Structural, mechanical, electronic, and thermodynamic properties of dense B3N4 under high pressure predicted from first principles

Five dense B3N4 structures are established by substitution of C atoms with B atoms in the five hypothetical dense C3N4 phases. The structural, elastic, and electronic properties of the five B3N4 polymorphs are investigated through the first-principles calculations. Our calculations indicate that c-B3N4 is energetically favorable in the five structures. Through the calculations of formation enthalpy, we conclude that the B3N4 polymorphs are thermodynamically stable at zero pressure except for the c(s)-B3N4. The elastic moduli of the five B3N4 polymorphs are calculated using the Voigt-Reuss-Hill approximation. The calculated bulk modulus of c(s)-B3N4 (337GPa) is the highest of them. The c-B3N4, with shear modulus of 248GPa, might be a potentially ultra incompressible and hard material. By the elastic stability criteria, it is predicted that the c(s)-B3N4, c-B3N4, and p-B3N4 are mechanically stable within 100GPa, while the -B3N4 and -B3N4 become mechanically unstable when pressure increases to 30 and 80GPa, respectively. The calculated B/G ratios indicate that c(s)-B3N4 and p-B3N4 possess a ductile nature within 100GPa. The c-B3N4 possess a brittle nature at 0GPa, as it begins to be prone to ductility, when the pressure increases to 90GPa. The calculated band structures and densities of states show that all the five B3N4 phases are insulative. Through the quasiharmonic Debye model, we also investigated the thermodynamic properties of these B3N4 polymorphs.