Structure, bonding, and hardness of CrB$_4$: a superhard material?
  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. 
Haiyang Niu, Jiaqi Wang, Xing-Qiu Chen, Dianzhong Li, Yiyi Li, Petr   Lazar, Raimund Podloucky, Aleksey N. Kolmogorov
cond-mat.mtrl-sci