First-principles investigation of technetium carbides and nitrides

Phase stabilities and mechanical properties of ideal stoichiometric technetium monocarbide (TcC) and technetium mononitride (TcN) in the tungsten carbide (WC), nickel arsenide (NiAs), rocksalt (NaCl), and zinc-blende (ZnS) structures, respectively, have been studied systematically by first-principles calculations. It is found that both TcC and TcN in two hexagonal phases (WC and NiAs) are not only elastically stable but also hard and ultrastiff materials. Remarkably, for the two hexagonal TcC phases, both bulk moduli and linear incompressibilities along the c axis exceed that of c BN and even rival with diamond. Their hardness can also match the known hard materials such as WC. The combination of good metallicity, strong stiffness, and high hardness suggests that the materials may find applications as hard conductors and cutting tools.