Origin of incompressibility and hardness from electronic and mechanical properties of hard material ruthenium diboride

We study structural, elastic, and electronic properties for three RuB2 phases using ab initio total-energy calculations within the density functional theory. The orthorhombic and hexagonal structures are mechanically stable. More precisely, the orthorhombic is more stable than the hexagonal form. Results of bulk modulus, which are in good agreement with experimental data, show that the considered structures are potentially highly compressible materials. This is confirmed by the calculation of the hardness, indicating that RuB2 is an ultracompressible material, but not a superhard material.