Metallic ruthenium ilmenites: First-principles study of MgRuO3 and CdRuO3

Ilmenites ABO3 provide a platform for electron correlation and magnetism on alternatively stacked honeycomb layers of edge-sharing AO6 or BO6 octahedra. When A and B are 3d transition metals, strong electron correlation makes the systems Mott insulators showing various magnetic properties, while when B is Ir with 5d electrons, competition between electron correlation and spin–orbit coupling realizes a spin–orbital coupled Mott insulator as a potential candidate for quantum spin liquids. Here, we theoretically investigate intermediate 4d ilmenites, ARuO3 with A = Mg and Cd, which have recently been synthesized and shown to be metallic, unlike the 3d and 5d cases. By using first-principles calculations, we optimize the lattice structures and obtain the electronic band structures. We show that MgRuO3 exhibits strong dimerization on RuO6 honeycomb layers, leading to the formation of bonding and anti-bonding bands for one of the three t2g orbitals; the lattice symmetry is lowered from R3̄ to P1̄, and the Fermi surfaces are composed of the other two t2g orbitals. In contrast, we find that CdRuO3 has a lattice structure close to R3̄, and all three t2g orbitals contribute almost equally to the Fermi surfaces. A comparison of our results with other Ru honeycomb materials, such as Li2RuO3, indicates that the metallic ruthenium ilmenites stand on a subtle balance among electron correlation, spin–orbit coupling, and electron–phonon coupling.