Synthesis of new high-entropy alloy-type Nb3 (Al, Sn, Ge, Ga, Si) superconductors

https://doi.org/10.1016/j.jallcom.2021.159233Get rights and content

Highlights

  • New high-entropy-alloy-type superconductors of Nb3M (M: Al, Sn, Ge, Ga, Si) were synthesized.

  • A superconducting transition at 11.0 K was observed for Nb3Al0.3Sn0.3Ge0.2Ga0.1Si0.1.

  • The HEA state in the M site of Nb3M results in a lower Tc.

Abstract

Studies on high-entropy alloy (HEA) superconductors have recently been increasing, particularly in the fields of materials science and condensed matter physics. To contribute to research on new HEA-type superconductors, in our study we synthesized polycrystalline samples of A15-type superconductors of Nb3Al0.2Sn0.2Ge0.2Ga0.2Si0.2 (#1) and Nb3Al0.3Sn0.3Ge0.2Ga0.1Si0.1 (#2) with an HEA-type site by arc melting. Elemental and structural analyses revealed that the compositions of the obtained samples satisfied the HEA state criteria. Superconducting transitions were observed at 9.0 and 11.0 K for #1 and #2, respectively, in the temperature dependence of magnetization and electrical resistivity. Specific heat measurements revealed that the Sommerfeld coefficient, Debye temperature, and ΔC/γTc for the obtained samples were close to those reported for conventional Nb3Sn family superconductors.

Introduction

High-entropy alloys (HEAs) are typically defined as alloys containing at least five elements with concentrations between 5 and 35 at% [1], [2], resulting in high configurational mixing entropy (ΔSmix), defined as ΔSmix = -RΣicilnci, where ci and R are the compositional ratio and the gas constants, respectively [2]. HEAs have recently attracted much attention in the fields of materials science and engineering because of their tunable properties as structural materials, such as excellent mechanical performance under extreme conditions [1], [2]. As HEA superconductors, simple alloys with bcc, α-Mn, CsCl, and hcp crystal structures have mainly been studied so far [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Among these, Ta0.34Nb0.33Hf0.08Zr0.14Ti0.11 with a superconducting transition temperature (Tc) of 7.3 K, exhibits robustness in the superconducting state under extremely high pressures up to 190 GPa [4]. This suggests that HEA superconductors can be applied under extreme conditions and the study of HEA superconductors can be accelerated.

Thus far, we have extended the concept of HEA to compounds [15], [16], [17], [18], [19], [20], [21], [22], wherein one of the crystallographic sites is high-entropy alloyed. Recently, we reported layered superconductors with HEA-type crystallographic sites: HEA-type cuprate RE123 (RE: rare earth elements) and REO0.5F0.5BiS2 with a HEA-type rare earth (RE) site [15], [16], [17]. In a RE123-type cuprate, no degradation of Tc was confirmed by increasing ΔSmix [15]. In addition, the emergence of bulk superconductivity has been observed in HEA-type REO0.5F0.5BiS2 [16], [17]. Notably, crystal structural analysis revealed suppression of the local structural disorder, which corresponds to the improvement of the bulk nature of superconductivity in this system, with an increase in ΔSmix [17]. Based on these findings, the HEA effects in layered superconductors seem to work positively or at least less negatively. To investigate the effect of HEA on non-layered compounds, we also investigated NaCl-type metal chalcogenide superconductors with high ΔSmix [20], [21], [22]. For instance, an HEA-type AgInSnPbBiTe5 superconductor with Tc = 2.6 K was reported. In contrast to layered systems, NaCl-type HEA tellurides exhibited lower Tc than low-entropy tellurides [21]. These results suggest that the effects of HEAs on the superconducting properties of compounds depend on their crystal structure and dimensionality.

To further investigate the effects of HEAs on superconducting properties, we focused on A15-type Nb3M (M: Al, Sn, Ge, Ga, Si) compounds, which have a relatively high Tc (above 18 K) and upper critical fields (Hc2(0)) of approximately 30 T [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. They are well known as practical materials for superconducting magnets higher than 10 T [33]. In this paper, we report on the syntheses and properties of new HEA-type Nb3M superconductors, wherein the composition at the M site satisfies the compositional criterion of the HEA and achieves ΔSmix above 1.5R.

Section snippets

Experimental details

Polycrystalline samples of Nb3Al0.2Sn0.2Ge0.2Ga0.2Si0.2 (#1) and Nb3Al0.3Sn0.3Ge0.2Ga0.1Si0.1 (#2) were synthesized by arc melting in an Ar atmosphere. Pure metal powders of Nb (99.9%), Al (99.9%), Sn (99.9%), Ge (99.99%), and Si (99.999%) were mixed with the above-mentioned composition and pelletized. Metal pellets and pure Ga (99.9999%) grains were used as starting materials for the arc melting process. Arc melting was repeated after turning the sample three times to ensure homogenization of

Results and discussion

The powder XRD patterns of Nb3Al0.2Sn0.2Ge0.2Ga0.2Si0.2 (#1) and Nb3Al0.3Sn0.3Ge0.2Ga0.1Si0.1 (#2) are shown in Fig. 1. The XRD peaks of the major phase can be indexed using the cubic Cr3Si-type (Pm_3m) model. The lattice constant was estimated using Rietveld refinement and is plotted in Table 1. A schematic image of the refined crystal structure of sample #2 is shown in the inset of Fig. 1. Impurity phases of Nb5Si3 were detected at 8.5% and 4.5% for samples #1 and #2, respectively. Nb5Si3 is

Conclusions

Herein, we have reported the synthesis and superconducting properties of new HEA-type superconductors, Nb3Al0.2Sn0.2Ge0.2Ga0.2Si0.2 and Nb3Al0.3Sn0.3Ge0.2Ga0.1Si0.1. Polycrystalline samples were prepared using pure metals via arc melting. The composition of the obtained samples satisfied the definition of HEA. Superconducting transitions were observed at 9.0 and 11.0 K for #1 and #2, respectively, through magnetization, electrical resistivity, and specific heat measurements. From the

CRediT authorship contribution statement

Aichi Yamashita: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Validation, Visualization, Writing - original draft, Writing - review & editing. Tatsuma D Matsuda: Formal analysis, Investigation, Methodology, Resources, Supervision, Validation, Writing - review & editing. Yoshikazu Mizuguchi: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank M. Yamashita, M. Katsuno, Y. Goto, and O. Miura for their assistance. This work was partly supported by JSPS-KAKENHI (Grant no. 18KK0076) and Tokyo Metropolitan Government Advanced Research (H31-1).

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