Group IV element allotropes in the Fmmm phase: First-principles calculations

Three new Group IV element allotropes Fmmm C72, Si72, and Ge72 are proposed in this paper, and their physical properties are researched by density functional theory (DFT). Each novel allotrope has mechanical, dynamical, and thermodynamic stability. The study of the elastic modulus shows that Fmmm Si72 has greater elastic anisotropy than Fmmm C72 and Ge72. Fmmm C72 is a superhard material with the hardness of 47.8 GPa. The investigation of the electronic band structures of Fmmm Si72 and Ge72 exhibits they have semiconducting properties, while Fmmm C72 shows metallicity. Fmmm Si72 has a direct band gap (1.41 eV), and it is very suitable for solar cells. In addition, Fmmm Si72 has good absorption capacity in the visible light region, so Si72 has possible applications in photovoltaic applications due to its direct band gap property and good absorption capacity in the visible light region.