The Importance of Metal Ion Impurity Bands in the Photoluminescence of Halide Double Perovskites

Double perovskite halides are attracting considerable attention due to their applications in solar cells and photovoltaics. It is observed that many (more than 20) publications in top journals have associated spectral features in emission in the region at 460-470 nm with "pristine" materials. This leads to not only false observations but also incorrect conclusions concerning the transformation of dark into bright excitons or boosting self-trapped exciton (STE) emission. The use of the phrase STE is commented upon. Most commonly, the spectra present correspond to those of the antimonate (III) ion, Sb3+. The emission spectra should be interpreted in terms of sp-s(2) orbital transitions rather than self-trapped excitonic transitions. The compounds Cs2NaRECl6 (RE = Y, Gd) are synthesized by two major methods, namely: the evaporation and hydrothermal syntheses, for various rare earth starting materials, in order to demonstrate the effect of antimony contamination. Contamination by the Mn2+ ion is also found in the samples and double perovskite halides are susceptible to attack by moisture. This study emphasizes the need to check sample synthesis and sample purity and carefully monitor the integrity of starting materials.