Oxygen-ion and electron conductivity in Sr2(Fe1-xGax)2 O5

Article abstract of DOI:10.1016/j.jssc.2006.05.028

High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and ⁵⁷Fe Moessbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr₂(Fe_1-xGa_x)₂O ₅, where x = 0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite (x = 0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.

Full text of DOI:10.1016/j.jssc.2006.05.028

ARTICLE IN PRESS
Journal of Solid State Chemistry 179 (2006) 3045–3051
www.elsevier.com/locate/jssc
Oxygen-ion and electron conductivity in Sr (Fe Ga ) O
x 2 5
2
1ꢀx
a
a
a
b
b
I.A. Leonidov , M.V. Patrakeev , J.A. Bahteeva , K.V. Poholok , D.S. Filimonov ,
c
K.R. Poeppelmeier , V.L. Kozhevnikov
a,
Ã
a
Institute of Solid State Chemistry, Ural Branch of RAS, Pervomaiskaia 91, Ekaterinburg 620219, Russia
b
Moscow State University, Vorob’evy gory, Moscow 119987, Russia
c
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
Received 11 January 2006; received in revised form 24 April 2006; accepted 30 May 2006
Available online 3 June 2006
Abstract
5
High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and Fe Mo
7
¨
ssbauer
spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr (Fe1ꢀxGa ) O ,
2
x 2 5
where x ¼ 0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the
orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as
in the parent ferrite (x ¼ 0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a
complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron
with gallium.
r 2006 Elsevier Inc. All rights reserved.
¨
Keywords: Strontium ferrite; Mossbauer spectroscopy; Electrical conductivity
˚
1
. Introduction
The strontium ferrite, SrFeO3 is known as a typical
unit parameter affi2a of about twice larger than a ffi3.9 A
p
p
in the non-stoichiometric perovskite-like SrFeO
(CP)
.5
2
which becomes stable on heating at about 950–1000 1C. It
is interesting to observe that the oxygen ion conductivity in
both OB and CB structures, though smaller than in the CP
phase, is nonetheless quite appreciable. Because vacancy
ordering in the OB and CB structures is far from ideal,
incipient disordering of oxygen vacancies and oxygen ions
manifests itself in noticeable changes of conductivity in the
ferrite at relatively low temperatures near 600 1C [11].
Partial replacement of iron in Sr Fe O for gallium,
ꢀg
mixed oxygen ion and electron conductor at elevated
temperatures [1]. Decreasing the partial pressure of oxygen
gives rise to an increase in the non-stoichiometry parameter
such that in a reducing environment g ¼ 0:5 can be
achieved [2,3]. At temperatures below 850 1C the reduced
ferrite SrFeO2.5 crystallizes with the orthorhombic brown-
millerite (OB) structure Sr Fe O where vacancy ordering
results in an alternation of layers of octahedra FeO6/2 (O)
and tetrahedra FeO4/2 (T) along the b-axis [4–7]. Raising
the temperature above 850 1C results in transformation of
the brownmillerite to a cubic structure that has long been
thought of as a non-stoichiometric perovskite-like SrFeO2.5
2
2
5
2
2
5
where gallium is known to have a strong preference for
tetrahedral coordination by oxygen, can have a consider-
able influence on the disordering of the oxygen vacancies
and the oxygen ions, the OB-CB phase transition
temperature, and, eventually, the oxygen ion and electron
transport. In this work, we studied the electrical con-
ductivity of the gallium-substituted oxides Sr (Fe
(
cubic perovskite (CP)) [1,8,9]. It was shown, however, by
in situ high-temperature neutron diffraction studies [10]
that this cubic modification of Sr Fe O (henceforth, cubic
brownmillerite or CB) is characterized by the elementary
2
2
5
2
1ꢀx
Ga ) O at different temperatures and partial pressures
x 2 5
¨
of oxygen. Additionally, Mossbauer spectroscopy was
utilized in order to study the local environment of the iron
ions. The contributions to the total conductivity from the
Ã
Corresponding author. Fax: +7 3432 74 00 03.
E-mail address: kozhevnikov@imp.uran.ru (V.L. Kozhevnikov).
0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2006.05.028

ARTICLE IN PRESS
I.A. Leonidov et al. / Journal of Solid State Chemistry 179 (2006) 3045–3051
3046
oxygen ions, electrons and electron holes were derived
based on their dependence on the oxygen partial pressure.
The variations of these partial contributions with tempera-
ture and gallium content are discussed.
¨
Mossbauer spectra were obtained with an electrody-
namic spectrometer in the mode of permanent acceleration.
The data were computer processed with standard software
for minimization of square functionals. The Co isotope
with activity of 0.45 GBq was imbedded in the rhodium
metal matrix and used as a gamma-rays source. The
obtained chemical shifts were referred to a-Fe at room
temperature.
5
7
2
. Experimental
The specimens SrFe1 Ga O , where x ¼ 0, 0.1 and
ꢀx
x
3ꢀg
0
either appropriate mixtures of high grade purity oxides
.2, were synthesized by heating in air at 900–1250 1C
3. Results and discussion
Fe O , Ga O and strontium carbonate SrCO , or mixtures
2
3
2
3
3
of dehydrated co-precipitates Fe(OH) /Ga(OH)3 with
3
3.1. Structural features
strontium oxide. The hydroxide precursors were obtained
by addition of ammonium hydroxide to water solutions of
iron and gallium nitrates. The co-precipitates were dried in
air at 250 1C and then mixed with appropriate amount of
freshly prepared strontium oxide in a dry box. The
strontium oxide was obtained immediately before use by
thermolysis under dynamic vacuum of strontium carbonate
at 950 1C. The synthesized powders were ball milled and
pressed under 1 kbar of uniaxial load in pellets with the
thickness of about 2 mm and the diameter of 20 mm. The
pellets were sintered for 10 h in the air at 1250 1C and
cooled down to room temperature with the rate of 21/min.
The density of the ceramic samples was about 90% of
theoretical. The specimens with reduced oxygen content
The X-ray powder diffraction patterns give evidence of a
brownmillerite-like structure in the samples Sr (Fe
2
1ꢀx
pffiffiffi
Ga ) O with x ¼ 0, 0.1 and 0.2 (a ꢁ 2a , b ꢁ 4a ,
p
p
x 2
5
pffiffiffi
c ꢁ 2a , where a ꢁ 3:9 [7]) (see Table 1). Attempts to
p
p
incorporate larger amount of gallium, x ¼ 0:3, resulted in
appearance of additional phases. It can be seen from Table 1
that the elementary cell parameters are appreciably smaller in
the sample x ¼ 0:1 than in x ¼ 0. This change can be
3
+
explained assuming that the smaller Ga
cations replace
3+
Fe cations [13] in the T-layers, which is consistent with the
strong preference of gallium for tetrahedral coordination.
¨
The Mossbauer study was undertaken in order to gain more
insight into distribution of gallium cations.
5
7
corresponding to the formula Sr (Fe Ga ) O5 were
2
¨
2 x 2 5
The Mossbauer spectra of Fe in Sr (Fe Ga ) O at
1ꢀx
obtained by firing the air synthesized materials in the flow
x 2
1ꢀx
100 K are shown in Fig. 1. The spectra can be described as
a superposition of two Zeeman sestets (Table 2). The sestet
with larger values of the isomer chemical shift
(dE0.50 mm/s) and of the hyperfine magnetic field
of commercial He at 900 1C, or in the gas mixture He (5%
H ) at 700 1C. Phase purity control and determination of
2
the crystal lattice parameters were carried out with X-ray
diffraction using a STADI-P (STOE) diffractometer in
Bragg–Brentano geometry (CuKa radiation, 2Y range of
3
þ
(HE540 kOe) corresponds to iron cations FeO in distorted
octahedral oxygen coordination, while the sestet with
smaller values (dE0.35 mm/s, HE453 kOe) is due to iron
5
–1201, step 0.021, acquisition time 4 s).
3
T
þ
Rectangular bars 2 Â 2 Â 18 mm were cut from the
cations Fe in a distorted tetrahedral oxygen environ-
sintered pellets for d.c. conductivity (s) measurements.
Current leads of platinum wire (0.3 mm) were tightly
wound to the sample at 14 mm spacing while the spacing
between the potential probes was 8 mm. The measurements
were carried out in a cell utilizing oxygen sensing and
pumping properties of cubic zirconia as described else-
where [12]. The cell was filled with a 50% O , 50% CO gas
ment. The strong distortions of the oxygen polyhedra
follow from large quadrupole shifts of the spectral lines,
which are close to those in Sr Fe O (ꢀ ¼ ꢀ0:28 mm=s for
2
2
5
3
O
þ
3þ
Fe and ꢀ ¼ 0:30 mm=s for Fe ). It should be noticed
T
3
þ
3þ
and FeT do not
that the chemical shifts for Fe
O
significantly change with gallium content, which demon-
strates the iron–oxygen chemical bonds do not change in
any significant way at the doping levels used in this work.
Comparison of the experimental spectra in Fig. 1 shows
2
2
mixture in the beginning of the experiment and sealed. The
electrical parameters were measured with a high-precision
SOLARTRON 7081 voltmeter. The measurements were
carried out in isothermal runs. The equilibration time after
a change of the oxygen pressure inside the cell varied from
several dozen minutes to several hours depending on
temperature and oxygen pressure in the cell. The measure-
ments were halted upon achievement of the desirable low-
pressure limit. Then the oxygen pressure was increased to
the starting upper limit and the measurements repeated to
confirm reversibility of the experiment; thereupon tem-
perature was changed thus enabling the next measuring
cycle. The errors in the experimental conductivity data
were related mainly to measurements of geometrical sizes
and did not exceed 10%.
3
+
3+
that partial replacement of Fe
cations results mainly in decrease of the spectral compo-
for diamagnetic Ga
3þ
nent corresponding to Fe . Considering the small
3
T
difference of the Lamb–Mossbauer factors for FeO and
þ
¨
Table 1
The elementary unit parameters in the samples of Sr1ꢀxGa
x
O
3ꢀg obtained
after heat treatment at 700 1C in 5% H
2
/95% He gas mixture
˚
a (A)
˚
b (A)
˚
c (A)
x
0
0
0
.0
5.668(5)
5.647(9)
5.646(1)
15.582(3)
15.561(1)
15.553(6)
5.526(5)
5.510(4)
5.505(7)
.1
.2

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I.A. Leonidov et al. / Journal of Solid State Chemistry 179 (2006) 3045–3051
3047
3
þ
3+
Fe in Sr Fe O [14], the assertion can be made that Ga
2
where O- and T- layers are shown with square brackets,
while parameters a and b are interrelated as a þ b ¼ 2x,
and a is substantially smaller than b. In particular, the
T
cations in Sr (Fe Ga ) O replace predominantly iron in
2
5
2
1ꢀx
x 2
5
the layers of tetrahedra, which is consistent with the
analysis of the concentration-dependent behavior of the
lattice parameters. At the same time, a more precise
3þ
3þ
T
spectral areas AðFe Þ and AðFe Þ in Table 2 enable one
O
to represent the oxide x ¼ 0:2 at the solubility border as
Sr2½ðFeO ðFeO ðGaO
3
O
þ
evaluation on the basis of the spectral areas AðFe Þ and
Þ
Þ
Þ
Š
3
T
þ
6=2 0:94
5=2 0:04
4=2 0:02 O
AðFe Þ for O- and T-sestets, respectively, gives additional
½
ðFeO4
Þ
ðFeO
Þ
ðGaO
Þ
Š
=2 0:54
5=2 0:08
4=2 0:38 T
evidence that a small (and increasing with doping) amount
of gallium substitutes for iron in the layers of octahedra
or, in shorter notation, as Sr2½Fe0:98Ga0:02Š ½Fe0:62Ga0:38Š O5.
O
T
(
see Table 2). It should be noticed also from parameter G in
Table 2 that gallium substitution results in considerable
3.2. Oxygen-ion conductivity
broadening of the Zeeman sestets. This broadening reflects
3
þ
the fact that the second coordination spheres of Fe and
3
O
Fe include different cation configurations. Generally, the
þ
The experimental isotherms of the total conductivity vs.
T
oxygen partial pressure are shown in Fig. 2. The
conductivity increase with the pressure to the right of the
minima shows the presence of the electron hole contribu-
experimental data suggest that the structural disordering,
which occurs in response to gallium incorporation in
Sr (Fe Ga ) O , can be represented by the crystallo-
2
1ꢀx
x 2
5
tion (s ), while a similar increase with the pressure decrease
chemical formula
p
to the left of the minima signals the electron contribution
(
sn). Rather small, and decreasing with the temperature
Sr2½ðFeO6 ðFeO Þ ðGaO Þ Š
Þ
=2 1ꢀ3a
5=2 2a
4=2 a O
increase, changes in conductivity with variations of oxygen
pressure are indicative of the appreciable oxygen-ion (si)
component. Therefore, the measured values for the
conductivity near the minima were modeled with the
known relation
½
ðFeO4
Þ
ðFeO Þ ðGaO Þ Š ,
ð1Þ
=2 1ꢀbꢀ4a
5=2 4a
4=2 b T
o
n
ꢀ1=4
o
p
þ1=4
s ¼ s þ s pO
þ s pO
,
(2)
i
2
2
where s is the pressure independent ion contribution and
i
o
n
o
p
s
and s represent electron and hole components,
respectively, extrapolated to pO ¼ 1 atm. The parameters
2
o
o
s , s and s , which provide an excellent fit of the
i
n
p
experimental data, are given in Table 3. The sum of
electron and hole contributions ðs
¼ s þ s Þ can be
nþp
n
p
obtained by subtracting the ion contribution from the total
conductivity (see Fig. 3). The plots in Fig. 3 confirm that
near the minima the electron and hole components change
ꢀ1=4
þ1=4
with the pressure as snꢂpO
and spꢂpO2 , respec-
2
tively.
The oxygen-ion conductivity is shown with Arrhenius
coordinates in Fig. 4. At temperatures below 800 1C, where
the OB structure is stable in all the samples (x ¼ 0, 0.1 and
0.2), the oxygen-ion conductivity is relatively small and
5
7
strongly varies with the gallium content. The very large
activation energy (ꢂ3 eV) can be interpreted as evidence of
Fig. 1. The Fe Mo
¨
ssbauer spectra for Sr
2
(Fe1ꢀxGa
x
)
O
2 5
at 80 K: ¼ 0
(
a) and ¼ 0:2 (b).
Table 2
The Mo
5
7
¨
ssbauer spectra parameters for Fe in Sr
2
(Fe1ꢀxGa
x
)
2
O
5
at 80 K
e (mm/s)70.02
Compound
SrFeO2.5
d (mm/s)70.02
H (kOe)73
G (mm/s)70.02
A (%)72
3
O
þ
Fe
Fe
0.52
0.31
ꢀ0.28
0.30
542
455
0.32
0.33
50
50
3
T
þ
3
O
þ
SrFe0.90Ga0.10
O
2.5
2.5
Fe
Fe
0.55
0.32
ꢀ0.29
0.29
536
450
0.42
0.43
54
46
3
T
þ
3
O
þ
SrFe0.80Ga0.20
O
Fe
Fe
0.53
0.31
ꢀ0.28
0.31
540
451
0.45
0.45
61
39
3
T
þ

ARTICLE IN PRESS
I.A. Leonidov et al. / Journal of Solid State Chemistry 179 (2006) 3045–3051
3048
Fig. 2. The experimental results for changes in the total conductivity of
Sr (Fe1ꢀxGa with oxygen partial pressure at different temperatures:
Fig. 3. The isothermal logarithmic plots for the sum of electron and hole
versus oxygen partial pressure: x ¼ 0
2
x 2 5
) O
contributions in Sr
(
2
(Fe1ꢀxGa
a), 0.1 (b) and 0.2 (c). Dashed lines show slopes proportional to pO2
x
)
2
O
5
x ¼ 0 (a), 0.1 (b) and 0.2 (c). Solid lines show results of the fitting
Æ1=4
.
according to Eq. (2).
Table 3
The conductivity parameters in Eq. (2) for Sr
2 x 2 5
(Fe1ꢀxGa ) O
x
T (1C)
Parameter
ꢀ1
s ðS cmꢀ1Þ
o
s ðS cmꢀ1Þ
o
s
i
(S cm
)
n
p
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
5
5
5
6
0
950
0.24
0.22
0.21
0.14
0.04
1.9 Â 10
8.9 Â 10
3.5 Â 10
1.2 Â 10
6.7 Â 10
16.9
20.1
25.4
36.1
16.7
9
8
8
7
00
50
00
50
ꢀ
ꢀ
5
5
0
.1
950
0.37
0.29
7.7 Â 10
3.1 Â 10
8.5 Â 10
6.0 Â 10
3.0 Â 10
8.4
8.4
2.4
2.7
7.9
9
8
8
7
00
50
00
50
ꢀ
ꢀ
ꢀ
2
2
3
ꢀ6
ꢀ6
ꢀ6
2.1 Â 10
1.1 Â 10
2.2 Â 10
Fig. 4. Arrhenius plots for the ion conductivity in Sr
(&), x ¼ 0.1 (’) and x ¼ 0.2 ( ).
2
(Fe1ꢀxGa
x
)
2
O
5
: x ¼
ꢀ
ꢀ
ꢀ
ꢀ
4
5
5
5
0
0.2
950
0.31
0.23
1.1 Â 10
4.6 Â 10
2.1 Â 10
1.1 Â 10
9.0 Â 10
8.5
10.7
5.5
900
850
800
750
0.07
0.03
the temperature driven disordering of the OB framework
that involves oxygen anions and vacancies in the layers of
octahedra and tetrahedra, respectively [11]. Moreover, it
4.7
6.1
ꢀ
3
ꢀ6
5.6 Â 10

ARTICLE IN PRESS
I.A. Leonidov et al. / Journal of Solid State Chemistry 179 (2006) 3045–3051
3049
follows from the conductivity values that most intense
disordering occurs in the parent ferrite Sr Fe O , while
2
2
5
substitution of gallium in the layers of tetrahedra (as
described before, see Section 3.1) results in a more rigid
structure and the exclusion of a number of oxygen
vacancies, possibly adjacent to gallium cations, from the
oxygen-ion transport network. Thus, the oxygen-ion
conductivity in the gallium-substituted derivatives is
inferior compared to Sr Fe O . As discussed before, the
2
2
5
¨
Mossbauer spectroscopy data indicate that a small number
of gallium tetrahedra are in the octahedral layers,
especially between x ¼ 0:1 and 0.2. This can help to
understand the changes in conductivity in the OB structure
at x ¼ 0:1 versus at x ¼ 0:2. Indeed, according to the
suggested structural disordering, and as it follows from the
crystallo-chemical formula (1) for the OB phase, replace-
ment of FeO6/2 octahedra with GaO4/2 tetrahedra is
accompanied with the simultaneous formation of twice
the number of square pyramids FeO5/2 and additional
oxygen vacancies in the octahedral layers. These ‘‘induced’’
vacancies apparently favor enhanced oxygen-ion conduc-
tivity when x ¼ 0:2 compared to the composition x ¼ 0:1.
Further heating results in an increase in the conductivity,
such that at temperatures near 900 1C the conductivity
values approach about 0.2 S/cm and then cease to viably
depend on the gallium content. Considering that the
change in the temperature dependence of conductivity in
Sr Fe O reflects the OB-CB transition [10], we surmise
Fig. 5. Arrhenius plots for the hole conductivity in Sr
ꢀ8
2
x 2 5
(Fe1ꢀxGa ) O at
1
0
atm: x ¼ 0 (&), x ¼ 0.1 (’) and x ¼ 0.2 ( ).
where DHp is the oxidation enthalpy. It follows from the
reaction equation that the activation energy E for the hole
p
conductivity is equal to E ¼ ꢀ þ DH =2, where e is the
p
p
p
p
hole mobility activation energy. Assuming the inequality
ꢀp5DHp, the reaction enthalpy can roughly be estimated
as DHp ꢁ 2Ep. The experimental data in Fig. 5 result in
Ep ¼ ꢀ0:45 eV, which gives DHp ꢁ ꢀ0:9 eV. This value for
DHp is rather close to the partial molar enthalpy for
oxygen incorporation (DHO ¼ ꢀ1:03 eV) obtained from
2
2
5
that preservation of the OB structure in the gallium-
substituted derivatives on heating becomes unfavorable
too, and on accumulation of a certain level of disorder the
orthorhombic structure transforms to some other structur-
al type. It is natural to suppose that this high-temperature
modification of the gallium-substituted samples bears a
close resemblance to the high-temperature CB structure of
Sr Fe O . The proposal is supported by and is consistent
the pO –T–g diagram for the non-stoichiometric ferrite
2
SrFeO3 at g-0.5 [15]. Values for DH and DHO should
ꢀg
p
be equal [16] when compared at equal stoichiometry.
Hence, the difference between DHp and DHO can be
ascribed to the hole mobility activation energy of about
0.06–0.07 eV.
The changes in the slope of the isobars in Fig. 5 on
cooling from high temperatures are likely interrelated with
the CB-OB transition and the formation of tetrahedral
layers unfavorable for electron hole transport. Because
vacancy ordering is more intensive in the samples that
contain gallium, ‘‘switching off’’ of the tetrahedral layers
from the hole transport manifests itself in a considerable
conductivity decrease with temperature in the samples
containing gallium compared to Sr Fe O . Further cooling
2
2
5
with the observation that the conductivity is practically
independent of the gallium substitution. Although the
exact structure of the CB phase was not established in [10]
it may be that essential structural features include five-fold,
nearly pyramidal oxygen coordination of some percentage
the iron/gallium cations and some ordering of oxygen
vacancies. This model is consistent with the reduced
conductivity of the CB phase compared to CP phase with
completely or nearly random oxygen vacancies [1].
2
2
5
of compounds with x ¼ 0:1 and 0.2 reveals an unusual
increase in the hole conductivity with temperature
decreases below about 830 1C. This too can be under-
stood as reflecting intercalation of a small amount of the
off-stoichiometric oxygen in the layers of tetrahedra, which
3
.3. Electron p- and n-type conductivity
ꢀ
8
in the gallium substituted samples does not exceed 1% of
ꢀ8
The hole conductivity at pO ¼ 10 atm was calculated
2
þ1=4
o
p
o
p
the total oxygen content at pO ¼ 10 atm and
as sp ¼ s pO
with the values for s from Table 3. The
temperature dependences obtained for different samples
2
2
To850 1C.
The n-type contribution to conductivity, calculated as
ꢀ16
are shown with Arrhenius coordinates in Fig. 5. The
ꢀ
ꢀ
1=4
1
o
n
s ¼ s pO
, at pO ¼ 10
atm exhibits tempera-
positive slope of the plot log sp vs. T for Sr Fe O at
n
2
2
2
2
5
ture activated character (Fig. 6). The structural phase
transition CB-OB on cooling results in the decrease of
the conductivity activation energy from 1.9 eV in the
cubic phase to about 1.0 eV in the OB phase. Because
high temperatures, i.e. in the CB phase, is indicative of the
exothermal oxidation reaction:
3
þ
1
2
4þ
2
Fe þ O2 Ð 2Fe þ O2ꢀ þ DHp,
(3)

ARTICLE IN PRESS
I.A. Leonidov et al. / Journal of Solid State Chemistry 179 (2006) 3045–3051
3050
demonstrated that replacement of iron with gallium in the
brownmillerite structure of Sr (Fe Ga ) O with x ¼ 0:1
2
1ꢀx
occurs as incorporation of GaO4/2 tetrahedra in the FeO
x 2
5
4/2
structural layers. The gallium cations in these tetrahedral
layers favor exclusion of the neighboring oxygen vacancies
from the transport process thus resulting in a strong
decrease of the oxygen-ion conductivity. However, when
substitution increases to x ¼ 0:2, the gallium–oxygen
tetrahedra begin to appear in the iron–oxygen octahedral
layers thus promoting oxygen vacancies in these layers and
an increase of the ion conductivity to the level higher than
in the ferrite with x ¼ 0:1. Heating of the gallium-
substituted samples results in the transition of the
orthorhombic to CB structure similar to that in the
undoped ferrite. It is argued that the high-temperature
cubic structure is characterized by five-fold oxygen
coordination of a small minority of the iron and gallium
cations. The high-temperature cubic phase displays
more thermodynamic stability than the low tempera-
ture orthorhombic phase. The introduction of gallium in
the B cation sublattice is shown to generally disfavor
electron–hole transport. However, electron and hole
components of conductivity do not change in a regular
manner with changes in gallium content and temperature.
This peculiar behavior is explained as reflecting a
complicated interplay of the disordering processes in the
oxygen sublattice in response to gallium substitution and
heating.
Fig. 6. Arrhenius plots for the electron conductivity in Sr
2
x 2 5
(Fe1ꢀxGa ) O
ꢀ16
at 10 atm: x ¼ 0 (&), x ¼ 0.1 (’) and x ¼ 0.2 ( ).
2+
non-intrinsic electrons (Fe cations) appear in Sr (Fe
2
1ꢀx
Ga ) O in response to the reaction
x 2
5
3
þ
1
2
2
Fe þ O2ꢀ Ð 2Fe2þ þ O2 þ DHn
(4)
where DHn is the reaction enthalpy, the apparent activa-
tion energy for conductivity En ¼ ꢀn þ DHn=2 includes
energy contributions necessary for migration (e ) and
n
formation ðDHn=2Þ of electron carriers. It can be seen
from the expression for E that one of the reasons for the
n
large difference in the activation energy in the CB and OB
phases can be related to the migration energy being larger
in the CB phase than in the more ordered brownmillerite
phase. However, more probable is that so large a difference
reflects a larger reduction enthalpy [17]. This may in fact be
the case because oxygen depletion in the cubic phase takes
Acknowledgments
Authors are grateful to the Ural Branch of the Russian
Academy of Science for support of this work under
cooperation agreement #7-1. The present work has been
also supported in part by the Grant #978002 from the
NATO SfP program, and by the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic
Energy Sciences, Office of Science, US Department of
Energy under Award #DE-FG02-03-ER15457.
3
+
place from a pyramidal environment of Fe cations, while
in the OB oxygen anions are scavenged from iron–oxygen
octahedra where the Coulomb binding energy per oxygen
ion is smaller than in the pyramid at environment. It
should be noticed that the partial components of the
conductivity s and s are both smaller in the gallium-
n
p
doped samples than in the parent ferrite Sr Fe O ,
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