Role of square planar coordination in the magnetic properties of Na4IrO4

Iridates supply fertile grounds for unconventional phenomena and exotic electronic phases.With respect to well-studied octahedrally-coordinated iridates, we pay our attention to a ratherunexplored iridate, Na4IrO4, showing an unusual square-planar coordination. The latter is key torationalize the electronic structure and magnetic property of Na4IrO4, which is here explored byfirst-principles density functional theory and Monte Carlo simulations. Due to the uncommonsquare-planar crystal field, Ir 5dstates adopt intermediate-spin state with double occupation of?z2orbital, leading to a sizable local spin moment, at variance with many other iridates. Thesquare-planar crystal field splitting is also crucial in opening a robust insulating gap in Na4IrO4,irrespective of the specific magnetic ordering or treatment of electronic correlations. Spin-orbitcoupling plays a minor role in shaping the electronic structure, but leads to a strongmagnetocrystalline anisotropy. The easy axis perpendicular to the IrO4plaquette, well explainedusing perturbation theory, is again closely related to the square-planar coordination. Finally, thelarge single-ion anisotropy suppresses the spin frustration and stabilizes a collinearantiferromagnetic long-range magnetic ordering, as confirmed by Monte Carlo simulationspredicting a quite low Néel temperature,expected from almost isolated IrO4square-planar units ascrystalline building blocks.