The role of valence electron concentration in the cohesive properties of YBxN1−x, YCxN1−x and YNxO1−x compounds

The mechanical properties are not yet understood at basic levels. Previous works shows that the greatest hardness for rock-salt structures (such as TiCxN1−x) is attained for a valence electron concentration (VEC) of 4.2 electrons per atom. The present work is aimed to explore this concept for yttrium-based compounds. By means of first principles calculations we did a systematical investigation where nitrogen in YN (VEC = 4) was supplanted by either of B, C or O to reduce or increase its VEC, forming YBxN1−x, YCxN1−x and YN1−xOx ternary compounds. We have calculated the cohesive energy (EO), cell volume (VO), bulk modulus (BO) and density of states (DoS) as a function of VEC. The Fermi level (Ef,) is shifted toward the valence band by substituting B or C in YN, and toward the conduction band by means of O. It is concluded that the optimal position for Ef (maximum BO) is linked to the saturation of electronic states with eg-symmetry. At this point the excess of electrons provided by O starts filling antibonding states with t2g-symmetry. That is, BO increases monotonically as a function of VEC until VEC ∼ 4.1, after that point BO decrease.