Extension of the 1201 family to strontium-rich chromite and ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5

Article abstract of DOI:10.1006/jssc.2002.9617

Two new Sr-rich 1201 -type oxides, Bi_0.4Sr_2.5Cr_1.1O_4.9 and Bi_0.4Sr_2.5Fe_1.1O₅ have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi,M) and Sr species within the intermediate rock-salt layer [Bi_0.4M_0.1Sr_0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe³⁺ and 50% Fe⁴⁺ (also confirmed by Moessbauer), and chromium could be a mixture of Cr³⁺ and Cr⁶⁺ sitting inthe perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite.

Full text of DOI:10.1006/jssc.2002.9617

Journal of Solid State Chemistry 167, 48–58 (2002)
doi:10.1006/jssc.2002.9617
Extension of the ‘‘1201’’ Family to Strontium-Rich Chromite
and Ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5
M. Allix, D. Pelloquin,1 F. Studer, N. Nguyen, A. Wahl, A. Maignan, and B. Raveau
! ! !
Laboratoire de Cristallographie et Sciences des Materiaux, UMR 6508-CNRS, ISMRAet Universit e de Caen 6, Boulevard du Marechal Juin,
14050 CAEN Cedex, France
Received February 15, 2002; accepted April 12, 2002
slabs. It is indeed observed that intermediate RS slab
Two new Sr-rich ‘‘1201’’-type oxides, Bi0.4Sr2.5Cr1.1O4.9 and exhibits a 1:1 ordering of the (Bi,M) and Sr atoms.
Bi0.4Sr2.5Fe1.1O5 have been synthesized. These compounds,
intergrowths of double rock-salt layers with single perovskite
layers, show a 1:1 ordering between (Bi,M) and Sr species within
the intermediate rock-salt layer [Bi0.4M0.1Sr0.5O]. The XANES
study shows that bismuth is mainly trivalent, whereas iron is
mixed valent containing 50% Fe3+ and 50% Fe4+ (also
In contrast to copper, cobalt and manganese, ferrites and
chromites do not show any Bi-2201 or Bi-1201 structures
although Bi2Sr3Fe2O9 2212-type ferrite was synthesized
(10), isotypic to the corresponding manganite (11). In order
to enlarge the chemistry of these layered compounds, we
have explored the systems Bi–Sr–Fe–O and Bi–Sr–Cr–O.
We describe herein the synthesis, crystal structure, and
magnetotransport properties of two new Sr-rich 1201
oxides, Bi0.4Sr2.5Fe1.1O5 and Bi0.4Sr2.5Cr1.1O4.9, character-
ized again by 1:1 ordering between bismuth and strontium
.
confirmed by Mossbauer), and chromium could be a mixture
of Cr3+ and Cr6+ sitting in the perovskite and rock-salt-type
sites, respectively. Both compounds exhibit antiferromagnetic
interactions. The Cr-phase is a strong insulator, whereas the Fe-
phase exhibits a semi-conductor-like resistivity whose value at
room temperature is close to that of isotypic cobaltite. # 2002 in the intermediate RS layer. We also show the existence of
Elsevier Science (USA)
continuous solid solutions between the pure iron 1201-
phase and other 1201 oxides (M=Mn, Cr, and Co).
Key Words: layered oxides; Xanes and Mossbauer spectro-
scopies; TEM analyses.
CHEMICAL SYNTHESES
INTRODUCTION
In a first step, the investigations of the 1201 phases in the
Bi–Sr–Fe–O and Bi–Sr–Cr–O systems were performed on
After the discovery of superconductivity in the ‘‘2201’’-
type structure of Bi2Sr2CuO6 (1), trivalent bismuth has
been considered as a potential element for the stabilization
of layered structures leading to closely related cuprates
Bi2An+1CunO2n+4 (2, 3) with higher Tc; and to isotypic
cobaltite, manganite and ferrite with the 2201 structure
(4–8). Recent investigations of the systems Bi–Sr–M–O
with M=Mn and Co, have shown the important role of
bismuth for the synthesis of new strontium-rich ‘‘1201’’-
type oxides Bi0.4Sr2.5M1.1O5ꢀd (8, 9). Like the classical
‘‘1201’’ structure, these compounds exhibit single perovs-
kite layers intergrown with double rock-salt (RS) layers.
However, they differ from the latter by the nature of the
RS layers, which are formed of an intermediate slab
containing mainly Bi3+ and Sr2+ and some additional
cobalt or manganese, sandwiched between two strontium
0
the basis of the nominal compositions Bi0.57xSr2.57x
M17x O5ꢀd (M=Cr, Fe), varying x; x0; and x00 from 0 to
00
0.2 and controlling d through the SrO/SrO2 ratio. In a
second step, syntheses were carried out with nominal
compositions corresponding to the average cation content
obtained from EDS analyses. The same method was used
0
to study the solubility range Bi0.5ꢀxSr2.5ꢀx Fe1.1ꢀyMyO5ꢀd
(M=Cr, Mn, Co).
Each sample was prepared in a dry box, starting from
the appropriate mixture of oxides Bi2O3, M2O3 (M=Fe,
Mn, Cr), Co3O4, SrO and SrO2. The presence of carbon
dioxide or carbonate groups must absolutely be avoided,
since it may indeed lead to the formation of oxycarbonates
as recently reported in the Sr–Fe–O system (12, 13). To
avoid the carbonate contamination, SrO was freshly
prepared by heating SrO2 or Sr(OH)2, 8H2O at 11001C.
The mixtures, intimately ground in an agate mortar, were
placed in an evacuated quartz ampoule and heated up to a
1 To whom correspondence should be addressed. Fax: 02-31-96-16-00.
E-mail: pelloquin@ismra.fr.
48
0022-4596/02 $35.00
r 2002 Elsevier Science (USA)
All rights reserved.

EXTENSION OF THE ‘‘1201’’ FAMILY
49
temperature close to 10001C at 151C hꢀ1, kept at this conditions, a point located at an energy of 800 eV from
temperature for 24 h and quenched to room temperature.
the edge, where no more EXAFS oscillations were still
observable, was set to unity. Then the intensity of a point
with an energy between 50 and 100 eV from the edge was
recorded on the EXAFS spectrum and reported on the
normalized height.
EXPERIMENTAL TECHNIQUES
The electron diffraction (ED) studies were carried out
using a JEOL 200CX microscope fitted with an eucentric
goniometer (7601) while the high-resolution electron
microscopy (HREM) images and convergent beam electron
diffraction (CBED) pattern were recorded with a TOP-
CON 002B microscope operating at 200 kV and having a
RESULTS AND DISCUSSION
Under the experimental conditions, the study of the
system with closing nominal compositions Bi0.4Sr2.5FeO5
and Bi2/3Sr2.5CrO5 has allowed to stabilize nearly pure
samples. During the synthesis of chromium-based sample,
the formation of bismuth metal as droplets could not be
avoided. The corresponding XRPD patterns (Fig. 1) show
that they are closely related to the tetragonal 1201-type
structure observed for T10.5Pb0.5Sr2CuO5 (15) and
(Bi0.4Sr0.45Co0.15)Sr2CoO4.85 (9). In a second step, the
solubility range Fe1.1ꢀyMy has been scanned substituting
cobalt, manganese, and chromium for iron in order to
study possible magnetic interactions.
(
point resolution of 1.8 A. Energy dispersive spectroscopy
(EDS) analyses were systematically carried out, both
electron microscopes being equipped with KEVEX analy-
zers.
Oxygen content was determined by chemical analysis
using redox titration.
The X-ray powder diffraction (XRPD) data were
collected at room temperature with two vertical diffract-
ometers: a Philips X’PERT PRO and a Philips PW 1830.
Both diffractometers are working with CuKa radiation.
Data were collected by step scanning over an angular range
101r2yr1201 by increments of 0.021 (2y) and were treated
by profile analysis with the program Fullprof (14).
Magnetic susceptibility wðTÞ measurements were inves-
tigated at low temperature (5–400 K) using an AC–DC
SQUID Quantum Design magnetometer and at high
temperature (300–700 K) using the Faraday method with,
in both cases, an applied field of 3 kG. Resistivity
measurements were recorded with a physical properties
measurements system (PPMS) from Quantum Design
(four-probe method).
The EDS analyses carried out on numerous microcrys-
tals of each sample lead to the same average cationic
composition, namely Bi0.4Sr2.5Fe1.1 and Bi0.4Sr2.5Cr1.1
.
This suggests that the structure of these compounds is
close to that previously reported for the cobaltite
Bi0.4Sr2.45Co1.15O5ꢀd (9), i.e., the excess of transition
element with regard to the theoretical 1201 formula is
located in the mixed (Bi,Sr)O plane leading to the actual
formula (Bi0.4M0.1Sr0.5)Sr2MO5ꢀd. On this basis, the
chemical analyses lead to an actual oxygen deficiency d
close to 0 for M=Fe and to 0.1 for M=Cr.
The 57Fe Mo.ssbauer spectrum at room temperature of
the powdered Bi0.4Sr2.5Fe1.1O5 sample was recorded using
a conventional spectrometer operating in the constant
acceleration mode. 57Co/Rh was used as the g-ray source.
The isomer shift (IS) is referred to metallic iron.
The X-ray absorption spectra at Fe K-, Cr K- and Bi L3-
edge were recorded at room temperature in a classical
transmission mode at the EXAFS I station (channel cut
monochromator) using the synchrotron radiation of the
DCI storage ring of LURE (Orsay, France). The energy
resolution at the Fe and the Cr K-edge is estimated to be
1.3 eV and at Bi L3-edge to 5.9 eV whereas the reproduci-
bility of the monochromator position is as high as 0.3 eV.
An iron metal K-edge, chromium metal K-edge or bismuth
metal L3-edge was run simultaneously for precise energy
calibration. The relative energies between various spectra
were established by a careful comparison of the derivative
metal transition spectra. The normalization procedure used
through this work was a standard one: after subtraction of
the same diffusion background on the XANES and
EXAFS spectra, recorded in the same experimental
FIG. 1. Experimental X-ray diffraction patterns for the 1201-ferrite:
Bi0.4Sr2.5Fe1.1O5 (b) and the 1201-chromite: Bi0.4Sr2.5Cr1.1O4.9 (c) in
comparison with the tetragonal 1201-cobaltite: Bi0.4Sr2.45Co1.15O4.85 (a).

50
ALLIX ET AL.
observed for the whole pattern. Thus, the tetragonal
I4=mmm space group can be proposed. Consequently, the
refinements of XRPD patterns of both phases, indexed in
this tetragonal cell, lead to the following parameters:
(
(
a ¼ 5:3833ð1Þ A and c ¼ 17:9639ð2Þ A for M ¼ Fe
and
(
(
a ¼ 5:3696ð1Þ A and c ¼ 17:6970ð3Þ A for M ¼ Cr:
In order to confirm the layer stacking mode of these new
iron- and chromium-based oxides and to determine the
origin of the superstructure, especially the doubling of the
c1201 parameter, an HREM study was performed. Typical
images, recorded in the case of Bi0.4Sr2.5Cr1.1O4.9, are given
in Fig. 3, where high electron density zones are highlighted.
The best orientation to image the layer stacking is the
[100]p, i.e., the [110] orientation in the supercell. At the edge
of the crystal, the contrast consists of three adjacent rows
FIG. 2. Experimental ED [100] oriented pattern (a) and CBED [001]
pffiffiffi
pffiffiffi
oriented pattern (b) indexed in the ðap 2 ꢁ ap 2 ꢁ 2c1201Þ lattice.
Structural Investigations
The reciprocal space was reconstructed for characteristic
crystallites from ED patterns by tilting around the crystal-
lographic axes. The analysis shows that these two new
phases are related to a perovskite-layered structure. The
system of reflections evidences a tetragonal cell with a ¼
pffiffiffi
bEap 2 (ap is the parameter of the cubic perovskite-type
(
structure) and c ¼ 2c1201E18 A while the main condition
limiting the reflections, h k l : h þ k þ l ¼ 2n implies a
I-type lattice. The characteristic ED [100] oriented pattern
is shown in Fig. 2a.
This tetragonal symmetry is confirmed by the comple-
mentary convergent beam electron diffraction (CBED)
study carried out on the [001] oriented patterns (Fig. 2b).
The analysis of direct beam (bright field mode) shows
clearly two perpendicular m mirrors and evidences the
FIG. 3. Experimental HREM image recorded along the [110] (a) and
four-fold axis existence. These symmetry elements are also the [100] (b) directions. Bright dots are correlated to Bi and Sr positions.

EXTENSION OF THE ‘‘1201’’ FAMILY
51
of bright dots separated by a single row of smaller bright
In order to check this structural model (Fig. 4),
dots (Fig. 3a). They are, respectively, correlated to triple structural refinements were carried out from XRPD data
[AO] layers and a single [CrO2ꢀd] layer, in agreement with a in the I4=mmm space group. Taking into account the
1201-type stacking mode. Here again, the cationic compo- partial occupancy of the Bi site by Fe or Cr atoms and
sition suggests the formation of one mixed Bi/Sr layer. The fixing B factors for oxygens, we obtain the final agreement
actual information about the superstructure is given by the factors: RBragg ¼ 0:077 and w2 ¼ 2:96 for iron-based 1201-
[100] oriented crystals. This point is shown in Fig. 3b. At type structure and RBragg ¼ 0:069 and w2 ¼ 2:51 for
the level of the single mixed layer sandwiched between two chromium-based 1201-type structure, respectively. These
[SrO]N layers, the alternation of one bright dot with one results confirm that these new layered oxides exhibit the
(
less bright dot is observed, leading to a periodicity of 5.4 A. 1201-type structure with a Bi–Sr ordering and a partial
pffiffiffi
This contrast is shifted by ap 2=2 in the adjacent RS block substitution of Bi by Fe or Cr in the intermediate layer
(see white arrows in Fig. 3b), explaining the doubling of the [Bi0.4M0.1Sr0.5O] in good agreement with EDS results. The
c1201 parameter. Taking into account the actual (Bi+Cr)/ final refined parameters are given in Table 1 while the main
Sr ratio determined by the EDS analyses and previous interatomic distances (Table 2) show a highly distorted
HREM observations reported in the Bi–Sr–M–O system octahedral environment for iron or chromium atoms in the
(6–10), such a contrast can be interpreted as the formation transition metal–oxygen planes with four short equatorial
of mixed Bi/Cr/Sr layers, especially by a 1:1 ordering distances (M–O(4)), one medium (M–O(3)) and one long
between (Bi,Cr) and Sr species within the intermediate RS (M–O(2)) apical bond distance. From these refinements,
layer, leading to alternating files of (Bi,Cr) and Sr atoms, the oxygen vacancies seem to be located at the level of the
running along the [100] direction. Each ordered layer is intermediate mixed [(Bi,Cr)0.5Sr0.5O] RS layer. Neverthe-
~
sandwiched between two [SrO] layers along c according to less, an accurate study of the structure by neutron
the [SrO][(Bi,Cr)0.5Sr0.5O][SrO] sequence. This structural diffraction will be necessary to clarify the oxygen content
feature leads to the doubling of the c1201-axis in agreement and positions in Bi0.4Sr2.5Cr1.1O4.9.
with the I symmetry. Note that this ordering at the level of In the same way, the partial substitutions of cobalt,
the intermediate [AO] layer has been also reported in manganese, and chromium for iron have been analyzed.
Bi0.4Sr2.45Co1.15O5ꢀd (9)1201-type phases while the quad- The cell parameters evolution versus the composition of
rupling of the c1201 is observed in the case of Bi0.4Sr2.6 the mixed Fe1.1ꢀyMy 1201 is shown in Fig. 5, while the
MnO4.7 (8).
corresponding EDS analyses and main structural para-
meters are listed in Table 3. The non-linear curves (Fe/Co
and Fe/Mn) are explained by the oxygen stoichiometry,
ranging from ‘‘O5’’ to ‘‘O4.8’’ for cobalt and being equal
to ‘‘O4.7’’ for Mn. Among these results, a perfect range of
solubility between iron and manganese, cobalt and
chromium, respectively, is observed. No specific ordering
has been detected at room temperature between the
transition elements whatever the Fe/M ratio.
XANES Study
In order to obtain more information about the main
formal charge and the environment of bismuth, chromium,
and iron species in Bi0.4Sr2.5Fe1.1O5 and Bi0.4Sr2.5Cr1.1O4.9
XANES investigations were carried out.
,
Bismuth L3-Edge Study
XANES spectra at Bi L3-edge for the reference
compounds (Bi2O3 for Bi3+ and NaBiO3 for Bi5+) and
for the 1201-type ferrite and chromite (the same curve has
been registered for both of them) are presented in Fig. 6.
The environments are regular octahedra with a single Bi–O
distance in NaBiO3 (16) and two types of distorted
octahedral coordination in Bi2O3 (17). As expected, an
energy shift between the two reference compounds can be
FIG. 4. Perspective view of the Bi0.4Sr2.5Fe1.1O5 ferrite and the
Bi0.4Sr2.5Cr1.1O4.9 chromite.

52
ALLIX ET AL.
TABLE 1
Refined Structural Parameters for Bi0.4Sr2.5Fe1.1O5 and Bi0.4Sr2.5Cr1.1O4.9
2
B (A )
(
Atom
Site
x
y
z
n
1201 Fe
Bi/Fe
Sr(1)
Sr(2)
Fe
2a
2b
8g
4e
8i
0.0
0.0
0.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.5
0.25
0.0
0.5
1.0(1)
0.2(1)
0.3(1)
0.1(1)
1.0a
1.58(2)/0.42(2)
2
8
4
4
4
4
8
0.1481(1)
0.2500
0.0
O(1)
O(2)
O(3)
O(4)
0.400(4)
0.0
0.5
4e
4e
8f
0.124(1)
0.133(1)
0.25
1.0a
1.0a
1.0a
0.25
1201Cr
Bi/Cr
Sr(1)
Sr(2)
Cr
2a
2b
8g
4e
8i
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.5
0.25
0.0
0.5
0.35(4)
0.0(1)
0.0(1)
0.0(1)
0.3(2)
0.3(2)
0.3(2)
0.3(2)
1.8(1)/0.2(1)
2
0.5
0.0
0.1490(1)
0.2500
0.0
8
4
3.8(1)
O(1)
O(2)
O(3)
O(4)
0.423(4)
0.0
0.5
4e
4e
8f
0.133(1)
0.131(1)
0.25
4
4
8
0.25
aParameter not refined.
seen, in connection with bismuth formal charge. The A shown that B and C peaks can be assigned to the specific
shoulder in the NaBiO3 spectra can be assigned to the transitions 2p3=2-6dðt2gÞ and 2p3=2-6dðegÞ; respectively,
specific transition 2p3=2-6s (18) which can only be seen in due to the splitting of the 6d states into non-bonding
the case of Bi5+ because 6s orbitals are assumed to be oribitals ðt2gÞ and antibonding orbital ðegÞ: These peaks are
empty. Considering now the Bi L3-edge of our 1201 phase, well separated in the two phases because of a better
it appears that the measured energy of the main jump concentration of Bi–O distances compared to the one
midheight (EMJM=6.2 eV) can be compared to the Bi3+ present in Bi2O3.
reference (EMJM=5.9 eV) and well separated from the
Bi5+ reference (EMJM=8.6 eV) as seen in Table 4. It
Iron K-Edge Study
indicates that the bismuth formal charge is nearly equal
to +3. But, considering the not-well-defined A shoulder,
Figure 7a shows the spectra at Fe K-edge registered for
due to the signal-to-noise ratio induced by the small the 1201 ferrite and three references: Fe2O3 for Fe3+ which
amount of Bi in the 1201-type structure, we cannot exclude exhibits
a
distorted octahedral coordination (19),
the presence of mixed formal charge Bi3+/Bi5+ with small SrFeO3ꢀx ðxE0:15Þ representing a mixed Fe3+/Fe4+
quantities of Bi5+. Moreover, Rao and Wang (18) have reference involving another distorted octahedral coordina-
tion (20) and Sr2FeCoO6 (21) for the Fe4+ reference. In a
first step, the measured energy of the midheight of the main
TABLE 2
peak of our 1201-ferrite (Table 4) compared with those
Main Interatomic Distances in Bi0.4Sr2.5Fe1.1O5
obtained for Fe2O3, SrFeO3ꢀx, Sr2FeCoO6 suggests a
and Bi0.4Sr2.5Cr1.1O4.9
mixed Fe3+/Fe4+ formal charge with equal amount of
Fe3+ and Fe4+. From the edge value of our ferrite given
in Table 4, an equivalent amount of Fe3+ and Fe4+ species
can be deduced. Nevertheless, a first pre-edge A can be
observed. It is due to a hybridization of Fe(4p)–O(2p)–
Fe(3d) orbitals. This hybridization is stronger for non-
centrosymmetric environment like tetrahedral environment
compared to octahedral environment (22), and allows
partly the forbidden 1s-3d transition to be visible (23).
Considering our structure, a tetrahedral environment can
be excluded from the small-intensity pre-edge A which is
comparable to the one of the other references, in agreement
with the structural results.
(
Distance (A)
1201-Fe
(
Distance (A)
1201-Cr
M–O
n
Bi/M–O(1)
Bi/M–O(2)
Sr(1)–O(1)
Sr(1)–O(3)
Sr(2)–O(1)
Sr(2)–O(2)
Sr(2)–O(3)
Sr(2)–O(4)
M–O(2)
2.16(2)
2.23(2)
2.745(4)
2.39(2)
2.714(4)
2.726(3)
2.705(2)
2.641(1)
2.39(2)
1.97(2)
1.908(1)
2.27(2)
2.35(2)
2.72(1)
2.32(2)
2.67(1)
2.70(1)
2.70(2)
2.61(0)
2.08(2)
2.11(2)
1.898(1)
ꢁ 4
ꢁ 2
ꢁ 8
ꢁ 2
ꢁ 2
ꢁ 2
ꢁ 2
ꢁ 4
ꢁ 1
ꢁ 1
ꢁ 4
M–O(3)
M–O(4)

EXTENSION OF THE ‘‘1201’’ FAMILY
53
FIG. 5. Evolution of parameters in the system: Bi0.4Sr2.5Fe1.1ꢀyO5ꢀd
.
(
Natoli and co-workers (24) have shown that the peak corresponding to the ferrite are calculated as 2.2 A ðBÞ and
(
energies on a XANES spectra can be linked empirically to 1.92 A ðCÞ; respectively. These values are in agreement with
the M–O distance through the simple relationship: XRPD results with four short equatorial distances at
ðE2E0Þ ꢁ R2 ¼ K which holds for transition metal com- 1.908 A and two longer apical distances (1.97 and 2.39 A).
pounds and high-energy transitions. E is corresponding to Peaks B and C correspond to 1s-4p transitions in the 3d5
the peak energy and R to the M–O distance, whereas E0 L configuration for non-bonding (long distances) and
(
(
%
and K are constants determined for a given edge and antibonding (short distances levels).
environment. To apply the so-called Natoli’s rule to the Fe
K-edge, one needs to determine E0 and K constants
Chromium K-Edge Study
through a precise knowledge of the Fe–O distances in the
reference oxides Fe2O3 (19) and SrFeO2.85 (20) as shown in
The chromium K-edge of Bi0.4Sr2.5Cr1.1O4.9 and of the
Table 4. The Fe–O distances (noted B and C in Fig. 7a) three reference oxides, NdCrO3 for Cr3+, CrO2 for Cr4+

54
ALLIX ET AL.
TABLE 3
EDS Results and Main Structural Parameters for the Series
Bi0.4Sr2.5Fe1.1ꢀyO5ꢀd (M=Co, Mn, and Cr)
TABLE 4
Formal Charge (FC), dM2O, and EMJM at Bi L3-edge, Fe
K-edge and Cr K-edge for references, Bi0.4Sr2.5Fe1.1O5 and
Bi0.4Sr2.5Cr1.1O4.9
(
a (A)
(
c (A)
EDS composition
Compounds
Fe2O3
FC
+3
EMJM (eV) 70.3 eV
dM2O
Bi0.4Sr2.5Fe1.1O5
5.3833(1)
5.3815(6)
5.3784(3)
5.3748(1)
5.3696(1)
5.3735(4)
5.3581(2)
5.3373(3)
5.3802(4)
5.3778(6)
5.3759(8)
17.964(0)
17.895(1)
17.835(3)
17.774(0)
17.6979(3)
17.929(6)
17.929(5)
17.949(5)
17.947(7)
17.924(1)
17.910(4)
Bi0.4Sr2.5Fe0.8Cr0.3O5ꢀd
Bi0.4Sr2.5Fe0.55Cr0.55O5ꢀd
Bi0.4Sr2.5Fe0.24Cr0.76O5ꢀd
Bi0.4Sr2.5Cr1.1O4.9
13.7
1.94 ꢁ 3
2.11 ꢁ 3
1.92 ꢁ 4
1.98 ꢁ 2
1.92 ꢁ 6
1.91 ꢁ 4
1.97 ꢁ 1
2.39 ꢁ 1
1.97 ꢁ 4
SrFeO2.85
+3.7
15.1
Bi0.4Sr2.5Fe0.8Co0.3O5ꢀd
Bi0.4Sr2.5Fe0.54Co0.56O5ꢀd
Bi0.4Sr2.5Fe0.27Co0.83O5ꢀd
Bi0.4Sr2.5Fe0.79Mn0.31O5ꢀd
Bi0.4Sr2.5Fe0.54Mn0.56O5ꢀd
Bi0.4Sr2.5Fe0.32Mn0.78O5ꢀd
Sr2FeCoO6
Bi0.4Sr2.5Fe1.1O5.0
+4
+3.4
15.6
14.5
NdCrO3
CrO2
+3
+4
+6
14.0
16.9
1.98 ꢁ 2
1.78 ꢁ 4
2.08 ꢁ 2
1.54–1.71 (Cr(1))
K2Cr2O7
18.3 in tetrahedral
configuration
and K2Cr2O7 for Cr6+ formal charges, are shown in
Fig. 7b. The energy of main jump midheight (Table 4)
allows the oxidation state of chromium in Bi0.4Sr2.5-
Cr1.1O4.9 to be fixed at a higher value than Cr3+, namely
close to +3.4. From this result, mixed valence states
between Cr3+ and Cr4+ or Cr3+ and Cr6+ can be
proposed for chromium species. In fact, as it has been
already seen in Hg0.5Cr0.5Sr2CuO4.82 (23), the Cr part
substituted to Bi ions might be hexavalent and the Cr
species in the [CrO2] layers could be trivalent (for instance,
10% of Cr6+ leads to a +3.3 formal charge). Moreover,
the intensity of the pre-peak is in agreement with a
1.55–1.86 (Cr(2))
1.59–1.84 (Cr(3))
1.51–1.75 (Cr(4))
1.90 ꢁ 4
Bi0.4Sr2.5Cr1.1O4.9
+3.4
15.2
2.08 ꢁ 1
2.11 ꢁ 1
NaBiO3
Bi2O3
Bi0.4Sr2.5Fe1.1O5.0
+5
+3
+3
8.6
5.9
6.2
(
distorted octahedral environment of chromium. The E1.98 A) generates a strong white line at 17.6 eV due to
differences between NdCrO3, Bi0.4Sr2.5Cr1.1O4.9 and the occurrence of electronic transitions 1s-4px;y;z at the
K2Cr2O7 arise from the distortion of the oxygen polyhedra same energy. Conversely, the octahedral distortion in
around chromium cation. The regular octahedron in Bi0.4Sr2.5Cr1.1O4.9 with three sets of Cr–O distances (1.90,
(
NdCrO3 with six similar Cr–O distances ((Cr–O) 2.08, and 2.10 A) induces a wide spread of the 1s-4p
transitions along the x-, y- and z-axis, respectively. By
applying Natoli’s rule, we have found two distances equal
(
to 1.91 and 2.2 A, which is in good agreement with the
distances calculated from X-ray diffraction. The pre-peak
of Bi0.4Sr2.5Cr1.1O4.9 (due to hybridization of Cr(3d)–
O(2p)–Cr(4p) orbitals) is observed (inset of Fig. 7b) with
a fine structure of two peaks A1 and A2 (2.48 and 3.96 eV),
due to the crystal field effect on the Cr 3d orbitals:t2g and
eg (separated by a gap of 1.48 eV) in agreement with
the distortion of the octahedron.
.
Mossbauer Study
The spectrum of Bi0.4Sr2.5Fe1.1O5 compound recorded
at room temperature is shown in Fig. 8. It consists of a
quadrupolar doublet characterizing pure electric interac-
tions, composed of two broad lines exhibiting an asym-
metry. This reveals the existence of different paramagnetic
Fe ions. The spectrum is well fitted by accounting for three
components with IS and quadrupolar splitting (QS) values
given in Table 5. The smallest component noted C
FIG. 6. Bismuth L3 edges for Bi2O3, NaBiO3, the chromite and the
ferrite.

EXTENSION OF THE ‘‘1201’’ FAMILY
55
FIG. 8. Mo.ssbauer spectrum of Bi0.4Sr2.5Fe1.1O5 reported at a room
temperature.
between the ZFC and FC mode (3000 G); this pronounced
irreversibility is indicative of a magnetic ordering transition
at low temperature for the magnetic ions. The inverse
susceptibility data (shown in the inset of Fig. 9a) exhibit
only a paramagnetic region within the range 540–690 K.
The curve can be fitted with a Curie–Weiss law:
C
w ¼
;
T ꢀ yp
where C is the Curie constant and yp the paramagnetic
Curie temperature independent susceptibility. The fit gave
C ¼ 1:91 cm3 K molꢀ1 and yp ¼ ꢀ950 K. The magnetic
moment estimated from the Curie–Weiss constant is
meff =3.9 mB f.u.ꢀ1, in good agreement with the value
calculated from the spin-only contribution and chemical
analyses, considering one Cr3+ H.S. in the [CrO2] layers
and 0.1 Cr6+ to the part substituted to Bi ions (23)
(meff ¼ 3:9 mB f.u.ꢀ1). The strongly negative value of yp is
indicative of antiferromagnetic interactions; the onset of
the magnetic order can be seen in Fig. 9a at 130 K. This
assynthesized material exhibits a strong insulating behavior
at room temperature.
FIG. 7. (a) XANES-normalized Fe K-edges at room temperature for
iron references and Bi0.4Sr2.5Fe1.1MyO5. (b) XANES-normalized Fe
K-edges at room temperature for chromium references and Bi0.4Sr2.5
Cr1.1O4.9
.
(IS=0.13 mm sꢀ1QS=0.25 mm sꢀ1), with relative intensity
about 1375%, can be attributed to the Fe4+ part
substituted to Bi ions. Otherwise, the A and B components
exhibit two IS values which are significantly different, and
correspond indeed, to Fe3+ (IS=0.21 mm sꢀ1) and Fe4+
(IS=0.08 mm sꢀ1) contribution, respectively, in the iron
site. The sum of B and C observed intensities lead to about
50% Fe4+ in Bi0.4Sr2.5Fe1.1O5 compound in agreement
with the chemical and XANES analyses.
TABLE 5
Fe Mossbauer Hyperfine Parameters of Bi0.4Sr2.5Fe1.1O5
Phase at Room Temperature
57
.
Physical Properties
Site
IS (mm sꢀ1
)
QS (mm sꢀ1
)
% (75%)
Ion charge
Bi0.4Sr2.5Cr1.1O4.9
A
B
C
0.21
0.08
0.13
0.75
0.67
0.25
47
40
13
+3
+4
+4
The temperature dependence of the magnetic suscept-
ibility is shown in Fig. 9a. One observes a strong difference

56
ALLIX ET AL.
FIG. 9.
solid line is the fit).
w
MðTÞ curves obtained from ZFC and FC MðTÞ curves recorded in 3000 G. Inset: wꢀ1ðTÞ curve (symbols are experimental points whereas
Bi0.4Sr2.5Fe1.1O5
tions and an effective paramagnetic moment meff of
5.0 mB molꢀ1 of product. Considering the ratio 50:50 of
The magnetic susceptibility measurements were per- Fe3+/Fe4+ determined by Mo.ssbauer spectroscopy,
formed in the range 5–600 K (Fig. 9b). The change of XANES and chemical titration, the meff value agrees well
slope in the zero-field cooling curve (ZFC) below 70 K with a combination of low- and high-spin configurations
indicates the existence of an antiferromagnetic transition for Fe4+ and Fe3+, respectively. Taking account of these
with TNE70 K. The hysteresis between ZFC and FC could usual electronic configurations, the meff value is calculated
result from the stronger spin canting induced by the FC as 4.9 mB f.u.ꢀ1
process. For T > 250 K, the linear part of wꢀ1 (inset of
Fig. 9b), fitted with the Curie–Weiss law leads to a negative resistivity of this phase is very high at low temperature,
.
In spite of presence of the Fe3+/Fe4+ mixed valence, the
yp values (ꢀ135 K), typical of antiferromagnetic interac- reaching the maximum measurable value with our setup

EXTENSION OF THE ‘‘1201’’ FAMILY
57
FIG. 10. T dependence of the resistivity registered during cooling in 0 and in 7 T for Bi0.4Sr2.5Fe1.1O5 and Bi0.4Sr2.45Co1.15O4.85 (9).
(107 O cm) at 100 K. The dramatic decrease of r from can be clarified for studying the transport properties.
2 ꢁ 105 O cm at 120 K to 2.2 O cm at 400 K (Fig. 10) shows This work is actually in progress in order to demonstrate
the semiconductor-like behavior of this phase with also some possible magnetic interactions between M
Ea ¼ 1:2 eV. More importantly, the resistivity of this elements.
bismuth ferrite (r300 K=18 O cm) is close to that of the
Bi–Sr 1201 cobaltite (r300 K=3 O cm), i.e., considerably
smaller than that of the manganite Bi0.4Sr2.6MnO4.7 (8)
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