Origin of the insulating state in honeycomb iridates and rhodates

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A burning question in the emerging field of spin-orbit driven insulating iridates, such as Na2IrO3 and Li2IrO3 is whether the observed insulating state should be classified as a Mott-Hubbard insulator derived from a half-filled relativistic jₑff=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 (lambda > W, where W is the band width) 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 non-relativistic one-electron origin (although Coulomb correlations assist in opening the gap), and can be traced down to formation of quasi-molecular orbitals, similar to those in Na2IrO3.