Electronic and magnetic ground states of the Kitaev material candidates $A_2$CeIrO$_6$ ($A$~$=$~Ba,Sr)

Quantum magnets with significant bond-directional Ising interactions, so-called Kitaev materials, have attracted tremendous attention recently in the search for exotic spin liquid states. Here we present a comprehensive set of measurements that enables us to elucidate the crystal structures, Ir$^{4+}$ single ion properties, and magnetic ground states of the double perovskite iridates $A_2$CeIrO$_6$ ($A$~$=$~Ba,Sr). The crystal structures of Ba$_2$CeIrO$_6$ and Sr$_2$CeIrO$_6$ are found to be cubic and monoclinic respectively, with a large nearest neighbor distance $>$~5~\AA~between Ir$^{4+}$ ions. X-ray absorption spectroscopy and resonant inelastic x-ray scattering were used to establish $J_{eff}$~$=$~1/2 spin-orbit-assisted Mott insulating states, while bulk characterization and neutron powder diffraction identify A-type antiferromagnetic order. These electronic and magnetic ground states are both consistent with expectations for face-centered-cubic magnets with dominant Kitaev exchange, which indicates that spacing magnetic ions far apart may be a promising design principle for uncovering new Kitaev materials.