Coupled anion and cation ordering in Sr3RFe4O 10.5 (R=Y, Ho, Dy) anion-deficientperovskites

Article abstract of DOI:10.1016/j.jssc.2010.09.039

The Sr₃RFe₄O_10.5 (R=Y, Ho, Dy) anion-deficient perovskites were prepared using a solid-state reaction in evacuated sealed silica tubes. Transmission electron microscopy and ⁵⁷Fe Mssbauer spectroscopy evidenced a complete A-cations and oxygen vacancies ordering. The structure model was further refined by ab initio structure relaxation, based on density functional theory calculations. The compounds crystallize in a tetragonal a≈2√2a_p≈11.3 A, ≈4_p≈16A unit cell (a_p: parameter of the perovskite subcell) with the P4₂/mnm space group. Oxygen vacancies reside in the (FeO_5/4□_3/4) layers, comprising corner-sharing FeO₄ tetrahedra and FeO₅ tetragonal pyramids, which are sandwiched between the layers of the FeO ₆ octahedra. Smaller R atoms occupy the 9-fold coordinated position, whereas the 10-fold coordinated positions are occupied by larger Sr atoms. The Fe sublattice is ordered aniferromagnetically up to at least 500 K, while the rare-earth sublattice remains disordered down to 2 K.

Full text of DOI:10.1016/j.jssc.2010.09.039

Journal of Solid State Chemistry 183 (2010) 2845–2854
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Coupled anion and cation ordering in Sr RFe O
anion-deficient perovskites
(R¼Y, Ho, Dy)
3
4 10.5
a,b,
n
a
a,c
d
Artem M. Abakumov
, Hans D’Hondt , Marta D. Rossell , Alexander A. Tsirlin ,
b
b
d
d
a
Olga Gutnikova , Dmitry S. Filimonov , Walter Schnelle , Helge Rosner , Joke Hadermann ,
a
b
Gustaaf Van Tendeloo , Evgeny V. Antipov
a
EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Department of Chemistry, Moscow State University, 119991 Moscow, Russia
Department of Materials, ETH Z u¨ rich, Wolfgang-Pauli-Strasse 10, 8093 Z u¨ rich, Switzerland
Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
4 10.5
The Sr RFe O
(R¼Y, Ho, Dy) anion-deficient perovskites were prepared using a solid-state reaction
57
3
Received 11 August 2010
Received in revised form
in evacuated sealed silica tubes. Transmission electron microscopy and Fe M o¨ ssbauer spectroscopy
evidenced a complete A-cations and oxygen vacancies ordering. The structure model was further
refined by ab initio structure relaxation, based on density functional theory calculations. The
2
2 September 2010
Accepted 24 September 2010
Available online 1 October 2010
˚
˚
compounds crystallize in a tetragonal aE2O2a
the perovskite subcell) with the P4 /mnm space group. Oxygen vacancies reside in the (FeO5/4
layers, comprising corner-sharing FeO tetrahedra and FeO tetragonal pyramids, which are
sandwiched between the layers of the FeO octahedra. Smaller R atoms occupy the 9-fold coordinated
p p p
E11.3 A, cE4c E16 A unit cell (a : parameter of
2
3/4
& )
Keywords:
4
5
Anion-deficient perovskite
Ferrite
6
position, whereas the 10-fold coordinated positions are occupied by larger Sr atoms. The Fe sublattice is
ordered aniferromagnetically up to at least 500 K, while the rare-earth sublattice remains disordered
down to 2 K.
Anion ordering
Cation ordering
Crystal structure
Magnetic properties
Transmission electron microscopy
&
2010 Elsevier Inc. All rights reserved.
1
. Introduction
4 5
[12,13]) or by replacing the FeO tetrahedra by FeO tetragonal
pyramids, which form ordered arrangements together with the FeO
octahedra in Sr Fe 11 and Sr Fe 23 [9]. Completely different
6
Anion-deficient perovskite-based ferrites demonstrate rich crys-
4
4
O
8
8
O
tal chemistry and diverse physical properties. The wide range of
oxygen non-stoichiometry associated with the variable Fe –Fe
situation is realized in the anion-deficient perovskite-based ferrites
with the lone pair A-cation (Pb ). Point oxygen vacancies are
2
+
4+
2+
oxidation state, the ability of iron to adopt octahedral, tetrahedral,
distorted five-fold (trigonal bipyramidal of tetragonal pyramidal),
and even square-planar coordination creates a variety of structures
with different ordering patterns of oxygen atoms and anion
vacancies. The simplest ordering pattern arises when all oxygen
eliminated from the structure of such materials by periodically
spaced crystallographic shear planes [14–16].
The ordering of oxygen vacancies can be stabilized by the
presence of ordered A cations with different sizes, as in YBaFe
and YBa Fe . Here, oxygen atoms are removed from a part of
the AO layers, thus transforming the neighboring FeO octahedra
into the FeO tetragonal pyramids [17–19]. The smaller Y
2 5
O
2
3 8
O
atoms are removed from the perovskite AO layers, leaving the BO
layers intact, as in the ’’infinite-layer’’ AFeO
(A¼Ca, Sr) structures
1,2]. Layered ordering of the oxygen vacancies is preserved at
2
6
3
+
2
5
˚
[
cations (r¼1.015 A, coordination number CN¼8) reside in the
higher oxygen content in the A
2
Fe
2
O
5
(A¼Ca, Sr) brownmillerites
anion-deficient layers and are 8-fold coordinated by the oxygen
2
+
˚
[3–9]. They consist of layers of FeO
6
octahedra interleaving with
atoms, whereas the larger Ba cations (r¼1.60 A) have CN¼12 in
the complete (AO) layers. Coupled oxygen vacancy and partial A-
4
layers of the FeO tetrahedra and demonstrate a variety of polytypes
because of the configurational flexibility of the tetrahedral layers
10,11]. A further increase in oxygen content is obtained either by
increasing the amount of the octahedral layers (LaA Fe
, A¼Ca, Sr
cation ordering is also observed in Ca
representing an original intergrowth of the brownmillerite and
YBa Fe -type structures [20].
The different anion-deficient perovskites host versatile physical
properties. Anion disorder leads to high oxygen conductivity and
supports potential applications in solid-oxide fuel cells [21]. The
2 2 2 6
Ba Nd Fe O15.6 compound,
[
2
O
3 8
2
3 8
O
n
Corresponding author at: EMAT, University of Antwerp, Groenenborgerlaan
peculiar layered structures of the AFeO
unusual orbital state of Fe [1,2,22] and a spin-state transition under
2
compounds induce an
1
0
71, B-2020 Antwerp, Belgium. Fax: +32 3 2653257.
2
+
E-mail address: artem.abakumov@ua.ac.be (A.M. Abakumov).
022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.09.039

2
846
A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
pressure [23]. Recent studies explored the thermoelectric properties
of Ca Fe and evidence a reduced thermal conductivity, compared
to the ideal CaFeO perovskite that lacks anion vacancies [24]. All
these findings motivate the search for new Fe-based anion-deficient
perovskites.
the carrier gas. The analysis yielded 20.270.2 wt% of oxygen, which
corresponds to a Sr RFe 10.370.1 composition. This formula is close
to Sr RFe 10.5 with all iron atoms having the oxidation state +3.
Since M o¨ ssbauer spectroscopy confirms the oxidation state of +3,
we further refer to the ideal Sr RFe 10.5 composition.
2
2
O
5
3
4
O
3
3
4
O
3
4
O
Our study was guided by a similarity in anion–vacancy ordering
Phase analysis and cell parameter determination were per-
formed using X-ray powder diffraction (XRPD) with a Huber G670
patterns of ferrites and cobaltites (compare the Fe- and Co-based
0
A
2
B O
2 5
brownmillerites or the anion-deficient AA’B
O
2 5
(A, A are the
1
Guinier diffractometer (CuKa -radiation, image plate detector).
cations of different size) perovskites) [25,26]. An original anion-
deficient perovskite structure was reported for cobaltites
Electron diffraction (ED) and high-resolution transmission
electron microscopy (HRTEM) investigations were performed on
crushed samples deposited on holey carbon grids. ED patterns
were obtained using a Philips CM20 microscope, while HRTEM
experiments were performed using a JEOL 4000EX microscope.
HRTEM image simulations were made using the JEMS software.
Energy-dispersive X-ray analysis was performed with a Philips
CM20 microscope equipped with an Oxford Instruments attach-
ment. High-angle annular dark field scanning transmission
electron microscopy (HAADF-STEM) images were recorded on a
Tecnai G2 microscope. HAADF-STEM image simulations were
made using the QSTEM software [37].
Sr4ꢀx
R
x
Co
4
O
10.5+
d
with R¼La, Y, Sm–Yb, 0.4rxr1.33, and –
0.02rdr0.74 (the cation homogeneity range and the oxygen
content vary depending on the R-type and might also depend on
synthesis conditions, causing the discrepancy between published
data) [27–29]. The idealized structure of these compounds with the
Sr
AO–CoO5/4
partially ordered in the (CoO5/4
five-fold coordinated Co atoms. However, one equatorial oxygen
position is only 25% occupied, resulting locally in CoO tetrahedra
and CoO distorted tetragonal pyramids in a 1:1 ratio. This structure
is realized in the frame of a tetragonal 2a ꢁ 4a supercell
ꢁ 2a
—a parameter of the perovskite subcell) and the I4/mmm space
4
RCo
4
O
10.5 composition corresponds to the layer sequence—CoO
3/4–AO–CoO —[30–32]. The anion vacancies are
3/4) layers, giving rise to formally
2
–
&
2
&
4
57
5
M o¨ ssbauer spectroscopy ( Fe) was performed in transmission
mode using
constant acceleration M o¨ ssbauer spectrometer
coupled with a 1024 multichannel analyzer. A Co/Rh
was used for the experiments. The velocity scale was calibrated
relative to -Fe. All isomer shift values ( ) are referred to -Fe. All
p
p
p
a
57
(
a
p
g-rays source
group, but further ordering and structural distortions resulting in the
unit cell expansion and symmetry lowering are also reported
a
d
a
[33,34]. The anion ordering is usually coupled with the ordering of
experimental data were resolved in sets, with Lorenzian lineshapes
the Sr and R cations: the partially occupied oxygen site is adjacent to
the R cations, thus their coordination number is decreased. This
coupling seems to be an essential prerequisite for the ordering,
although anion ordering can exist even with a random placement of
using an iterative least square fitting program.
The magnetic susceptibility was measured with a Quantum
Design MPMS SQUID magnetometer in the field range 0.1–5 T and
in the temperature range 2–750 K. For measurements above room
temperature, the powder sample was placed into a sealed quartz
tube filled with He.
2
+
the A cations, as in the case of R¼La with similar sizes of Sr
3
+
˚
˚
(
r¼1.32 A, CN¼10) and La (r¼1.28 A, CN¼10) [29].
An ordered pattern of anion vacancies, similar to that in the
Co solid solutions, was also observed at partial
replacement of Co by Ga or Fe, or even in the completely Co-free
10.53 compound [35,36]. However, up to
now attempts to prepare purely iron-based anion-deficient
analogs of the Sr RCo 10.5 phases were unsuccessful. A replace-
ment of Co with Fe in the Sr YCo4ꢀxFe solid solution
3 4 10.5
For ab initio structure relaxation, we chose the Sr YFe O
Sr4ꢀx
R
x
4
O
10.5+
d
compound to avoid the treatment of the partially filled and strongly
correlated 4f shells of Dy and Ho. The relaxation was based on density
functional theory (DFT) band structure calculations in the VASP code
[38,39] with the projected-augmented-wave basis set (plane wave
energy cutoff 400 eV) [40,41] and the generalized gradient
approximation (GGA) for the exchange-correlation potential [42].
The Brillouin zone was sampled by 16 k-points. We performed both
the spin-unpolarized (GGA) and the spin-polarized (GGA+U) calcula-
tions, the latter including the mean-field correction for correlation
effects in the Fe 3d shell (the GGA+U input parameters U¼5 eV and
J¼1 eV, according to previous studies [43]). Since the Fe–O–Fe angles
are close to 1801 and the experimental data evidence antiferromag-
Sr2.93
Y
1.07Mn2.67Ga1.33O
4
4
O
3
x 10.5+d
O
allows to maintain the anion–vacancy ordering only up to x¼2.5
and yields polyphasic samples for x43.5 [35]. In the present
study, we avoid this limitation by using a different preparation
method and report on the crystal structure and magnetic
properties of Sr
contrast to cobaltites, the Sr
cation ordering at room temperature.
3
RFe
4
O
10.5 compounds with R¼Y, Ho, and Dy. In
3
RFe 10.5 ferrites show anion and
4
O
3 4
netic (AFM) ordering in Sr RFe O10.5, a simple AFM ground state with
antiparallel spins on neighboring Fe atoms was assumed. The energy
3
+
3 4
spectrum of Sr YFe O10.5 is typical for Fe oxides, with valence bands
2
. Experimental
formed by Fe 3d and O 2p states. GGA leads to a metallic spectrum
due to an underestimate of the correlation effects, while GGA+U
The Sr
solid-state reaction in evacuated sealed silica tubes. SrO, Fe
and R
(R¼Y, Ho, Dy) were used as the starting materials. SrO
was prepared by a decomposition of SrCO at 900 1C in a dynamic
3
RFe
4
O
10.5 (R¼Y, Ho, Dy) samples were prepared using a
restores the insulating ground state (energy gap E
with the red-brownish color of the samples). The magnetic moment
on the Fe sites is 4.0–4.3 , in agreement with experimental
estimates for other Fe anion-deficient perovskites [19]. During the
relaxation, the atom positions were varied, while the unit cell
parameters were fixed at the experimental values. Residual forces in
g
¼1.8 eV, consistent
2 3
O ,
2
O
3
m
B
3
+
3
ꢀ
3
vacuum of 10 mbar and further handled in an Ar-filled glove
box. The starting materials were mixed according to the required
cation ratio, placed into alumina crucibles and then into silica
tubes. The tubes were evacuated to a residual pressure of
˚
the relaxed structures are below 0.01 eV/A.
ꢀ
2
1
0
mbar and sealed. The samples were annealed at 1000 1C
3. Results
for 36 h. After furnace cooling, the tubes were opened, the
samples were reground, pressed into pellets, sealed, and annealed
again at 1100 1C for 48 h. Finally the samples were furnace cooled.
Some samples were additionally annealed at 1150 1C for 24 h and
then cooled to room temperature with a rate of 3 1C/h.
3.1. Preliminary investigation
Typical XRPD patterns of the Sr
samples are shown in Fig. 1. They can be indexed in a primitive
cubic unit cell with the parameters given in Table 1. The lattice
3
RFe
4
O
10.5 (R¼Y, Ho, Dy)
The oxygen content was analyzed by the carrier-gas-hot-
extraction technique using a TCH600 LEKO instrument and He as

A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
2847
ionic radius of the R cation. Superstructure reflections and
splitting or broadening of the basic perovskite reflections, which
could indicate a possible symmetry reduction or an increase in the
unit cell volume, were not observed.
EDX analysis demonstrated a Sr:R:Fe atomic ratio close to the
nominal 3:1:4 one (Sr:R:Fe¼2.9(1):1.17(9):3.9(1) for R¼Y;
3
.0(1):1.0(1):3.9(1) for R¼Ho; 2.95(5):1.17(6):3.87(7) for R¼Dy).
The M o¨ ssbauer spectra of Sr
3 4
DyFe O10.5 revealed that the com-
pound is in a magnetically ordered state at room temperature. In
order to better resolve the signals from inequivalent Fe atoms, the
spectrum was measured at T¼78 K (Fig. 2). The spectrum consists
of three sets of Zeeman sextets with different hyperfine
parameters, which were attributed to three distinct atomic
positions Fe(1), Fe(2), and Fe(3). The observed isomer shifts
3
+
IS¼0.26–0.43 mm/s correspond to Fe
cations exclusively
(
Table 2). According to the isomer shift values, the Fe(1) atoms
adopt an octahedral oxygen environment, whereas tetrahedral
oxygen coordination is realized for the Fe(3) atoms. The isomer
shift for the Fe(2) atoms is in between the isomer shifts of Fe(1) and
Fe(3), indicating that the Fe(2) atom is in a five-fold coordination.
The observed Fe(1), Fe(2), and Fe(3) sextet areas correspond to the
ꢂ2:1:1 ratio of the octahedrally, five-fold, and tetrahedrally
coordinated Fe atoms.
We conclude that the Sr
3
RFe
4
O
10.5 (R¼Y, Ho, Dy) compounds
adopt an anion-deficient perovskite structure. The presence of the
Fe atoms in three distinct coordinations points to an ordered
arrangement of the oxygen atoms and anion vacancies, as further
verified by electron diffraction.
3.2. Electron diffraction and structure model
All the Sr RFe
3
4
O
10.5 (R¼Y, Ho, Dy) compounds demonstrate
similar ED patterns (Fig. 3). The brighter reflections on the ED
patterns belong to the primitive cubic perovskite structure with
p
cell parameter a . Besides these subcell reflections, numerous
weaker superstructure reflections are present. The most
frequently observed pattern along the cubic axis of the
perovskite subcell is shown in Fig. 3a. The reflection positions
on this pattern match well with a tetragonal symmetry. However,
the intensity distribution does not demonstrate four-fold
Fig. 1. Le Bail fit of the X-ray powder diffraction patterns of Sr
3
RFe
4
O
10.5 (R¼Y, Ho,
Dy). The ticks mark the positions of the cubic perovskite subcell reflections.
Table 1
Perovskite subcell parameters for Sr
3
RFe
4
O
10.5 (R¼Y, Ho, Dy).
57
Fig. 2. The Fe M o¨ ssbauer spectrum of Sr
3 4
DyFe O
10.5 at T¼78 K.
Sample composition
˚
˚
Ionic radius of R (A)
a (A)
Table 2
Sr
Sr
Sr
3
3
3
YFe
HoFe
DyFe
4
O
10.5
10.5
10.5
1.155
1.16
1.17
3.8850(7)
3.8856(5)
3.8871(6)
5
7
The Fe M o¨ ssbauer hyperfine parameters of Sr
3 4
DyFe O10.5: I, relative area; IS,
4
O
isomer shift relative to
a
-Fe;
G
, line-width.
4
O
Fe(1)
Fe(2)
Fe(3)
I (%) (71)
53
25
22
parameters and the intensity distribution of the reflections on the
XRPD pattern reflect that all compounds adopt the perovskite-
based structure. The lattice parameter decreases with decreasing
IS (mm/s) (70.02)
0.43
0.72
0.39
0.50
0.26
0.77
!
(mm/s) (70.02)

2
848
A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
Fig. 3. Electron diffraction patterns of Sr
3 4
DyFe O
10.5 taken from a twinned region (a) and single domain regions (b–f). The indexation is performed on a tetragonal
pffiffiffi
pffiffiffi
2
2a
p
ꢁ 2 2a
p
p
ꢁ 4a supercell.
symmetry. A closer inspection of different areas of the crystals in
the /100S orientations revealed that they are heavily twinned
2 2
the P4 /mnm space group or its subgroups P4 n2 and P4 nm. The
p
forbidden h00: ha2n, 0k0: ka2n and 00l: la2n reflections on
the [001], [110] and [101] ED patterns are caused by multiple
diffraction.
and the ED pattern of Fig. 3a is a superposition of the ED patterns
o
in Fig. 3b, c and its 90 -rotated counterpart. Single-domain ED
patterns are represented in Fig. 3b–f. They can be completely
The lack of superstructure reflections on the XRPD patterns is
likely related to the small size of the twinned domains that makes
the structure determination from XRPD data essentially impos-
sible. To construct a structural model, the results of electron
diffraction, electron microscopy, and M o¨ ssbauer spectroscopy
were taken into account. The model was further refined via
DFT-based ab initio structure relaxation. Our proposed model
stems from the following experimental observations:
indexed on
a tetragonal unit cell with lattice parameters
pffiffiffi
˚
˚
aE2 2a
p
E11.3 A, cE4a
p
E16 A. The absence of any reflection
splitting on the ED patterns from twinned regions, as well as
the absence of the splitting and broadening of reflections on the
XRPD patterns, indicates that the tetragonal strain in the
Sr
3
RFe
4
O
10.5 (R¼Y, Ho, Dy) structures is negligibly small. The
observed reflection conditions h0l: h+l¼2n, 0kl: k+l¼2n suggest

A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
2849
ꢃ
ꢃ
unit cell parameters and space symmetry determined from
electron diffraction;
the Sr RFe 10.5 composition, confirmed by EDX analysis,
chemical analysis and M o¨ ssbauer spectroscopy. This composi-
tion implies that all Fe atoms are in the oxidation state +3;
three crystallographically different positions of Fe atoms with
coordination numbers 6, 5, and 4 are present in a 2:1:1 ratio.
3
4
O
ꢃ
Although our experimental data do not allow us to exclude
partial cation and oxygen ordering, we have constructed the
structural model assuming a complete ordering of oxygen atoms
and vacancies, as well as of the Sr and R cations. The most
symmetric P4
positions in the tetragonal supercell were calculated from the
atomic positions in the perovskite structure using the a¼2a –2b
b¼2a +2b , c¼4c relation between the subcell and the super-
cell. The cation composition, the oxygen content, and the unit cell
parameters of the Sr RFe 10.5 compounds are similar to those of
2
/mnm space group was chosen, and the atomic
p
p
,
p
p
p
3
4
O
their Co-based analogs (aFe¼O2aCo, cFe¼cCo), thus pointing to a
similarity between these crystal structures.
The Sr
alternation of perovskite-type layers along the c-axis of the
tetragonal supercell: –FeO5/4 3/4–AO–FeO –AO–FeO5/4
A¼Sr, R). The Fe cations in the (FeO ) layers adopt an octahedral
oxygen coordination. There are two (FeO5/4 3/4) layers per unit
cell, symmetrically related by the 4 screw axis. The Fe atoms in
the (FeO5/4 3/4) layers reside in two 8-fold positions (0,1/4,0) and
1/2,1/4,0), resulting in four Fe atoms of every position per one
3/4) layer. For the oxygen atoms in the (FeO5/4
3 4
RFe O10.5 structure can be represented by an ordered
&
2
3/4
& –
Fig. 4. The structure model of Sr
3
RFe
4
O
10.5 (R¼Y, Ho, Dy). The FeO
6
octahedra,
(
2
FeO tetragonal pyramids and FeO tetrahedra are light blue, green and brown,
5
4
&
respectively. The Sr and R cations are shown as large yellow and blue spheres. The
oxygen atoms at the corners of polyhedra are marked as small blue spheres. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
2
&
(
(
FeO5/4
&
3/4
& )
layers, four 4-fold and two 8-fold positions are available,
providing 16 oxygen positions per single (FeO5/4 3/4) layer. Six
of these 16 positions should be vacant, in agreement with the
Fe 3/4)) layer composition. In order to get
10) (¼8 ꢁ (FeO5/4
such composition of the (FeO5/4 3/4) with a complete ordering of
the oxygen atoms and vacancies, either three out of four 4-fold
positions or one 4-fold and one 8-fold position should be vacant.
Only the last possibility creates tetrahedral coordination for half
&
3 4
HAADF-STEM image of Sr DyFe O10.5 (Fig. 5). In the HAADF-STEM
image, the contrast has a relation to the average atomic number of
(
8
O
&
n
an atomic column as ꢂZ (n¼1–2). Thus, the most prominent
&
dots on the image correspond to the projections of the A-cation
columns, which form a square pattern, and each square is
centered with a weaker dot corresponding to the FeO column.
The pure oxygen columns are not visible due to the low atomic
number of oxygen. The brightness of the A-cation dots is not
uniform: the brighter dots, corresponding to the mixed Sr (Z¼38)
and Dy (Z¼66) columns, and the less bright dots of pure Sr
columns alternate, forming a chess-board pattern, consistent with
the proposed structure model. The ordered alternation of the Sr
and (Sr,Dy) columns is clearly seen from the intensity plot along
the row of the A-cation dots (Fig. 5). The [001] HAADF-STEM
image, simulated using the proposed structure model, is in
excellent agreement with the experimental one (Fig. 5).
of the Fe atoms in the (FeO5/4
&3/4) layer and tetragonal pyramidal
coordination for another half of the Fe atoms in this layer. This
ordering pattern of oxygen atoms and vacancies results in a 2:1:1
ratio of the Fe atoms in octahedral (CN¼6), tetragonal-pyramid
(
CN¼5), and tetrahedral (CN¼4) coordination, in agreement with
the results of the M o¨ ssbauer spectroscopy.
The anion ordering modifies the coordination environment for
the A cations. In the framework of this model, the A cations can be
placed at one 8-fold and one 4-fold position with CN¼12, one 8-
fold and one 4-fold position with CN¼10, and one 8-fold position
with CN¼9. Continuing the analogy with the Co-based phases, the
smaller R atoms can be placed into the 8-fold position with the
lowest coordination number, whereas other positions are occu-
pied by larger Sr cations. The complete model of the ordering in
As suggested by the ED data, the HRTEM images along the
p
/100S direction show several twinned regions (Fig. 6). The
Fourier transforms taken from the left and right part of the image
in Fig. 6 show domains in the [001] and [110] orientation; the
domain boundary is not well defined though. The major type of
3 4
the Sr RFe O10.5 anion-deficient perovskites is shown in Fig. 4.
o
the twinning in Sr
3
RFe
4
O
10.5 is a 90 rotation twin that aligns the
The atomic coordinates are given in Table S1 of the
Supplementary data. At this point, the atomic positions are
derived from the parent perovskite structure and do not take into
account atomic displacements, which necessarily occur in order
to optimize the coordination polyhedra and interatomic distances
and relieve the strain, imposed by the ordered arrangement of the
anion vacancies.
c-axis of the tetragonal supercell along the three /100S
symmetrically equivalent in the parent perovskite structure.
The [001] and [110] HRTEM images of Sr DyFe 10.5 are shown
p
axes,
3
4
O
in Figs. 7 and 8. The Fourier transforms confirm that the images
refer to single domains in the [001] and [110] orientation,
respectively. The bright dots in the [001] HRTEM image
correspond to projections of the A-cation and FeO columns. The
tetragonal supercell is centered with four brighter dots marking
the projections of the mixed (Sr,Dy) columns, whereas the dots of
pure Sr columns appear less bright. The simulated [001] HRTEM
3.3. HAADF-STEM and HRTEM observations
˚
˚
The proposed model of the anion and cation ordering in
image at a focus value f¼ ꢀ450 A and a thickness t¼48 A is in
agreement with the experimental contrast. The [110] HRTEM
image was taken at imaging conditions where the bright dots
Sr
3 4
RFe O10.5 was confirmed by HAADF-STEM and HRTEM obser-
vations. The A-cation ordering is clearly observed in the [001]

2
850
A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
3 4
Fig. 7. [001] HRTEM image of Sr DyFe O10.5. The Fourier transform is shown as
inset at the left bottom corner. The perovskite subcell and the tetragonal
pffiffiffi
pffiffiffi
2
2a
p
ꢁ 2 2a
p
ꢁ 4a
p
supercell are marked with white small and large rectangles,
respectively. The simulated HRTEM image (f¼ ꢀ450 A˚ , t¼48 A˚ ) is superimposed
and outlined with a white line.
3 4
Fig. 5. [001] HAADF-STEM image of Sr DyFe O10.5. The plot below the image is the
intensity profile along the A-cation columns as marked with the white rectangle
on the image. The simulated image is shown as inset.
3 4
Fig. 8. [110] HRTEM image of Sr DyFe O10.5. The Fourier transform is added in the
bottom left corner. The simulated HRTEM image (f¼ ꢀ800 A˚ , t¼32 A˚ ) is super-
imposed and outlined by a white line.
3 4
Fig. 6. HRTEM image of a twinned area in Sr DyFe O10.5. The left and right parts of
the image correspond to the [001] and [110] orientation, respectively. The Fourier
transforms from both domains are shown as insets.
The superstructure induced by the anion and cation ordering is
3 4
clearly visible on the [101] HRTEM image of Sr DyFe O10.5, where
dots of different brightness are grouped in rectangles forming a
chess-board pattern (Fig. 9). Here the structure is viewed along
the close-packed perovskite layers which are parallel to the b-axis
of the tetragonal supercell. The Sr and Dy atoms are arranged into
separated columns in this structure projection. The bright dots
correspond to projections of the columns of oxygen atoms which
correspond to regions of low projected potential (columns of
oxygen atoms and/or anion vacancies). In this projection, the
oxygen-deficient columns, consisting of oxygen vacancies only or
of vacancies and oxygens in a 50:50 ratio, alternate with the
vacancy-free oxygen columns. According to the simulated [110]
˚
˚
HRTEM image (f¼ ꢀ800 A, t¼32 A), the ordered arrangement of
the oxygen-deficient columns is visible on the image as a pattern
of less bright dots surrounded by a darker gray halo.
6
are at the corners of the FeO octahedra. The difference in
brightness arises from different surroundings of these oxygen
atoms by A cations: the dots of the oxygen columns close to the

A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
2851
3 4 10.5
Fig. 9. [101] HRTEM image of Sr DyFe O . The simulated HRTEM image
(
f¼ ꢀ600 A˚ , t¼39 A˚ ) is superimposed and outlined by a white line.
Dy columns are darker than the dots corresponding to oxygen
columns positioned next to Sr atoms only. The simulated [101]
˚
˚
HRTEM image (f¼ ꢀ600 A, t¼39 A) is in excellent agreement with
the experimental one.
3.4. Ab initio structure relaxation
To improve the accuracy of the structural model, we performed
the relaxation of the atomic positions. The DFT-based relaxation
accounts for the chemical nature of the system and for all the
bonding interactions. However, it has an inherent shortcoming
related to the treatment of correlation effects in the Fe 3d shell. To
test the relevance of such effects, we performed the relaxation
within two different approaches: GGA (a nearly free-electron
approximation, spin-unpolarized calculation) and GGA+U (mean-
field correction for the correlation effects, spin-polarized calcula-
tion). The results are collected in the Supporting information,
Table S1. Both approaches modify the atomic positions, compared
to the initial model. In particular, we find a remarkable tilting of
6
the FeO octahedra (Fig. 10) that leads to several short Y–O bonds
˚
˚
(
2.35–2.45 A). The Sr–O distances exceed 2.5 A, in agreement with
Fig. 10. The crystal structure of Sr
3
YFe
4
O
10.5 relaxed within GGA+U. The FeO
tetrahedra are light blue, green and
6
the larger size of the Sr cation (Table 3). The tilting of the
octahedra modifies the local environment of the 12-fold coordi-
nated Sr(1) and Sr(4) positions. In the relaxed structure, two Sr–O
distances exceed 3.5 A, and the CN is effectively reduced to 10
Table 3). Despite the different treatment of the Fe 3d shell, GGA
octahedra, FeO tetragonal pyramids and FeO
5
4
brown, respectively. The Sr and R cations are shown as large yellow and blue
spheres. The oxygen atoms at the corners of polyhedra are marked as small blue
spheres. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
˚
(
and GGA+U lead to a similar oxygen coordination of the iron
positions, with slight differences in the Fe–O bond lengths. GGA
underestimates the Coulomb repulsion on the Fe site and,
consequently, overbonds the iron atoms, so that the GGA+U-
relaxed structure looks more realistic. The interatomic distances
for the GGA+U-relaxed structure are listed in Table 3.
in the 2–750 K temperature range. The measurements above
500 K led to irreversible changes that are likely caused by a slight
change in the oxygen content upon heating. In the following, we
focus on the data collected below 500 K only.
For both compounds, the susceptibility increases with decreas-
ing temperature (Fig. 11). The data above 50 K were fitted with a
3.5. Magnetic properties
modified Curie–Weiss law
temperature-independent contribution, C is the Curie constant,
and is the Curie–Weiss temperature. The fits yielded
w
¼
w
0
+C/(T+
y
), where
w
0
is the
3 4 10.5
Magnetic measurements were performed for Sr RFe O
compounds with R¼Dy and Ho. The susceptibility was measured
y
0
w ¼

2
852
A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
Table 3
Selected interatomic distances (in A˚ ) in the GGA+U optimized structure of Sr
3 4 10.5
YFe O .
Y–O(2)
2 ꢁ 2.44
2 ꢁ 2.75
2.70
Sr(2)–O(2)
Sr(2)–O(3)
Sr(2)–O(4)
Sr(2)–O(4)
Sr(2)–O(9)
Sr(3)–O(5)
Sr(3)–O(7)
Sr(3)–O(8)
Sr(3)–O(9)
Sr(4)–O(1)
Sr(4)–O(2)
Sr(4)–O(5)
Sr(4)–O(7)
2 ꢁ 2.97
2 ꢁ 2.83
2 ꢁ 2.63
2 ꢁ 2.87
2 ꢁ 2.76
2 ꢁ 2.73
2 ꢁ 2.55
2 ꢁ 2.90
4 ꢁ 3.11
2 ꢁ 2.76
4 ꢁ 2.74
2 ꢁ 2.69
2 ꢁ 2.96
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–O(6)
Fe(2)–O(2)
Fe(2)–O(4)
Fe(2)–O(4)
Fe(2)–O(5)
Fe(2)–O(7)
Fe(2)–O(9)
Fe(3)–O(3)
Fe(3)–O(8)
Fe(3)–O(9)
1.95
Y–O(4)
2 ꢁ 2.01
2.01
Y–O(5)
Y–O(6)
2.42
2.07
Y–O(7)
2.82
2.04
Y–O(9)
2 ꢁ 2.36
3.05
1.99
Sr(1)–O(1)
Sr(1)–O(2)
Sr(1)–O(3)
Sr(1)–O(4)
Sr(1)–O(5)
Sr(1)–O(7)
Sr(1)–O(8)
2.02
2 ꢁ 3.01
2 ꢁ 3.06
2 ꢁ 2.60
2.60
2.02
2.02
2.43
1.93
2.63
1.93
3.10
2 ꢁ 1.88
4. Discussion
Synthesis by
a solid-state reaction at 1100–1150 1C in
evacuated sealed silica tubes allowed us to prepare the single-
phase Sr RFe
3
4
O
10.5 (R¼Y, Dy, Ho) compounds with an ordered
arrangement of oxygen atoms and anion vacancies, as well as Sr
and R cations. This result can be compared with the attempts to
partially replace Co by Fe in Sr
state reaction at nearly the same temperature (1100 1C) and
higher partial oxygen pressure (in air). Initially, in Sr YCo
the oxygen vacancies are partially ordered in the (CoO5/4
layers. This ordering gives rise to a tetragonal 2a
ꢁ 2a
3 4
YCo O10.5, performed by a solid-
3
4 10.5
O
&
3/4
)
p
p
ꢁ 4a
p
superstructure, where one position of the oxygen atom, occupied
by 25%, forms a common vertex of two CoO distorted tetragonal
5
pyramids (also considered as being more close to trigonal
bipyramids) (Fig. 12a). This type of ordering is maintained in
the Sr
between 3rxr3.5
3
YCo4ꢀxFe
x
O
10.5+
d
solid solution only up to x¼2.5, and
a
disordered perovskite structure was
observed. The synthesis of the fully substituted Sr
yielded a multiphase sample [30]. Suppression of the anion
ordering in the Sr YCo4ꢀxFe solid solution is presumably
related to increasing oxygen content because of the thermal
treatment in air. Indeed, for the x¼2 sample, was found to be
.304 [30]. Synthesis of Sr RFe 10.5 in a sealed silica tube,
performed in the present work, prevents the partial oxidation of
3 4 10.5+d
YFe O
3
x 10.5+d
O
Fig. 11. Magnetic susceptibility of Sr
3
RFe
4
O
10.5 with R¼Dy (top) and Ho (bottom).
Experimental data, shown with open circles, were measured in an applied field of
d
0.1 T. Solid lines are fits of the data with the modified Curie–Weiss law.
0
3
4
O
–
3
3+
4+
7
.5(1) ꢁ 10 emu/mol, C¼14.74(1) emu K/mol, and
y
¼9.8(1) K
Fe
ordered structure of the (FeO5/4
Partial anion ordering in the anion-deficient layers with the
idealized (CoO5/4 3/4) composition (Fig. 12a) associated with the
2a ꢁ 4a superstructure was reported for a wide range of
ꢁ 2a
Co compositions [22–24]. The broad homogeneity
to Fe
and maintains the chemical composition and the
–3
for R¼Dy and
w
0
¼7.4(1) ꢁ 10 emu/mol, C¼15.04(1) emu K/mol,
&
3/4) layers.
and
y¼6.3(1) K for R¼Ho. The effective moments of 10.86(1) and
1
0.97(1)
m
B
for R¼Dy and Ho, respectively, are in reasonable
. Thus,
&
agreement with the expected values of 10.65 and 10.61
m
B
p
p
p
the temperature dependence of the magnetic susceptibility can be
attributed to the paramagnetic behavior of the rare-earth cations.
According to the results of the M o¨ ssbauer spectroscopy, the Fe
sublattice is in a magnetically ordered state at room temperature.
To find out the type of the magnetic ordering, we measured
magnetic susceptibility in different applied fields. At low tem-
peratures, we observed a pronounced suppression of the suscept-
ibility by the applied field, typical for paramagnets. Above 40 K,
the susceptibility does not depend on the field. Hence, the ordering
in the Fe sublattice is antiferromagnetic and persists up to at least
Sr4ꢀxR
x
4 10.5+d
O
range of these solid solutions indicates that the ordering in these
compounds is intrinsically incomplete. In the frame of the
2a
p
ꢁ 2a
p
ꢁ 4a
p
unit cell and the I4/mmm space group, the
occupancy factors are the same for the partially vacant oxygen
sites at the trans- and cis-positions with respect to the R cations
above and beneath the layer (viewed along [001] axis) (Fig. 12a). An
extra ordering was reported to result in either an orthorhombic
C-centered 2 2a
monoclinic 2 2a
orthorhombic superstructure, the oxygen sites at trans-positions
acquire different occupancies (Fig. 12b) [28]. The A-centered
monoclinic supercell arises as a result of the order–disorder phase
transition redistributing oxygen atoms in such a way that one pair
of the oxygen trans-positions becomes completely vacant, and
another pair of trans-positions is 50% occupied (Fig. 12c) [29]. The
pffiffiffi
pffiffiffi
p
ꢁ 4a
p
ꢁ 2 2a
p
superstructure or in an A-centered
p
superstructure. In the C-centered
pffiffiffi
pffiffiffi
p
ꢁ 2 2a
p
ꢁ 4a
5
00 K. The Fe contribution to the magnetic susceptibility is low
and can be roughly approximated by the constant term.
w
0
The smooth temperature dependence of the magnetic suscept-
ibility indicates the lack of a magnetic ordering transition in the
2
–500 K temperature range. Positive Curie–Weiss temperatures y
correspond to weak and predominantly antiferromagnetic inter-
actions between the rare-earth atoms. Indeed, the susceptibility
shows a negative deviation from the Curie–Weiss law below 20 K,
revealing the onset of antiferromagnetic correlations between the
magnetic moments on the rare-earth atoms.
pffiffiffi
pffiffiffi
tetragonal 2 2a
symmetry of Sr
ordering of the oxygen atoms. The perovskite (BO
p
ꢁ 2 2a
p
ꢁ 4a
p 2
supercell and the P4 /mnm space
3
RFe
4
O
10.5 provide a possibility for complete
2
) layer can be
formally considered as a –O–BO–O–BO– sequence of atomic chains

A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
2853
Fig. 12. The anion-vacancy ordering schemes in the Sr4ꢀx
square). The partially occupied oxygen positions are shown as small open circles. (b) The anion-deficient layer in the C-centered 2 2a
R
x
B
4
O
10.5+
d
(B¼Co, Fe) compounds: (a) the (CoO5/4 3/4) layer in the 2a
p
ꢁ 2a
p
ꢁ 4a
p
unit cell (outlined with a
pffiffiffi
pffiffiffi
p
ꢁ 4a
p
p
ꢁ 2 2a supercell. Only part
of the unit cell is shown. The partially occupied oxygen positions with different occupancies are marked with black and open small circles. (c) The anion-deficient layer in
pffiffiffi
pffiffiffi
the A-centered monoclinic 2 2a
p
ꢁ 2 2a
p
ꢁ 4a
p
supercell. Only part of the unit cell is shown. The partially occupied oxygen positions are marked by small open circles. (d)
pffiffiffi
pffiffiffi
The (FeO5/4 3/4) layer in the tetragonal 2 2a
p
ꢁ 2 2a
p
ꢁ 4a
p
supercell (outlined with a solid square). The dashed square corresponds to the part of the unit cell shown in (a).
On all figures the BO
5
tetragonal pyramids and BO
4
tetrahedra (B¼Co, Fe) are green and brown, respectively; the Sr and R cations are shown as yellow and blue circles. The
atomic displacements are neglected for clarity in all structures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
this article.)
running along [100]
Sr RFe 10.5 structure every second O chain is absent and every
fourth oxygen position in the BO chain is vacant resulting in the –
–FeO3/4 1/4–&– sequence (Fig. 12d). This
ordered arrangement of oxygen atoms and vacancies transforms
the infinite net of FeO octahedra into a pattern of blocks consisting
of four FeO tetrahedra and four FeO tetragonal pyramids sharing
corners with each other. The completely ordered (FeO5/4 3/4) layer
does not have a four-fold symmetry, and the overall tetragonal
symmetry of the structure is maintained only by the 4 screw axis
p
and alternating along [010]
p
.
In the
are occupied by the larger Sr atoms. This ordering pattern is
virtually the same as the A-cation ordering in nearly stoichiometric
3
4
O
3 4 10.5+d
Sr RCo O compounds.
&
&
1/4–O–FeO3/4
&
The magnetic behavior of Sr RFe O10.5 is typical for anion-
3 4
3
+
deficient perovskites containing Fe . The dominating couplings in
the Fe sublattice arise from the Fe–O–Fe superexchange. Since Fe
polyhedra share corners, the Fe–O–Fe angles are close to 1801 and
lead to strong antiferromagnetic interactions, the antiferromag-
netic ground state with antiparallel spins on nearest neighbors,
and the magnetic ordering temperatures in the 500–1000 K range
[13,19]. Indeed, M o¨ ssbauer spectroscopy experiments and
magnetic measurements evidenced antiferromagnetic ordering in
the Fe sublattice up to at least 500 K. The shortest distance
6
4
5
&
2
relating the anion-deficient layers at z¼0 and 1/2. Along the c-axis,
such anion-deficient layers alternate with the layers consisting
6
of corner sharing FeO octahedra (Fig. 4). Thus, the Fe atoms
˚
with coordination numbers 6, 5, and 4 are present in the structure
in a 2:1:1 ratio. The complete ordering in the anion sublattice is
accompanied by an A-cation ordering: the smaller R atoms occupy
the positions with CN¼9, whereas the positions with CN¼10, 12
between the R cations is about 4.5 A (along c) and may be
responsible for the weak antiferromagnetic interactions between
the rare-earth cations. However, the separations along a and b
exceed 6.5 A, thus impeding a three-dimensional (long-range)
˚

2
854
A.M. Abakumov et al. / Journal of Solid State Chemistry 183 (2010) 2845–2854
ordering in the R sublattice. According to our observations, the
magnetic moments of the R cations remain disordered down to 2 K.
Despite the complete cation and anion ordering at room
temperature, the oxygen atoms in the anion-deficient layers
should become mobile at high temperatures, similar to the
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