Stability of corundum- versus rutile-type structures of ruthenium and rhodium oxides

The relative stability of ruthenium and rhodium oxides in the corundum- vs rutile-type structures was studied combining density-functional theory with classical thermodynamics. RuO2 in the rutile-type structure is thermodynamically preferred compared to Ru2O3 in the corundum-type structure upon compression and in contact with ambient oxygen. Whereas, the thermal stability of Rh2O3 in the corundum-type structure was found to be dependent on the ambient oxygen pressure. The estimated free energies are consistent with the observed distinct preference of rhodium and ruthenium oxides for corundum- vs rutile-type structures as a function of oxygen pressure. The harder isotropic bulk modulus is obtained for the oxide with the stronger metal-oxygen bonds in, both, rutile- and corundum-type structures, respectively. Whereas, the isotropic shear modulus is softer for the corundum-type oxide (Ru2O3) with stronger metal-oxygen bonds.