Abstract Body

Laves phases are a type of intermetallic compounds with the chemical composition AB₂, and classified into cubic MgCu₂-type, hexagonal MgZn₂-type, and MgNi₂-type based on the differences in the B₄ tetrahedral network within the crystal structure. Laves phases are of considerable interest due to their diverse crystal structures and combinations resulting in intriguing superconducting properties. Examples include the high upper critical field in MgCu₂-type (Hf,Zr)V₂ [1], and the time-reversal symmetry breaking which suggests unconventional superconductivity in MgZn₂-type HfRe₂ [2].

In recent years, the introduction of third elements into binary Laves phases has been investigated to induce superconductivity and to improve the superconducting transition temperature (T_c) associated with structural changes. For example, the emergence of superconductivity associated with the formation of derived structures has been observed in Mg₂Cu₃Si-type Mg₂Ir₃Si (T_c = 7 K) [3] and Mg₂Ir_2.3Ge_1.7 (T_c = 5.3 K) [4] which were derived by introducing Si and Ge into MgZn₂-type MgIr₂ [3]. A significant increase in T_c has been observed inMg₂Cu₃Si-type Ta₂V_3.1Si_0.9 (T_c = 7.5 K) [5] achieved by introducing Si into MgCu₂-type TaV₂ (T_c = 3.2 K) [6]. Moreover, the nonmonotonic dependence of Tc on Si substitution in MgCu₂-type Sc₂Ir_4-xSi_x (x = 0 ~ 0.7) [7]have been reported. Thus, the extension to the ternary Laves phase provides a promising route to explore and improve superconductors.

In this study, we attempted to introduce various third elements into BaIr₂ (T_c = 2.7 K) [8], which we recently discovered by high-pressure synthesis, to stabilize the new superconducting phase at ambient pressure by inducing structural modifications. As a result, we found that the new compound Ba₅Ir₇Ge₄ crystallizes at ambient pressure by adding Ge. Structural analysis demonstrated that Ba₅Ir₇Ge₄ belongs to the tetragonal space group I 4₁/a with a = 12.9726(2) Å, c = 8.2703(2) Å, and that 40% of the Ir₄ tetrahedron of BaIr₂ (= Ba₅Ir₁₀) are replaced by Ge₄Ir (Ba₅Ir₆Ge₄Ir = Ba₅Ir₇Ge₄), resulting in a corner-sharing network of Ge₄Ir and Ir₃Ge tetrahedron. Magnetic susceptibility, electrical resistivity (Fig.1), and specific heat measurements revealed that Ba₅Ir₇Ge₄ is a type-II superconductor with T_c = 3.2 K.

References

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