Structure, bonding, and hardness of CrB$_4$: a superhard material?

TL;DR

This study clarifies the crystal structure of CrB4, confirming it as a potentially superhard material with a specific structure, and provides theoretical evidence of its exceptional hardness and bonding characteristics.

Contribution

The paper establishes the correct crystal structure of CrB4 and demonstrates its potential as a superhard material through experimental and theoretical analysis.

Findings

CrB4 crystallizes in the Pnnm structure.

CrB4 has a Vickers hardness estimated at 48 GPa.

The structure features a distorted 3D boron network.

Abstract

By electron and X-ray diffraction we establish that the CrB$_4$ compound discovered over 40 years ago crystallizes in the $oP10$ (\emph{Pnnm}) structure, in disagreement with previous experiments but in agreement with a recent first-principles prediction. The 3D boron network in the new structure is a distorted version of the rigid carbon $sp^3$ network proposed recently for the high-pressure C$_4$ allotrope. According to our density functional theory calculations and the analysis of the bonding, CrB$_4$ is a potentially superhard material. In fact, the calculated weakest shear and tensile stresses exceed 50 GPa and its Vickers hardness is estimated to be 48 GPa.