Key Takeaways & Executive Findings
- •• • NaB8 achieves a superconducting critical temperature (Tc) of 24.4 K at ambient pressure, among the highest reported for boron cage compounds, enabling potential energy-efficient power applications. • • ZrB8 exhibits the highest hardness of 21.9 GPa among the studied MB8 compounds, approaching superhard thresholds and offering mechanical robustness for industrial coatings. • • s-block metals (e.g., Na, Mg) increase the electron-phonon coupling constant (λ) and Tc but reduce hardness, whereas d-block metals (e.g., Zr, Ti) enhance hardness at the expense of Tc, revealing a tunable trade-off. • • The study systematically maps the influence of metal centers on p-orbital occupancy of boron bands, providing a design rule for balancing superconductivity and hardness in cage-like borides.
Abstract
Borides, exhibiting complex bonding and structural diversity, are promising materials in the fields of ambient superconductivity and hard materials. Although some cage borides with either superhard or superconductivity have been reported, there is still a lack of systematic tuning of the metal centers in regulating their superconductivity and hardness. This study investigates the impact of s-block/d-block metal elements on the superconducting and hardness properties of boron cage lattices under ambient conditions. Using first-principles calculations, we predict a novel class of stable cage-like metallic borides, MB8 (M = Na, Be, Mg, Sr, Sc, Y, Ti, Zr, Hf, V, Nb, Ta) characterized by metal embedded in B–B sublattices composed of 4/8 member B-rings. Superconductivity, hardness, and electronic structure calculations indicate that s-block and d-block metals influence the p-orbital occupancy of B bands, affecting both the electron-phonon coupling (EPC) constant and the bonding strength. In general, within the I422 MB8 (B16 cage) structural family investigated here at ambient pressure, s-block metals enhance the EPC constant (λ) and favour higher superconducting critical temperature (Tc) but weaken the hardness, whereas d-block metals contribute the opposite. Therefore, the Tc of NaB8 reached 24.4 K, which is among the highest values reported for boron cage compounds, while ZrB8 from d-block metals exhibits the highest hardness (21.9 GPa) among. This work proposes a pathway for designing novel superconductors with robust mechanical performance under ambient pressure and elucidates the distinct roles of s-block and d-block metals in modulating the superconductivity and hardness of cage-like borides.
1. Introduction
The pursuit of materials that simultaneously exhibit high electrical conductivity and exceptional hardness has long been a central challenge in materials science, with applications spanning energy transmission, electronics, and aerospace. Traditional superhard materials such as diamond and cubic boron nitride achieve hardness through strong covalent bonds, but these bonds inherently suppress electron mobility, resulting in wide band gaps or insulating behavior. Conversely, conventional metals offer excellent conductivity but lack mechanical robustness. This intrinsic trade-off has hindered the development of materials that can withstand extreme conditions while efficiently transporting charge.
Metal borides (MBs) have emerged as a promising class of compounds that can potentially reconcile these conflicting properties by combining covalent, ionic, and metallic bonding. Boron atoms form robust B–B covalent networks that contribute to hardness, while metal atoms with lower electronegativity donate electrons via M–B bonds, facilitating electron transfer. This synergy enables diverse electronic behaviors, from semiconducting to superconducting. For instance, MgB2 exhibits a superconducting critical temperature of 39 K at ambient conditions, with electronic states dominated by boron orbitals coupling strongly to specific phonon modes. However, systematic tuning of metal centers to optimize both superconductivity and hardness remains underexplored. This study addresses this gap by investigating a novel class of cage-like borides MB8 (M = s-block or d-block metals) under ambient pressure, using first-principles calculations to elucidate the distinct roles of metal type in modulating electron-phonon coupling and bonding strength, thereby proposing a pathway for designing hard superconductors.
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Jingkun Yu, Xue Yong, Siyu Lu (2026). Engineering ambient superconductivity and hardness in cage-like borides with s-block and d-block metals. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3878-5
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Frequently Asked Questions
What is the maximum superconducting critical temperature (Tc) achieved in this study, and which compound exhibits it?
The maximum Tc is 24.4 K, achieved in NaB8 at ambient pressure. This is among the highest values reported for boron cage compounds, indicating potential for practical superconducting applications without external pressure.
How does the choice of s-block versus d-block metal influence the trade-off between superconductivity and hardness?
s-block metals (e.g., Na, Mg) enhance the electron-phonon coupling constant (λ) and thus Tc, but they weaken hardness. Conversely, d-block metals (e.g., Zr, Ti) contribute to higher hardness but lower Tc. For example, ZrB8 exhibits the highest hardness of 21.9 GPa, while NaB8 has the highest Tc of 24.4 K.
What is the structural family of the predicted compounds, and what are the key structural features?
The compounds belong to the I422 MB8 (B16 cage) structural family, where metal atoms are embedded in B–B sublattices composed of 4/8 member B-rings. This cage-like arrangement is stable under ambient conditions and allows for systematic variation of metal centers.
What computational methods were used to predict these properties, and how reliable are they?
First-principles calculations based on density functional theory (DFT) were employed to evaluate stability, superconductivity, and hardness. The methods are well-established for predicting material properties, and the results align with known trends in boride superconductors, providing confidence in the predictions.
What are the potential industrial applications of these cage-like borides, and what challenges remain for their synthesis?
These materials could be used in applications requiring both high conductivity and mechanical robustness, such as cutting tools, wear-resistant coatings, and superconducting wires. However, synthesis of these predicted compounds has not yet been demonstrated, and challenges include achieving phase purity and scaling up production. Further experimental validation is needed.
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