Theoretical study of the crystal and electronic properties of $\alpha$-RuI$_3$

TL;DR

This theoretical study investigates the structural and electronic properties of the newly synthesized $ extalpha$-RuI$_3$, revealing its metallic nature due to strong hybridization effects, contrasting with the insulating behavior of $ extalpha$-RuCl$_3$.

Contribution

The paper provides a detailed first-principles analysis of $ extalpha$-RuI$_3$, identifying its stable stacking structure and explaining its metallic behavior through hybridization and reduced effective Coulomb repulsion.

Findings

$ extalpha$-RuI$_3$ is most likely ABC-stacked in R$ar{3}$ structure.

$ extalpha$-RuI$_3$ is dynamically stable with no imaginary phonon modes.

$ extalpha$-RuI$_3$ exhibits metallic behavior due to decreased effective Coulomb repulsion $U$.

Abstract

The material $\alpha$-RuCl$_3$, with a two-dimensional Ru-honeycomb sublattice, has attracted considerable attention because it may be a realization of the Kitaev quantum spin liquid (QSL). Recently, a new honeycomb material, $\alpha$-RuI$_3$, was prepared under moderate high-pressure and it is stable under ambient conditions. However, different from $\alpha$-RuCl$_3$, $\alpha$-RuI$_3$ 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 $\alpha$-RuI$_3$ are theoretically studied. First, based on first-principles density functional theory (DFT) calculations, the ABC stacking honeycomb-layer $R\overline{3}$ (No. 148) structure is found to be the most likely stacking order for $\alpha$-RuI$_3$ along the $c$-axis. Furthermore, both $R\overline{3}$ and $P\overline{3}1c$ are dynamically stable because no imaginary frequency modes were obtained in the phononic dispersion spectrum. Moreover, the different physical behavior of $\alpha$-RuI$_3$ compared to $\alpha$-RuCl$_3$ can be understood naturally. The strong hybridization between Ru $4d$ and I $5p$ orbitals decreases the effective atomic Hubbard repulsion $U$, leading the electrons of RuI$_3$ to be less localized than in RuCl$_3$. As a consequence, the effective repulsion $U$ is reduced from Cl to I, leading to the metallic nature of $\alpha$-RuI$_3$. Based on the DFT+$U$ ($U_{\rm eff} = 2$ eV), plus spin-orbital coupling (SOC), we obtained a spin-orbit Mott insulating behavior for $\alpha$-RuCl$_3$ and, by the same procedure, a metallic behavior for $\alpha$-RuI$_3$, in good agreement with experimental results. Furthermore, when introducing a large (unrealistic) $U_{\rm eff} = 6$ eV, the spin-orbit Mott gap opens in $\alpha$-RuI$_3$ as well, supporting the physical picture we are proposing.