Electronic control of the stability of rutile- versus corundum-type structures of ruthenium and rhodium oxides

The distinct preference of ruthenium and rhodium oxides for corundum- versus rutile-type structures was studied with density-functional theory at the generalized gradient level of approximation (DFT-GGA). The optimal filling of bonding states in the metal-oxygen hybridization complex controls the thermodynamic preference of ruthenium and rhodium oxides for a rutile- versus a corundum-type structure. The relative metal-oxygen bond strengths were quantified in terms of the bond formation energy per oxide formula-unit. The energetically most efficient bonding mechanism was found for ruthenium oxide in the rutile-type structure (RuO2), which is a metal with the Fermi level at a minimum in the density of states separating bonding occupied Ru-dt2g states and empty Ru-deg states. While in the corundum-type structure, the Fermi level falls within peaks in the DOS of partially filled Ru-d states, mostly of t2g symmetry and partly of non-bonding character. α-Rh2O3 in the corundum-type structure, on the other hand, becomes a stable insulator opening a gap at the Fermi level between filled Rh-4d-O-p states, and empty anti-bonding states of predominantly Rh-deg-O-py character. While upon replacing ruthenium by rhodium in the rutile-type structure, additional rhodium 4d electrons populates Rh-deg-O(px + pz) bands, which were unoccupied in RuO2, and of mostly anti-bonding character. As a result, rutile-type RhO2 is a metal with a lower Rh-O bond formation energy per formula-unit as compared to that in corundum-type α-Rh2O3. The elastic bulk modulus was found to correlate to the metal-oxygen bond strength for ruthenium and rhodium oxides in both, corundum- and rutile-type structures, respectively.