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Low Temperature Crystal Structure and 57Fe Mössbauer
Spectroscopy of Sr3Sc2O5Fe2As2
Marcus Tegela, Inga Schellenbergb, Franziska Hummela, Rainer Pöttgenb and Dirk
Johrendta
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
Reprint requests to D. Johrendt: Email johrendt@lmu.de
Z. Naturforsch. 2009, …
The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was
determined between 300 and 10 K. The lattice parameters of the tetragonal cell
decrease anisotropically according to , which results in a slight
flattening of the As–Fe–As bond angle within the FeAs layers. No indication of a
structural instability could be detected. 57Fe Mössbauer spectroscopic data show a
single signal at 4.2, 77, and 298 K, respectively, subjected to quadrupole splitting.
The isomer shift increases from 0.36(1) mm/s at 298 K to 0.49(1) mm/s at 4.2 K.
No indication for magnetic ordering was found.
Key words: Superconductors, Iron-arsenides, Crystal Structure, 57Fe Mössbauer
Spectroscopy
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Introduction
The discovery of high-Tc superconductivity in a number of iron arsenides with
ZrCuSiAs [1], ThCr2Si2 [2] and Cu2Sb type structures [3] has raised an enormous
and growing interest in these class of materials [4,5]. Especially the quest for new
members of this family of superconductors with even higher critical temperatures
(Tc) attracts the attention of many research groups. Despite the substantial
progress that has been made within only one year, the exact recipe that produces
higher Tc‘s is far from being clear. An empirical relation between the
dimensionality of the crystal structures and the critical temperatures has been
proposed. The largest Tc of up to 55 K appears in the ZrCuSiAs-type compounds
like Sm(O,F)FeAs [6], where the [FeAs] layers are well separated by [SmO]
layers. In the ThCr2Si2-type materials like (Ba,K)Fe2As2, where the FeAs layers
are only separated by barium atoms, the highest observed Tc was 38 K [2] and
finally in LiFeAs with even an smaller layer separation, Tc decreases further down
to 18 K [3]. However, the relation between Tc and the separation of the FeAs
layers, i. e. the two-dimensional character, is not justified by any theoretical
argument and may have its origin in some rather artificial relationships to the
cuprate superconductors. Nevertheless, as long as no other signpost is available,
the search for new iron arsenide materials with low dimensional structures is a
promising task. Recently, new compounds derived from structures with
isoelectronic copper sulfide (CuS) layers [7-9] were reported [10,11], among them
the superconductors Sr2ScO3FeP (Tc = 17 K) [12] and Sr2VO3FeAs (Tc = 37 K)
[13]. However, these compounds become superconducting without doping and
they also lack the supposed preconditions for superconductivity in iron arsenides,
because they show neither structural distortions nor antiferromagnetic ordering.
Thus, the origin of superconductivity remains unclear in these new compounds
just as its absence in the previously reported compound Sr3Sc2O5Fe2As2 [10]. In
order to complete the structural and magnetic data of this compound, we present
the low temperature crystal structure and a 57Fe Mössbauer spectroscopy study of
(Sr3Sc2O5)Fe2As2 in this paper. We also give some crystallographic relationships
that may be useful in the search of new layered iron based superconductors.
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Experimental
Synthesis
(Sr3Sc2O5)Fe2As2 was synthesized by heating a stoichiometric mixture of
strontium, scandium, iron (II) oxide and arsenic oxide in an alumina crucible
sealed in a silica ampoule under an atmosphere of purified argon. The mixture
was heated to 1323 K at a rate of 200 K/h, kept at this temperature for 60 h and
cooled down to room temperature. The product was homogenized in an agate
mortar, pressed into a pellet and sintered at 1323 K for 60 h.
Crystal structure determination
Powder patterns were recorded on a Huber G670 Guinier imaging plate
diffractometer (Cu-Kα1 radiation, Ge-111 monochromator) equipped with a
closed-cycle He-cryostat. Rietveld refinements were performed with the TOPAS
package [14] using the fundamental parameters approach as reflection profiles
(convolution of appropriate source emission profiles with axial instrument
contributions as well as crystallite microstructure effects). In order to describe
small peak half width and shape anisotropy effects, the approach of Le Bail and
Jouanneaux [15] was implemented into the TOPAS program and the according
parameters were allowed to refine freely at 300 and 10 K. Preferred orientation of
the crystallites was described with a March-Dollase function. An empirical 2θ-
dependent absorption correction for the different absorption lengths of the Guinier
geometry was applied. In order to get the accurate course of the lattice parameters,
powder patterns between 10 and 300 K were refined using a similar approach as
described in Ref. [16]. As the background between 10 and 25 degrees 2θ shows
artifacts from the low-temperature configuration of the Guinier diffractometer,
small sections of this range were excluded from the refinements.
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57Fe Mössbauer spectroscopy
A 57Co/Rh source was available for the 57Fe Mössbauer spectroscopy
investigations and the quoted values of the isomer shifts are given relative to this
material. The (Sr3Sc2O5)Fe2As2 sample was placed in a thin-walled PVC
container. The measurement was run in the usual transmission geometry at
temperatures between 4.2 and 298 K. The source was kept at room temperature.
The total counting time was approximately 1 day per spectrum. Fitting of the
spectra was performed with the NORMOS-90 program system [17].
Results and discussion
Crystal chemistry
The refined crystal structure of Sr3Sc2O5Fe2As2 (Fig. 1) at room temperature is
in good agreement with Ref. [10]. A detailed description can also be found in Ref.
[18]. No evidence of any structural instability at low temperatures was detected.
The crystallographic data of Sr3Sc2O5Fe2As2 at 300 and 10 K are compiled in
Table 2. The course of the lattice parameters and As–Fe–As angles on cooling
reveals no anomaly as shown in Figure 3. While the a lattice parameter only
decreases by 0.61 pm, the c lattice parameter decreases by 16.7 pm on cooling
down to 10 K. Thus the thermal contraction of the unit cell is anisotropic
according to . The more pronounced shrinkage of the c axis leads to
a slight increase of the vertical As–Fe–As angle (ε of Fig. 1) by about 1°, i. e. the
FeAs layers become flatter.
The structure of Sr3Sc2O5Fe2As2, space group I4/mmm, Wyckoff sequence
ge3dba is closely related to other structures, which however have different
composition. The different site occupancy variants and the corresponding free z
parameters for the 8g (0, ½, z) and 4e (0, 0, z) sites are listed in Tables 1 and 2.
The first representative of this structure type was the mineral chalcothallite
(K/Tl)2Cu7SbS4 [19]. Later on, the ternary germanides SmNi3Ge3 [20] and
U3Co4Ge7 [21] and the quaternary and quinary compounds listed in Table 2 have
been reported.
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The large structural diversity of this structure type enables formation of
[Fe2As2], [Cu2S2], [Co2Ge2], [Ni2Ge2], and [Pt2P2] tetrahedral layers, which can be
separated by either intermetallic or oxide layers, leading to different bonding
patterns. The tetrahedral layers are built up from the atoms at Wyckoff positions
4e (4th column of Table 2) and 4d (6th column of Table 2). It is interesting to note
that for some representatives the transition metal atoms switch between the 4e and
4d sites, but this does not scale with the course of the electronegativities.
Although the atoms occupy the same Wyckoff positions, nature allows for
variance in the different compounds, i. e. the lattice parameters and the free z
parameters of the 8g and 4e sites. This allows large flexibility for this structural
arrangement. A careful inspection of the z parameters listed in Table 2 readily
reveals differences between the eight compounds. Based on this comparison, we
can regroup the eight compounds into two groups, (i) Sr3Sc2O5Fe2As2,
Sr3Fe2O5Cu2S2, (Sr3Sc2O5)Cu2S2, and (K/Tl)2Cu7SbS4, and (ii) U3Co4Ge7,
SmNi3Ge3, Eu2Pt7AlP2.95, and Eu2Pt7.3Mg0.7P3. Since these structural differences
significantly affect the chemical bonding, these two groups of compounds are
isopointal [22,23] rather than strictly isotypic. The short overview on these eight
compounds manifests the large potential of these and other stacking variants of
tetrahedral layers and one can expect rich crystal chemistry.
57Fe Mössbauer spectroscopy
The 57Fe Mössbauer spectra of the Sr3Sc2O5Fe2As2 sample at various
temperatures are presented in Figure 4 together with transmission integral fits.
The corresponding fitting parameters are listed in Table 3. In accordance with the
presence of a single Fe site we observe a single signal at an isomer shift of δ =
0.36(1) mm/s and an experimental line width Γ = 0.34(1) mm/s subject to
quadrupole splitting of ΔEQ = 0.19(1) mm/s at room temperature. The non-cubic
site symmetry (4–m2) of the iron atoms is reflected in the quadrupole splitting
value. For 77 and 4 K we observe also a single signal at an isomer shift of δ =
0.47(1) mm/s, respectively 0.49(1) mm/s. The increase of the isomer shift with
decreasing temperature can be considered as a second order Doppler shift
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