Pressure-induced variation of structural, elastic, vibrational, thermodynamic properties and hardness of C11N4 polymorphs

Abstract

The pressure-induced structural, elastic, vibrational, thermodynamic properties and the hardness of C11N4 polymorphs were investigated by first-principles calculations based on the density functional theory. Our calculations reveal that the g-C11N4 is the most stable structure at ambient pressure among the four C11N4 polymorphs and that α-C11N4 and d-C11N4 are the most energetically preferred structures. The transition from g-C11N4 to α-C11N4 and d-C11N4 with low pressure phases occurs around 3.557 and 9.468 GPa, respectively, and the transition from g-C11N4 to β-C11N4 with high pressure phase occurs around 46.032 GPa. The elastic properties and anisotropic mechanical properties of C11N4 polymorphs are then discussed in detail. Importantly, the hardnesses of C11N4 polymorphs are estimated using a semi-empirical model for the hardness, and the results indicate that α-C11N4, β-C11N4, and d-C11N4 are superhard materials. Finally, the mechanical and thermodynamic properties, including the bulk moduli, heat capacity and the thermal expansion coefficient, of C11N4 polymorphs depending on pressure are predicted.