Chemical Control of Spin‐Orbit Coupling and Charge Transfer in Vacancy‐Ordered Ru(IV) Halide Perovskites

Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru IV halides are presented here; A 2 RuCl 6 and A 2 RuBr 6 , where A is K, NH 4 , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behaviour can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand to metal charge-transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature-dependent due to a non-magnetic ground state with J eff = 0 caused by SOC. Ru- X covalency and, consequently, the delocalization of metal d -electrons result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.