The pressure-induced chemical structures and properties trend for compressed iron-boride compounds

Pressure is one of the most effective fundamental elements that can change the bonding patterns of materials and lift the reactivity of elements, leading to the synthesis of unconventional compounds with fascinating properties. Using unbiased structure search method based on particle-swarm optimization algorithm in combination with density functional theory calculations, we investigate the stable compositions of iron-boride (Fe-B) system at pressures ranging from 0 to 300 GPa. We obtained the high pressure Fe-B phase diagram with six pressure-induced stable compositions (Fe3B, Fe2B, FeB, Fe2B3, FeB2, FeB4) under their desired pressure range. In their stable structure, it is found that the coordination number of Fe atoms, the Fe-B spherical packing efficiency, the number of B-B bonds for each B atom, and the calculated hardness almost increase with the increasing of B content. Through the characterization of bonds in these stable Fe-B structures, it reveals that the ionic gradually become weak while the covalence becomes strong as B content increasing. Due to this, we attribute the excellent mechanical properties of the FeB2 and FeB4 structure to the more strong covalent bonds dominantly by providing coulomb repulsion between the B atoms. Our current results expand the knowledge and provide further insight into the pressure-induced structure and properties of Fe-B compounds.