Pressure induced evolution of structures and properties of iron tetraboride

Abstract

Iron tetraboride (FeB₄) is attracting increasing attention due to its attractive electronic and mechanical properties (e.g., superconductivity and superhard). Using particle swarm optimization (PSO) combined with first-principles calculations, we systemically studied the structural phase and properties of FeB₄ up to 1 TPa. Our results revealed, unexpectedly, that pressure stabilizes two hitherto unknown I4/m (420–542 GPa) and Imma (>542 GPa) structures. Under compression (0–1 TPa), for the first time, we identify that the structural phases of FeB₄ transform initially from Pnnm to I4₁/acd, I4/m, and finally to Imma. During these transformations, the evolution of a boron framework is from a graphene-like layer to a tetrahedral B₄ cluster, one-dimensional B₄ cluster-chain, and interconnected graphene-like network, where the number of B–B bonds for each B atom gradually increases from 3 to 4, 5 and 6, respectively. Among these phases, a remarkable feature of the Fe atoms is that their coordination number changes from an 8-fold coordination to unexpected 12-fold coordination with increasing pressure. By the analysis of electronic structures, a feature is revealed that the electrical properties of FeB₄ evolve from those of a superconductor to a semiconductor, and a conductor in the range of 0–1 TPa. Moreover, the mechanical property calculations indicate that these phases of FeB₄ all exhibit excellent mechanical behaviors and are expected to be superhard materials. These results expand the knowledge of FeB₄ and stimulate further interest in exploring it from a high pressure perspective.