Transition metals and their carbides and nitrides:  Trends in electronic and structural properties

A study of the structural and electronic properties of selected transition metals and their carbides and nitrides is presented. We focus on assessing trends of possible importance for understanding their hardness. Lattice constants, bulk moduli ${(B}_o),$ and charge densities are calculated using the local density approximation with a pseudopotential plane wave approach. An fcc lattice is employed for the transition metal elements in order to make comparisons and study trends relateable to their carbides and nitrides. Our results show that both increasing the number of valence $d$ electrons and the presence of $f$ electrons in the core lead to larger ${(B}_o).\mathrm{}$ Charge density plots and histograms enable us to explain the nature of the charge distribution in the interstitial region for the different compounds considered. In addition, we include the heavier elements seaborgium, bohrium, and hasnium in order to test further trends. Surprisingly, the calculated $B_o$ for Hs is comparable to that of diamond.