Integrating intrinsic lightweight, superhard, and magnetic properties in 3D covalent fullerene C₂₄ networks: a...

Multifunctional materials that simultaneously possess intrinsic magnetic and superhard properties, particularly those composed of light elements, have a wide range of applications in advanced sensors, shielding, durable devices, and other fields. However, research on the development and understanding of such materials remains limited. In this study, a series of 3D C₂₄ covalent networks derived from the D_6d C₂₄ fullerene precursor were theoretically designed. These networks exhibit dynamical and mechanical stability across various building configurations. As anticipated, two networks, bqHexa-II and qHexa-II, demonstrate both superhard and magnetic properties. The magnetism in these networks originates from the isolated three-fold coordinated carbon atoms, with magnetic moments of approximately 0.16/0.24 and 0.3μ_B on each magnetic carbon atom. The Vickers hardness H_V of bqHexa-II and qHexa-II is estimated to be 37.7 GPa and 41.9 GPa, respectively, indicating quasi superhard or superhard characteristics. Owing to the hollow structure of the C₂₄ fullerene building block, the 3D C₂₄ networks have a significantly lower mass density compared to conventional superhard materials such as diamond and cubic boron nitride. Electronic calculations reveal that bqHexa-II and qHexa-II networks exhibit features of quasi-direct and direct band gap semiconductors, each with nearly identical band gaps of ∼0.47 eV, considerably lower than those of traditional superhard materials. The other networks behave as direct or indirect band gap semiconductors, with relatively larger energy gaps ranging from 1.51 to 3.50 eV. Furthermore, external biaxial strain can effectively modulate the magnetic and electronic structures of these C₂₄ networks, including the transformation from antiferromagnetism to nonmagnetism. This study presents a promising pathway for exploring lightweight magnetic superhard carbon materials, which have significant potential to advance the development of durable and energy-efficient materials tailored for aerospace and defense applications.