Non-stoichiometry and the magnetic structure of Sr2CrO3FeAs

epl draft
Non-stoichometry and the magnetic structure of Sr2CrO3FeAs
Marcus Tegel1, Franziska Hummel1, Yixi Su2, Tapan Chatterji3, Michela Brunelli4 and Dirk
Johrendt1
1 Department Chemie, Ludwig-Maximilians-Universita¨t Mu¨nchen, Butenandtstraße 5-13 (Haus D), 81377 Mu¨nchen,
Germany
2 Ju¨lich Centre for Neutron Science, IFF, Forschungszentrum Ju¨lich, Outstation at FRM II, Lichtenbergstraße 1,
D-85747 Garching, Germany
3 Ju¨lich Centre for Neutron Science, Forschungszentrum Ju¨lich, Outstation at Institut Laue-Langevin, BP 156, 38042
Grenoble Cedex 9, France
4 Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France
Dedicated to Dr. Klaus Ro¨mer on the occasion of his 70th birthday
PACS 74.10.+v – Superconductivity, potential candidate
PACS 71.27.+a – Strongly correlated electron systems
PACS 61.05.fm – Neutron diffraction
PACS 75.50.Ee – Antiferromagnetics
Abstract. - The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was
synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to
previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found
within the FeAs4/4 layer (93 ± 1% Fe : 7 ± 1% Cr). We suggest that the partial Cr-doping at
the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity
in this compound. Additional experiments via neutron polarization analysis revealed short-range
spin correlations below ∼ 100 K and long-range antiferromagnetic ordering below TN = 36 K
with a magnetic propagation vector of q = ( 12 ,
1
2 , 0). The Cr
3+ ions form a collinear magnetic
structure of the C-type in the magnetic space group CPmma′ (a′ = a− b,b′ = a + b, c′ = c),
where Cr3+-ions occupy the 4g (0, 14 , z) Wyckoff position. The magnetic moments are aligned
along the orthorhombic a′-axis. At 3.5 K, an ordered magnetic moment of 2.75± 0.05 µB for the
Cr3+-sublattice was refined.
Introduction. – Since the discovery of superconduc-
tivity in tetragonal layered iron arsenides with ZrCuSiAs-
type, ThCr2Si2-, or PbFCl-type structures [1–3] and crit-
ical temperatures up to 55 K [4,5], immense progress has
been made regarding the rich physical and structural phe-
nomena occurring in this new class of superconductors.
[6, 7]. But beyond great efforts to understand the under-
lying physics, the search for new compounds with similar
FeAs layers is also important in order to widen the ma-
terial basis and perhaps to increase the critical tempera-
tures.
The Tc’s of the known iron arsenides increase with
the anisotropy of their crystal structures, which is rather
small [8] compared with the cuprates, whose Tc’s are also
higher. Even though we should be very careful in transfer-
ring principles from the cuprates, FeAs-compounds with
larger inter-layer distances are of particular interest and
recently a number of new compounds with structures de-
rived from copper sulfides with perowskite-like blocks were
synthesized and studied. The first was Sr3Sc2O5Fe2As2
[9] with the known structure of Sr3Fe2O5Cu2S2 [10]. The
iron arsenide is not superconducting and shows neither a
structural anomaly nor magnetic ordering as found in the
ZrCuSiAs- and ThCr2Si2-type parent compounds [11,12].
Superconductivity at 17 K has been discovered in the iron
phosphide Sr2ScO3FeP [13] with the Sr2GaO3CuS-type
structure [14]. This Tc is considerably higher in compari-
son with the ZrCuSiAs-type phosphide oxides like LaFePO
(4-7 K) and may promise even higher values in the anal-
ogous arsenides. Indeed, the arsenide Sr2VO3FeAs with
p-1
ar
X
iv
:0
91
1.
04
50
v2
[
co
nd
-m
at.
su
pr
-co
n]
2
4 F
eb
20
10

M. Tegel et al.
Tc = 37 K was found [15] which proved the potential
of such compounds. The isotypic chromium compound
Sr2CrO3FeAs (Figure 1) [16, 17] is not superconducting,
but exhibits antiferromagnetic ordering of the Cr3+ mo-
ments according to susceptibility measurements, whereas
57Fe-Mo¨ssbauer spectra revealed non-magnetic iron atoms
in this compound [17,18].
Fig. 1: Crystal structure of Sr2CrO3FeAs. Space group:
P 4nmm, origin choice 1.
The synthesis of single phase samples of the ’21311’-
compounds has turned out to be difficult and most
of the published X-ray powder patterns reveal signif-
icant amounts of impurity phases. This is especially
true in the case of the superconducting compound
Sr2VO3FeAs and very pronounced in the recently reported
Sr2(Mg,Ti)O3FeAs (Tc = 39 K) [19], which is hardly the
main phase of the sample. Such multi-phase samples cast
serious doubts about the true chemical composition of the
superconducting fractions.
We have synthesized almost single phase samples of the
chromium 21311-compound Sr2CrO3FeAs, which allow a
more precise determination of the structure. By neutron
scattering we are able to distinguish between chromium
and iron very well in contrast to X-ray diffraction. In
this letter we report the re-determination of the crys-
tal structure and the antiferromagnetic spin structure of
Sr2CrO3FeAs. Our results shed light on the absence of su-
perconductivity in Sr2CrO3FeAs and the chemical nature
of the 21311-type compounds containing similar d-metals
in general.
Experimental. – Sr2CrO3FeAs was synthesized by
heating stoichiometric mixtures of strontium, chromium,
iron oxide and arsenic oxide in alumina crucibles sealed in
silica ampoules under an atmosphere of purified argon in
four separate batches of 1 gram. Each mixture was heated
to 1173 K at a rate of 80 K/h, kept at this temperature
for 60 h and cooled down to room temperature. The prod-
ucts were homogenized in an agate mortar, pressed into
pellets and sintered at 1323 K for 60 h. The batches were
then united, reground, pressed into pellets of 14 mm in
diameter and sintered together at 1323 K for 50 h. The
obtained black crystalline product Sr2CrO3FeAs is stable
in air.
Powder diffraction patterns at various temperatures
were recorded at the high flux powder diffractometer
D20 at Institut Laue-Langevin (Grenoble, France) with
0.187 nm incident wavelength. Rietveld refinements of
the D20 nuclear scattering patterns were performed with
the TOPAS package [20] using the fundamental parameter
approach as reflection profiles (convolution of appropriate
source emission profiles with axial instrument contribu-
tions as well as crystallite microstructure effects). In or-
der to describe small peak half width and shape anisotropy
effects, the approach of Le Bail and Jouanneaux [21] was
implemented into the TOPAS program and the according
parameters were allowed to refine freely. Preferred ori-
entation of the crystallites was described with the March
Dollase function. The Fe:Cr ratio on both the iron and
the chromium site were also allowed to refine freely. Ad-
ditionally, powder diffraction patterns were recorded using
polarized neutrons (0.474 nm incident wavelength) at the
polarized spectrometer DNS at FRM II (Garching, Ger-
many) at different temperatures. An unambiguous sepa-
ration of nuclear coherent, spin incoherent and magnetic
scattering contributions simultaneously over a wide scat-
tering angle has been achieved via neutron polarization
analysis from the well established xyz-method [22]. Both
the nuclear and magnetic DNS powder patterns were in-
terpolated, scaled and converted to a format with a con-
stant step width and refined with the GSAS package [23].
The standard Gaussian profile function with asymmetry
corrections (CW profile function 1) was used as reflection
profiles. To obtain accurate changes of the ordered mag-
netic moments at different temperatures, only the gaus-
sian parameters U and W were refined using the 3.5 K
pattern and held constant for all other temperatures. The
scaling factor obtained from the nuclear scattering contri-
bution was corrected for the number of formula units per
unit cell and taken as reference for the magnetic scatter-
ing contribution. A shifted Chebychev series of 9th or-
der was used as background function and the parameters
were allowed to refine freely for each temperature. The
structural parameters for all refinements were taken from
an additional refinement (combined nuclear and magnetic
scattering) performed with GSAS using the low tempera-
ture diffraction data taken at D20.
p-2

Non-stoichometry and the magnetic structure of Sr2CrO3FeAs
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 00 . 00 . 5
1 . 01 . 52 . 0
2 � ( d e g r e e s )

Intensity (105 co
unts) D 2 0 , 3 0 0 K , λ = 0 . 1 8 7 n m S r 2 C r O 3 F e A sF e A sS r O
Fig. 2: (Color online) D20 neutron powder pattern (blue)
and Rietveld fit (red) of Sr2CrO3FeAs at 300 K (space group
P 4nmm). The angle range from 54.0 to 55.1
◦ 2θ was excluded
from the refinement due to the presence of an unknown impu-
rity phase and the angle range from 73.9 to 75.7◦ 2θ due to a
faulty detector segment.
Results and Discussion. – Figure 2 shows the neu-
tron powder pattern of Sr2CrO3FeAs measured at the
D20 diffractometer. It could be fitted successfully with
a tetragonal Sr2GaO3CuS-type Sr2CrO3FeAs main phase
and FeAs, as well as SrO as minor impurity phases. Yet
another small impurity phase could not be identified and
its largest peak was therefore excluded from the refine-
ment. No structural phase transition was observed down
to 6.5 K. The crystallographic data at 300 K (Table 1)
are in good agreement with our previously published X-
ray data [17]. However, the refinement of the neutron
data unambiguously shows a mixed occupancy of iron and
chromium at the iron site 2a (93 ± 1% Fe : 7 ± 1% Cr),
while no mixed occupancy at the chromium site or any
oxygen deficiency were detected within one standard de-
viation. Within one standard deviation, full occupancy
of all sites was found in X-ray diffraction experiments.
These findings suggest partial interchangeability of the 3d-
metals Fe and Cr in the FeAs layers. This Cr-doping of
the Fe-site could explain why Sr2CrO3FeAs does neither
display a spin-density-wave anomaly nor superconductiv-
ity. The situation is very similar to that in Cr-doped
BaFe2As2, where small amounts of chromium at the iron
site in BaFe2−xCrxAs2 strongly effect the SDW anomaly
and it is apparently detrimental to superconductivity [24].
Such mixing of iron with other d-metals in the FeAs
layers may also occur in other 21331 or 32522-systems,
and we do not rule out that the alleged stoichiometric
37 K superconductor Sr2VO3FeAs [15] is in fact a doped
compound likewise. Our data do not show any oxygen
deficiencies, unlike Sr2VO3−δFeAs, which was recently
reported [25]. However, no detailed structural data of
these compounds were published.
Table 1: Crystallographic data of Sr2CrO3FeAs (D20).
temperature (K) 300
wave length (nm) 0.187
space group P 4nmm (o1)
a (pm) 391.71(7)
b (pm) = a
c (pm) 1578.0(2)
V (nm3) 0.2421(1)
Z 2
data points 1446
excluded regions (◦ 2θ) 54.0-55.1, 73.9-75.7
reflections (main phase) 118 (1 excluded)
profile variables (main phase) 6
anisotropy variables 24
atomic variables (main phase) 15
background variables 12
variables of impurity phases 21
other variables 6
d range 0.966− 15.780
RP , wRP 0.0304, 0.0452
Rbragg 0.0110
wght. Durbin-Watson d stat. 0.812
Atomic parameters:
Sr1 2c (0, 12 , z)
z = 0.8059(3)
Uiso = 131(9)
Sr2 2c (0, 12 , z)
z = 0.5856(3)
Uiso = 145(12)
Cr/Fe - occ. 0.99(1):0.01(1) 2c (0, 12 , z)
z = 0.3104(4)
Uiso = 69(19)
Fe/Cr - occ. 0.93(1):0.07(1) 2a (0, 0, 0)
Uiso = 85(9)
As 2c (0, 12 , z)
z = 0.0883(2)
Uiso = 192(13)
O1 4f (0, 0, z)
z = 0.2943(2)
Uiso = 93(7)
O2 2c (0, 12 , z)
z = 0.4308(3)
Uiso = 172(12)
Sel. bond lengths (pm):
Sr–O 244.3(6)×1; 251.7(3)×4
272.6(4)×4; 278.2(1)×4
Cr–O 190.0(8)×1; 197.5(1)×4
Fe–Fe 277.0(1)×4
Fe–As 240.4(3)×4
Sel. bond angles (deg):
As–Fe–As 109.6(1)×4; 109.1(2)×2
O–Cr–O 89.0(1)×4; 97.4(2)×4;
165.2(4)×2
p-3

M. Tegel et al.
Recently reported susceptibility measurements on
Sr2CrO3FeAs [17] revealed Curie-Weiss behavior above
150 K with an effective magnetic moment µexpeff =
3.83(3) µB . As this is typical for Cr3+ ions in the 4F3/2
state (µcalceff = 3.87 µB), we expected that the observed
magnetism comes from the chromium atoms only, whereas
the iron sites carry no magnetic moments. Furthermore,
a drop of the χ(T ) plot below 31 K together with a large
negative Weiss-constant Θ = −141(3) K indicated anti-
ferromagnetic ordering. Neutron patterns measured at
the D20 diffractometer showed additional peaks appear-
ing below 35 K (not shown). These magnetic reflections
could be indexed with the primitive tetragonal cell |a′| =
|a−b| =
√
2 · |a|, |c′| = |c| according to a magnetic prop-
agation vector q = ( 12 ,
1
2 , 0) based on the original tetrag-
onal unit cell. In order to separate nuclear and magnetic
scattering, we performed experiments with polarized neu-
trons at DNS. Sharp magnetic reflections are discernible
below TN , indicating long-range antiferromagnetic order-
ing of the Cr-sublattice (Figure 3). Furthermore, mag-
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0
2 8 K
1 6 0 K5 0 K3 5 KRelative intensity

2 θ ( d e g r e e s ) 3 . 5 K
1 0 0 K
Fig. 3: (Color online) Evolution of the magnetic scattering
contribution of Sr2CrO3FeAs at different temperatures.
netic diffuse scattering due to short-range spin correla-
tions can be clearly observed above TN . At temperatures
above 120 K, Sr2CrO3FeAs displays Curie-Weiss-like para-
magnetic scattering only. Short-range antiferromagnetic
spin correlations begin to emerge below ∼ 100 K. The
observed asymmetric diffuse scattering profile strongly
suggests that the short-range spin correlations are two-
dimensional in nature.
From the observed q-vector ( 12 ,
1
2 , 0) we assumed a
checkerboard-like spin arrangement of the C-type, which
is reversed between the adjacent chromium layers along c.
Since the layers are at the coordinates z = ±0.31, no G-
type pattern is possible. A first expected spin alignment
along c did not reproduce the observed data, therefore we
developed models with orientations within the (ab)-plane.
The by far best fit was found with the alignment along
[a− b] and reversed along a and b based on the original
tetragonal cell. This arrangement required the orthorhom-
bic magnetic space group CPmma′ (Litvin No. 67.15.591)
[26], where Cr3+ occupies the 4g (0, 14 , z) Wyckoff posi-
tion and the magnetic moments of Cr3+ align along the
orthorhombic a′-axis, building up a checkerboard arrange-
ment in each Cr layer at both heights z and z (Figure 4).
By testing different magnetic space groups and different
Fig. 4: (Color online) Magnetic ordering of the Cr atoms. Cr
atoms at height z are depicted as solid, Cr atoms at height z
as checkered spheres. The tetragonal crystallographic unit cell
is depicted as solid black, the orthorhombic magnetic cell as
dashed blue square. The transformation from the tetragonal
to the orthorhombic cell is a′ = a− b,b′ = a + b, c′ = c with
an origin shift of − 14 ,
1
4 , 0.
spin orientations, any other model for the magnetic order-
ing in a direct magnetic subgroup derived from the crys-
tallographic space group Cmme could be unambiguously
ruled out. Sections of the nuclear and magnetic powder
patterns recorded at 3.5 K and corresponding Rietveld re-
finements are depicted in Figure 5. The ordered magnetic
moment of a Cr3+-ion was refined to be 2.75(5) µB at
3.5 K, which is close to the expected 3 µB . The evolution
of the magnetic ordering becomes evident from the or-
der parameter and its temperature dependence is depicted
in Figure 6. It follows the power law a ∗ (TN−TTN )
β with
a = 2.84(3), TN = 36.0(5) K and β = 0.22(2). The expo-
nent β is between the idealized 2D and 3D Ising values ( 18
and 516 , respectively), which agrees with the fact that the
magnetic arrangement of each Cr-layer at height z is cou-
pled with the corresponding z-layer and the magnetism
therefore can be explained neither strictly two- nor three-
dimensionally.
p-4

Non-stoichometry and the magnetic structure of Sr2CrO3FeAs
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0- 2 . 00 . 02 . 0
4 . 06 . 08 . 01 0 . 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0
- 1 . 0- 0 . 50 . 00 . 51 . 0
1 . 52 . 02 . 5
��
��� ����������������� 2 ��������������
��
�� ���
�
Intensity (arb. units)
����������������������� ������λ�
���
�
������������� �� ��

Fig. 5: (Color online) Magnetic and nuclear reflections of
Sr2CrO3FeAs (blue) and Rietveld fit (red) at 3.5 K measured
at the polarized spectrometer DNS. The magnetic space group
is CPmma′ and the crystallographic space group P 4nmm. The
miller indices of the magnetic reflections were transformed to
the tetragonal cell for comparability. Red markers: Reflection
conditions of both space groups.
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 50 . 00 . 51 . 01 . 52 . 02 . 5
3 . 0 ordered magnetic moment (µ B
)
T e m p e r a t u r e ( K )
Fig. 6: Variation of the refined ordered magnetic moment of the
Cr-sublattice with temperature. The temperature dependence
follows the simple power law a ∗ (TN−TTN )
β (red curve).
Conclusion. – We have synthesized the iron arsenide
oxide Sr2CrO3FeAs and re-determined its crystallographic
and magnetic structure by neutron diffraction experiments
at the D20 and DNS diffractometers. A mixed occupancy
of chromium and iron in the FeAs layers was found, which
points up the ability of substitution between similar 3d-
metals in these compounds. We suggest that this Cr-
doping may also be the reason for the absence of a SDW
anomaly and superconductivity. Such non-stoichiometries
may also occur in similar compounds like the supercon-
ducting Sr2VO3FeAs and may also be responsible for the
different physical behavior of these compounds when com-
pared with the 1111- and 122-iron arsenides. Deviations
from the ideal stoichiometry has especially to be taken
into account when discussing their electronic structures.
Sr2CrO3FeAs shows short-range spin correlations from the
Cr3+-ions below ∼ 100 K and long-range antiferromag-
netic ordering below TN = 36.0(5)K. The magnetic struc-
ture is of the C-type with the Cr-spins oriented parallel to
[a− b] with all nearest-neighbor Cr3+ moments antifer-
romagnetically aligned, thus forming a checkerboard ar-
rangement.
∗ ∗ ∗
M. T. would like to thank Dr. Klaus Ro¨mer for finan-
cial support. This work was financially supported by the
German Research Foundation (DFG).
REFERENCES
[1] Kamihara Y., Watanabe T., Hirano M. and Hosono
H., J. Am. Chem. Soc, 130 (2008) 3296.
[2] Rotter M., Tegel M. and Johrendt D., Phys. Rev.
Lett., 101 (2008) 107006.
[3] Wang X. C., Liu Q. Q., Lv Y. X., Gao W. B., Yang
L. X., Yu R. C., Li F. Y. and Jin C. Q., Solid State
Commun., 148 (2008) 538.
[4] Johrendt D. and Po¨ttgen R., Angew. Chem. Int. Ed.,
47 (2008) 4782.
[5] Ren Z.-A. et al., Chin. Phys. Lett., 25 (2008) 2215.
[6] Ishida K., Nakai Y. and Hosono H., J. Phys. Soc. Jpn.,
78 (2009) 062001.
[7] Chu P. C. W., Koshelev A., Kwok W., Mazin I.,
Welp U., Wen H.-H., (Eds.), Superconductivity in Iron-
Pnictides, Vol. 469 (Elsevier B.V. , Amsterdam) 2009.
[8] Ni N., Bud’ko S. L., Kreyssig A., Nandi S., Rustan G.
E., Goldman A. I., Gupta S., Corbett J. D., Kracher
A. and Canfield P. C., Phys. Rev. B, 78 (2008) 014507.
[9] Zhu X. Y., Han F., Mu G., Zeng B., Cheng P., Shen
B. and Wen H. H., Phys. Rev. B, 79 (2009) 024516.
[10] Zhu W. J. and Hor P. H., J. Solid State Chem., 134
(1997) 128.
[11] de la Cruz C. et al., Nature, 453 (2008) 899.
[12] Rotter M., Tegel M., Schellenberg M., Hermes
W., Po¨ttgen R. and Johrendt D., Phys. Rev. B, 78
(2008) 020503(R).
[13] Ogino H., Matsumura Y., Katsura Y., Ushiyama K.,
Horii S., Kishio K. and Shimoyama J.-i., Supercond. Sci.
Tech., 22 (2009) 075008.
[14] Zhu W. J. and Hor P. H., Inorg. Chem., 36 (1997) 3576.
[15] Zhu X., Han F., Mu G., Cheng P., Shen B., Zeng B.
and Wen H.-H., Phys. Rev. B., 79 (2009) 220512(R).
[16] Ogino H., Katsura Y., Horii S., Kishio K. and Shi-
moyama J.-i., Superconductor Science and Technology, 22
(2009) 085001.
p-5