  A sp2-sp3 hybrid carbon allotrope named HSH-carbon is proposed by the
first-principles calculations. The structure of HSH-carbon can be regarded as a
template polymerization of [1.1.1]propellane molecules in a hexagonal lattice,
as well as, an AA stacking of recently reported HSH-C10 consisting of carbon
trigonal bipyramids. Based on calculations, the stability of this structure is
demonstrated in terms of the cohesive energy, phonon dispersion, Born-Huang
stability criteria, and ab initio molecular dynamics. HSH-carbon is predicted
to be a semiconductor with an indirect energy gap of 3.56 eV at the PBE level
or 4.80 eV at the HSE06 level. It is larger than the gap of Si and close to the
gap of c-diamond, which indicates HSH-carbon is potentially an ultrawide
bandgap semiconductor. The effective masses of carriers in the VB and CB edge
are comparable with wide bandgap semiconductors such as GaN and ZnO. The
elastic behavior of HSH-carbon such as bulk modulus, Young's modulus and shear
modulus is comparable with that of T-carbon and much smaller than that of
c-diamond, which suggests that HSH-carbon would be much easier to be processed
than c-diamond in practice.
