Electronic structures and mechanical properties of iron borides from first principles

• FeB4 is incompressible and hard, but not superhard.
• FeB2 rivals FeB4 in hardness.
• The underlying origins of physical properties are elucidated.

The structural properties, mechanical behaviors and electronic structures of FeB4 and FeB2 have been studied systematically by first-principles calculations considering the strong correlation effect. Our results show that FeB4 is incompressible and hard, but the recently reported superhard feature [Phys. Rev. Lett. 111 (2013) 157002] is not supported by the present calculations. Interestingly, we find that FeB2 rivals FeB4 in hardness. By analyzing their crystal geometries, band structures and density of states, we elucidate the underlying origins of the related physical properties.