Abstract
The material α−RuCl3, with a two-dimensional Ru honeycomb sublattice, has attracted considerable attention because it may be a realization of the Kitaev quantum spin liquid. Recently, a new honeycomb material, α−RuI3, was prepared under moderately high pressure, and it is stable under ambient conditions. However, different from α−RuCl3, α−RuI3 was reported to be a paramagnetic metal without long-range magnetic order down to 0.35 K. Here, the structural and electronic properties of the quasi-two-dimensional α−RuI3 are theoretically studied. First, based on first-principles density functional theory calculations, the ABC stacking honeycomb-layer R¯3 (No. 148) structure is found to be the most likely stacking order for α−RuI3 along the c axis. Furthermore, both R¯3 and P¯31c are dynamically stable because no imaginary frequency modes were obtained in the phononic dispersion spectrum without Hubbard U. Moreover, the different physical behavior of α−RuI3 compared to α−RuCl3 can be understood naturally. The strong hybridization between Ru 4d and I 5p orbitals decreases the “effective” atomic Hubbard repulsion, leading the electrons of RuI3 to be less localized than in RuCl3. As a consequence, the effective electronic correlation is reduced from Cl to I, leading to the metallic nature of α−RuI3. Based on the DFT+U (Ueff=2 eV) plus spin-orbital coupling, we obtained a spin-orbit Mott insulating behavior for α−RuCl3 and, with the same procedure, a metallic behavior for α−RuI3, in good agreement with experimental results. Furthermore, when introducing large (unrealistic) Ueff=6 eV, the spin-orbit Mott gap opens in α−RuI3 as well, supporting the physical picture we are proposing. Our results provide guidance to experimentalists and theorists working on two-dimensional transition metal tri-iodide layered materials.
