Original Orthorhombic Tetrahedral-Trigonal Hybrid Allotropes C_n (n = 8, 10, 12, 14) with Ethene–Like and Propadiene–Like Units: Crystal Engineering and Quantum Mechanical Calculations

Original carbon allotropes, orthorhombic C₈, C₁₀, C₁₂ and C₁₄ presenting mixed sp²/sp³ carbon hybridizations with C=C ethene-like and C=C=C propadiene-like embedded units are proposed from crystal engineering and calculations within the framework of 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 stackings along the a-orthorhombic direction (vertical) close to but different from “isoglitter”. The carbon allotropes (C₈, C₁₀, C₁₂, C₁₄) were shown to be cohesive and stable both mechanically (elastic properties) and dynamically (phonons), with calculated Vickers hardness (H_V) magnitudes ranging between 25 and 52 GPa, the latter magnitude assigned to C₁₂ possessing the largest number of tetrahedral versus trigonal stackings. High phonon frequencies ω ∼ 50 THz were attributed to stretching mode of C=C (in C₈ and C₁₂) and C=C=C (in C₁₀ and C₁₄) units with good agreement with experiment for Raman molecular identifications. Observed magnitudes of ω ∼ 40 THz were proposed as signatures of C–C simple bonds in the tetrahedra as in diamond. The electronic band structure and electronic density of states DOS shown exemplarily for C₈ point to metallic-like behavior assigned mainly to the itinerant role of trigonal carbon π-electrons.