Recent shock compression experiments produced clear evidence of a new carbon phase, but a full structural identification has remained elusive. Here we establish by ab initio calculations a bodycentered cubic carbon phase in Ia3d (O 10 h ) symmetry, which contains twelve atoms in its primitive cell, thus termed BC12, and comprises all-sp 3 six-membered rings. This structural configuration places BC12 carbon in the same bonding type as cubic diamond, and its stability is verified by phonon mode analysis. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak found in shock compression experiments. Electronic band and density of states calculations reveal that BC12 is a semiconductor with a direct band gap of ∼ 2.97 eV. These results provide a solid foundation for further exploration of this new carbon allotrope.
Design and synthesis of three-dimensional denser carbons are one of the hot issues in condensed matter physics because of their fascinating properties. Here we identify by ab initio calculations several tetragonal and monoclinic polymorphs of carbon that adopt the t32, t32*, m32, and m32* structures in P421c, P 43212, P 21/c, and C2 symmetry, respectively. These carbon polymorphs have large 32-atom unit cells in all-sp 3 bonding networks comprising five-and six-membered rings that are dynamically stable as verified by a phonon mode analysis. Electronic band structure calculations show that they are insulators with band gaps in the range of 5.19 ∼ 5.41 eV, close to the calculated band gap of 5.34 eV for diamond. Remarkably, these new carbon phases possess extremely high atom number density exceeding that of diamond. The present results establish a new type of carbon phases and offer insights into their outstanding structural and electronic properties.
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