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Origin of Luminescence in Sb3+- and Bi3+-Doped Cs2SnCl6 Perovskites: Excited State Relaxation and Spin-Orbit Coupling.

Sb3+- and Bi3+-doped Cs2SnCl6 zero-dimensional perovskites are emerging as stable and nontoxic phosphors for light emitting diodes. The outermost s-electrons (ns2) of the dopants are responsible for both light absorption (ns2 to ns1np1) and emission (ns1np1 to ns2). At cryogenic temperatures, the Sb3+ dopant shows two emission peaks, but Bi3+ shows only one emission peak. Why? Here we address such questions, revealing the origin of luminescence in Sb3+- and Bi3+-doped Cs2SnCl6. We find that the emitting excited state ns1np1 is a triplet state 3T1u*. The notation "*" implies spin-orbit coupling between the 3T1u and 1T1u states. After light absorption, 3T1u* is occupied with one electron, which then undergoes Jahn-Teller distortion yielding a relaxed excited state (RES). For the Sb3+ dopant, the combination of Jahn-Teller distortion and spin-orbit coupling gives rise to two minima in RES 3T1u*, resulting in two emission peaks, whereas for the Bi3+ dopant, the spin-orbit coupling significantly dominates over the Jahn-Teller splitting yielding a single minimum in RES 3T1u* and, therefore, a single emission peak.