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The layered iron arsenide oxides Sr2CrO3FeAs and Ba2ScO3FeAs
Marcus Tegela, Franziska Hummela, Sebastian Lacknera, Inga Schellenbergb, Rainer
Pöttgenb, and Dirk Johrendt*a
a Department Chemie und Biochemie, Ludwig-Maximilians-Universität München,
Butenandtstrasse 5–13 (Haus D), 81377 München, Germany
b Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-
Universität Münster, Corrensstrasse 30, 48149 Münster, Germany
Abstract. The iron arsenide oxides Sr2CrO3FeAs and Ba2ScO3FeAs were
synthesized by high temperature solid state reactions and their crystal structures
determined by the X-ray powder diffraction. Their structures are tetragonal (P4/nmm;
Sr2CrO3FeAs: a = 391.12(1) pm, c = 1579.05(3) pm; Ba2ScO3FeAs: a = 412.66(5) pm,
c = 1680.0(2) pm, Z = 2) and isotypic to Sr2GaO3CuS. Iron arsenide layers are
sandwiched between perowskite-like metal oxide layers and separated by ~1600 pm,
which is much larger compared to the ZrCuSiAs-type ‘1111’ iron arsenide
superconductors. The bond lengths and angles within the FeAs layers are adapted to the
space requirements of the oxide blocks. Measurements of the magnetic susceptibility,
electrical resistivity and temperature-dependent crystal structure show no hint for a
structural phase transition or magnetic anomaly in both compounds. Sr2CrO3FeAs
shows Curie-Weiss paramagnetism above 160 K with an effective magnetic moment of
3.83(3) µB in good agreement with the theoretical value of 3.87 µB for Cr
3+ (S = 3/2).
Antiferromagnetic ordering was detected below TN ~ 31 K.
57Fe Mössbauer spectra of
Sr2CrO3FeAs show one single signal that broadens below the Néel temperature due to a
small transferred hyperfine field induced by the magnetic ordering of the chromium
atoms. 57Fe-Mössbauer spectra of Ba2ScO3FeAs show single signals which are only
subject to weak quadrupole splitting.
Running title: Sr2CrO3FeAs and Ba2ScO3FeAs
Keywords: Iron arsenides, Superconductors, Crystal structure, Magnetism, 57Fe
Mössbauer-spectroscopy
*Prof. Dr. Dirk Johrendt
Department Chemie und Biochemie, Universität München
Butenandtstrasse 5–13 (Haus D), D-81377 München, Germany
e-mail: dirk.johrendt@cup.uni-muenchen.de
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Introduction
The discovery of high-Tc superconductivity in iron arsenides [1] has generated an
enormous and growing tide of interest [2], which is reflected in about 600 papers
already published within one year. Besides outstanding superconducting properties with
Tc’s up to 55 K [3] and very high critical fields of at least 70 T [4], it is certainly also
the richness of crystal chemical and physical properties, that has pushed this class of
materials in the spotlight of interest.
Up to now, the family of iron-based superconductors consists of rather simple ternary
or quaternary compounds with long known crystal structures, namely LaOFeAs with the
ZrCuSiAs-type structure [1, 5], BaFe2As2 with ThCr2Si2-type structure [6-8], LiFeAs
with PbFCl-type structure [9, 10] and β-FeSe with the anti-PbO structure [11, 12]. The
currently by far most investigated compounds are the REFeAsO (‘1111’, RE = rare
earth) and AFe2As2 (‘122’, A = alkaline earth) based systems.
Superconductivity emerges in the FeAs layers built up by edge-sharing FeAs4/4
tetrahedra, and is assumed to be unconventional [13, 14] because of the relatively high
critical temperatures and the proximity to structural and magnetic transitions. The latter
ones only occur in the non-superconducting ‘parent compounds’ like LaFeAsO [15] or
BaFe2As2 [6] and have to be at least partially suppressed by doping or by external
pressure before superconductivity arises [1, 8]. This scenario is reminiscent of the high-
Tc cuprates, where also an antiferromagnetic ground-state of a layered parent compound
like La2CuO4 becomes unstable upon doping before superconductivity appears [16].
However, this analogy is limited by the fact, that the doping of a Mott-insulator as in the
case of the cuprates is of a very different nature compared to doping the metallic iron
arsenides. Indeed, the metallic property of the parent compounds and the multi-band
character of the Fermi-surfaces constitute significant differences between the cuprate
and iron arsenide superconductors. It has also been shown that the anisotropy of the iron
arsenide superconductors is significantly lower compared to the cuprates, which have
even higher Tc’s [17].
Even though one should be careful in transferring principles from the cuprates, it is
an important task to search for new iron arsenides with lower dimensional structures,
i. e. with larger FeAs interlayer distances. Several pnictide oxide structures are
considerable candidates [18], but also materials derived form copper sulfide oxides with
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isoelectronic CuS layers [19, 20] are very promising. The first reported compound was
Sr3Sc2O5Fe2As2 [21] with the known structure of Sr3Fe2O5Cu2S2 [20]. The iron arsenide
is not superconducting and shows no structural anomaly or magnetic ordering. Then
superconductivity at 17 K [22] has been found in the iron phosphide Sr2ScO3FeP with
the Sr2GaO3CuS-type [19] structure. This Tc is considerably higher in comparison with
the ZrCuSiAs-Type phosphide oxides like LaFePO (4-7K) and may promise even
higher Tc’s in analogue arsenides.
In this paper, we report on the iron arsenide oxides Sr2CrO3FeAs and Ba2ScO3FeAs,
both sharing the key feature of tetragonal iron arsenide layers with other iron arsenide
superconductors, but the interlayer distances are much larger due to separation by
perowskite-like metal oxide blocks. Their crystal structures were determined by X-ray
powder diffraction and the physical properties were characterized by magnetic
susceptibility and electrical resistivity measurements and 57Fe Mössbauer spectroscopy
experiments. The electronic structure of the new compound Ba2ScO3FeAs is compared
with the parent compounds of the superconducting iron arsenides.
Results
Crystal structure
The crystal structures of Sr2CrO3FeAs and Ba2ScO3FeAs are isotypic to the oxide sulfide
Sr2GaO3CuS [19] in the space group P4/nmm. Figure 1 shows X-ray powder patterns,
which could be completely fitted with one single phase in the case of Sr2CrO3FeAs using
starting parameters from ref. [19]. Minor impurities of Sc2O3 and FeAs were detected in
the Ba2ScO3FeAs sample. Crystallographic data, selected bond lengths and parameters of
the Rietveld fits are compiled in Table 1. Further details of the structure determinations
may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-
Leopoldshafen (Germany) by quoting the Registry No’s CSD-###### (Sr2CrO3FeAs) and
CSD-###### (Ba2ScO3FeAs).
The crystal structure of Sr2CrO3FeAs and Ba2ScO3FeAs is shown in Figure 2. The
FeAs layers perpendicular to the c-axis are separated by strontium atoms from
chromium oxide and strontium oxide layers according to the stacking order
(CrO2)(SrO)(SrO)(CrO2). The Fe−As bonds are shorter and the FeAs4/4 tetrahedra are
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less distorted in Sr2CrO3FeAs in comparison with Ba2ScO3FeAs. Also the Fe−Fe
distance is significantly shorter in the strontium compound (276.6 pm) than in the
barium compound (291.8 pm). This may be attributed to the larger required space of the
barium ions and to a smaller extent the scandium ions, and it shows that the geometry of
the FeAs layer is flexible and can be adapted to the oxide blocks. The interlayer distance
of the iron arsenide layers equals the c lattice parameter for this structure type, thus
Ba2ScO3FeAs (c = 1680 pm) exhibits the highest iron arsenide interlayer distance so far.
The powder patterns recorded at lower temperatures revealed no structural phase
transitions for both compounds down to 10 K. At 10 K, the corresponding a lattice
parameter is decreased by 0.73 pm (0.92 pm) and the c lattice parameter by 8.4 pm
(6.0 pm) for Sr2CrO3FeAs (Ba2ScO3FeAs).
Magnetism and resistivity
Figure 3 shows the magnetic susceptibility of Sr2CrO3FeAs between 1.8 and 360 K.
The compound obeys the Curie-Weiss law above ~ 160 K. An effective magnetic
moment of 3.83(3) µB was found, which is in good agreement with the theoretical spin-
only value of 3.87 µB for Cr
3+ (S = 3/2). The paramagnetic temperature is –141(3) K.
Antiferromagnetic ordering is discernible, the highest susceptibility was reached at TN ~
31 K. The origin of the anomaly around 120 K is not yet clear, but probably due to
traces of ferromagnetic contamination. Ba2ScO3FeAs is Pauli-paramagnetic in the
temperature range between 1.8 and 300 K (not shown). Neither compound shows any
sign of anomaly in the course of the magnetic susceptibility at any applied external
magnetic field. The resistivity of both Sr2CrO3FeAs and Ba2ScO3FeAs is depicted in
Figure 4. Both compounds are poor metals over the whole measured temperature range
and show no anomaly. Both magnetism and resistivity therefore indicate that there is no
occurrence of a spin-density-wave anomaly in any of the compounds.
Mößbauer spectroscopy
The 57Fe Mössbauer spectra collected for the Sr2CrO3FeAs sample at various
temperatures are shown in Figure 5 together with transmission integral fits. The
corresponding fitting parameters are summarized in Table 2. As expected from the
crystal structure (one single Fe site), the spectra at 298, 77, and 40 K are well
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reproduced with single signals, which are subject to weak quadrupole splitting due to
the non-cubic site symmetry (4–m2) of the iron atoms. Within the standard deviations,
the observed isomer shift, the experimental line width, and the quadrupole splitting of
Sr2CrO3FeAs are almost identical with those obtained for Sr3Sc2O5Fe2As2 [23],
indicating a similar electronic situation for the iron atoms in both arsenide oxides. The
observed isomer shift is in line with the recently reported 57Fe data for LaFePO [24],
SrFe2As2 [25], and BaFe2As2 [26]. The increase of the isomer shift with decreasing
temperature (0.29 → 0.45 mm/s) is due to a second order Doppler shift (SODS), well
known for iron compounds.
In contrast to Sr3Sc2O5Fe2As2 [21], where no magnetic ordering is observed down to
4.2 K, the chromium atoms in Sr2CrO3FeAs reveal antiferromagnetic ordering. This is
also reflected in the 57Fe Mössbauer spectra at 20 and 4.2 K, i. e. well below the Néel
temperature. In the magnetically ordered regime we observe significant broadening of
the Mössbauer signal and the fits revealed increased line width and quadrupole splitting
parameters. Since the transferred field (Bhf) is small, independent refinement of all
parameters, δ, Γ, ΔEQ, and Bhf showed strong correlations. In order to get an estimate for
the transferred field, the 4.2 K spectrum was fitted with various fixed values for the
hyperfine field. A reasonable fit was obtained for a fixed hyperfine field of 0.5 T and
the refined values δ = 0.45(1) mm/s, Γ = 0.84(2) mm/s, and ΔEQ = 0.24(1) mm/s. From
these fitting tests we estimate a transferred hyperfine field of 0.5±0.2 T at 4.2 K.
Figure 6 shows the 57Fe Mössbauer spectra of the Ba2ScO3FeAs sample at various
temperatures together with transmission integral fits. The fitting parameters are listed in
Table 2. In agreement with the single Fe site in the crystal structure, the spectra at 298,
77, and 4.2 K show single signals which are subject to weak quadrupole splitting. The
77 K isomer shifts of Ba2ScO3FeAs (Table 2), Sr3Sc2O5Fe2As2 [23], and Sr2CrO3FeAs
are almost identical. We can thus assume a similar electronic situation within the
tetrahedral [Fe2As2] layers of the three structures. They are also comparable to LaFePO
[24] and SrFe2As2 [25]. The increase of the isomer shift with decreasing temperature
(0.35 → 0.50 mm/s) results from a second order Doppler shift. The spectra give no hint
for magnetic ordering of the iron moments down to 4.2 K. Ba2ScO3FeAs and
Sr3Sc2O5Fe2As2 show a lower quadrupole splitting parameter (0.20 mm/s for both
compounds) than Sr2CrO3FeAs (0.42 mm/s) at 4.2 K. Even smaller values of ΔEQ =
0.19 and -0.04 mm/s occur for LaFePO and BaFe2As2, respectively.
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