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

Three new Group IV element allotropes Fmmm C-72, Si-72, and Ge-72 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 Si-72 has greater elastic anisotropy than Fmmm C-72 and Ge-72. Fmmm C-72 is a superhard material with the hardness of 47.8 GPa. The investigation of the electronic band structures of Fmmm Si-72 and Ge-72 exhibits they have semiconducting properties, while Fmmm C-72 shows metallicity. Fmmm Si-72 has a direct band gap (1.41 eV), and it is very suitable for solar cells. In addition, Fmmm Si-72 has good absorption capacity in the visible light region, so Si-72 has possible applications in photovoltaic applications due to its direct band gap property and good absorption capacity in the visible light region.