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

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Sc-Zr-Nb-Rh-Pd and Sc-Zr-Nb-Ta-Rh-Pd HighEntropy Alloy Superconductors on a CsCl-type lattice Karoline Stolze, Jing Tao, Fabian O. von Rohr, Tai Kong, and Robert J. Cava Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04578 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Chemistry of Materials

Karoline Stolze*†, Jing Tao‡, Fabian O. von Rohr§, Tai Kong†, and Robert J. Cava*†

†Department

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

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 (Tcs) depending on the valence electron count (VEC); the Tcs 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.

interest due to recent high pressure studies, as it shows 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-Ti-based 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 separately on one or the other position, building interpenetrating primitive cubic lattices. A direct result of this atomic arrangement is mostly brittle mechanical

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 hexagonal-closed 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 single-phase 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

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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)1x[RhPd]x HEA superconductors with a CsCl-type 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 dramatically influences the superconducting properties of the materials.

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 to prepare the X-ray samples did not cause any sample contamination, as no indication was seen in the pXRD patterns, the TEM studies or the EDX spectra for steel-related phases or the elements from the bolt cutter or file. The pXRD patterns were obtained on a Bruker D8 Advance Eco in Bragg-Bretano Geometry with Cu-Kradiation and a LynxEye-XE detector. LeBail fits for determining the average cell parameters of all samples and Rietveld refinement for selected samples were performed using the FullProf Suite26 with Thompson-Cox-Hastings pseudo-Voigt peak shapes. Transmission Electron Microscopy and EDX measurements. The high-resolution transmission electron microscopic (HRTEM) experiment was carried out using a JEOL ARM 200F microscope with the accelerating voltage of 200 kV of a cold field-emission gun and double Cs-correctors at Brookhaven National Laboratory. The HRTEM samples were prepared by distributing tiny amounts of the filed HEA samples directly on a holey carbon grid, as it was not possible to grind the alloys to obtain thin fragments. Compositions were verified for selected materials by performing energy dispersive X-ray spectroscopy (EDX) studies with the accelerating voltage of 15 kV on surfaces of flattened sheets with a SU8020 (Hitachi) scanning electron microscope equipped with a (SDD) X-MaxN (Oxford) spectrometer, collected and analyzed employing the INCA suite of programs. Physical Property Measurements. The magnetizations and resistivities were studied on bulk sample pieces using a Quantum Design Physical Property Measurement System (PPMS) DynaCool equipped with a Vibrating Sample Magnetometer (VSM) Option. Zero-field cooled (ZFC) temperature-dependent magnetization measurements were carried in a field of µ0H = 2 mT. For the resistivity measurements, a standard four-probe technique was used with an applied current of I = 3 mA.

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 K.23,24 Structure

Compound

Tc (K)

BCC elements

V

5.4

Nb

9.3

Ta

4.5

MoxRe1−x

11.8

NbxTa1−x

9.0

NbxTi1−x

9.9

NbxZr1−x

11.1

AuSc

2.0

RuV

5.1

HfOs

2.4

RhZr

2.4

BCC alloys

Cubic primitive CsCl-type

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Syntheses. All samples were prepared by mixing stoichiometric amounts of scandium pieces (distilled, 99.8 % purity, stored under Ar in glove box), 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 co-heating of a zirconium sponge at each melting step. The samples were re-melted 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


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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 4

5

6

Normalized Sum Formula

x

Composition

Δm(%)*

VEC (e/a)**

(ScZr)0.50[RhPd]0.50

0.50

Sc0.250(2)Zr0.250(1)Rh0.250(1)Pd0.250(1)

0.98

6.50

(ScZrNb)0.55[RhPd]0.45

0.45

Sc0.184(1)Zr0.184(1)Nb0.184(1)Rh0.224(1)Pd0.224(1)

0.64

6.47


0.42

Sc0.194(1)Zr0.194(1)Nb0.194(1)Rh0.210(1)Pd0.210(1) †

0.52

6.31


0.40

Sc0.200(2)Zr0.200(1)Nb0.200(1)Rh0.200(1)Pd0.200(1)

0.92

6.20


0.39

Sc0.203(1)Zr0.203(1)Nb0.203(1)Rh0.195(1)Pd0.195(1)

0.36

6.15


0.38

Sc0.206(1)Zr0.206(1)Nb0.206(1)Rh0.191(1)Pd0.191(1)

0.76

6.10


0.37

Sc0.209(1)Zr0.209(1)Nb0.209(1)Rh0.186(1)Pd0.186(1)

0.76

6.04


0.36

Sc0.213(1)Zr0.213(1)Nb0.213(1)Rh0.181(1)Pd0.181(1) ††

0.44

5.99


0.35

Sc0.216(2)Zr0.216(1)Nb0.216(1)Rh0.176(1)Pd0.176(1)

0.84

5.94

(ScZrNbTa)0.57[RhPd]0.43

0.43

Sc0.143(1)Zr0.143(1)Nb0.143(1)Ta0.143(1)Rh0.214(1)Pd0.214(1)

0.24

6.50


0.38

Sc0.155(1)Zr0.155(1)Nb0.155(1)Ta0.155(1)Rh0.190(1)Pd0.190(1)

0.48

6.24


0.35

Sc0.162(2)Zr0.162(1)Nb0.162(1)Ta0.162(1)Rh0.176(1)Pd0.176(1)

1.04

6.09


0.33

Sc0.167(1)Zr0.167(1)Nb0.167(1)Ta0.167(1)Rh0.167(1)Pd0.167(1)

0.68

6.00


0.328

Sc0.168(3)Zr0.168(1)Nb0.168(1)Ta0.168(1)Rh0.164(1)Pd0.164(1)

1.32

5.98


0.32

Sc0.169(2)Zr0.169(1)Nb0.169(1)Ta0.169(1)Rh0.162(1)Pd0.162(1)

0.92

5.95


0.315

Sc0.171(1)Zr0.171(1)Nb0.171(1)Ta0.171(1)Rh0.158(1)Pd0.158(1)

0.48

5.91


0.31

Sc0.173(1)Zr0.173(1)Nb0.173(1)Ta0.173(1)Rh0.154(1)Pd0.154(1)

0.76

5.86

*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 to even small changes in the composition. Thus, the broad transition of the sample with x = 0.32 can be explained due to small chemical inhomogeneities.

(1)

(Figure 7). The model, which goes back to the

Ginzburg-Landau Theory and was also used to explain the temperature dependence of the upper critical field of PbTaSe2,34 Nb0.18Re0.82,35 Mo3Al2C,36 and the borocarbide intermetallic superconductors,37 for example, gave very good fits to all experimental data (R2: 0.9946, 0.9992, 0.9999, and 0.9997 for x: 0.40, 0.38, 0.37, and 0.35, respectively). The upper critical fields at zero temperature µ0Hc2(0) for (ScZrNb)1-x[RhPd]x with x = 0.40, 0.38, 0.37 and 0.35 are exhibited in Figure 7. The upper critical fields increase with decreasing x, in accord with the trend of the critical temperatures. Hence, the HEA sample with the lowest Rh/Pd-content, i.e. x = 0.35 and VEC = 5.94, displays the highest Tc ≈ 9.3 K as well as the largest µ0Hc2(0) = 10.7 T. Both parameters fulfill two of the three critical materials characteristics for practical superconductors: reasonably high critical temperatures and large upper critical fields. Paired with their advantageous mechanical properties, such as ductility and hardness, the superconducting (ScZrNb)1-x[RhPd]x HEA may be of interest for potential applications. Composition dependence of the superconducting transition and upper critical fields in (ScZrNbTa)1-x[RhPd]x HEAs. We also obtained the hexanary (ScZrNbTa)1x[RhPd]x HEA by adding tantalum in equi- or near-



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Figure 9. Composition dependence of the superconducting transition in (ScZrNbTa)1-x[RhPd]x with x = 0.43, 0.38, 0.35, 0.33, 0.328, 0.32, 0.315, and 0.31. Normalized temperaturedependent ZFC magnetization, measured in an external magnetic field of µ0H = 2 mT. Inset: Temperature-dependent upper critical fields µ0Hc2(T) of (ScZrNbTa)1-x[RhPd]x HEA with x = 0.33 and 0.315. The open circles are the 50%-values obtained from the magnetic field dependent ρ(T) plots; the dotted lines show the fits for the determination of µ0Hc2(0) according to the equation (1).

Figure 8. PXRD patterns of filed powder samples of (ScZrNbTa)1-x[RhPd]x with x = 0.43, 0.38, 0.35, 0.33, 0.328, 0.32, 0.315, and 0.31. The black h,k,l-indices represent the peak position for the reflections indexing a cubic primitive CsCltype lattice (Pm3̅m). The grey bars emphasize the peak shift towards smaller 2θ values with increasing (ScZrNbTa)content indicating an increase of the lattice parameter a.

The transition temperatures of the (ScZrNbTa)1-x[RhPd]x HEAs plotted as a function of their VEC follow a similar trend like the pentanary CsCl-type HEAs with the same steep slope, but with a more pronounced, almost s-like curvature (Fig. 5, orange circles and orange trend line). Compared to the behaviors for crystalline and amorphous alloys, as well as for the bcc [TaNb]1-x(HfZrTi)x HEA superconductors, the more complex hexanary CsCl-type (ScZrNbTa)1-x[RhPd]x HEA superconductors follow their own trend. The temperature dependence of the upper critical fields µ0Hc2(T) of (ScZrNbTa)1-x[RhPd]x with x = 0.33 and 0.315 are depicted in the inset of Figure 9. For the approximation of the upper critical field at zero temperature µ0Hc2(0) the µ0Hc2(T) data was fitted with the formula (1)33 (R2: 0.9936 and 0.9976 for x: 0.33 and 0.315, respectively) also used for the (ScZrNb)1-x[RhPd]x HEAs. In accord with the trend of the critical temperatures, the upper critical fields increase with decreasing x. An overview for the Tc derived from the magnetization and resistivity, and the upper critical field at zero temperature µ0Hc2(0) for (ScZrNb)1x[RhPd]x and for (ScZrNbTa)1-x[RhPd]x is given in Table 3. The critical temperatures observed in the transport measurement (Tc,ρ) for x = 0.33 is significantly higher than the Tc from the magnetization. This may be due to the higher sensitivity of the transport experiment to filamentary superconductivity. Influence of the chemical makeup. In order to investigate the influence of the chemical makeup on the superconductivity of the CsCl-type HEAs, the quaternary (ScZr)0.50[RhPd]0.50, pentanary (ScZrNb)0.60[RhPd]0.40 and

hexanary (ScZrNbTa)0.67[RhPd]0.33 alloys, all with equimolar ratios of their constituent elements, were selected. The pXRD patterns of the three samples are shown in the inset of Figure 10. (ScZr)0.50[RhPd]0.50 also crystallizes on a cubic primitive CsCl-type lattice (Pm3̅m) with broad diffraction peaks. The peak positions shift slightly towards smaller 2θ values with increasing number of principle elements indicating an increase of the lattice parameter a [from 3.274(1) Å 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 Tc ≈ 9.2 K and [TaNb]0.67 (HfZrTi)0.33 with Tc ≈ 7.6.15


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Table 3. Superconducting critical temperatures derived from the magnetization and resistivity, and upper critical field at zero temperature µ0Hc2(0) for (ScZrNb)1-x[RhPd]x (5) and for (ScZrNbTa)1-x[RhPd]x (6). No. of elements x

5

6

Tc, magnetization Tc, resistivity µ0Hc2(0) (K)

(K)

(T)

0.40

4.2

5.2

2.1

0.38

6.6

9.2

8.9

0.37

8.1

9.3

9.6

0.35

9.3

9.7

10.7

0.33

3.2

4.8

2.1

0.315 6.2

6.4

8.8

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 CsCltype lattice (Pm3̅m).

This shows that especially the presence of the element Nb in a certain concentration as well as the elemental makeup in the HEA is important for obtaining an optimal superconducting transition temperature.

We have synthesized the new pentanary (ScZrNb)1x[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 CsCltype lattice (Pm3̅m) with mixed site occupancy and the 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 transitions temperatures of the pentanary and hexanary HEAs is neither that of crystalline nor amorphous transition metal superconductors, as the CsCl-type HEAs 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.

Supporting Information. Rietveld refinement plots, Tables with crystallographic data and details of the structure determination. This material is available free of charge via the Internet at http://pubs.acs.org.

*K. Stolze, E-mail: [email protected] *R. J. Cava, E-mail: [email protected] The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 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. The authors declare no competing financial interest.

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

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