New Superconducting and Semiconducting Fe-B Compounds Predicted with an Ab Initio Evolutionary Search

New candidate ground states at 1∶4, 1∶2, and 1∶1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB₂ stabilizes by a break up of 2D boron layers into 1D chains while oP10-FeB₄ stabilizes by a distortion of a 3D boron network. The uniqueness of these configurations gives rise to a set of remarkable properties: oP12-FeB₂ is expected to be the first semiconducting metal diboride and oP10-FeB₄ is shown to have the potential for phonon-mediated superconductivity with a T_c of 15-20 K.