Exploring novel transition metal ceramics M2X (M = Mo, Tc, Ru, Rh, Os; X = B, C,N) via ab-initio calculations

The heavy transition metal borides, carbides, and nitrides are still difficult to synthesize, and certain compositions are not even possible to stabilize. With the help of existing materials databases, we assemble 240 materials of chemical formula M2X, that we intend to investigate via density-functional theory (DFT). Our calculations demonstrate the stability of several novel compounds, which are highly likely to be synthesized in experiment, widening the potential applications as structural ceramics. The X site plays the primary role in determining the structural stability, with boron leading to many more stable ceramics than carbon and nitrogen. The chemical bonding is found to arise from a mixture of covalent bonding (M-X) and weak metallic bonding (M-M), which ensures varied elastic properties. Among the newly predicted materials, Tc2B, Os2B, and Os2C are shown to be ultra-incompressible. A low shear modulus makes these systems prone to plasticity, which leads to an interesting coexistence of hardness and ductility for novel Os2B and Os2C, as well as existing Mo2B and Mo2C. Chemically, replacing 4d with 5d elements is shown to enhance bulk and shear moduli, which could be used for hardness engineering by selective doping. This report lays the foundations for further experimental research on the mechanical merits of the late transition metal borides, carbides, and nitrides of chemical formula M2X.