Superhard and Superconducting B6C

TL;DR

This study predicts a new metastable B6C structure that is both superhard and superconducting, with high mechanical stability, thermal stability, and a superconducting transition temperature of around 12.5 K.

Contribution

The paper introduces a novel metastable B6C structure with superhard and superconducting properties, identified through crystal structure searching and ab initio calculations.

Findings

B6C is more stable than B4C and boron at high pressure.

The B6C structure has a Vickers hardness of around 48 GPa.

It exhibits superconductivity with Tc approximately 12.5 K.

Abstract

Crystal structure searching and ab initio calculations have been used here to explore low-energy structures of boron carbides under high pressure. Under pressures of 85-110 GPa, a metastable B6C with R-3m symmetry is found to be energetically more stable than the mixture of previous B4C and elemental boron. This B6C is a rhombohedral structure and contains mooncake-like B24 clusters with stuffing of C2 pairs. The mechanical and dynamical stabilities of this structure at ambient pressure are confirmed by its elastic constants and phonon dispersions. The bulk modulus and shear modulus of this newly predicted B6C at ambient pressure reach high values of 291 GPa and 272 GPa, respectively. The Vickers hardness is calculated to be around 48 GPa, and its melting temperature at ambient pressure is estimated to be around 2500 K, indicating that this metastable B6C is a potential superhard material with very good thermal stability. Interestingly, this superhard B6C structure is also found to be superconducting and its superconducting critical temperature (Tc) is evaluated to be around 12.5 K at ambient pressure by electron-phonon coupling.