J$_{\mathrm{eff}}=$1/2 Mott spin-orbit insulating state close to the cubic limit in Ca$_{4}$IrO$_{6}$

The J$_{\mathrm{eff}}=$1/2 Mott spin-orbit insulating state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and recently strong evidence for this state in Sr$_{2}$IrO$_{4}$, and additional iridates, has been presented. All the candidates, however, have distorted octahedra, such as the elongation along the c-axis in Sr$_{2}$IrO$_{4}$, and consequently a non-cubic local crystal field environment. Consequently the materials form a mixed J$_{\mathrm{eff}}=$1/2,3/2 ground state. The lack of a material with an unmixed J$_{\mathrm{eff}}=$1/2 has impacted the development and testing of robust models of this novel insulating and magnetic state. We present neutron diffraction, resonant x-ray scattering and DFT calculations that not only reveal Ca$_{4}$IrO$_{6}$ is a new candidate J$_{\mathrm{eff}}=$1/2 material with long-range magnetic order, but furthermore resides close to the required cubic limit. Both our experimental and theoretical investigation indicate Ca$_{4}$IrO$_{6}$ is uniquely positioned to act as a canonical system to investigate of the J$_{\mathrm{eff}}=$1/2 state.