Growth and Characterization of Millimeter-sized Single Crystals of CaFeAsF
Yonghui Ma, Hui Zhang, Bo Gao, Kangkang Hu, Qiucheng Ji, Gang Mu, Fuqiang Huang, Xiaoming Xie
GGrowth and Characterization of Millimeter-sized Single Crystals of CaFeAsF
Yonghui Ma,
1, 2
Hui Zhang, Bo Gao, Kangkang Hu,
2, 4
QiuchengJi, Gang Mu, ∗ Fuqiang Huang, and Xiaoming Xie
1, 2 School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China State Key Laboratory of Functional Materials for Informatics and Shanghai Center for Superconductivity,Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences, Shanghai 200050, China CAS Key Laboratory of Materials for Energy Conversion,Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China College of Sciences, Shanghai University, Shanghai 200444, China
High-quality and sizable single crystals are crucial for studying the intrinsic properties of uncon-ventional superconductors, which are lacking in the 1111 phase of the Fe-based superconductors.Here we report the successful growth of CaFeAsF single crystals with the sizes of 1-2 mm using theself-flux method. Owning to the availability of the high-quality single crystals, the structure andtransport properties were investigated with a high reliability. The structure was refined by usingthe single-crystal x-ray diffraction data, which confirms the reports earlier on the basis of powderdata. A clear anomaly associated with the structural transition was observed at 121 K from theresistivity, magnetoresistance, and magnetic susceptibility measurements. Another kink-feature at110 K, most likely an indication of the antiferromagnetic transition, was also detected in the resis-tivity data. Our results supply a basis to propel the physical investigations on the 1111 phase ofthe Fe-based superconductors.Keywords: CaFeAsF, Single Crystals, Fe-based Superconductors
I. INTRODUCTION
The F-doped LnFeAsO (Ln = rare-earth elements),which has been abbreviated as 1111 phase, is the firstreported family with the highest critical transition tem-perature T c in bulk in the Fe-based superconductors(FeSCs). However, investigations on the physical prop-erties of 1111-type FeSCs are restricted remarkably, com-pared with the 122 phase and 11 phase, due to the diffi-culties in obtaining sizable single crystals. As we know, itis essential to have high-quality single crystals when car-rying out many experiments, including the measurementsof electrical transport, inelastic neutron diffraction, angleresolved photoemission spectroscopy, and so on. Duringthe past several years, many efforts have been made toimprove the quality and size of the single crystals. NaCland KCl were first used as the flux and small single crys-tals with sizes of 20-70 µ m can be obtained. Then moreattempts, including the high-pressure method and theNaAs-flux method, were made to further improve thegrowth processes. Up to now, the two goals, sizable andhigh-quality, are still not achieved commendably. Re-cently, single crystals with the size of several millimeterswere reported to be accessible in F-vacant and Na-dopedCaFeAsF, which is another type of 1111 phase withoutoxygen, possibly due to the decrease of melting pointin this fluorine-based system. As we know, a rather high T c above 50 K can also be achieved by doping in thisfluorine-based 1111 system. More important informa-tion can be obtained owning to the availability of the siz-able single crystals. To our knowledge, the investigationson the single crystals of the parent phase CaFeAsF arestill lacking.Here we present the growth, structure, and transport measurements of the high-quality CaFeAsF single crys-tals with the sizes of 1-2 mm. The single crystals weregrown by the self-flux method. The structure detailswere obtained from the refinement of the single-crystalx-ray diffraction data. The structural transition at 121 Kwas confirmed by the resistivity, magnetoresistance, andmagnetic susceptibility measurements. A feature comingfrom the antiferromagnetic transition was also observedin the resistivity data.
II. EXPERIMENTAL DETAILS
High quality CaFeAsF single crystals were grown us-ing the self-flux method with CaAs as the flux. First,the starting materials Ca granules (purity 99. 5%, AlfaAesar) and As grains (purity 99.995%, Alfa Aesar) weremixed in 1: 1 ratio. Then the mixture was sealed in anevacuated quartz tube and followed by a heating processat 700 ◦ C for 10 h to get the CaAs precursor. CaAs, FeF2powder (purity 99%, Alfa Aesar) and Fe powder (purity99+%, Alfa Aesar) were mixed together in the stoichio-metric ratio 10: 1: 1, and the mixture were placed in acrucible. Finally, the crucible was sealed in a quartz tubewith vacuum. All the weighing and mixing procedureswere carried out in a glove box with a protective argonatmosphere. The quartz tube was heated at 950 ◦ C for 40hours firstly, and then it was heated up to 1230 ◦ C andstay for 20 hours. Finally it was cooled down to 900 ◦ Cat a rate of 2 ◦ C /h and followed by a quick cooling downto room temperature.The microstructure was examined by the scanning elec-tron microscopy (SEM, Zeiss Supra55). The composi-tion of the single crystals was checked and determined a r X i v : . [ c ond - m a t . s up r- c on ] M a y TABLE I: Compositions of the crystal characterized by EDSmeasurements.Element Weight (%) Atomic (%)F 14.90 34.52Ca 19.58 21.50Fe 27.31 21.52As 38.22 22.46 by the energy dispersive x-ray spectroscopy (EDS) mea-surements on an Oxford Instruments. The crystals werefirst checked using a DX-2700 type powder x-ray diffrac-tometer. The detailed structure was characterized andanalyzed by the single-crystal x-ray diffraction measure-ments on a Bruker D8 Focus diffractometer equippedwith the graphite-monochromatized Mo K α radiation.The magnetic susceptibility measurement was carried outon the magnetic property measurement system (Quan-tum Design, MPMS 3). The electrical resistance andmagnetoresistance (MR) were measured using a four-probe technique on the physical property measurementsystem (Quantum Design, PPMS) with magnetic fieldup to 9 T. For the MR measurements, the magnetic fieldwas oriented parallel to the c axis of the samples and thedata were measured for both positive and negative fieldorientations to eliminate the effect of the Hall signals. III. RESULTS AND DISCUSSIONS
A typical dimension of the single crystals is1.2 × × . The morphology was examined bythe scanning electron microscopy. An SEM picture forthe CaFeAsF single crystal can be seen in Fig. 1(a),which shows the flat surface and some terrace-like fea-tures. An enlarged view of this picture can be seen inFig. 1(b). The composition of the crystals was char-acterized by energy-dispersive x-ray spectroscopy (EDS)measurements. We measured the EDS at different po-sitions of the sample. Here we show a typical result inFig. 1(c) and Table I, which revealed that the ratio ofCa: Fe: As is close to the stoichiometric ratio. The con-tent of the light element F is difficult to determine pre-cisely based on EDS measurements. The structure of thecrystals was first check by a powder x-ray diffractome-ter, where the x-ray was incident on the ab-plane of thecrystal. The diffraction pattern is shown in Fig. 2. Allthe diffraction peaks can be indexed to the tetragonalZrCuSiAs-type structure (see the inset of Fig. 2). Onlysharp peaks along (00l) orientation can be observed, sug-gesting a high c-axis orientation. The full width at halfmaximum (FWHM) of the diffraction peaks is only about0.10 ◦ after deducting the K α contribution, indicating arather fine crystalline quality. The c-axis lattice constantwas obtained to be 8.584 ˚A by analyzing the diffractiondata.We used the high-resolution single-crystal x-raydiffraction to study the structural details of our sam- FIG. 1: (color online) (a) An SEM picture of a CaFeAsFcrystal with the lateral size larger than 1 mm. (b) The en-larged view of the SEM picture. (c) The EDS microanalysisspectrum taken on one crystal.FIG. 2: (color online) X-ray diffraction pattern measured onthe CaFeAsF single crystal with the x-ray incident on the ab -plane. The inset is the schematic of the crystal structure ofCaFeAsF. ple. The diffraction data were collected at room temper-ature by the ω - and ϕ -scan method. The crystal struc-ture was solved by SHELXS-2014 and refined by SHELX-2014. The parameters for the data collection and struc-ture refinement are listed in Table II. The values of R and wR are much smaller than the previously reportedpolycrystalline results, and also small compared to theNa-doped single crystalline system, indicating the high-quality of our sample and the reliability of our refine-ments. As shown in Table III, the final cell constants aredetermined to be a = b = 3.8774(4) ˚A, c = 8.5855(10) ˚A.It is clear that the c-axis lattice constants are very closeto that obtained from the data in Fig. 2. In addition,the a- and c-axis lattice constants determined from ourexperiment are consistent with the polycrystalline sam- TABLE II: Parameters for the data collection and structurerefinement of CaFeAsF.Theta range for data collection 4.749 to 27.508 ◦ Index ranges -4 ≤ h ≤ ≤ k ≤ ≤ l ≤ Refinement program SHELXL-2014 (Sheldrick, 2014)Data /restraints /parameters 114 /0 /12Goodness-of-fit on F = 0.0139wR = 0.0318Weighting scheme w=1/[ σ (F o )+0.6368P]where P=(F o +2F c )/3Extinction coefficient 0.014(3)Largest diff. peak and hole 0.655 and -0.364 e˚A − R.M.S. deviation from mean 0.122 e˚A − TABLE III: Refined lattice constants for the CaFeAsF singlecrystal.Chemical formula CaFeAsFFormula weight 189.85 g/molTemperature 296(2) KWavelength 0.71073 ˚ACrystal system tetragonalSpace group P4/nmm (No. 129)Z 2Unit cell dimensions a = 3.8774(4) ˚A, α = 90 ◦ b = 3.8774(4) ˚A, β = 90 ◦ c = 8.5855(10) ˚A, γ = 90 ◦ Volume 129.076(4) ˚A Bond angle ( δ As − Fe − As ) 107.82(6) ◦ × ◦ × h As = 1.413 ˚ADensity (calculated) 4.885 g/cm Absorption coefficient 20.223 mm − F(000) 176 ples reported previously.
Compared to the Na-dopedsingle crystalline samples, the a-axis lattice constant issimilar while the c-axis constant is clearly smaller. Theanion (As) height relative to Fe layer is a bit larger thanthe optimal value (1.38 ˚A) for the highest T c in FeSCs. The atomic coordinates from the refinement are shownin Table IV, which also confirm the structure obtainedearlier on the basis of powder data, with a difference of
TABLE IV: Atomic coordinates and equivalent isotropicatomic displacement parameters (˚A ) for CaFeAsF.Atom x y z U(eq)As 1/4 1/4 0.16461(8) 0.0067(3)Fe 3/4 1/4 0 0.0069(3)Ca 3/4 3/4 0.34801(16) 0.0080(4)F 3/4 1/4 1/2 0.0092(8) FIG. 3: (color online) Temperature dependence of the in-plane resistivity (a), magnetoresistance (b), and the magneticsusceptibility (c). The field of 1 T was applied along the c-axis of the crystal during the magnetic susceptibility mea-surement. The MR data were collected under the field of 9T. The dashed lines are guides for eyes. about 0.16% for the c-axis position of As element. Thebond angles δ As − F e − As deviate a bit from the optimalvalue of about 109.47 ◦ .The resistivity, MR, and magnetic susceptibilitychange the variation tendency at the same temperature121 K on the temperature dependent curves, as revealedin Figs. 3(a), (b), and (c). This seems to be a com-mon feature in most of the FeSCs, associated with thestructure and the spin-density-wave (SDW)-type anti-ferromagnetic transition. This transition temperatureis a little higher than the polycrystalline results (118-120 K). Temperature dependence of resistivity areshown in Fig. 3(a). Above 121 K, the resistivity in-creases almost linearly with the decrease of temperature.We note that it is rather conflicting about this behav-ior, among different repots based on polycrystalline sam-ples.
Only one result reported on SrFeAsF by Tegel etal. shows similar tendency, compared with our data. Weargue that the data from single-crystal samples revealsthe intrinsic properties since the scattering processes arenot affected by the grain boundaries. Moreover, the tran-sition at 121 K is sharper than the results from polycrys-talline samples. Below that temperature, the resistivitydecreases with cooling and a clear kink can be observedat about 110 K, as indicated by the green dashed line.These two characteristic temperatures with the intervalof 11 K are reminiscent of the reported T str = 150 K and T N = 138 K in the another 1111 phase LnFeAsO (Ln =La, Ce), where T str and T N are the transition tem-peratures from tetragonal to orthogonal structure andthat from paramagnetic to SDW-type antiferromagneticphase, respectively. We note that such two distinct tran-sition temperatures have also been detected from the re-sistivity data in the Co-doped BaFe As system. So itis very likely that these two transition temperatures are T str = 121 K and T N = 110 K for the present CaFeAsFsystem.In is paper, MR is expressed as ∆ ρ/ρ = [ ρ (9T) − ρ ] /ρ , where ρ (9T) and ρ are the resistivity under thefield 9 T and zero field, respectively. In Fig. 3(b), weshow temperature dependence of MR. The magnitudeof MR decreases with the increase of the temperaturemonotonously until T str and vanishes above this tem-perature. These observations suggest that the MR inthis system is associated with the magnetic and elec-tronic structures, which are affected by the structureand the SDW-type antiferromagnetic transitions remark-ably. The transition on the M − T curve shows thefeature of an antiferromagnetic transition, as shown inFig. 3(c). In the high temperature non-magnetic nor-mal state, a linear-temperature-dependent behavior canbe observed. This is a non-Curie-Weiss-like paramag- netic behavior and cannot be understood within a sim-ple mean-field picture. This behavior should be veryimportant to understand the mechanism of high- T c su-perconductivity because it was also observed in undopedand highly underdoped cuprates. In the pnictide com-pounds, this feature was interpreted by the antiferromag-netic fluctuations with the local SDW correlations. IV. CONCLUSIONS
In summary, high-quality and sizable single crystalsof CaFeAsF were grown successfully by the self-fluxmethod. The single-crystal x-ray diffraction measure-ments were carried out and the structure details wererefined based on the data. The resistivity, magnetore-sistance, and magnetic susceptibility show clear differentbehaviors below and above 121 K. The critical temper-atures of the structure and antiferromagnetic transitionwere determined to be T str = 121 K and T N = 110 K,respectively. Our results supply a platform to study theintrinsic properties of the 1111 phase of FeSCs. Acknowledgments
This work is supported by the National Natural Sci-ence Foundation of China (No. 11204338), the “StrategicPriority Research Program (B)” of the Chinese Academyof Sciences (No. XDB04040300, XDB04040200 andXDB04030000) and Youth Innovation Promotion Associ-ation of the Chinese Academy of Sciences (No. 2015187). ∗ [email protected] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am.Chem. Soc. , 3296-3297 (2008). L. Fang, P. Cheng, Y. Jia, X. Zhu, H. Q. Luo, G. Mu, C.Z. Gu, H. H. Wen, J. Cryst. Growth , 358 (2009). J.-Q. Yan, S. Nandi, J. L. Zarestky, W. Tian, A. Kreyssig,B. Jensen, A. Kracher, K. W. Dennis, R. J. McQueeney,A. I. Goldman, R. W. McCallum, T. A. Lograsso, Appl.Phys. Lett. , 222504 (2009). N. D. Zhigadlo, S. Weyeneth, S. Katrych, P. J. W. Moll, K.Rogacki, S. Bosma, R. Puzniak, J. Karpinski, B. Batlogg,Phys. Rev. B , 214509 (2012). J. Tao, S. Li, X. Y. Zhu, H. Yang, H. H. Wen, Sci. China , 632 (2014). L. Shlyk, K. K. Wolff, M. Bischoff, E. Rose, Th. Schleid,R. Niewa, Supercond. Sci. Technol. , 044011 (2014). F. Han, X. Zhu, G. Mu, P. Cheng, H. H. Wen, Phys. Rev.B , 180503(R) (2008). M. Tegel, S. Johansson, V. Weiss, I. Schellenberg, W. Her-mes, R. Poettgen, D. Johrendt, Europhys. Lett. , 67007(2008). P. Cheng, B. Shen, G. Mu, X. Zhu, F. Han, B. Zeng, H.H. Wen, Europhys. Lett. , 67003 (2009). S. Matsuishi, Y. Inoue, T. Nomura, H. Yanagi, M. Hirano, H. Hosono, J. Am. Chem. Soc. , 14428-14429 (2008). G. Wu, Y. L. Xie, H. Chen, M. Zhong, R. H. Liu, B. C.Shi, Q. J. Li, X. F. Wang, T. Wu, Y. J. Yan, J. J. Ying,X. H. Chen, J. Phys.: Condens. Mat. , 142203 (2009). X. Zhu, F. Han, P. Cheng, G. Mu, B. Shen, L. Fang, H.H. Wen, Europhys. Lett. , 17011(2009). Bruker (2007).
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