Prediction of pure carbon crystals with intrinsic antiferromagnetism: polymerized from C₂₀ fullerenes

Pure carbon materials with magnetic properties have attracted considerable research interest due to their advantages over traditional magnetic materials. Nevertheless, such materials are exceedingly rare. Disrupting the Kekulé valence structures in carbon materials potentially leads to the emergence of magnetism. In this study, using first principles calculations, we developed a range of pure carbon allotropes derived from the smallest fullerene C₂₀ which potentially disrupts the Kekulé valence structures after polymerization. The results indicate that some of the allotropes disrupting the Kekulé valence structures exhibit intrinsic antiferromagnetic ordering, and the magnetism originates from the presence of isolated three-fold coordinated C atoms. The other allotropes adhering to the Kekulé valence structures show non-magnetism with all three-fold coordinated C atoms forming dimers. In all magnetic polymers, magnetism arises from unpaired electrons on the isolated three-fold coordinated carbon atoms, with magnetic moments of about 0.40μ_B at these sites. The adsorption of dopant atoms can significantly alter the magnetic properties of polymers, for instance, the C₂₀-71 polymer with Immm symmetry undergoes a transition from non-magnetic to anti-magnetic ordering upon adsorption of hydrogen atoms. Electronic calculations indicate that these polymers display a range of electronic properties, encompassing both metallic and semiconducting characteristics. Notably, certain magnetic phases exhibit superhard properties, with the hardness value exceeding 40 GPa. This study presents a potential method for designing magnetic carbon materials. Specifically, certain compounds address the gap in magnetic superhard materials composed of light elements, and can be utilized in the field of spintronics where traditional superhard materials are unsuitable.