High-entropy alloys (HEAs) are a new class of materials which are being energetically studied around the world. HEAs are characterized by a multicomponent alloy in which five or more elements randomly occupy a crystallographic site. The conventional HEA concept has developed into simple crystal structures such as face-centered-cubic (fcc), body-centered-cubic (bcc) and hexagonal-closed packing (hcp) structures. The highly atomic-disordered state produces many superior mechanical or thermal properties. Superconductivity has been one of the topics of focus in the field of HEAs since the discovery of the bcc HEA superconductor in 2014. A characteristic of superconductivity is robustness against atomic disorder or extremely high pressure. The materials research on HEA superconductors has just begun, and there are open possibilities for unexpectedly finding new phenomena. The present review updates the research status of HEA superconductors. We survey bcc and hcp HEA superconductors and discuss the simple material design. The concept of HEA is extended to materials possessing multiple crystallographic sites; thus, we also introduce multisite HEA superconductors with the CsCl-type, α-Mn-type, A15, NaCl-type, σ-phase and layered structures and discuss the materials research on multisite HEA superconductors. Finally, we present the new perspectives of eutectic HEA superconductors and gum metal HEA superconductors.

Mn 5 Si 3 -type structure has been offering an interstitial chemistry. Recent report of enhancement of superconductivity in a Nb-based Mn 5 Si 3 -type compound by addition of an interstitial atom motivated us to investigate the effect of oxygenaddition in Mn 5 Si 3 -type Zr 5 Pt 3 . The superconducting critical temperature of 6.4 K in the parent Zr 5 Pt 3 is monotonously reduced to 3.2 K in Zr 5 Pt 3 O x (x=0.6) with increasing oxygen-content. As x is further increased from 0.6 to 2.5, exceeding the full occupancy of oxygen site (x=1.0), samples become multi-phases composed of Zr 5 Pt 3 O ∼0.5−0.6 , ZrPt and ZrO 2 . However, the superconducting critical temperature slightly increases to 4.8 K at x=2.5. The metallographic observation has revealed a change of microstructure at x ≥1.0. The change of microstructure and/or the composition effect would be responsible for the enhancement of superconductivity.

We have investigated the superconducting critical temperature T c of solid solution Nb 5 Ir 3−x Pt x O with the Ti 5 Ga 4-type structure. The both end-members of Nb 5 Ir 3 O and Nb 5 Pt 3 O are reported to be superconductors with T c of 10.5 K and 3.8 K, respectively. Particularly Nb 5 Ir 3 O is considered as a two-gap superconductor. The entire series of alloy hold the Ti 5 Ga 4-type structure and show the linear x dependence of lattice parameters. On the other hand, a nonlinear T c vs x plot was obtained, suggesting that T c is not determined by only lattice parameters. The experimental result is discussed based on the Matthias rule and compared with those of the other two-gap superconductors such as MgB 2 , Mo 8 Ga 41 and Nb 3 Sn.

The first purpose of this chapter is materials research on face-centered-cubic (fcc) high-entropy alloy (HEA) superconductors, which have not yet been reported. We have investigated several Nb-containing multicomponent alloys. Although we succeeded in obtaining Nb-containing samples with the dominant fcc phases, no superconducting signals appeared in these samples down to 3 K. The microstructure analyses revealed that all samples were multi-phase, but the existence of several new Nb-containing HEA phases was confirmed in them. The second purpose is the report of materials research on the Mn5Si3-type HEA superconductors. This hexagonal structure offers various intermetallic compounds, which often undergo a superconducting state. The Mn5Si3-type HEA is classified into the multisite HEA, which possesses the high degree of freedom in the materials design and is good platform for studying exotic HEA superconductors. We have successfully found a single-phase Mn5Si3-type HEA, which, however, does not show a superconducting property down to 3 K. The attempt of controlling the valence electron count was not successful.

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