Carbon-rich superhard ruthenium carbides from first-principles

- Ruthenium carbides with various stoichiometries were studied by density functional theory calculations.
- Ground states of Ru2C, RuC, Ru2C3, RuC2, RuC3 and RuC4 are found, pressure-induced phase transitions are uncovered.
- Two carbon-rich structures are found to have large Vickers hardness 45.1 GPa and 41.5 GPa.

Compounds formed by transition metals and light elements have attracted increasing attention owing to superior functionalities. Here, high throughput first-principles calculations are employed to investigate the crystal structures and physical properties of ruthenium carbides with various stoichiometries. It is found that the R3¯m-Ru2C, R3¯m-RuC, P3¯m1-Ru2C3, P3¯m1-RuC2, P3¯m1-RuC3 and C2/c-RuC4 are the ground states for the respective chemical compositions at ambient pressure, from a systematical investigation of both thermodynamic and mechanical stabilities, as well as phonon dispersions. Further calculations indicate that P3¯m1-RuC3 and P63/mmc-RuC4 are ultra-incompressible with high bulk and shear modulus. Subsequent empirical calculations predict that the carbon-rich P3¯m1-RuC3 and P63/mmc-RuC4 are superhard materials with a large Vickers hardness of 45.1 GPa and 41.5 GPa, respectively. In addition, a strong covalent CC bonding was observed from the electronic localization function contours of all the ground states, which is crucial for their excellent mechanical properties.

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