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

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