Abstract
Based on literature tetragonal ths topology C8 characterized by trigonal C(sp2), selective insertions of carbon induce large structural changes leading to C(sp3) C12 with dia topology after geometry optimizations to ground state structure using calculations within the quantum density functional theory DFT. Further crystal engineering operated on C12 lead to complete the series with novel C16 and C20. Such allotropes respectively characterized by ene (C=C) and propadiene (C=C=C) -like units beside C4 tetrahedra reveal hybrid C(sp3)/C(sp2) with complex stc topology. Alike ths C8, all three allotropes (C12, C16, and C20) identified in high symmetry I41/amd (No. 141) space group are cohesive with larger magnitude for C12 than for ths C8 and close energies for C16 and C20. Mechanical properties show diamond-like Vickers hardness HV for C12 (HV = 95 GPa) and smaller magnitudes for mixed C(sp3)/C(sp2) C16 (HV = 26 GPa) and C20 (HV = 47 GPa), noting that C8 with C(sp2) was found with the largest compressibility with HV = 19 GPa. Dynamically, all allotropes are stable from positive phonon acoustic and optic frequencies with the largest frequency magnitudes for C16 and C20 corresponding to CC and C=C=C vibrations respectively. Thermodynamically, the temperature changes of the specific heat CV show close agreement with diamond experimental values for C12 and larger magnitudes for the three other allotropes. Electronic band structure analyses reveal conductive C8 and C16, insulating C12 with a large band gap close to diamond's and semi-conducting C20 with small band gap.
Published Version
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