The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173–1373 K in a resistance furnace and characterized by X-ray powder diffraction with Rietveld analysis: Sr3Fe2O5Cu2S2 type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm. Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like oxide blocks. No structural transition occurs in the temperature range from 10 to 300 K. Magnetic measurements have revealed Curie–Weiss behavior with an effective magnetic moment of 7.79 μB per europium atom in agreement with the theoretical value of 7.94 μB for Eu2+. A drop in the magnetic susceptibility at 5 K indicates possible antiferromagnetic ordering. 151Eu and 57Fe Mössbauer spectroscopic measurements have confirmed a beginning cooperative magnetic phenomenon by showing significantly broadened spectra at 4.8 K compared to those at 78 K.
Since the discovery of superconductivity in LaO1-xFxFeAs  (so-called 1111 type) many other structure types of iron-based superconductors have been identified [2–7]. Among these the 122 type , where layers of edge-sharing FeAs4/4 tetrahedra are separated by a monolayer of ions, and the 32522 type , which exhibits larger perovskite-like interlayers between the FeAs layers. Despite the variety of compounds reported, only few europium compounds are known. Particularly EuFe2As2 has been intensively studied, as its properties are strongly related to those of the isostructural alkaline earth compounds. Similar to AEFe2As2 (AE = Ca, Sr, Ba), the parent compound undergoes a structural transition (at 190 K in the case of EuFe2As2 ), and superconductivity can be induced by suppressing the transition either by applying pressure  or by doping [12–18]. However, EuFe2As2 is a special case among the 122 parent compounds of iron-based superconductors due to the magnetism of europium. In addition to the antiferromagnetic stripe-like ordering of the iron moments at 190 K, it exhibits a second magnetic transition at 19 K where the europium moments order antiferromagnetically (A type) . Although the ordering of the Eu2+ moments is not suppressed when superconductivity is induced, it seems to influence the superconducting properties. In EuFe2As2  under pressure, and in Eu1–xSrxFe2–yCoyAs2 , a small but significant increase in resistivity has been observed at the ordering temperature of the europium moments.
Here we report the existence of the first europium compound of the 32522-type iron pnictides, where the FeAs layers are widely separated by perovskite-like oxide layers. Up to now, the only known iron pnictides with this structure are Sr3Sc2O5Fe2As2 , Ba3Sc2O5Fe2As2 , and Ca3Al2O5–yFe2Pn2 (Pn = As, P) . Only the calcium compounds show superconductivity, and by doping the strontium compound with titanium, traces of superconductivity have been observed in Sr3Sc2–xTixO5Fe2As2 .
A polycrystalline sample of Eu3Sc2O5Fe2As2 was synthesized by heating a stoichiometric mixture of Eu, Sc, FeO and As2O3. The reaction mixture was transferred into an alumina crucible and sealed in a silica ampoule under argon atmosphere. The sample was heated up to 1173 K for 20 h at rates of 50 and 200 K h–1 for heating and cooling, respectively. Afterwards, the sample was ground in an agate mortar, pressed into a pellet, and sintered for 60 h at 1373 K (heated at a rate of 100 K h–1 and cooled at 200 K h–1). This sintering step was performed twice.
Temperature dependent X-ray powder-diffraction patterns between 300 and 10 K (10 K step size) were recorded using a Huber G670 Guinier Imaging Plate diffractometer (CoKα1 radiation). The data were pre-processed with the program hconvert . For Rietveld refinements of the data the topas package  was used applying the fundamental parameter approach for generating the reflection profiles. Spherical harmonics functions were used to describe the preferred orientation of the crystallites. Shape anisotropy effects were described by the approach of Le Bail and Jouanneaux .
Further details of the structure determination may be obtained from: Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, by quoting the Registry No. CSD-429704 [fax: (-49)7247-808-666; e-mail: firstname.lastname@example.org, http://www.fiz-karlsruhe.de/request_for_deposited_data.html].
Magnetic measurements were performed using a SQUID magnetometer (MPMS-XL5, Quantum Design Inc., San Diego, CA, USA).
The 21.53 keV transition of 151Eu with an activity of 130 MBq (2 % of the total activity of a 151Sm:EuF3 source) and a 57Co/Rh source were used for the Mössbauer spectroscopic characterizations. The measurements were conducted in transmission geometry with a commercial nitrogen bath (78 K) and helium flow (5 K) cryostat, while the sources were kept at room temperature. Sample amounts of 100 (151Eu measurements) and 40 mg (57Fe measurements) of Eu3Sc2O5Fe2As2 were placed in thin-walled PVC containers with optimized thicknesses of about 16.4 mg Eu cm–2 and 1.6 mg Fe cm–2. Fitting of the spectra was performed with the normos-90 program system .
Eu3Sc2O5Fe2As2 was obtained as a black polycrystalline air-stable sample. The crystal structure and the sample composition were analyzed by Rietveld refinements of the X-ray powder data at 300 K (Fig. 1): 91 % Eu3Sc2O5Fe2As2, 7 % EuFe2As2, 1 % Sc2O3, and 1 % FeO (wt-%). Crystallographic data are compiled in Table 1.
|Space group||I4/mmm (no. 139)|
|Molar mass, g mol–1||887.334|
|Lattice parameter a, pm||406.40(1)|
|Lattice parameter c, pm||2646.9(1)|
|Cell volume, nm3||0.43717(2)|
|Density, g cm–3||6.74(1)|
|Data points||19 001|
|Reflections (all phases)||161|
|Eu1||2b (0, 0, 1/2)||Uiso = 186(6) pm2|
|Eu2||4e (0, 0, z); z = 0.3583(1)||Uiso = 62(5) pm2|
|Sc||4e (0, 0, z); z = 0.0722(1)||Uiso = 58(7) pm2|
|O1||8g (0, 1/2, z); z = 0.0838(2)||Uiso = 285(13) pm2|
|O2||2a (0, 0, 0)||Uiso = 285(13) pm2|
|Fe||4d (0, 1/2, 1/4)||Uiso = 56(8) pm2|
|As||4e (0, 0, z); z = 0.1991(1)||Uiso = 253(6) pm2|
Eu3Sc2O5Fe2As2 crystallizes in the Sr3Fe2O5Cu2S2-type structure  with lattice parameters a = 406.40(1) pm, c = 2646.9(1) pm, and consists of tetrahedral iron-arsenide layers separated by a perovskite-like separating oxide layer (Fig. 2). Eu1 is coordinated by 8+4 oxide ions similar to perovskites like EuTiO3 , where all 12 Eu-O contacts are equal. Eu2 is eightfold coordinated by four oxide and four arsenide neighbors. Scandium is surrounded by five oxide ions, which form a square pyramid. The Fe–As bonds [243.76(6) pm] and As–Fe–As angles [2 × 112.9(1)°, 4 × 107.8(1)°] in the FeAs4/4 tetrahedra are close to the values found in related compounds like Sr3Sc2O5Fe2As2 [Fe–As bonds: 243.56(9) pm, As–Fe–As angles: 2 × 113.3(1)°, 4 × 107.6(1)°] . However, the tetrahedra are strongly compressed as compared to those in EuFe2As2 [Fe–As 256.45(5) pm, As–Fe–As 2 × 99.4(1)°, 4 × 114.7(1)°]  which becomes superconducting upon doping.
Temperature-dependent X-ray data revealed no signs of a structural transition down to 10 K (Fig. 3). While the lattice parameter a is contracted by only 0.2 % upon cooling, c decreases by 0.6 % but the course of both lattice parameters as a function of temperature does not imply any anomaly which would indicate a structural transition. The reflections at 2θ = 15, 25, and 29° appearing below 250 K are due to the sample environment and the reflection at about 42° in 2θ belongs to the impurity phase EuFe2As2. This phase undergoes a structural transition below 190 K (tetragonal I4/mmm to orthorhombic Fmmm ).
Magnetic susceptibility measurements show Curie–Weiss behavior between 5 and 300° K (Fig. 4). The fit reveals an effective magnetic moment μeff = 13.49(1) μB per formula unit, equivalent to 7.79 μB per europium atom, close to the theoretical value of Eu2+ of 7.94 μB . The negative Weiss constant θ = −9.5(3) K together with the drop in χ(T) near 5 K suggests antiferromagnetic ordering at low temperatures.
151Eu and 57Fe spectra of Eu3Sc2O5Fe2As2 are presented in Figs. 5 and 6 together with transmission integral fits (Table 2). In accordance with the determined crystal structure, the 151Eu signal could be well reproduced by a superposition of two resonances with isomer shifts of –13.26(3) and –10.84(2) mm s–1 indicating Eu2+. The areas of both contributions were kept fixed with the ideal values derived from the multiplicity of the atomic Eu1 and Eu2 sites. With respect to its nearly ideal cuboctahedral coordination by 8+4 oxygen atoms, Eu1 was assumed to be the nucleus of less electron density (more ionic) compared to Eu2 and therefore the origin of the resonance at –13.26(3) mm s–1. The isomer shifts of ionic Eu1 compare well with the values of the perovskites EuTiO3 (–13.5 mm s–1) and EuZrO3 (–14.1 mm s–1) [34, 35] as well as of several borate glasses containing divalent europium . The Eu2 signal is close to the value observed for EuFe2As2 (–11.3 mm s–1) .
|Compound||δ(mm s–1)||ΔEQ (mm s–1)||Γ (mm s–1)||ratio (%)|
δ = isomer shift, ΔEQ = electric quadrupole splitting, Γ = experimental line width. Parameters marked with an asterisk were kept fixed during the fitting procedure. The bold text highlights the element which has been measured: the first 4 lines refer to 151Eu measurements; the next 3 lines refer to 57Fe measurements.
Furthermore, a quadrupole splitting for Eu1 was excluded because of the symmetric arrangement around the nucleus (almost cubic site symmetry). A small resonance at an isomer shift of 0.80(4) mm s–1 indicates trivalent europium possibly due to extrinsic Eu3+ because of surface oxidation.
The 57Fe spectrum of Eu3Sc2O5Fe2As2 shows a single signal with isomer shifts of 0.35(1) and 0.47(1) mm s–1 (intermetallic iron [32, 38, 39]) at ambient temperature and 78 K, respectively, and a small interaction between quadrupole and electric field gradient resulting in a splitting of 0.20(1) mm s–1.
With regard to the suggested magnetic ordering, 151Eu and 57Fe Mössbauer spectroscopic measurements were repeated at 4.8 K (Figs. 5 and 6), quite close to the observed drop in the magnetic susceptibility data. Both the 151Eu and 57Fe spectra clearly show broadened resonances confirming a cooperative magnetic phenomenon. A fitting of the obtained spectra in order to determine the magnitude of the hyperfine field was difficult. On the one hand, there are too many parameters that cannot be fitted confidentially; on the other hand, at 4.8 K our experimental set up is at its limit. The temperature varies in the range of about ±0.3 K which has an influence on the exact splitting, especially for measurements close to the ordering temperature, leading to too many correlations within the refinement procedures.
Eu3Sc2O5Fe2As2 is a new member of the 32522-type iron pnictides with the Sr3Fe2O5Cu2S2-type structure. Similar to the already known members of this group, we found no signs for a structural phase transition down to 10 K. Magnetic measurements revealed an effective magnetic moment of 7.79 μB per europium atom and possible antiferromagnetic ordering below 5 K. 151Eu and 57Fe Mössbauer spectroscopic experiments have confirmed the magnetic data by showing resonances for Eu2+ and intermetallic iron. Low-temperature measurements showed a beginning magnetic ordering. As europium and alkaline earth compounds show a strong similarity for the 122-type iron pnictides, and traces of superconductivity were reported for Sr3 Sc2–xTixO5Fe2As2, one could expect that Eu3Sc2O5Fe2As2 might become superconducting upon doping.
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