Strong Effect of Hydrogen Order on Magnetic Kitaev Interactions in ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$

Very recently a quantum liquid was reported to form in ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$, an iridate proposed to be a close realization of the Kitaev honeycomb model. To test this assertion we perform detailed quantum chemistry calculations to determine the magnetic interactions between Ir moments. We find that weakly bond dependent ferromagnetic Kitaev exchange dominates over other couplings, but still is substantially lower than in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. This reduction is caused by the peculiar position of the interlayer species: removing hydrogen cations next to a ${\mathrm{Ir}}_2{\mathrm{O}}_2$ plaquette increases the Kitaev exchange by more than a factor of 3 on the corresponding $\mathrm{Ir}─\mathrm{Ir}$ link. Consequently, any lack of hydrogen order will have a drastic effect on the magnetic interactions and strongly promote spin disordering.

Figure 1

Layered honeycomb network of ${\mathrm{IrO}}_6$ octahedra in ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$. For the stacking pattern proposed in Ref. [22], interlayer connectivity is realized through linear $\mathrm{O}─\mathrm{H}─\mathrm{O}$ links (top); the different types of magnetic couplings on a given hexagonal ring are also shown (bottom).

Figure 2

Phase diagram obtained by ED for the effective spin model (1). The ab initio NN interactions listed in Table 1 and variable second- and third-neighbor isotropic couplings $J_2$ and $J_3$ were used. Schematic spin configurations are also shown.