J = 0 nonmagnetic insulating state in K$_2$Os$X_6$ (X = F, Cl and Br)

TL;DR

This study uses ab initio calculations to reveal that K$_2$Os$X_6$ compounds with X=F, Cl, Br exhibit a nonmagnetic insulating state driven by strong spin-orbit coupling and electron localization, highlighting the role of crystal field and hybridization effects.

Contribution

The paper demonstrates the emergence of a J=0 nonmagnetic insulating state in K$_2$Os$X_6$ due to strong spin-orbit coupling and structural factors, providing new insights into 5d transition-metal oxides.

Findings

K$_2$Os$X_6$ exhibits a J=0 nonmagnetic insulating state.

Strong spin-orbit coupling separates effective j_eff states.

Electron localization increases from F to Br, affecting bandwidth.

Abstract

In $4d/5d$ transition-metal systems, many interesting physical properties arise from the interplay of bandwidth, electronic correlations, and spin-orbit interactions. Here, using {\it ab initio} density functional theory, we systematically study the double-perovskite-like system K$_2$Os$X_6$ (X = F, Cl, and Br) with a $5d^4$ electronic configuration. Our main result is that the $J = 0$ nonmagnetic insulating state develops in this system, induced by strong spin-orbital coupling. Specifically, the well-separated Os$X_6$ octahedra lead to the cubic crystal-field limit and result in dramatically decreasing hoppings in nearest neighbor Os-Os sites. In this case, the three degenerate $t_{2g}$ orbitals are reconstructed into two ``effective'' $j_{\rm eff}$ ($j_{\rm eff} = 1/2$ and $j_{\rm eff} = 3/2$ states) states separated by the strong SOC, opening a gap with four electrons occupying the $j_{\rm eff} = 3/2$ orbitals. Furthermore, the hybridization between the Os $5d$ orbitals and the $X$ ($X$ = F, Cl, and Br) $p$ orbitals increases from F to Br, leading the electrons in K$_2$OsF$_6$ to be more localized than in K$_2$OsCl$_6$ and K$_2$OsBr$_6$, resulting in a smaller bandwidth for K$_2$OsF$_6$ than in the Cl- or Br- cases. Our results provide guidance to experimentalists and theorists working on this interesting family of osmium halides.