Transition-metal dioxides with a bulk modulus comparable to diamond

Recently it has been reported that a high-pressure cubic phase of ruthenium dioxide has an unusually large bulk modulus, and consequently is a most interesting candidate as a very hard material. Based on ab initio calculations it is shown that the high bulk modulus is a result of a strong covalent bonding between ruthenium d states and oxygen p states in combination with the favorable geometry of the orbitals in the fluorite structure. In addition an even higher bulk modulus is predicted for the isoelectronic and isostructural compound OsO2.