Physical properties of B₄N₄-I and B₄N₄-Ⅱ: First-principles study

The structural, mechanical, electronic, mechanical anisotropy, and thermal properties of boron nitride (BN) polymorphs, such as B₄N₄-I and B₄N₄-Ⅱ, are investigated under ambient pressure utilizing first-principles generalized gradient approximation calculations using an ultrasoft pseudopotential scheme. The phonon spectra and elastic constants reveal that B₄N₄-I is dynamically and mechanically stable at the pressure of 0 GPa and temperature of 0 K. Anisotropic calculations indicate that both B₄N₄-I and B₄N₄-Ⅱ exhibit higher anisotropy of Young's modulus than cubic BN (c-BN). B₄N₄-Ⅱ and B₄N₄-I present indirect and wide band gaps of 5.32 eV and 4.86 eV, respectively. In addition, B₄N₄-I is more brittle than B₄N₄-Ⅱ. Moreover, the minimum thermal conductivity, κ_min, of B₄N₄-Ⅱ at 300 K is 1.92 W/(cm·K), which is slightly higher than those of B₄N₄-I and c-BN (1.84 W/(cm·K) and 1.83 W/(cm·K), respectively. However, κ_min of B₄N₄-I is slightly higher than that of c-BN.