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

Abstract

The relative stability of ruthenium and rhodium oxides in the corundum- vs rutile-type structures was studied combining density-functional theory with classical thermodynamics. $\mathrm{Ru}{\mathrm{O}}_{2}$ in the rutile-type structure is thermodynamically preferred compared to ${\mathrm{Ru}}_{2}{\mathrm{O}}_{3}$ in the corundum-type structure upon compression and in contact with ambient oxygen. Whereas, the thermal stability of ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}$ 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 $({\mathrm{Ru}}_{2}{\mathrm{O}}_{3})$ with stronger metal-oxygen bonds.