DOI: 10.1021/cg101128w
Single Crystal Growth of the CeCu2Si2 Intermetallic Compound by a Vertical Floating Zone Method
2011, Vol. 11 431–435
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Chongde Cao,*,‡,† Micha Deppe,§ G€ unter Behr,† Wolfgang L€ oser,*,† Nadja Wizent,† Olga Kataeva, ,† and Bernd B€ uchner† †
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IFW Dresden, Leibniz-Institut f€ ur Festk€ orper- und Werkstoffforschung, Helmholtzstrasse 20, D-01171 Dresden, Germany, ‡Department of Applied Physics, Northwestern Polytechnical University, Xi’an, ur Chemische Physik fester Stoffe, N€ othnitzer Shaanxi 710072, PR China, §Max-Planck-Institut f€ Strasse 40, 01187 Dresden, Germany, and A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences, Arbuzov Stranitsa 8, Kazan 420088, Russia Received August 26, 2010; Revised Manuscript Received December 3, 2010
ABSTRACT: Large CeCu2Si2 single crystals were grown by a vertical floating zone method with optical heating at slow zone traveling rates of 1-3 mm/h. Elevated Ar pressure of 3 MPa in the growth chamber was applied and floating zone was controlled by an in situ temperature measurement in order to minimize Cu evaporation. The compound exhibits incongruent melting behavior with Ce2CuSi3 primary phase precipitates in the initial growth phase. Gradual accumulation of Cu in the floating zone finally led to the TSFZ growth mode of CeCu2Si2 single crystals homogeneous over the whole cross section. The obtained CeCu2Si2 crystal was proven to be single phase with no sign of Cu depletion in the final part, the X-ray single crystal diffraction study proved its excellent quality. It exhibited the S-type behavior with heavy fermion superconducting properties at T < Tc = 0.58 K.
1. Introduction The CeCu2Si2 intermetallic compound with tetragonal ThCr2Si2-type structure has attracted great interest after heavy fermion superconductivity was discovered by Steglich et al. in this Kondo lattice system.1 Later on, it turned out that the actual composition determines whether the ground state properties show antiferromagnetic order (A-type), superconductivity (S-type), or both antiferromagnetism and superconductivity (A/S-type).2 It is reported that CeCu2Si2 single crystals prepared from stoichiometric melts do not show superconductivity at ambient pressure3-6 but surprisingly do so under an external pressure above 1 kbar.3 While Steglich et al. found that the nominal stoichiometry “1:2:2” led to A/Stype samples, Cu excess resulted in S-type and Si rich samples showed A-type behavior.7 This implies that the preparation path and parameters of CeCu2Si2 crystals also have a great impact upon the physical behavior. So far, the crystal preparation, magnetism, and superconductivity of CeCu2Si2 have been the most important subjects in the field of heavy fermion superconductors and still attract much attention.8-10 The preparation of CeCu2Si2 single crystals with welldefined composition has contributed much to understanding the exotic behavior of this compound. Crystal growth attempts have been accomplished with various methods. CeCu2Si2 were grown from an In flux by Stewart et al.6 and from Sn, In, or Cu flux by Batlogg et al.11 The Bridgman technique using alumina and BN crucibles was applied by Assmus et al.12 and Kletowski,5 respectively. Onuki et al.13 applied the Czochralski technique by using a W crucible. One serious drawback of all these trials is that conventional crucibles are severely corroded by the melt, therefore, the crystals are unavoidably
contaminated.14 A crucible-free horizontal zone melting technique in a cold boat was successfully applied by Sun et al.15 to produce large superconducting CeCu2Si2 crystals from melts with different amounts of excess copper at low growth velocities (1-4 mm/h). Additional Ce2CuSi3 phase reflections were detected X-ray powder diffraction patterns of crystals grown from melts with less excess copper. It was also demonstrated that the copper concentration of copper deficient crystals with low Tc can be increased by annealing in an atmosphere of copper. A Hukin-type cold crucible was applied by N€ uttgens et al. to grow large crystals by the NackenKyropoulos method from levitated melts with excess copper.16,17 One shortcoming of this method is that the solidified button still consists of multiple grains with segregated grain boundaries and even contains secondary phases. So far, the maximum single crystals sized by only 2 3 6 mm3 15 or 3 3 3 mm3 16 have been obtained. It was reported that the application of the vertical floating zone (FZ) technique with radio frequency (RF) induction heating failed because of poor stability of the molten zone.18 The FZ method would be ideally suited for crystal growth of CeCu2Si2 because any contact with crucibles is avoided and the steep temperature gradients enable short zone length for limiting the evaporation of copper. Therefore, the aim of the present work is the growth of CeCu2Si2 crystals by the vertical FZ technique with optical heating at elevated Ar pressure. 2. Experimental Section
*To whom correspondence should be addressed. For C.D.C.: phone/fax, þ86-29-88431664; E-mail,
[email protected]. For W.L.: phone, þ49 351 4659 647; fax, þ49 351 4659 313; E-mail,
[email protected]. Website: http://www.ifw-dresden.de/.
Crystal Growth. Feed rods of CeCu2Si2 were prepared in a twostep melting process, which reduces possible evaporation losses. First, a binary Cu-Si alloy was arc melted on a water-cooled copper hearth under a Zr-gettered Ar atmosphere from bulk pieces of high purity 99.999 wt % Cu and of 99.9999 wt % Si. The preparation of 99.9 wt % pure Ce bulky pieces (Changsha Developing Center of Rare Earth Science and Technology, Changsha, P.R. China) was carried out in a glovebox under Ar to avoid any contact with the
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Crystal Growth & Design, Vol. 11, No. 2, 2011
oxygen atmosphere. The Cu-Si button alloy was comelted with an appropriate portion of Ce in a Hukin-type copper cold-crucible induction melting facility under 0.1 MPa Ar and cast into a copper mold to get feed rods 6 mm in diameter and 55-70 mm in length. Ce20Cu41Si39 feed rods and a previously grown coarse-grained seeds are used. The FZ crystal growth process with radiation heating was performed in a laboratory type apparatus URN-2-ZM (MPEI, Moscow)19 with a vertical double ellipsoid optical configuration and a 5 kW air-cooled xenon lamp positioned at the focal point of the lower mirror.20 The growth process was conducted in a vacuum chamber under 3 MPa flowing Ar atmosphere purified with a Ti-getter furnace, leaving oxygen traces 2σ(I)); Nvar, 9; R1(F), 0.0194; wR2(F2), 0.0199; GoF(all), 1.292; ΔFmin,max, -1.454, 1.777 e- A˚-3.
3. Results and Discussion In Figure 1, the image of a CeCu2Si2 crystal grown by the FZ method is shown. The crystal was grown from a polycrystalline feed rod with a composition Ce20Cu41Si39 containing excess copper. A coarse crystalline seed from a previous growth experiment was used. The traveling speed was reduced from 2.3 mm/h in the starting phase to 1.4 mm/h when the growth process had lasted 15.5 h. The growth process is more stable at the lower growth velocity. These conditions greatly differ from former vertical FZ experiments with RF heating,18
Cao et al.
Figure 1. Image of a CeCu2Si2 single crystal grown by the FZ method with two growth velocities v = 2.3 mm/h and 1.4 mm/h from a polycrystalline seed (left). The quenched last zone (LZ) is visible immediately before the sample holder (right).
Figure 2. Operating temperature of the floating zone during the growth of a CeCu2Si2 crystal (dashed line) and time averaged temperature course (solid line). The length coordinate z = t 3 v is related to the processing time t and the zone traveling velocity v. Inset: Recorded axial temperature profile across the floating zone measured from crystal (left) to feed rod after the start of the growth process. The zone length L and the liquidus temperature TL = 1550 C are denoted.
where much higher speeds of 18 and 38 mm/h, respectively, and CeCu2.5-2.9Si2 feed rods with huge excess Cu were applied. Obviously, for those parameters, large columnar CeCu2Si2 grains separated by an interdendritic Cu-rich phase were received instead of a crystal homogeneous over the whole crosssection. According to the literature, the evaporation is reduced by an inert gas at elevated pressure and large diameter of the molecule.27 To limit Cu evaporation during the extensive time of the FZ growth process elevated pressure, 3 MPa flowing Ar atmosphere was applied and the floating zone was kept small, typically
