Pnma-BN: Another Boron Nitride Polymorph with Interesting Physical Properties

Abstract

Structural, mechanical, electronic properties, and stability of boron nitride (BN) in Pnma structure were studied using first-principles calculations by Cambridge Serial Total Energy Package (CASTEP) plane-wave code, and the calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew-Burke-Ernzerhof. This BN, called Pnma-BN, contains four boron atoms and four nitrogen atoms buckled through sp³-hybridized bonds in an orthorhombic symmetry unit cell with Space group of Pnma. Pnma-BN is energetically stable, mechanically stable, and dynamically stable at ambient pressure and high pressure. The calculated Pugh ratio and Poisson's ratio revealed that Pnma-BN is brittle, and Pnma-BN is found to turn brittle to ductile (~94 GPa) in this pressure range. It shows a higher mechanical anisotropy in Poisson's ratio, shear modulus, Young's modulus, and the universal elastic anisotropy index A^U. Band structure calculations indicate that Pnma-BN is an insulator with indirect band gap of 7.18 eV. The most extraordinary thing is that the band gap increases first and then decreases with the increase of pressure from 0 to 60 GPa, and from 60 to 100 GPa, the band gap increases first and then decreases again.

Keywords: BN polymorph; anisotropic properties; electronic properties; mechanical properties.

Figure 1

Unit cell crystal structures of BN in Pnma structure.

Figure 2

The lattice constants a/a₀, b/b₀, c/c₀ compression as functions of pressure for Pnma-BN (a), and primitive cell volume V/V₀ for Pbca-BN, Pnma-BN, c-BN, and diamond (b).

Figure 3

The phonon spectra of Pnma-BN at 0 GPa (a) and 100 GPa (b); Mixing enthalpy ΔH of BN alltropes calculated using PBE (c).

Figure 4

Elastic constants and elastic modulus (a) and B/G ratio (b); Poissons’ ratio v (c) of Pnma-BN as a function of pressure.

Figure 5

The surface construction of Poisson’s ratio (a); shear modulus (b); and Young’s modulus (c) for the Pnma-BN.

Figure 6

2D representation of Poisson’s ratio (a); shear modulus (b) and Young’s modulus (c) in the main plane for Pnma-BN, respectively.

Figure 7

The band structures under pressure of Pnma-BN, (a) 0 GPa, (b) 30 GPa, (c) 60 GPa, (d) 100 GPa.

Figure 8

The band gap under pressure of Pnma-BN (a), the Fermi level and the energy of G high-symmetry points along valence band maximum (VBM) (b); the energy of T and Y high-symmetry points along conduction band minimum (CBM) (c).