Tetrahedral coordination and low-spin configuration in a 5d oxide

Spin state plays a key role in a plethora of physical, biological, and geological phenomena. In octahedral coordination, spin state of 3d transition metals can be high spin or low spin, and can be tuned with external stimuli. In a tetrahedral coordination environment, with the smaller crystal field splitting energy, only high spin is considered possible for oxides. Herein we report the realization of low spin state in tetrahedral coordination in a novel oxide, Sr9Ir3O17. The low spin configuration was confirmed with density functional theory (DFT) calculations, including the effect of spin-orbit coupling (SOC), electron-electron repulsion (U), magnetic data, and crystal chemical consideration. This exotic electronic configuration was realized with the confluence of extended 5d orbitals, short bond lengths, and an electron count maximizing the crystal field stabilization energy. The discovery of low spin state for a tetrahedral environment provides a novel platform for spin state manipulation in solid state materials.