Superconductivity of the platinum doped 122 iron arsenide SrFe 2-xPtxAs2

Article abstract of DOI:10.1016/j.physc.2011.05.005

Pt doped 122 iron arsenide SrFe_1-xPt _xAs₂ (0 ≤ x ≤ 0.4) was successfully synthesized. The tetragonal unit-cell volume and the lattice constant a increase with increasing the Pt content, while c decreases, suggesting that the Fe ions are indeed replaced by Pt ions. By the Pt doping, the magnetic order of the parent phase is suppressed, and superconductivity emerges at approximately x = 0.15. T _c reaches the maximum of 16 K at x = 0.2. The compounds series can be a suitable subject to investigate role of the doped 5d state in the superconducting 3d Fe-As layer.

Full text of DOI:10.1016/j.physc.2011.05.005

Physica C 471 (2011) 600–602
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Physica C
journal homepage: www.elsevier.com/locate/physc
Superconductivity of the platinum doped 122 iron arsenide SrFe2ÀxPtxAs2
a
S.B. Zhang a, , Y.F. Guo , J.J. Li b,c, X.X. Wang b,c, K. Yamaura b,c,d, E. Takayama-Muromachi a,c,d
⇑
a International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
b Superconducting Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
c Department of Chemistry, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
d JST, Transformative Research-Project on Iron Pnictides (TRIP), Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Accepted 5 January 2011
Available online 13 May 2011
Pt doped 122 iron arsenide SrFe1ÀxPtxAs2 (0 6 x 6 0.4) was successfully synthesized. The tetragonal unit-
cell volume and the lattice constant a increase with increasing the Pt content, while c decreases, suggest-
ing that the Fe ions are indeed replaced by Pt ions. By the Pt doping, the magnetic order of the parent
phase is suppressed, and superconductivity emerges at approximately x = 0.15. Tc reaches the maximum
of 16 K at x = 0.2. The compounds series can be a suitable subject to investigate role of the doped 5d state
in the superconducting 3d Fe–As layer.
Keywords:
Iron based-superconductor
Doping
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
studied, separately. The maximum Tc is about 17 K in our samples,
which is almost same as in above two works. There is an interesting
In the past several years, the Fe-based superconductor has at-
tracted broad attentions. Superconductivity (SC) has been reported
one after another for the family members, like REFeAsO1ÀxFx (RE = -
rare-earth metal) [1–3], doped AEFe2As2 (AE = Ca, Sr, Ba) [4–7],
AFeAs (A = Li, Na) [8,9], FeSex [10,11], and Sr4V2O6Fe2As2 [12]. All
of them have a common structure piece, the Fe–As layer, which
is directly responsible for the SC.
In the materials system, AEFe2As2 has several advantages for
investigating nature of the FeAs layer [1]; there is no complication
caused by oxygen nonstoichiometry [2]. AEFe2As2 widely accepts
chemical doping to the AE, Fe, or As sites [4–7]. Some doped ele-
ments eventually induce the SC [3]. Besides, applied pressure can
also induce the SC; CaFe2As2 indeed shows crossover to the SC at
12 K at pressure as low as 0.4 GPa [13–15] as does SrFe2As2 at
27 K at 3 GPa and BaFe2As2 at 29 K at 3.5 GPa [16].
In this paper, we report SC induced by replacing Fe of SrFe2As2
with the 5d element Pt. The doped compound crystallizes into the
ThCr2Si2-type structure with the space group I4/mmm as well as
the host compound. The lattice parameters variation over the Pt sub-
stitution demonstrates that Fe is successfully replaced by Pt up to
20 at.%. The spin-density-wave (SDW) transition of the parent com-
pound is suppressed gradually with the Pt doping, and the SC
emerges. A comparable feature was observed for the Pt doped
SrFe2As2 by other independent groups very recently [17,18]. Single-
crystal and polycrystal samples of Pt-doped SrFe2As2 had been
phenomenon that the Tc is almost independent with Pt doping con-
tent in Ref. [18]. However, the Tc is dependent Pt doping content
strongly in our work and Ref. [17]. Stoichiometry and homogeneity
of Pt element should been the key point for this behavior.
2. Experiments
Polycrystalline sample was prepared by a solid-state reaction
method with two steps. First, SrAs and FeAs were prepared by
the method from Sr pieces (99.9%, Strem Chemicals), Fe powder,
and As grains (99.999%, High Purity Chem.). Second, they were
mixed with Pt (99.9%, Strem Chemicals) and Fe powder (99.9%,
100 mesh, Rare Metallic Co.) with the proportions of SrFe2ÀxPtxAs2
(x = 0, 0.1, 0.15, 0.2, 0.3, 0.4). The mixture was pressed into a Ta
capsule. The capsule was sealed in an evacuated quartz tube, fol-
lowed by heating at 1000 °C for 48 h. The weighing, grounding,
and pressing were processed in a glove box, which was constantly
maintained under a protective argon atmosphere.
Phase identification was carried out by an X-ray diffraction
(XRD) method in Philips X’pert PRO X-ray diffractometer using a
h–2h scan with Cu K
a radiation at room temperature. Structural
refinement was performed by a standard Rietveld technique [19].
Electronic resistivity measurements were conducted in physical
property measurements system, Quantum Design, by a standard
four-probe method. DC magnetization measurements were done
in magnetic property measurements system, Quantum Design.
3. Results and discussion
⇑
Corresponding author. Address: National Institute for Material Science (NIMS),
International Center for Materials Nanoarchitectonics (MANA), 1-1 Namiki, Tsukuba,
Ibaraki 305-0044, Japan. Tel.: +81 29 851 3354x8024; fax: +81 29 860 4674.
E-mail address: ZHANG.Shoubao@nims.go.jp (S.B. Zhang).
Fig. 1 shows the XRD patterns of the SrFe2ÀxPtxAs2 samples at
x = 0, 0.1, 0.15, 0.2, 0.3 and 0.4. Obvious impurity peaks were
0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2011.05.005

S.B. Zhang et al. / Physica C 471 (2011) 600–602
601
undetected, indicating that all the samples are of single phase. We
indexed all peaks with a tetragonal lattice model [I4/mmm; Sr 2a
(0, 0, 0), Fe/Pt 4d (0.5, 0, 0.25), As 4d (0.25, 0, 0.3608)] [17] by a
standard Rietveld method. As a representative, fit to the pattern
for SrFe1.85Pt0.15As2 is shown in Fig. 2. The lattice constants a, c,
a/c and the unit-cell volume V obtained by the fits are shown in
Fig. 3. It reveals that the lattice constants a, a/c, V increase with
the Pt content, while c decreases. Distance between the Fe–As lay-
ers is slightly shortened, however the increase of unit-cell volume
indicates a negative chemical pressure is actually introduced by
the Pt-doping.
Fig. 4 shows the temperature dependence of the electrical resis-
tivity of SrFe2ÀxPtxAs2 at x = 0, 0.1, 0.15, 0.2, 0.3 and 0.4. It can be
seen that the parent compound SrFe2As2 exhibits a clear anomaly
at 210 K, which corresponds to the SDW transition [17]. The
SDW transition temperature decreases by the Pt doping; 143 K at
x = 0.1, 94 K at x = 0.15, and the anomaly disappears at x = 0.2
and higher. Meantime, SC appears at x = 0.1 and higher. The zero
resistivity was observed at x = 0.15, 0.2 and 0.3, but not at x = 0.4.
The x = 0.4 sample is probably in the ‘‘over doped’’ region. It is
noteworthy that coexistence of SC and SDW was observed for the
samples at x = 0.1 and 0.15. Because the coexistence directly re-
flects nature of the unusual SC, further investigation of the samples
is in progress.
Fig. 2. X-ray diffraction pattern of SrFe1.85Pt0.15As2. Crosses indicate the experi-
mental data and the calculated data is the continuous line overlapping them. The
lowest curve shows the difference between experimental and calculated patterns.
Fig. 5a–f. show temperature dependence of the magnetic sus-
ceptibility for all the samples of SrFe2ÀxPtxAs2 (x = 0–0.4) measured
in a magnetic field of 3 kOe at zero field cooling (ZFC) and field
cooling (FC) conditions. Inset to Fig. 5a is an expanded view focus-
ing on the anomaly at 210 K. Other insets to Fig. 5b–f. show addi-
tional data measured at 10 Oe below 30 K.
The 3 kOe data indicate that all of the samples are paramag-
netic-like. The small gap in the ZFC and FC curves at x = 0.1, 0.2,
0.3 indicate magnetic contributions from undetected impurities,
masking the SC diamagnetism. In fact, such the magnetic contribu-
tions are often observed for many iron-based superconducting
compounds [20]. We thus conducted the measurements in a much
weak field of 10 Oe.
Fig. 3. Pt-substitution dependence of lattice constants a, c, a/c and unit cell volume V.
As shown in the inset to Fig. 5a, a small anomalous peak was ob-
served at 210 K, which is likely due to the SDW transition. With
increasing the Pt content, the SDW gradually disappears. For the
x = 0.1 sample, a weak downturn feature in the ZFC curve was de-
tected, while the samples at x = 0.15, 0.2, 0.3 show much larger dia-
magnetic signals below 12 K, 16.5 K and 16 K, respectively.
Because the temperatures are comparable with the zero resistivity
temperature, it is reasonable to attribute the large diamagnetic sig-
nals to the SC. At the maximum Pt concentration x = 0.4, the SC is
suppressed probably due to the over doped carriers.
Fig. 4. Temperature dependence of resistivity for SrFe2ÀxPtxAs2 with x = 0, 0.1, 0.15,
0.2, 0.3 and 0.4 at zero magnetic field.
In summary, Pt doped 122 iron arsenide SrFe1ÀxPtxAs2 was suc-
cessfully synthesized up to x = 0.4. The structure, electric transport,
and magnetic properties were investigated. With increasing the Pt
content, the SDW transition of the parent phase is gradually sup-
pressed and SC emerges at a doping level of approximately
x = 0.15. Although the Pt doping eventually applies a negative
Fig. 1. X-ray diffraction patterns of SrFe2ÀxPtxAs2 (x = 0, 0.1, 0.15, 0.2, 0.3 and 0.4).

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S.B. Zhang et al. / Physica C 471 (2011) 600–602
tectonics from MEXT, Japan; the Grants-in-Aid for Scientific
Research (20360012, 22246083) from JSPS, Japan; and the Funding
Program for World-Leading Innovative R&D on Science and Tech-
nology (FIRST Program) from JSPS.
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Acknowledgments
This research was supported in part by the World Premier Inter-
national Research Center (WPI) Initiative on Materials Nanoarchi-