Title: Original orthorhombic tetrahedral and trigonal hybrid allotropes Cn with n= 8, 10, 12, 14 possessing ethene -like and propadiene -like units: Crystal chemistry and first principles

Abstract: Original carbon allotropes, orthorhombic C8, C10, C12 and C14 presenting mixed sp2 and sp3 carbon hybridizations exhibiting ethene like and propadiene like embedded units are proposed from crystal chemistry and calculations within the quantum density functional theory DFT. The carbon allotropes with topologies related with jeb, mog, as well as new topologies, show alternating tetrahedral and trigonal carbon stacking along the a orthorhombic direction (vertical) close to but different from isoglitter. The carbon allotropes were shown to be cohesive and stable both mechanically (elastic properties) and dynamically (phonons), with calculated Vickers hardness (HV) magnitudes ranging between 25 GPa and 52 GPa, the latter magnitude assigned to C12 possessing the largest number of tetrahedral versus trigonal stacking. High phonon frequencies close to 50 THz were attributed to stretching mode of ethene (in C8 and C12) and propadiene (in C10 and C14) components with good agreement with experiment for Raman molecular C double bond C intense stretching mode. Frequencies close to 40 THz were proposed as signatures of C...C simple bonds in the tetrahedra. The electronic band structure and electronic density of states DOS shown exemplarily for C8 point to metallic-like behavior assigned to the itinerant role of trigonal carbon pi-electrons.