Origin of the insulating state in honeycomb iridates and rhodates

A burning question in the emerging field of spin-orbit driven insulating iridates, such as Na₂IrO₃ and Li₂IrO ₃, is whether the observed insulating state should be classified as a Mott-Hubbard insulator derived from a half-filled relativistic j _eff=1/2 band or as a band insulator where the gap is assisted by spin-orbit interaction or Coulomb correlations or both. The difference between these two interpretations is that only for the former strong spin-orbit coupling (λâ‰W, where W is the bandwidth) is essential. We have synthesized the isostructural and isoelectronic Li₂RhO₃ and report its electrical resistivity and magnetic susceptibility. Remarkably, it shows insulating behavior together with fluctuating effective S=1/2 moments, similar to Na₂IrO₃ and Li₂IrO₃, although in Rh4⁺ (4d5) the spin-orbit coupling is greatly reduced. We show that this behavior has a nonrelativistic one-electron origin (although Coulomb correlations assist in opening the gap) and can be traced to the formation of quasimolecular orbitals, similar to those in Na₂IrO ₃.