The Anomalous Effect of a Sample Holder in the Photoluminescence of Halide Double Perovskites

Double-perovskite halides are attracting considerable attention due to their potential applications in solar cells and photovoltaics. The Cs₂NaMCl₆ (M = Sc, Y, La, Lu) compounds are transparent in the visible region. However, their reported emission spectra often show bands due to contamination by minute amounts of impurities, such as Cr or Sb. We found the presence of another spurious spectral feature with broad green emission, also in Cs₂NaRECl₆, RE = Y, Gd. In particular, the CsCl starting material also exhibits an analogous strong, broad emission band at room temperature at about 530 nm. It was discovered that this emission is due to Cu¹⁺ from the use of a copper sample holder, and the band is absent when using a stainless-steel sample holder. Emission from the Mn²⁺ ion is also found in some samples due to the presence of manganese in the starting materials. Comparison of our results with those of ternary copper chlorides shows that the impurity compound formed is Cs₃Cu₂Cl₅. An alternative viewpoint for the energy level scheme of this 0-dimensional system is included. This study emphasizes the need to check sample synthesis and sample purity, carefully monitor the integrity of starting materials, and check for incorporation of impurities during sample handling.