd2O3, CoO and 1/2Fe2O3 that
exist under these conditions, is more convenient for
practical use. The oxygen content in the condensed
phases cannot be determined from the diagram. The
resulting image is a projection of the isobaric–isother-
mal phase diagram of the Nd–Co–Fe–O system hav-
ing a tetrahedron shape on the Nd–Co–Fe face.
473–523
523–673
673–1073
0.32
0.69
0.22
367–417
417–916
916–1369
0.51
0.72
0.32
NdCo0.7Fe0.3O3
NdCo0.3Fe0.7O3
NdFeO3
Present work
[44]
417–916
916–1369
0.63
0.83
323–398
398–690
690–1073
0.461
0.276
0.242
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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2017

PHASE EQUILIBRIA, CRYSTAL STRUCTURE AT 1373 K
1/2(Nd2O3)
1097
0
1.0
0.1
0.9
0.2
0.8
0.3
0.7
0.4
0.6
1
0.5
0.5
NdCoO3
0.6
NdFeO3
0.4
2
0.7
0.8
0.3
0.2
6
3
4
0.9
5
0.1
1.0
CoO
0
0.1 0.2
0.3
0.4
0.5 0.6
0.7
0.8
0.9
1.0 1/2(Fe2O3)
Fig. 7. The isobaric-isothermal cross-section of the phase diagram for the Nd–Co–Fe–O system at 1373 K in air. The phase
composition inside the fields are: (1) Nd O , NdCo Fe O (0.0 ≤ x ≤ 1.0); (2) NdCo Fe O (0.0 ≤ x ≤ 0.9), Co Fe O (0.0 ≤ y
2
3
4
1 – x
0.1 0.9
x
3
3
3
1 – x
x
3
1 – у
1 – x x 3
y
≤ 0.15); (3) Co Fe O, Co Fe O , NdCo Fe O ; (4) Co Fe
O (0.84 ≤ z ≤ 1.38), NdCo
Fe O (0.9 ≤ x ≤ 1.0);
0.85 0.15
1.38 1.62
z
3 – z
4
(5) NdFeO , Co Fe O , Fe Co O ; (6) NdFeO , Fe
Co O (0.0 ≤ u ≤ 0.06).
u 3
3
0.84 2.16
4
1.94
0.06
3
2 – u
temperature and oxygen pressure means that it is non- sition range x = 0.0–0.1 and x = 0.1–0.2, respec-
variant, and therefore the compositions of all the tively. All oxides are stoichiometric in oxygen content
coexisting phases within a given phase field are fixed. within the temperature range 298–1373 K in air. The
The composition of the solid solution NdCo0.1Fe0.9O3 average values of thermal expansion coefficients for
NdCo1 – xFexO3 (x = 0.3, 0.7) are calculated. It was
found that NdCo1 – xFexO3 solid solutions are semi-
coexisting with Co0.85Fe0.15O and Co1.38Fe1.62O4 in the
field 3 of the isobaric-isothermal section of the phase
diagram was determined as follows: the refined by the conductors, and the substitution of cobalt for iron
Rietveld method unit cell parameters of the orthor- leads to a significant decrease of electrical conductiv-
hombic perovskite solid solution coexisting with other ity. The calculated values of activation energies indi-
phases in the samples from field 3 were compared with cate the small polaron conductivity mechanism. An
the concentration dependence of the unit cell param- isobaric–isothermal section of the phase diagram of
eters that have been obtained in the study of the the quasi-ternary Nd–Co–Fe–O system at 1373 K in
NdCo1 – xFexO3 cross-section (0.0 ≤ x ≤ 1.0). In a sim- air has been constructed.
ilar way, it was shown that the perovskite phase coex-
isting with Co0.84Fe2.16O4 and Fe1.94Co0.06O3 in the
ACKNOWLEDGMENTS
field 5 is the practically unsubstituted neodymium fer-
rite NdFeO3.
The work has been done within the framework of the
project part of the State task of the Ministry of Educa-
tion and Science of the Russian Federation and with
partial financial support by Act 211 Government of the
Russian Federation, (agreement no. 02.A03.21.0006).
CONCLUSION
It is shown that a continuous series of solid solu-
tions of NdCo1 – xFexO3 (0.0 ≤ x ≤ 1.0) is formed in the
Nd–Co–Fe–O system, orthorhombic distortions
increase with increasing of iron content. Structural
transitions of the О → O'→О type within the orthor-
hombic syngony framework occur within the compo-
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