A Simple Method for Hardness Prediction of Transition Metal Compounds

We developed an empirical approach to predict the hardness of transition metal (TM) compounds by considering the metallicity of chemical bonds within them defined by the delocalized valence electrons. Particularly, the compounds with light elements linking to both TM and light elements are separated into two subsystems according to the different coordinated atoms of light elements, by which the hardness of such a catalogue of compounds is successfully predicted. The calculated hardness values of TM borides, carbides, nitrides and oxides agree well with the available experimental data. The current calculations indicate that the hardness of TM compounds is strongly affected by the crystal structure rather than monotonically increases with the light element content, which points out an essential guidance to explore novel superhard materials.