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

Zhenyang Ma,Dayun Wang,Yi Tian,Zheng Han,Xuhong Liu,Xinhai Yu

doi:10.3390/nano7010003

Zhenyang Ma, Dayun Wang + Show 4 more

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https://doi.org/10.3390/nano7010003

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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 AU. 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.

Highlights

In recent years, with the development of technology the interest in theoretical design and experimental synthesis of new superhard materials has increased
There is a range of phases, usually referred to as turbostratic boron nitride (t-BN) [7,8], which are located between highly ordered Hexagonal boron nitride (h-BN) and an amorphous material
Plane-wave code, and the calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew–Burke–Ernzerhof

Summary

Introduction

With the development of technology the interest in theoretical design and experimental synthesis of new superhard materials has increased. Such materials are in great demand in material science, electronics, optics, and even jewelry. Borides, nitrides, and the covalent compounds of light elements (B, Be, O, C, N, etc.) are regarded as candidates of superhard materials [1,2,3,4,5]. Besides the well-known cubic diamond-like phase (c-BN) [9], wurtzite-like phase (w-BN) [7], layered graphite-like phase (h-BN or r-BN) [6,10,11], BN nanosheet [12], and BN nanotubes (BNNTs) [13], many new BN polymorphs have been experimentally prepared or theoretical predicted, including P-BN [14], BC8 -BN [15], T-Bx Nx [16], Z-BN [17], I-BN [18], cT8 -BN [19], B4 N4 [20], o-BN [21], bct-BN [22], zeolite-like microporous BN [23,24], turbostratic BN [25], and BN fiber [10]

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