Electronic and magnetic properties of the RuX₃ (X = Cl, Br, I) family: two siblings—and a cousin?

Motivated by reports of metallic behavior in the recently synthesized RuI₃, in contrast to the Mott-insulating nature of α-RuCl₃ and RuBr₃, we present a comparative analysis of the electronic and magnetic properties of this family. Combining first-principles calculations and effective-model considerations, we conclude that RuI₃, like its siblings, is most probably on the verge of a Mott insulator, but with much smaller magnetic moments and strong magnetic frustration. We predict pristine RuI₃ to have a nearly vanishing nearest-neighbor Heisenberg interaction and to be a quantum spin liquid candidate distinct from the Kitaev spin liquid. To reconcile this with reported resistivity, we analyze experimental evidence for all three compounds and propose that the observed metallicity in existing RuI₃ samples arises from sample-quality issues (e.g., metallic grain boundaries). For the Mott insulator RuBr₃, we derive a magnetic Hamiltonian similar to α-RuCl₃ and show it agrees with experiments.