P212121-C16: An ultrawide bandgap and ultrahard carbon allotrope with the bandgap larger than diamond

Ultrawide bandgap semiconductor, e.g., diamond, is considered as the next generation of semiconductor. Here, a new orthorhombic carbon allotrope (P212121-C16) with ultrawide bandgap and ultra-large hardness is identified. The stability of the newly designed carbon is confirmed by the energy, phonon spectrum, ab-initio molecular dynamics and elastic constants. The hardness ranges from 88 GPa to 93 GPa according to different models, which is comparable to diamond. The indirect bandgap reaches 6.23 eV, which is obviously larger than that of diamond, and makes it a promising ultra-wide bandgap semiconductor. Importantly, the experimental possibility is confirmed by comparing the simulated X-ray diffraction with experimental results, and two hypothetical transformation paths to synthesize it from graphite are proposed.