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.

Cutting Edge of High-Entropy Alloy Superconductors from the Perspective of Materials Research

We propose an efficient way of increasing the entropy of mixing in high-entropy-alloy-type compounds, which can be achieved by multi-site alloying. As an example of this concept, we report the synthesis and observation of polycrystalline samples of new high-entropy-alloy-type metal chalcogenides (Ag,In,Pb,Bi)Te1-xSex (x = 0.0, 0.25, and 0.5) with a NaCl-type structure. The samples were synthesized using high pressure synthesis. Superconductivity with transition temperatures of 2.7, 2.5, and 2.0 K was observed with x = 0.0, 0.25, and 0.5, respectively. To investigate the multi-site alloying effect on the entropy of mixing (ΔSmix) for the examined samples, we calculated the total ΔSmix for two crystallographic sites. For the samples with x = 0.25 and 0.5, ΔSmix reaches 1.89R and 2.00R, respectively, which exceed the ΔSmix of 1.79R for a simple (single-site) high-entropy alloy containing six different elements.

An efficient way of increasing the total entropy of mixing in high-entropy-alloy compounds: a case of NaCl-type (Ag,In,Pb,Bi)Te1-xSex (x = 0.0, 0.25, 0.5) superconductors.

Research on high-entropy-alloy (HEA) superconductors is a growing field in material science. In this study, we explored new HEA-type superconductors and discovered a CuAl2-type superconductor Co0.2...

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2020.1860147

Superconductivity in CuAl2-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 with a high-entropy-alloy transition metal site

We report on the synthesis and superconductivity of high-entropy-alloy-type (HEA-type) compounds TrZr2 (Tr = Fe, Co, Ni, Rh, Ir), in which the Tr site satisfies the criterion of HEA. Polycrystalline samples of HEA-type TrZr2 with four different compositions at the Tr site were synthesized by arc melting method. The phase purity and crystal structure were examined by Rietveld refinement of X-ray diffraction profile. It has been confirmed that the obtained samples have a CuAl2-type tetragonal structure. From analyses of elemental composition and mixing entropy at the Tr site, the HEA state for the Tr site was confirmed. The physical properties of obtained samples were characterized by electrical resistivity and magnetization measurements. All the samples show bulk superconductivity with various transition temperature (Tc). The Tc varied according to the compositions and showed correlations with the lattice constant c and Tr-Zr bond lengths. Introduction of an HEA site in TrZr2 is useful to achieve systematic tuning of Tc with a wide temperature range, which would be a merit for superconductivity application.

Synthesis of high-entropy-alloy-type superconductors (Fe,Co,Ni,Rh,Ir)Zr2 with tunable transition temperature

Research on high-entropy-alloy (HEA) superconductors is a growing field in material science. In this study, we explored new HEA-type superconductors and discovered a CuAl2-type superconductor Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 with a HEA-type transition metal site. A superconducting transition was observed at 8.0 K after electrical resistivity, magnetization, and specific heat measurements. The bulk characteristics of the superconductivity were confirmed through the specific heat measurements. The discovery of superconductivity in HEA-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 will provide a novel pathway to explore new HEA-type superconductors and investigate the relationship between the mixing entropy and superconductivity of HEA-type compounds.

High-entropy-alloy-type tellurides M-Te, which contain five different metals of M = Ag, In, Cd, Sn, Sb, Pb, and Bi, were synthesized using high pressure synthesis. Structural characterization revealed that all the obtained samples have a cubic NaCl-type structure. Six samples, namely AgCdSnSbPbTe5, AgInSnSbPbTe5, AgCdInSnSbTe5, AgCdSnPbBiTe5, AgCdInPbBiTe5, and AgCdInSnBiTe5 showed superconductivity. The highest transition temperature (T c) among those samples was 1.4 K for AgInSnSbPbTe5. A sample of AgCdInSbPbTe5 showed a semiconductor-like transport behavior. From the relationship between T c and lattice constant, it was found that a higher T c is observed for a telluride with a larger lattice constant.

https://iopscience.iop.org/article/10.35848/1882-0786/ab7482/pdf

Superconducting properties of high-entropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure

High-entropy-alloy-type tellurides M-Te, which contain five different metals of M = Ag, In, Cd, Sn, Sb, Pb, and Bi, were synthesized using high pressure synthesis. Structural characterization revealed that all the obtained samples have a cubic NaCl-type structure. Six samples, namely AgCdSnSbPbTe5, AgInSnSbPbTe5, AgCdInSnSbTe5, AgCdSnPbBiTe5, AgCdInPbBiTe5, and AgCdInSnBiTe5 showed superconductivity. The highest transition temperature (Tc) among those samples was 1.4 K for AgInSnSbPbTe5. A sample of AgCdInSbPbTe5 showed a semiconductor-like transport behavior. From the relationship between Tc and lattice constant, it was found that a higher Tc is observed for a telluride with a larger lattice constant.

Superconducting properties of high-entropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure.

We have investigated the Pb-substitution effect upon the superconductivity of NaCl-type In1−xPbxTe. Polycrystalline samples with x = 0–0.8 were synthesized using high-pressure synthesis. The lattice parameter was systematically increased by Pb substitution. For x ≤ 0.6, bulk superconductivity was observed, and the superconducting transition temperature increased from 3 K (for InTe) to 5 K by Pb substitutions. From analyses of specific heat jumps at the superconducting transition, conventional (phonon-mediated) weak-coupling pairing mechanisms were suggested for In1−xPbxTe.

High-Pressure Synthesis and Superconducting Properties of NaCl-Type In1−xPbxTe (x = 0–0.8)

We have investigated the Pb-substitution effect on the superconductivity of NaCl-type In1-xPbxTe samples obtained using high-pressure synthesis. Polycrystalline samples with x = 0-0.8 were obtained, and the lattice parameter systematically increased by Pb substitution. The superconducting transition temperature (Tc ~3 K for x = 0) increased to 5.3 K (for x = 0.4) by Pb substitutions. According to the comparison of the electronic states between NaCl-type InTe and PbTe, we consider that the Pb substitution increases electron carriers. Possible mechanisms of the enhancement of Tc in this system have been discussed on the basis of conventional pairing mechanism and the possibility of the relation to the valence-skipping state of In.

Masayoshi Katsuno

Papers

Superconducting properties of high-entropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure

Superconducting properties of high-entropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure.

High-Pressure Synthesis and Superconducting Properties of NaCl-Type In1−xPbxTe (x = 0–0.8)

High-pressure synthesis and superconducting properties of NaCl-type In1-xPbxTe (x = 0-0.8)