Synthesis of New Layered Oxypnictides Sr2CrO2(FeAs)2
Naoya Eguchi, Fumihiro Ishikawa, Michihiro Kodama, Takeshi Wakabayashi, Atsuko Nakayama, Ayako Ohmura, Yuh Yamada
aa r X i v : . [ c ond - m a t . s up r- c on ] F e b Journal of the Physical Society of Japan
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Synthesis of New Layered Oxypnictides Sr CrO (FeAs) Naoya E guchi ∗ , Fumihiro I shikawa , Michihiro K odama , Takeshi W akabayashi , AtsukoN akayama , Ayako O hmura , and Yuh Y amada Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan Department of Physics, Niigata University, Niigata 950-2181, Japan Center for Transdisciplinary Research, Niigata University, Niigata 950-2181, Japan
KEYWORDS: Fe-Based superconductor, layered oxypnictides
Since the discovery of LaFeAs(O, F) superconductor, a large number of iron-oxypnictides with superconductivity have been investigated. In the oxypnictide supercon-ductors, blocking layers, which exist between FeAs layers, characterize series of compounds;LaO block in LaFeAs(O, F), for example. A series of oxypnictides having perovskite-typeblocking layers is attractive because of their highly two dimensional crystal structure. In fact,Sr V O (FeAs) shows superconducting transition at 37 K under ambient pressure withoutdoping, while most of FeAs-based superconductors needs doping or applying pressure.As reviewed by Ozawa and Kauzlarich, several layered oxypnictides structures are con-siderable candidates as a new superconductor. Sr Mn As O -type oxypnictide is one of thelayered oxypnictides introduced in the review. Nath et al. reported their attempt to synthe-size Sr Mn As O -type oxypnictides and succeed to prepare several kinds of polycrystallinesamples with MnAs layers. They also reported that they could not obtain the single phasesamples with Fe-substituted compositions, for example, Sr Fe As O .Recently, we successfully synthesized Fe-substituted Sr CrO (FeAs) , which is isostruc-tural with Sr Mn As O except for Cr-substitution of Mn-site in the blocking layer. The crys-tal structure of Sr CrO (FeAs) is shown in the inset of Fig. 1. The FeAs layers are separatedby two Sr-layer and CrO -layer according to the stacking order of (Sr)(CrO )(Sr). In thispaper, we represent our results of the crystal structure determined by x-ray powder di ff rac-tion and the physical properties of Sr CrO (FeAs) . Briefly, Sr CrO (FeAs) does not showsuperconductivity in analogy with Sr Cr O (FeAs) .
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Polycrystalline sample of Sr CrO (FeAs) is prepared by solid-state-reaction techniquesusing SrO (98% pure), FeAs (99.5% pure), Cr (99.9% pure), as starting materials. Composi- ∗ E-mail: naoya [email protected] 1 /
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SHORT NOTES tion of starting mixture is nominal. A pellet of the ground mixtures of the starting materialswrapped with tantalum foil was sealed inside double quartz tubes with argon gas. All thesample handling was carried out inside an Ar-filled glove box. The samples were heated to800 ◦ C, 900 ◦ C and 1000 ◦ C at a rate of 50 ◦ C / h, held there for 96 h, respectively. Each samplemainly consists of Sr CrO (FeAs) phase, which was confirmed by powder x-ray di ff raction.However, the samples sintered at 800 ◦ C and 1000 ◦ C also contain FeAs phases, clearly. Inaddition, the sample sintered at 800 ◦ C has a large amount of other impurity phases such asSrFe As and Sr Cr O (FeAs) . Thus, below in this paper, we only describe the sample ofSr CrO (FeAs) sintered at 900 ◦ C.Powder x-ray di ff raction patterns were measured using Cu K α radiation in the 2 θ rangeof 20-90 ◦ (RINT-2000, Rigaku, Japan) . Rietveld refinements were made by the analysisprogram RIETAN-FP. In Fig. 1, the x-ray di ff raction pattern for the sample sintered at 900 ◦ Cis shown with the results of Rietveld refinement profile. Sr CrO (FeAs) is obtained as themain phase with a small amount of impurities, such as Fe − x Cr x and SrFe As . According tothe results of multi-phases peak-fitting with RIETAN-FP, total amount of impurity phases areless than 2%. Refined results by Rietveld analysis are summarized in Table I . The space groupof the compound is I / mmm and the lattice constants are a = c = I n t e n s it y [ a r b it r a r y un it ]
2 [degree]Cu K α Sr CrO (FeAs) sintered at 900°C bca SrCrFeAsO ca SrSrSrCrCrCrCrFeFeFeFeFeAsAsOOO
Fig. 1. (Color online) X-ray powder di ff raction pattern and Rietveld fit for Sr CrO (FeAs) sintered at 900 ◦ C.Small amount of Fe-Cr alloy ( < As ( < As , respectively. Solid line at the bottom shows the residual error. Inset shows the crystalstructure of Sr CrO (FeAs) drawn by VESTA program. /
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Structure parameters for Sr CrO (FeAs) refined from powder x-ray di ff raction data with the spacegroup of I / mmm ; R wp = R p = S = e a c d e The electrical resistivity was measured by a conventional four-probe method with aGi ff ord-McMahon cryocooler between 3 and 300 K. Temperature dependence of the re-sistivity for Sr CrO (FeAs) is shown in Fig. 2. Over whole measured temperature range,Sr CrO (FeAs) shows semiconductor-like behavior. Additionally, the value of the resistiv-ity is smaller than that of Sr Cr O (FeAs) , which also shows nonmetallic behavior with theresistivity of about 60 m Ω cm at room temperature. While many kinds of iron-oxypnictidesuperconductor and related compounds show the anomaly due to structural and magnetic tran-sition, Sr CrO (FeAs) did not show any trace of such transition in the resistivity variationwith temperature. R e s i s ti v it y [ m Ω c m ] CrO (FeAs) sintered at 900°C Fig. 2. (Color online) Temperature dependence of the resistivity for Sr CrO (FeAs) . The magnetization up to 10 kG was measured using with a SQUID magnetometer (MPMSof Quantum Design Co.). Temperature dependence of the magnetization for Sr CrO (FeAs) at several magnetic fields is shown in Fig. 3. As shown in this figure, variation in magnetiza-tion is very small and shows no Curie-Weiss-like behavior. This is in contrast to the previouslyreported results for Sr Cr O (FeAs) .
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Tegel et al. reported antiferromagnetic order of Cr + moment. The inset in Fig. 3 shows the magnified view of curves between 240 K and 300 Kwith vertical o ff set. In the magnetic field of 1 and 10 kG, small kinks appeared in the curves; /
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SHORT NOTES the origin of these kinks is not yet clear. Additionally, temperature dependence at low tem-perature at 10 G does not show any trace of the Meissner e ff ect even under high pressure upto 1 GPa (not shown here). M a gn e ti za ti on [ e m u / g ] CrO (FeAs) sintered at 900°C B = 10 kG B = 1 kG B = 10 G M [ a r b it r a r y un it ] T [K] B = 10 kG B = 1 kG B = 10 G Fig. 3. (Color online) Temperature dependence of the magnetization for Sr CrO (FeAs) at several magneticfields. Inset shows enlarged view near room temperature. Note that each curve is o ff set and plotted in arbitraryunit in the inset. Arrows indicate kinks in the curves. As shown in the magnetization curves in Fig. 4, the present sample shows apparent fer-romagnetic behavior even at room temperature. With increasing temperature, magnetizationslightly decreases, but coercive force is almost same between 5 K and 300 K. These resultssuggest that the present sample contains some ferromagnetic phases having higher Curie tem-perature than room temperature. This ferromagnetic behavior is probably due to small amountof the Fe-Cr alloy which exists as an impurity phase. We may estimate that the amount of theFe-Cr alloy phase is about 1% using our magnetization value and the reported Fe-Cr al-loy magnetization. For example, bulk magnetization value of Fe Cr was experimentallydetermined to be about 138 emu / g. This estimated content of Fe-Cr alloy phase from mag-netization data is consistent with the Rietveld refinement result as mentioned above.In conclusion, we successfully prepared new layered iron oxypnictide Sr CrO (FeAs) ,which has no superconducting transition down to 3 K. This is the first report on the Fe-substituted Sr Mn As O -type oxypnictides to our knowledge. However, our present samplecontains the strong ferromagnetic Fe-Cr alloy as impurity. Purification is required to investi-gate intrinsic magnetic properties for Sr CrO (FeAs) . Moreover, we consider this compoundcan form a new family of the oxypnictide superconductor. Doping to the compound or sub-stitution of Cr by other element probably lead to the superconducting phase. At this time, /
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SHORT NOTES M a gn e ti za ti on [ B / f . u . ] M a gn e ti za ti on [ e m u / g ]
5K 300K incresing decreasing Sr CrO (FeAs) sintered at 900°C M [ B / f . u . ]
10 5 0 5 10 B [kG] T
300 K T Fig. 4. (Color online) Magnetization curves for Sr CrO (FeAs) at 5 K and 300 K. Inset shows the hysteresisloops at 5 K and 300 K. applying pressure to this compound does not cause the superconducting phase. Further studyof this series is being undertaken. Acknowledgment
This research was supported by JSPS KAKENHI Grant Number 23684024. /
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