A Novel Mechanism of Optical Spectrum Formation in the Cs₂NaInCl₆ Doped with Sb and Er

We present a mechanism describing the formation of the optical spectrum in double halide perovskites (DHPs), specifically Sb- and Er-doped Cs₂NaInCl₆. Optical measurements conducted at the Deutsches Elektronen-Synchrotron (DESY) facility between 10 and 300 K have revealed that the bandgap of Cs₂NaInCl₆ is significantly larger than previously estimated, measuring 6.7 eV. Based on density-functional theory (DFT) calculations, we conclude that the formation of In⁺ (5s² optical center) is crucial to the luminescence observed in Cs₂NaInCl₆ microcrystals produced by precipitation techniques. The formation of In⁺ is associated with the transfer of electron density from Cl^- to In³⁺, which occurs due to defects in Cs₂NaInCl₆, resulting in increased In content. This process is linked to the self-trapped exciton mechanism and the formation of defect-trapping levels within the Cs₂NaInCl₆ bandgap. Additionally, embedding Sb³⁺ (5s² doped ion) and Er³⁺ affects both the crystal structure and absorption properties of Cs₂NaInCl₆ samples. These findings shed light on the intricate interactions between the synthesis, composition, and the optical behavior of both undoped and Sb³⁺, Er³⁺-doped Cs₂NaInCl₆ powder samples.