Discovery of FeBi₂

Recent advances in high-pressure techniques offer chemists access to vast regions of uncharted synthetic phase space, expanding our experimental reach to pressures comparable to the core of the Earth. These newfound capabilities enable us to revisit simple binary systems in search of compounds that for decades have remained elusive. The most tantalizing of these targets are systems in which the two elements in question do not interact even as molten liquids-so-called immiscible systems. As a prominent example, immiscibility between iron and bismuth is so severe that no material containing Fe-Bi bonds is known to exist. The elusiveness of Fe-Bi bonds has a myriad of consequences; crucially, it precludes completing the iron pnictide superconductor series. Herein we report the first iron-bismuth binary compound, FeBi₂, featuring the first Fe-Bi bond in the solid state. We employed geologically relevant pressures, similar to the core of Mars, to access FeBi₂, which we synthesized at 30 GPa and 1500 K. The compound crystallizes in the Al₂Cu structure type (space group I4/mcm) with a = 6.3121(3) Å and c = 5.4211(4) Å. The new binary intermetallic phase persists from its formation pressure of 30 GPa down to 3 GPa. The existence of this phase at low pressures suggests that it might be quenchable to ambient pressure at low temperatures. These results offer a pathway toward the realization of new exotic materials.