Strong antiferromagnetic interaction owing to a large trigonal distortion in the spin-orbit-coupled honeycomb lattice iridate $\mathrm{CdIr}{\mathrm{O}}_3$

We investigated the magnetic properties of the ilmenite-type iridate $\mathrm{CdIr}{\mathrm{O}}_3$ with a honeycomb lattice formed by $\mathrm{I}{\mathrm{r}}^{4+}$ ions prepared via a solid-state metathesis. The magnetization measurements made using the powder sample reveal a large effective magnetic moment and a fairly strong antiferromagnetic interaction, indicating a deviation from the Kitaev model. Considering the relationship between magnetism and crystal structure in $\mathrm{CdIr}{\mathrm{O}}_3$, and comparing it with the other ilmenite-type iridates $\mathrm{ZnIr}{\mathrm{O}}_3$ and $\mathrm{MgIr}{\mathrm{O}}_3$, we conclude that insulating $\mathrm{CdIr}{\mathrm{O}}_3$ cannot be described as a $J_{\mathrm{eff}}=1/2$ Mott state owing to a metathetically stabilized large trigonal distortion of $\mathrm{Ir}{\mathrm{O}}_6$ octahedra.

Figure 1

Powder x-ray diffraction patterns of $\mathrm{CdIr}{\mathrm{O}}_3$. The observed intensities (red), calculated intensities (black), and their differences (blue) are shown. Vertical bars indicate the positions of Bragg reflections. The peak marked with an asterisk is an impurity peak due to unwashed $\mathrm{Ir}{\mathrm{O}}_2$ remaining in the final product.

Figure 2

Crystal structure of $\mathrm{CdIr}{\mathrm{O}}_3$ (a) viewed along the $c$ axis and (b) perpendicular to the $ab$ plane. (c) The local environment of a pair of edge-sharing $\mathrm{Ir}{\mathrm{O}}_6$ octahedra with the two different bond lengths of Ir-O and the bond angle of Ir-O-Ir. The vesta program is used for visualization [22].

Figure 3

(a) Temperature dependences of magnetic susceptibility $M$/$H$ and its inverse of powder samples $\mathrm{CdIr}{\mathrm{O}}_3$ under a magnetic field of 1 T. The measurements were conducted upon heating after zero field cooling (ZFC) and then upon cooling (FC), as shown by the arrows. The dashed line is a result of Curie-Weiss fitting. The values are those of estimated magnetic interactions. The inset shows the isothermal magnetization curve measured at $T=1.8\mathrm{K}$ for $\mathrm{CdIr}{\mathrm{O}}_3$. (b) ZFC and FC $M$/$H$ curves of $\mathrm{CdIr}{\mathrm{O}}_3$ under low magnetic field ${\mu }_{0}H=0.01\mathrm{T}$. The black dashed line is the result of fitting near $T_{\mathrm{N}}$ as described in the text. The inset shows temperature dependences of FC $M$/$H$ curves measured for several magnetic fields.

Figure 4

A schematic view of (a) splitting of $J_{\mathrm{eff}}=1/2$ and 3/2 states under cubic crystal field into (b) three Kramers doublets ${\phi }_0$, ${\phi }_1$, and ${\phi }_2$ under a trigonal crystal field. (c) The temperature dependence of inversed magnetic susceptibility $H$/$M$ in three ilmenite-type iridates. (d) Relationship between effective magnetic moments ${\mu }_{\mathrm{eff}}$ and bond angle variances σ in three ilmenite-type iridates.