A larger-cell tetragonal carbon allotrope tP176 carbon with superhardness

•A new larger carbon allotrope tP176 carbon with P4/nbm phase is first proposed.•The tP176 carbon is a potentially superhard material with hardness of 55 GPa.•tP176 carbon is a wide and quasi-direct band gap semiconductor material. A new allotrope of sp3 hybrid superhard carbon, tP176 carbon, is proposed and investigated in this work. The tP176 carbon is dynamically, thermodynamically and mechanically stable, respectively. The bulk modulus (B) and shear modulus (G) of tP176 carbon are 242 GPa, and 192 GPa, respectively. In addition, tP176 carbon is a wide band gap semiconductor material with quasi-direct band gap, and the band gap is 2.892 eV. The hardness of tP176 carbon is 55 GPa, which implies that it is a potentially superhard material.