Recently, new superconductors have been discovered in A₆MX₂ compounds with the hexagonal Zr₆CoAl₂-type crystal structure. Figure 1 shows this crystal structure, which is an ordered Fe₂P type with the noncentrosymmetric space group $P$-$62m$. The discovery of superconductors in the A₆MX₂ family was triggered by Sc₆MTe₂, which was found to exhibit superconductivity in the cases of seven transition metals M (M = Fe, Co, Ni, Ru, Rh, Os, and Ir) [1]. The superconducting transition temperature $T_{\mathrm{c}}$ in these compounds is approximately 2 K for M = 4$d$ and 5$d$ transition metals. However, for M = 3$d$, $T_{\mathrm{c}}$ increases in the order of Ni, Co, and Fe, reaching the highest value of $T_{\mathrm{c}}$ = 4.7 K for Sc₆FeTe₂. A remarkable feature of the A₆MX₂ superconductor family is its compositional flexibility, as the A, M, and X atoms can be substituted with various elements. The number of combinations of these A, M, and X atoms is considerably large, with more than 100 materials synthesized to date, raising expectations for discovering additional $d''-''$electron superconductors within this family.

We focused on Zr₆MBi₂ crystallizing in the Zr₆CoAl₂-type structure and found that Zr₆RuBi₂ and Zr₆FeBi₂ show bulk superconductivity, based on the electrical resistivity, magnetization, and heat capacity measurements of polycrystalline samples [2]. As shown in Fig. 2, Zr₆RuBi₂ and Zr₆FeBi₂ samples exhibit zero resistivity and strong diamagnetic signals owing to superconductivity. Considering the midpoint of the resistivity drop and the onsets of the magnetization drop and heat capacity jump, $T_{\mathrm{c}}$ was determined as 4.9 and 1.4 K for M = Ru and Fe, respectively.

Superconductivity of Zr₆RuBi₂ is most likely conventional; however, its $T_{\mathrm{c}}$ is considerably higher than that of Zr₆FeBi₂. We synthesized Zr₆MBi₂ samples with other M atoms, including Rh, Ir, and Co. However, no material other than M = Ru was confirmed to exhibit bulk superconductivity above 2 K, indicating that an anomalously high $T_{\mathrm{c}}$ is present in Zr₆RuBi₂. This trend in $T_{\mathrm{c}}$ differs from that observed in Sc₆MTe₂, where Sc₆FeTe₂ displayed the highest $T_{\mathrm{c}}$ of 4.7 K [1]. The fact that Zr₆RuBi₂ showed the highest $T_{\mathrm{c}}$ among the current A₆MX₂ members suggests that M being a 3$d$ transition metal element is not always favorable for superconductivity and that the combination of A, M, and X elements is crucial. Recently, Zr₆RuTe₂ is found to show bulk superconductivity below $T_{\mathrm{c}}$ = 1.1 K [3], which exceeds that of Zr₆MTe₂ with other transition metals. We hope that the exploration of new superconductors in the A₆MX₂ family will lead to the discovery of new superconductors with higher $T_{\mathrm{c}}$ or unusual properties.

References

• [1] Y. Shinoda, Y. Okamoto, Y. Yamakawa, H. Matsumoto, D. Hirai, and K. Takenaka, J. Phys. Soc. Jpn. 92, 103701 (2023).
• [2] K. Yuchi, D. Nishio-Hamane, K. Kojima, K. Moriyama, R. Okuma, and Y. Okamoto, J. Phys. Soc. Jpn. 94, 013701 (2025).
• [3] K. Yuchi, H. Matsumoto, D. Nishio-Hamane, K. Moriyama, K. Kojima, R. Okuma, J. Yamaura, and Y. Okamoto, J. Phys. Soc. Jpn. 94, 085001 (2025).

Authors

• Y. Okamoto, K. Yuchi, H. Matsumoto, D. Nishio-Hamane, K. Kojima, K. Moriyama, R. Okuma, and  J. Yamaura