Novel ultrahard sp2/sp3 hybrid carbon allotrope from crystal chemistry and first principles: Body-centered tetragonal C6 ('neoglitter')

A novel ultrahard carbon allotrope, body-centered tetragonal C-6 (space group I-4m(2)) presenting mixed sp(2)/sp(3) carbon hybridizations is proposed by crystal chemistry approach and studied for the ground state structure and stability (both dynamic and mechanical) within density functional theory (DFT). The structure characterized by C-4 tetrahedra connected with C-C trigonal carbons shows close relationship with C-6 'glitter' devised about 30 years ago. While 'glitter' has underlying net labeled 'tfi' according to RCSR nomenclature, novel C-6 called 'neoglitter' is identified as 'tfa' underlying net. 'Neoglitter' is characterized by large bulk and shear moduli approaching those of diamond, and very high Vickers hardness (H-V = 85 GPa). High phonons frequencies omega similar to 40 THz, close to diamond, are observed and attributed to trigonal C-C pairs stretching. The electronic band structure points to metallic-like behavior assigned mainly to the itinerant role of trigonal carbon pi-electrons. The presence of both trigonal (sp(2)) and tetrahedral (sp(3)) carbon in 'neoglitter' is relevant to an extensive research domain involving nanodiamonds and various electronic applications.