Fe-X (X = B, N) binary compounds: First-principles calculations of electronic structures, theoretic hardness and magnetic properties

The first-principles calculations are implemented to investigate the electronic structures, theoretic hardness and magnetic properties of iron borides and nitrides with four different crystal systems containing hexagonal (FeB2, epsilon-Fe3N), tetragonal (Fe2B, alpha"-Fe16N2), orthorhombic (alpha-FeB, theta-Fe3B, lambda-Fe2N), and cubic (zb-FeN, rs-FeN, gamma'-Fe4N, gamma-Fe23B6) phase. The calculated lattice parameters using RPBE meet well with the experimental results. The cohesive energy and formation enthalpy values indicate the Fe-X (X= B, N) binary compounds are thermodynamically stable. Meanwhile, the h-FeB2 is most difficult phase for experimental synthesis among these interstitial compounds. Moreover, magnetic properties are discussed and show that the mean magnetic moments of o-Fe3B and c-Fe23B6 with the values of 2.227 mu(B) and 2.256 mu(B) per iron atom are approaching to that of pure iron (2.32 mu(B)) while the c-Fe4N and t-Fe16N2 with the values of 2.51 and 2.48 mu(B) are beyond that of pure alpha-Fe. The c-FeN phase shows nonmagnetic in zb-style while rs-type shows antiferromagnetic with a value of 2.52 mu(B). Furthermore, the average bonding length and Mulliken population combined with electronic structures are also analysed in this paper which provide that strong Fe-X and X-X covalent bonds are responsible for high hardness. Finally, the theoretic hardness of X-X, Fe-X and Fe-Fe bonds is predicted by semi empirical hardness theory. (C) 2017 Elsevier B.V. All rights reserved.