Sc-Zr-Nb-Rh-Pd and Sc-Zr-Nb-Ta-Rh-Pd High-Entropy Alloy

High-Entropy Alloys (HEAs) are systems composed of five or more metallic ... definition, as mixtures of solid solutions with the same or different lat...
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Cite This: Chem. Mater. 2018, 30, 906−914

Sc−Zr−Nb−Rh−Pd and Sc−Zr−Nb−Ta−Rh−Pd High-Entropy Alloy Superconductors on a CsCl-Type Lattice Karoline Stolze,*,† Jing Tao,‡ Fabian O. von Rohr,§ Tai Kong,† and Robert J. Cava*,† †

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Chemistry, University of Zürich, CH-8057 Zürich, Switzerland ‡

S Supporting Information *

ABSTRACT: We have synthesized previously unreported high-entropy alloys (HEAs) in the pentanary (ScZrNb)1−x[RhPd]x and hexanary (ScZrNbTa)1−x[RhPd]x systems. The materials have CsCl-type structures and mixed site occupancies. Both HEAs are type-II superconductors with strongly varying critical temperatures (Tc’s) depending on the valence electron count (VEC); the Tc’s increase monotonically with decreasing VEC within each series, and do not follow the trends seen for either crystalline or amorphous transition metal superconductors. The (ScZrNb)0.65[RhPd]0.35 HEA with the highest Tc, ∼9.3 K, also exhibits the largest μ0Hc2(0) = 10.7 T. The pentanary and hexanary HEAs have higher superconducting transition temperatures than their simple binary intermetallic relatives with the CsCl-type structure and a surprisingly ductile mechanical behavior. The presence of niobium, even at the 20% level, has a positive impact on the Tc. Nevertheless, niobium-free (ScZr)0.50[RhPd]0.50, as mother-compound of both superconducting HEAs found here, is itself superconducting, proving that superconductivity is an intrinsic feature of the bulk material.

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INTRODUCTION High-entropy alloys (HEAs) are systems composed of five or more metallic elements in equimolar or near-equimolar ratios. They can crystallize as a single-phase solid solution, mainly in simple face- or body-centered cubic (fcc, bcc) or hexagonalclosed packed (hcp) lattices,1,2 or, in a broader definition, as mixtures of solid solutions with the same or different lattice type.1,3 Some HEAs contain multiple phases rather than a single-phase solid solution,4,5 e.g., in eutectic HEAs.6,7 The simple structures of HEAs are stabilized by high mixing entropy, whereas the criteria for the formation of single-phase solid solutions in HEAs are complex and continually discussed.8−11 Parameters with significant influence on singlephase formation in HEAs seem to be atomic size differences, the mixing enthalpy, and the mixing entropy. HEAs have recently attracted much attention due to their combinations of tunable properties, such as excellent mechanical performance at high and cryogenic temperatures,12 high hardness,13 simultaneous strength and ductility,14 oxidation resistance,15 and magnetism.16,17 Among the many studied HEAs only one superconducting system has been reported so far, namely, Ta−Nb−Hf−Zr−Ti,18−22 a type-II bulk superconductor with body-centered cubic structure and a maximum Tc ≈ 7.6 K for the composition [TaNb]0.67(HfZrTi)0.33 and an upper critical field μ0Hc2 at zero temperature of 7.75 T.19 This material is of particular interest due to recent high-pressure studies, as it shows © 2018 American Chemical Society

extraordinarily robust zero-resistance superconductivity under pressure up to 190.6 GPa.20 In the solid solution series [TaNb]1−x(HfZrTi)x the superconducting transition temperature increases with a general decrease of x down to 0.3. Furthermore, isoelectronic substitutions lead to strong changes in the critical temperature, showing that not only the VEC but also the elemental makeup of the alloy is crucial for the superconductivity.21 Many of the elemental metal superconductors with transition temperatures over 2 K crystallize on a bcc lattice, among them niobium with the highest elemental Tc = 9.3 K at ambient pressure (Table 1).23 Binary transition metal alloys with simple bcc structures provide the largest number of superconducting candidates as well. Technically important examples, which are predominantly ductile, are shown in Table 1.24 The Nb−Tibased solid solutions still dominate all commercial applications for superconducting materials, and are used for superconducting electromagnets, e.g., in NMR and MRI devices or in the Large Hadron Collider. A very common cubic structure type for simple intermetallic AB compounds is the CsCl-type structure, which is related to the bcc lattice, but instead of randomly mixing the atoms on the corners and the center of the cell, the A and B atoms order Received: October 31, 2017 Revised: January 15, 2018 Published: January 17, 2018 906

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dramatically influences the superconducting properties of the materials.

Table 1. Transition Metal Element and Binary Alloy Superconductors with the Body-Centered Cubic Structure, and Superconducting Binary Intermetallic AB Compounds with the CsCl-Type Cubic Primitive Structure, Tc ≥ 2 K23,24 structure BCC elements

BCC alloys

cubic primitive CsCl-type

compound

Tc (K)

V Nb Ta MoxRe1−x NbxTa1−x NbxTi1−x NbxZr1−x AuSc RuV HfOs RhZr

5.4 9.3 4.5 11.8 9.0 9.9 11.1 2.0 5.1 2.4 2.4

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EXPERIMENTAL SECTION

Syntheses. All samples were prepared by mixing stoichiometric amounts of scandium pieces (distilled, 99.8% purity, stored under Ar in glovebox), zirconium foil (99.8% purity), niobium pieces (99.99% purity, excluding tantalum), tantalum foil (99.9% purity), rhodium pieces (99.9% purity), and palladium granules (99.95% purity). The elemental metals were carefully arc-melted in 250 mg sample sizes to a single metallic nugget under an argon atmosphere with high current (T > 2500 °C, I < 100 A) and quenched on a water-chilled copper plate. The argon atmosphere was cleaned of residual oxygen and moisture by coheating of a zirconium sponge at each melting step. The samples were remelted three times and turned over each time, in order to ensure the optimal mixing of the constituents. Long heating treatments at high current were avoided, and only samples with a weight loss smaller than 1% (pentanary) or 1.4% (hexanary) were characterized. An overview of the samples studied, with the normalized sum formulas, corresponding molar composition, total weight loss from starting weight during preparation, and valence electron concentrations (VECs, i.e., electrons per atom, e/a), is given in Table 2. No amorphous phases are present in the HEA samples, because, for such simple systems, cooling rates of 105 degrees per second are required to form amorphous phases: several orders of magnitude faster than those employed here. X-ray Crystallography. For qualitative and quantitative structural characterization using powder X-ray diffractometry (pXRD) the samples had to be converted into powder form. The metallic nuggets with the composition (ScZrNb)0.55[RhPd]0.45 and (ScZrNbTa)1−x[RhPd]x with x ≥ 0.38 were brittle enough to be ground in a mortar and pestle. All other samples, namely, (ScZr)0.50[RhPd]0.50, (ScZrNb)1−x[RhPd]x, and (ScZrNbTa)1−x[RhPd]x with x ≤ 0.42 and x < 0.38, respectively, exhibit high hardness as well as ductility, and therefore could not be groundthey had to be cut into smaller pieces with a large bolt cutter (Cr−Mo steel) and then filed into powder with a high-carbon steel file to obtain X-ray diffraction (XRD) patterns. The mechanical handling

separately on one or the other position, building interpenetrating primitive cubic lattices. A direct result of this atomic arrangement is mostly brittle mechanical behavior. In contrast to the bcc alloy superconductors, the known simple intermetallics with the CsCl-type structure show comparably lower transition temperatures (Table 1). It seems that the bcc structure favors superconductivity whereas “the CsCl structure is probably one of the least favorable among those with cubic symmetries”, according to Matthias et al.25 In contrast, here we present the new pentanary (ScZrNb)1−x[RhPd]x and hexanary (ScZrNbTa)1−x[RhPd]x HEA superconductors with a CsCltype structure. These materials exhibit higher superconducting transition temperatures than their simple binary intermetallic relatives with the CsCl-type structure and also a surprisingly ductile mechanical behavior. We found that variation of the stoichiometry as well as of the chemical constituents

Table 2. Sample Overview with Sum Formula, Corresponding Composition, VEC, and Total Mass Loss during Sample Preparation for the CsCl-Type HEAs Studied no. of elements

normalized sum formula

x

composition

Δma (%)

VECb (e/a)

4 5

(ScZr)0.50[RhPd]0.50 (ScZrNb)0.55[RhPd]0.45 (ScZrNb)0.58[RhPd]0.42 (ScZrNb)0.60[RhPd]0.40 (ScZrNb)0.61[RhPd]0.39 (ScZrNb)0.62[RhPd]0.38 (ScZrNb)0.63[RhPd]0.37 (ScZrNb)0.64[RhPd]0.36 (ScZrNb)0.65[RhPd]0.35 (ScZrNbTa)0.57[RhPd]0.43 (ScZrNbTa)0.62[RhPd]0.38 (ScZrNbTa)0.65[RhPd]0.35 (ScZrNbTa)0.67[RhPd]0.33 (ScZrNbTa)0.672[RhPd]0.328 (ScZrNbTa)0.68[RhPd]0.32 (ScZrNbTa)0.684[RhPd]0.316 (ScZrNbTa)0.69[RhPd]0.31

0.50 0.45 0.42 0.40 0.39 0.38 0.37 0.36 0.35 0.43 0.38 0.35 0.33 0.328 0.32 0.315 0.31

Sc0.250(2)Zr0.250(1)Rh0.250(1)Pd0.250(1) Sc0.184(1)Zr0.184(1)Nb0.184(1)Rh0.224(1)Pd0.224(1) Sc0.194(1)Zr0.194(1)Nb0.194(1)Rh0.210(1)Pd0.210(1)c Sc0.200(2)Zr0.200(1)Nb0.200(1)Rh0.200(1)Pd0.200(1) Sc0.203(1)Zr0.203(1)Nb0.203(1)Rh0.195(1)Pd0.195(1) Sc0.206(1)Zr0.206(1)Nb0.206(1)Rh0.191(1)Pd0.191(1) Sc0.209(1)Zr0.209(1)Nb0.209(1)Rh0.186(1)Pd0.186(1) Sc0.213(1)Zr0.213(1)Nb0.213(1)Rh0.181(1)Pd0.181(1)d Sc0.216(2)Zr0.216(1)Nb0.216(1)Rh0.176(1)Pd0.176(1) Sc0.143(1)Zr0.143(1)Nb0.143(1)Ta0.143(1)Rh0.214(1)Pd0.214(1) Sc0.155(1)Zr0.155(1)Nb0.155(1)Ta0.155(1)Rh0.190(1)Pd0.190(1) Sc0.162(2)Zr0.162(1)Nb0.162(1)Ta0.162(1)Rh0.176(1)Pd0.176(1) Sc0.167(1)Zr0.167(1)Nb0.167(1)Ta0.167(1)Rh0.167(1)Pd0.167(1) Sc0.168(3)Zr0.168(1)Nb0.168(1)Ta0.168(1)Rh0.164(1)Pd0.164(1) Sc0.169(2)Zr0.169(1)Nb0.169(1)Ta0.169(1)Rh0.162(1)Pd0.162(1) Sc0.171(1)Zr0.171(1)Nb0.171(1)Ta0.171(1)Rh0.158(1)Pd0.158(1) Sc0.173(1)Zr0.173(1)Nb0.173(1)Ta0.173(1)Rh0.154(1)Pd0.154(1)

0.98 0.64 0.52 0.92 0.36 0.76 0.76 0.44 0.84 0.24 0.48 1.04 0.68 1.32 0.92 0.48 0.76

6.50 6.47 6.31 6.20 6.15 6.10 6.04 5.99 5.94 6.50 6.24 6.09 6.00 5.98 5.95 5.91 5.86

6

a

Total weight measured to 1 part in 2500; error from measured weight loss is Tc with a very shallow slope before the curvature turns into a steep slope at ∼Tc for x = 0.33 and 0.328. Especially in this range, the critical temperatures of (ScZrNbTa)1−x[RhPd]x are extremely sensitive

Figure 10. Normalized temperature-dependent magnetization of (ScZr)0.50[RhPd]0.50, (ScZrNb)0.60[RhPd]0.40, and (ScZrNbTa)0.67[RhPd]0.33, measured in an external magnetic field of μ0H = 2 mT. Inset: PXRD pattern of filed powder samples of (ScZr)0.50[RhPd]0.50, (ScZrNb)0.60[RhPd]0.40, and (ScZrNbTa)0.67[RhPd]0.33. The black h,k,l-indices represent the peak position for the reflections indexing a cubic primitive CsCl-type lattice (Pm3̅m). 912

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increasing number of principle elements indicating an increase of the lattice parameter a [from 3.274(1) to 3.286(1) Å] as can be expected from the increased proportion of the atom set with the larger average Van der Waals radii,38 that are (ScZr), 50%; (ScZrNb), 60%; and (ScZrNbTa), 67%, respectively. The normalized ZFC magnetization curves (Figure 10) show that the Tc changes significantly with the number of the constituent elements, although a systematic comparison is difficult as the three alloys are not isoelectronic, and the superconducting transition also strongly depends on the VEC. (ScZr)0.50[RhPd]0.50 becomes superconducting around 2.2 K even without niobium, which implies that the superconducting properties of these HEAs are not only a compositional average of the properties of the constituent elements (i.e., due to the presence of very dilute niobium in an inert matrix), but rather that of a single superconducting phase. The presence of tantalum decreases the Tc from 4.2 K in (ScZrNb)0.60[RhPd]0.40 to 3.0 K in (ScZrNbTa)0.67[RhPd]0.33, although the hexanary compound reveals the smaller VEC. The same influence of the chemical makeup on the Tc was observed for isoelectronic [Nb] 0.67 (HfZrTi) 0.33 with T c ≈ 9.2 K and [TaNb] 0.67 (HfZrTi)0.33 with Tc ≈ 7.6.15 This shows that especially the presence of the element Nb in a certain concentration as well as the elemental makeup in the HEA are important for obtaining an optimal superconducting transition temperature.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04578. Rietveld refinement plots, tables with crystallographic data, and details of the structure determination (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Karoline Stolze: 0000-0002-1909-4313 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by the Gordon and Betty Moore Foundation EPiQS program, grant GBMF-4412. Work done at Brookhaven National Laboratory was supported by the DOE BES, by the Materials Sciences and Engineering Division under Contract DE-SC0012704, and through the use of the Center for Functional Nanomaterials.

SUMMARY AND CONCLUSION

Notes

We have synthesized the new pentanary (ScZrNb)1−x[RhPd]x HEAs with x = 0.45, 0.42, 0.40, and 0.39−0.35 and the hexanary (ScZrNbTa)1−x[RhPd]x HEAs with x = 0.43, 0.38, 0.35, 0.33, 0.328, 0.32, 0.315, and 0.31, as well as (ScZr)0.50[RhPd]0.50, by arc-melting of the elements. The pXRD patterns of all samples reveal a cubic primitive CsCl-type lattice (Pm3m ̅ ) with mixed site occupancy and broad diffraction peaks characteristic of a generally low crystallinity. These HEAs are materials partway between an ordered intermetallic compound and a random solid solution. HRTEM experiments show that the average domain size of the crystallites decreases with decreasing x for (ScZrNb)1−x[RhPd]x. All compounds, except for the most [RhPd]-rich samples of both solution series, are type-II superconductors with strongly changing critical temperatures depending on the VEC as well as the number of constituent elements. The critical temperatures increase monotonically with decreasing VEC within a solution series. The (ScZrNb)0.65[RhPd]0.35 HEA with the highest Tc ≈ 9.3 K also exhibits the largest μ0Hc2(0) = 10.7 T. The valence electron count dependence of the superconducting transition temperatures of the pentanary and hexanary HEAs is neither that of crystalline nor amorphous transition metal superconductors, as the CsCl-type HEA systems seem structurally more complex and follow their own trend. The critical temperature also changes depending on the chemical composition of the HEAs, and especially the presence of niobium, even in the small amounts present (on the order of 20%), has a positive impact on the superconductivity of the HEAs. Nevertheless, also samples without any niobium are superconducting proving that superconductivity is an intrinsic feature of the bulk material and not just an average of the constituent’s element properties.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Tomasz Klimczuk at the Technical University of Gdansk, Poland, for helpful discussion about critical fields in superconducting alloys.

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REFERENCES

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