Hard Superconducting Borides: Phase‐Pure Ca/Sr _x Y _1‐x B ₆ Solid Solutions with Tunable Mechanical and Electronic Properties

Abstract The search for materials combining high hardnessand superconductivity is highly significant for fundamental science and technologicalapplications. However, achieving both properties within a single phase remainschallenging, because their underlying mechanisms are fundamentally conflicting. Here, the phase‐pure Ca/Sr x Y 1‐x B 6 solid solutions are reported, prepared viaborothermal reduction followed by high‐pressure sintering.Structural analyses confirm successful Ca/Sr incorporation into YB6, forming single‐phase solid solutions whose Vickers hardness increases up to 22.35 GPa with higher Ca/Sr content due to solid‐solution strengthening. Electronic transport measurements reveal metallic behavior in Y‐rich phases, whereas enhanced defect scattering leads to nearly temperature‐independent resistivity in Ca/Sr‐rich compositions. Importantly, superconductivity emerges in intermediate compositions with critical temperatures up to 5.8 K, displaying a dome‐like dependence on Sr content. Theoretical calculations show that dopingtunes a balance between the electronphonon coupling constant (λ) and thelogarithmic average phonon frequency (ω log ), yielding the observed superconducting dome, while introduction of dopings suppresses B─B antibonding states and strengthens B─B bonding, thus enhancing hardness. This coexistence in borides offers a newparadigm for designing multifunctional quantum materials with both mechanicalrobustness and superconducting functionality.