Crystal structures of FeB₄. (a) oP10-FeB₄ with an orthorhombic structure, which was first predicted by Kolmogorov et al.,^[127] and was later successfully synthesized at HPHT conditions.^[125] This type of FeB₄ is a superconductor with measured T_c below 2.9 K; its hardness was first measured to be 62 GPa^[125] and later proven to be ∼10–30 GPa.^[128–130] (b) tP10-FeB₄ as a predicted high-pressure phase. This phase is a superhard semiconductor with band gap of approximately 1.85 eV and calculated hardness of 45 GPa.^[122] The B and Fe atoms are painted in gray and green colors, respectively.